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22C H A P T E R 4 Utility Locating TechnologiesIntroduction âWhile it is hard to find a black cat in a dark coal bin by opti- cal means, it is easy to distinguish its furry outline from hard coal by touchâ (1). In other words, a property contrast is nec- essary to effectively differentiate an object from its environ- ment, as pointed out in the referenced NRC study. In utilities, property contrasts occur because the utilityâs material, interior product (such as gas or fluid), or backfill material is somehow different from the surrounding earth. Measuring electrical conductivity, magnetism, heat, mass, electrical capacity, and rigidity can detect property contrasts. Sometimes these con- trasts may be altered or enhanced through the use of conduc- tors, magnets, miniature transmitters, fluids, or gases to better differentiate the object from its environment. âGeophysical methodsâ describes the detection, imaging, and tracing of util- ities through property contrasts. Geophysical methods of identifying property contrasts are not foolproof, and the results often require professional inter- pretation in much the same way that medical electromagnetic imaging tools, such as CAT scans, MRIs, and X-rays, require a doctorâs interpretation. However, unlike medical imaging, where the aim may be to identify an objectâor organâ within organic material at a relatively shallow depth, imaging a utility may require looking through many feet of materials that are of varied composition, making the utility-imaging task more difficult than that of medical imaging in some ways. What follows are descriptions of some terms and methods common to buried utility detection, with indications of how the terms are used and how the methods are applied. Terminology The subsurface utility engineering profession has developed its own terminology over the past 25 years to address its particu- lar needs and those of designers. The Federal Highway Admin- istration (FHWA), the American Society of Civil Engineers(ASCE), and others have adopted this terminology, but the utility and damage-prevention communities have not. Thus, some terms differ in usage and meaning between the com- munities. It is important, therefore, to define how this report applies these terms, as opposed to how others might use them. Included in the title of this report is the term âlocating,â which has different meanings to different communities. To a subsurface utility engineer, âlocatingâ is the process of expos- ing a utility to precisely and accurately measure and document its three-dimensional location. To a contractor, âlocatingâ is the process of getting a utility owner or someone else to place a mark on the ground to prevent damage. For the purpose of this report, âlocatingâ is defined, as stated in chapter 1âs Back- ground to the Report, as indicating the determination of a util- ity position. The terminology used in this report is shown in Table 4.1. Geophysical Methods Geophysical methods for utility detection fall into one of two broad categories: passive or active. Passive geophysical meth- ods use energies produced by nature, society, utility struc- tures, or their products to detect contrast. Usually, passive detection instrumentation consists only of an energy receiver. For example, the passive method might detect the iron in a pipe, a buried magnet, the 60 Hz electromagnetic (EM) wave from an energized power line, the 87 Hz EM wave of the U.S. Navyâs submarine communications, or the noise produced from a leaking pipe as its contents escape through the leak orifice. Active geophysical methods, on the other hand, typi- cally use both a transmitting device and a receiving device. The transmitter produces energy that is broadcast into the ground or directly into the utility or its product through a physical connection. This energy interacts with the utility and the environment. The instrumentâs receiver detects the sub- sequent change in the transmitted energy. Examples of active geophysical utility detection methods include introducing
23Designating The process of using a surface geophysical method or methods to interpret the presence of a subsurface utility and to mark its approximate horizontal position (its designation) on the ground surface. (Note: Utility owners and contractors sometimes call this process âlocating.â) Locating The process of exposing and recording the precise vertical and horizontal location of a utility (or, see âdesignatingâ). The term is also used in a more general context within this report. Conflict analysis The engineering process of using a conflict matrix to evaluate and compare depicted designating infor- mation with proposed plans (highway, bridge, drainage, and other) in order to inform all stakeholders of potential conflicts, potential resolutions (including avoidance of utility relocations where possible), and costs to cure. Data management The process involving the physical surveying of designating and locating information for project planning and recordkeeping purposes and transferring it into a CAD system, GIS files, or project plans Minimally intrusive A method of excavation that minimizes the potential for damage to the structure being uncovered. Factors excavation method such as utility material and condition may influence specific techniques. Typical techniques for utility exposures include air-entrainment/vacuum-extraction systems, water-jet/vacuum-extraction systems, and careful hand tool usage. One-call notification center An entity that administers a system through which a person can notify utility owners and operators of proposed excavations. Typically, the one-call center notifies member utility owners that they may send records to the designer or designate and mark on the ground surface the existing indications of some or all of the utilities that may be present. One-call statute A local or state requirement that an excavator or designer of excavation call a central number to notify some or all existing utility owners of that planned excavation. State of the art The latest and most sophisticated use of equipment and procedures in regular commercial practice by at least one entity. State of the practice The most common level of equipment and procedures in regular commercial practice, on average, within the industry. Subsurface utility A branch of engineering practice that involves managing certain risks associated with utility mapping at engineering (SUE) appropriate quality levels, utility coordination, utility relocation design and coordination, utility condition assessment, communication of utility data to concerned parties, utility relocation cost estimates, imple- mentation of utility accommodation policies, and utility design. Subsurface utility engineer A person who by education and experience is qualified to practice subsurface utility engineering. Surface geophysical method Any of a number of methods designed to utilize and interpret ambient or applied energy fields for the purpose of identifying properties of, and structure within, the earth. Such methods typically include variants of electromagnetic, magnetic, elastic wave, gravitational, and chemical energies. Survey datum The points of reference used to define a specific geographic location in three-dimensional space. Test hole The excavation made to determine, measure, and record the presence of a utility structure. (Contractors may call this a âpotholeâ) Utility A privately, publicly, or cooperatively owned line, facility, or system for producing, transmitting, or distributing communications, cable television, power, electricity, light, heat, gas, oil, crude products, water, steam, waste, or any other similar commodity, including any fire or police signal system or street-lighting system. Utility attribute A distinctive documented characteristic of a utility that may include but is not limited to elevation, horizontal position, configurations of multiple nonencased pipes or cables, shape, size, material type, condition, age, quality level, and date of documentation. Utility depiction A visual image of existing utility information on project plan sheets or other media. Utility quality level A professional opinion of the quality and reliability of utility information. Such reliability is determined by the means and methods of the professional. Each of the four existing utility data quality levels is established by different methods of data collection and interpretation. Utility quality level A Precise horizontal and vertical location of utilities obtained by the actual exposure (or verification of previously exposed and surveyed utilities) and subsequent measurement of subsurface utilities, usually at a specific point. Minimally intrusive excavation equipment is typically used to minimize the potential for utility damage. A precise horizontal and vertical location, as well as other utility attributes, is shown on plan documents. Accuracy is typically set to approximately 0.6-in. vertical and to applicable horizontal survey and mapping accuracy as defined or expected by the project owner. Utility quality level B Information obtained through the application of appropriate surface geophysical methods to determine the existence and approximate horizontal position of subsurface utilities. Quality level B data should be repro- ducible by surface geophysics at any point of their depiction. This information is surveyed to applicable tolerances defined by the project and reduced onto plan documents. Table 4.1. Utility Locating Terminology (continued on next page)
24Table 4.1. Utility Locating Terminology (continued) Utility quality level C Information obtained by surveying and plotting visible above-ground utility features and by using professional judgment in correlating this information to quality level D information. Utility quality level D Information derived from existing records or oral recollections. Utility relocation policy A policy (typically of the project owner or utility owner) for the relocation of utility facilities required by the project. This policy includes but is not limited to establishing provisions for compensating utility owners; for removing and reinstalling utility facilities; for acquiring or permitting necessary rights of way at the new location; for moving, rearranging, or changing the type of existing facilities; and for taking necessary pro- tective measures. Utility search The search for a specific or unknown utility or utilities using a level of effort in accordance with the specified quality level, within a defined area. Utility trace The process of using surface geophysical methods to image and track a particular utility.sound into a water system through a fire hydrant, broadcast- ing a specific EM frequency into the ground, inserting a miniaturized transmitter into a sewer pipe, or introducing radioactive particles into a product line. Both passive and active geophysical methods allow us to distinguish an underground utility. They allow us to distin- guish but not to see; to calculate indirectly but not to measure directly; to infer the existence but not to confirm. These are important distinctions. The presence of a mark on the ground, made using geophysical methods, does not indicate with absolute accuracy the existence of a utility. Likewise, the absence of a mark on the ground is not assurance that there will be no utilities in that area. Currently, there are many commercially available instru- ments that use different methods to detect and trace utilities. Detecting a utility (that is, determining a contrast) and trac- ing that utility (that is, determining its direction and continu- ity) are sometimes accomplished through different methods. The effectiveness of the instruments employing these tech- niques varies as a result of the following: site geology, backfill type and homogeneity, utility material type, methods of and materials for joining/splicing utility sections, utility condition, depth, soil moisture, nearby buried objects, type of ground surface and its âsmoothness,â ambient noise and temperature, surface objects or site conditions, stray/interfering energies, and built-in equipment biases. In the subsequent sections, the principles and applications of various geophysical methods are described, and through a discussion of these methods, the complexity of the utility- detection tracing problem becomes apparent. Electromagnetic Methods The electromagnetic spectrum is broad, and a portion of it can be used for utility detection. Electromagnetic methods fall into four categories, each within a particular frequency range: visible light, radio waves, infrared, and X-rays.Visual Range This is the portion of the electromagnetic spectrum between 1014 and 1015 Hertz (Hz). Utilities that are above ground and not hidden by an opaque object can be positively identified. Evidence that a utility may exist includes a repair patch, natu- ral gas leakage âpinpointingâ bore, or vault cover. Utilities that are exposed through excavation can also be positively identi- fied. However, utilities hidden by an opaque surface, such as those below ground, behind a wall, or beneath a floor, cannot be visually identified. Radio Waves Radio waves are composed of frequencies ranging between 30 gigahertz (GHz) and 30 hertz (Hz) (see Table 4.2). For util- ity detection purposes, frequencies for commercially available equipment range from about 50 Hz to 1 GHz. Radio waves are a popular and versatile technology for looking into the ground. Pipe and Cable Locators (Time-Domain Electromagnetics) Pipe and cable locators are the most common instruments for detecting and tracing underground utilities. This equipment has many manufacturers and hundreds of separate pieces. It varies in antenna size, shape, and number; in the frequency and threshold of output; in types of attachments; in its depth mea- surements and current-flow direction indicating; in its signal strength displays; and in the shape, size, and weight of trans- mitters and receivers. The authors have witnessed firsthand instances in which instruments with identical frequencies, similar-looking antennas, and signal outputs were not equally able to detect a signal from a particular utility. The selection of the appropriate equipment is, thus, not a simple task based on easily defined characteristics. There are trade-offs with pipe and cable locators. Emissions are limited by Federal Communications Commission (FCC)
25Frequency Range Acronym Common Name Type of Instrumentation 30 GHz â 300 GHz EHF Extremely high frequency (microwaves) None yet 3 GHz â 30 GHz SHF Super high frequency (microwaves) None Yet 300 MHz â 3 GHz UHF Ultrahigh frequency Ground-penetrating radar 30 MHz â 300 MHz VHF Very high frequency Ground-penetrating radar 3 MHz â 30 MHz HF High frequency None yet 300 kHz â 3 MHz MF Medium frequency Pipe & cable locator 30 kHz â 300 kHz LF Low frequency Pipe & cable locator 3 kHz â 30 kHz VLF Very low frequency Pipe & cable locator, terrain conductivity meter 300 Hz â 3 kHz VF Voice frequency Pipe & cable locator 30 Hz â 300 Hz ELF Extremely low frequency Pipe & cable locator Table 4.2. Radio Wave Frequencies and Applicationsregulations. Lower-frequency locators can penetrate deeper into the ground, but more power is needed to do so. When the frequency is lower, the less that frequency will tend to excite other conductors nearby, increasing the likelihood of a correct interpretation. Less-conductive utilities need higher frequency to propagate over distance. However, with higher frequency, the signal will travel less distance. A larger homogenous con- ductor can be more difficult to detect and trace than a smaller one, since the radio waves spread out and travel on the pipe surface. A larger surface means more signal attenuation. A particular point along a utilityâs path will have a particular, most-efficient frequency. The most difficult conductive (metal- lic) utility to detect is typically a large and deep one with rela- tively poor conductivity (for example, a cast iron pipe with lots of nonwelded joints). All pipe and cable locators have one or more receiving antennas, and if the locators are active devices, they will also have one or more transmitting antennas. The size, shape, and type of antenna are directly related to its effi- ciency in receiving a signal of a certain shape and frequency. Antenna theory will not be discussed in detail here, other than to indicate the following concepts: â¢ The most efficient antenna for a cylindrical signal (as from a long straight pipe or cable) is a loop, the larger the better. â¢ Pipe- and cable-locating antennas are most frequently not loops but dipoles. â¢ Multifrequency instruments may compromise the efficiency of their antennas to receive/transit more than one frequency range. â¢ An underground pipe or cable network will act as a com- plex antenna, or group of antennas, in the ground. â¢ Radiation from one conductor can induce onto a nearby conductor, which will reradiate to another nearby conduc- tor and so forth, until the signal at the ground surface (nearthe transmitter) is no longer a good indication of the loca- tion of the initial conductor. It can be highly distorted, shift location, and dissipate. â¢ Each transmitter will have a distinctive shape and density of signal from its antenna; orientation of that antenna to a desired conductor is crucial. â¢ Antennas can be oriented to pick up a maximum signal (peak mode) or a minimum signal (null mode). When using any particular pipe and cable locator, there are three important performance elements. The first is get- ting the signal onto a conductor (either a specific one or one as yet undiscovered). The second is propagating the signal along the conductor to some point where the utility location is to be detected. The third is propagating that signal back through the ground and into the receiving antenna. Control over all three variables is initially available, but once a par- ticular frequency and power have been selected, the control over the signal propagation along the conductor (utility) ceases. Pipe and cable locators work in one or more of four distinct ways, each having strengths as well as weaknesses, and each a potentially useful tool for detecting and tracing a utility, depending on specific conditions. As it relates to conventional pipe-location equipment, a current is introduced onto a pipe or cable, and the currentâs magnetic field is measured at the surface through two detection methods. The first detection method uses passive utility detection. An antenna is used to detect a radio-frequency (RF) trans- mission source emanating from the utility without the use of a matching transmitter. There is no control over the signal strength or frequency. Some instruments detect only a small frequency band near 50 to 60 Hz. This frequency is found in underground power systems, and detection senses the load
26imposed on the wires by the consumer using the power. As the load changes, so does the signal strength. These changes can be used to advantage by a trained technician to determine the presence of more than one conductor in different but nearby locations. Additional passive power frequencies are used for cathodic protection, and there are some manufactur- ers with these available frequencies. A third source of passive signals is AM/FM commercial radio stations broadcasting in the medium frequency/high frequency/very high frequency (MF/HF/VHF) ranges. A fourth source is military communi- cations in the extremely low frequency (ELF) range. The second detection method uses active utility detection, and it comes in three varieties. The first turns the underground utility system into a broadcast antenna through a direct con- nection to the utility and to a grounding mechanism. This method will generally allow a maximum signal onto a particu- lar utility, at least at that initial contact point. Placement of the grounding mechanism is critical for maximum system isola- tion. Even with a perfect grounding location, the signal from one underground conductor can generate a secondary signal to other nearby conductors. The lower the frequency, the less signal âbleed-offâ will occur. The second uses a toroid clamp to direct a signal onto an exposed pipe or cable. A direct con- tact between the broadcast antenna and the utility does not occur, but the shape of the clamp isolates and directs the broadcast signal onto the clamped utility at that location. The third, and perhaps the most used, turns the geophysical instru- ment into a broadcasting station, similar to that of a radio or television station. The antenna in the transmitter broadcasts a radio wave. Signal density and shape is dependent on the antenna; therefore, propagation to underground conductors is highly variable depending on antenna placement, orientation, and the surrounding environment. The first pipe and cable locator of the transistor age had a single loop antenna in both its transmitting and receiving devices. It used a single frequency, the âtuningâ of which was based on finding a long continuous conductor in average soils. That frequency was about 150 kHz. Other instrumenta- tion quickly followed with different single frequencies both higher or lower than 150 kHz. It then became possible to connect the transmitting source directly to the underground utility at a surface feature, such as at a pipe riser or fire hydrant. This active mode came in two forms. One form used either a direct connection to the utility with a ground stake placed somewhere to complete a circuit or a toroid clamp antenna placed around a pipe or cable, as long as the pipe or cable was grounded from the toroid clamp in both directions. Small waterproof transmitters were devel- oped that could be sent inside sewer pipes or plastic conduits. A passive receiving device was developed to use commercial AM/FM radio station broadcast signals, some of which could be induced into long conductors in the ground.Antenna shapes became varied, and dipole antennas became popular because of their directionality and size. Eventually, some manufacturers began to use two receiving antennas to indicate to the operator that the signal peak was to the right or left. This was a great development that allowed for the obser- vation of signal symmetry, perhaps one of the most critical interpretation factors. A significant development occurred in the 1980s when multiple frequency transmitters and receivers in a single unit became available. A single piece of equipment could now have the frequency advantages of two or more separate pieces of equipment. Additional features such as signal strength readouts, depth estimates, and current direc- tion followed. Now there are many instruments available in the general fre- quency ranges of 60 Hz, 512 Hz, 1 kHz, 8 kHz, 29 kHz, 33 kHz, 80 kHz, 83 kHz, 110 kHz, 250 kHz, 300 kHz, 480 kHz, and so forth. It may be necessary to have equipment in all of these fre- quency ranges to effectively detect and trace a particular util- ity, although in some cases, a single frequency might be all that is needed. It depends on factors previously noted. Pipe and cable locators are suitable instruments for both detecting and tracing utilities, depending on frequency and method of signal generation. This can be effective up to a depth of 20 ft, although, more frequently, the maximum depth for effective detection is somewhat less. High-power sondes (trans- mitters) that can be placed into and pulled through a utility may increase the effective depth of detection to 50 ft or more, although the chances that the peak surface will be directly over the utility decreases with depth. Depth estimation with pipe and cable locators is possible, but it is fraught with potential for error due to site conditions. Under ideal conditions, such as a single conductor in homoge- neous soil with a recently calibrated instrument, depths can be quite precise and accurate. Some manufacturer specifications state 2.5% to 5% accuracy to a depth of 10 ft. In the congested utility arena of an urban or suburban street, depth estimations are frequently wrong by a significant amount. Depth estima- tions can also be useful for determining signal symmetry. To summarize current practice, competent use is made of a wide variety of instruments covering the broadest possible frequency spectrum, types and shapes of antennas, and acces- sory features such as signal strength meters, depth estimation, and current flow direction. A wide variety of direct coupling methods, with extended grounding wires, multiple ground- ing systems, and remote pipe attachment devices, including clamps, magnets, and spikes, are in use. Toroid clamps, with nonconductive extension poles are used, as are composite core reels for metallic insertions into nonmetallic pipes or conduits. Sondes of many varieties are available. Confined- space entry equipment, with dewatering pumps, is available. Crews are used for confined-space entry and electromagnetic âsweepingâ of the defined area. Procedures are available to
27maximize utility detection and tracing, and for safe opening of utility appurtenances. Practitioners employ several pieces of equipment with direct connect wire and toroid clamps. Terrain Conductivity (Frequency-Domain Electromagnetics) Another utility detection method in the radio wave range is that of terrain conductivity (TC). TC measures the average electrical conductivity of a cone-spaced volume of earth beneath the transmitting and receiving antennas. When less of a utility is within a cone, there will be less of an effect on the average resistivity. Maximum depth penetration of the bottom of the cone can be as deep as 150 ft; however, greater depths require greater antenna spacing and power and decrease res- olution. Utility detection thresholds are currently limited by manageable antenna spacing and the resolution required for utility detection to depths of 20 ft. Success is more often lim- ited to the first 10 ft of cover. Factors affecting ground conduc- tivity include earth materials, such as rock and soil, and the water and its solutes in the interstitial pore spaces. Most rocks and soils have high electric resistivity. Most of the water and solutes have low resistivity. Utilitiesâ resistivities can range from extremely low (metallic) to very high (large empty clay pipe). Attempts to look for contrasts between the utility and the earth can vary from highly effective, to somewhat effective, to impos- sible, depending on the type of utility and the soil. In the northern United States, road salt placed on the pave- ment for deicing purposes is absorbed into the soil around the roadway, increasing the ground conductivity. This can make detecting a metallic utility difficult, but conversely it may make detecting a nonmetallic utility easier. In areas that have high moisture or a high water table, it may be impossi- ble to detect any kind of utility, unless it is watertight, empty, large, and relatively shallow. Regardless of the conditions, to be adequately interpreted, there must be sufficient collected data with different antenna orientations or within a tightly spaced grid search pattern to clearly identify an anomaly that a utility might produce. Some utilities lend themselves to detection by TC methods. Tracing is more difficult than detection and requires massive amounts of data. TC methods may be applied to trace septic systems, especially in dry soil. The moisture and chemical composition from the septic waste might produce a detectable anomaly, as might the gravel bedding or metallic drain pipes. TC methods might also trace buried tanks, well shafts, or vault covers. Since TC instrumentation, which includes the receiver and transmitter, is basically carried by one person, it can be an effective detection or search tool to find metallic utilities in a noncongested, dry environment. However, in general, inter- preting TC data is much more complex than interpreting data from pipe and cable locators. Although data interpretationcan be performed remotely by others, accurate positional data must be available at all times during the data collection process. Data loggers linked to GPS help reduce this problem. Currently, depth estimation with TC methods is not consid- ered realistic. Competent practitioners select frequency, search and trace techniques, and survey methods on appropriate projects after review of records and site conditions, but this method is rarely used. Ground-Penetrating Radar (GPR) With perhaps the best-funded ongoing research, GPR is another geophysical electromagnetic tool in the radio wave range. Use of GPR research for utility detection began in the 1960s with the advent of plastic gas pipes. Since then, hundreds of millions of dollars have probably been poured into GPR research. Early versions of this tool required a full-time repair technician, expensive âthermalâ paper, much power, long cables, and a highly experienced geophysicist to interpret the data. Initial equipment costs were over $50,000. The results of these early GPR efforts were disappointing but showed prom- ise. Now there are many commercially available GPR devices. They are easier to use and interpret than ever before, and they are coming down significantly in price. GPR works by sending an electromagnetic pulse into the ground. For utilities, the frequency range is typically between about 50 MHz and 500 MHz. Some ratio of this pulse signal is transmitted through boundaries and some ratio is reflected from the boundaries back to the receiving antenna. The boundaries are formed as a function of a particular particleâs dielectric properties. Overall propagation into the ground is a factor of power, frequency, and soil resistivity. The FCC limits the power, and for each base GPR unit, the manufacturer selects the frequency and matches it to a particular antenna. Manufacturers offer several antennas, each at an extra cost and possibly requiring additional hardware and software. The operator cannot change the soil resistivity or the dielectric con- stant of the particle boundaries. In some ways then, GPR seems easier for an untrained technician to use in the field than pipe and cable locators, because GPR introduces fewer variables for the equipment operator to address. However, referencing the machineâs location becomes critical, and data interpretation becomes more difficult and time consuming, but attempts are being made to overcome these challenges to using GPR. With GPR, detection occurs when the utilityâs dielectric constant differs from that of the surrounding soil. A dielectric constant that differs significantly from the soil around it would produce the best reflection. This occurs, for instance, when the utility is metallic and the ground is dry sand. Although pipe and cable locators might detect the same utility faster and cheaper and the result would be easier to interpret, GPR may perform
28better in those situations in which the utility is metallic but its joints are not, precluding a good circuit for the pipe and cable locator method. A small clay sewer pipe is not much different in dielectric constant than the surrounding soil. If the pipe is empty, a reflection may occur at the pipe/air interface. If the pipe is filled with water and the surrounding ground is satu- rated, there will be little or no differentiating reflection. However, detection of a buried utility is constrained by the signal wavelength-to-pipe cross-sectional size ratio. This means that the smaller the utility, the higher the frequency needed to image it. Therefore, identifying a small utility becomes increasingly difficult the deeper it is placed. The diameter-to-depth ratio of a single fiber optic cable that is sur- rounded by soil with similar qualitiesâfor instance, if it has no metal, very small air space, or plastic sheathingâmakes such a cable virtually undetectable using GPR or anything else. A rule of thumb for the current technology in practice is that, under ideal conditions, a 12 to 1 depth-to-diameter ratio pro- vides reliable utility detection down to the first 6 ftâthat is, a 1-in. utility at a 1-ft depth, or a 3-in. utility at a 3-ft depth. Competent methodologies may improve this ratio. However, tightly spaced pavement reinforcing steel will effectively stop the penetration of any signal. Road deicing salt, which increases the conductivity of soil near roadways, may do the same. A very rough surface may create too much signal noise, effec- tively drowning out any signal. Highly conductive soil, as in areas with high iron content or in prepared roadway base material, will cause depth penetration problems, and utilities under salt water are virtually impossible to image, unless they are so large that a sufficiently low frequency can be used to penetrate the water. Such interferences preclude the imaging of any utility in cer- tain conditions. In fact, one of this reportâs authors observed Virginia marine clay preventing several commercially available GPRs from imaging a metallic storm-drain pipe that was 1 ft in diameter and less than 8 in. below ground at one site. At another site, GPRs failed to detect the signals of every buried utility. Some parts of the country have great success with GPR imaging. Florida, with its dry nonconductive sands, is an ideal setting for GPR. In Bellevue, Washington, a recent GPR survey detected about 50% of the utilities known to exist, but in nearby downtown Seattle, the success rate dropped to below 5%. GPR has been oversold in the past. GPR is expensive ($10,000 to $100,000, including integrated GPS and associated computer programs), and as demonstrated, its value may vary significantly depending on the surrounding conditions. Yet, GPR has several advantages over pipe and cable locators. Its biggest advantage is that it can detect nonmetallic utilities. A second advantage is that, even if the utility itself cannot be imaged, GPR can sometimes detect the sides or the materials of the trench in which the utility was placed. A third advantageis its depth determination. The radar data display is directly proportional to the electromagnetic waveâs speed in the soil. Given that the soilâs properties are relatively uniform and con- sistent in relation to wave speed, the depth of the utility can be easily measured. With a few test holes to calibrate wave prop- agation speed, the depth determination can be quite accurate and precise, as opposed to the variability and unreliability of the pipe and cable locator methods. Opinions vary widely on how useful GPR is as a utility detec- tion tool because of how disparate the success can be depend- ing on the geological factors at play. Those who have seen GPR work in one place but are not trained in the toolâs physics may believe it should work everywhere, but it will not. For others, GPR failed to work once, so they will not try it again. And while GPR is a good search and a good trace tool, the GPR instrument must be pulled along a grid pattern while data is collected, unlike pipe and cable locators that produce a continuous EM signal output. It is important to keep the grid spacing small to collect enough data to âconnect the dots,â especially in a utility-congested environment with many linear changes in utility direction. Grid spacing should not exceed the width of the antenna size or else there may be gaps in the data, and gaps in the data invariably result in guesses, which can lead to errors. Such grid spacing issues get insufficient attention from GPR users. Obstacles in the survey area can also present a significant challenge that could result in incomplete data. Recent research advances in data processing, GPS integra- tion, laser-based referencing, data migration, multiple antenna arrays, stepped frequency capabilities, and image recognition software are in commercial development. These technological advances will help address GPRâs challenges, but they will not turn GPR into a total utility-detection and -tracing solution. The current state of the art has competent practitioners review each project site for adequate soil conditions and employ GPR when it is suitable. They use multiple frequencies and use GPR in conjunction with other techniques. A site- appropriate survey and data referencing methods are selected. Data is collected in closely spaced parallel profiles and com- bined in a 3-D volume of data for postprocessing and time- or depth-slice interpretation. While GPR is still rarely used for conventional locating, it is becoming more common as equip- ment costs drop and ease of use improves. Infrared Electromagnetic Waves (Heat) Some utilitiesâincluding steam pipes and sanitary sewersâ have operating temperatures distinguishable from the temper- ature of the surrounding soil. If the utility is shallow enough or if the temperature difference is large enough, a temperature differential can be detected at the surface using an infrared camera. In fact, after a snowfall, a utility line can sometimes be detected by the difference in the speed with which the
29snow on the ground above it is melting because of the tem- perature difference. Some utilitiesâincluding steam lines, energized power cables, sanitary sewer lines, and industrial process linesâgive off significantly more heat than others, but the deeper a util- ity is buried, the less chance there is that the heat signature can be detected at the surface. Infrared methods are difficult to use and interpret in a congested urban environment with lots of cement paving. Solar gain during the day heats the ground to the point where infrared is mostly useless. Pavementâs large thermal mass retains daytime heat, spreads it out, and releases it uni- formly at night. Climate, site, geology, and utility conditions must be just right to make this technique work effectively with current technology to locate utilities. Infrared has no depth-estimation capabilities. The current state of the art employs infrared cameras or thermistors for very specific situations, but the infrared method is rarely used. Resistivity Measurements Resistivity measurements are taken by injecting a direct current (DC) into the ground using two or more electrodes, measur- ing the resultant voltage at other electrodes and calculating the average resistivity. The spacing of electrodes controls measure- ment depth. The many different types of electrode geometries each produce specific results. The detection is that, if enough data is collected, a utility with a resistivity different from that of the surrounding area will show up as an anomaly. Resistiv- ity may be useful as a search technique but not as a trace tech- nique. Data setup and collection is cumbersome and not easily delegated to a technician. This method is rarely used. When it is used, it is most often employed for an unrelated reason, with the advantage that it can also detect the presence of a utility. Magnetic Methods Iron is a material commonly used in pipes. The magnetic properties of iron or nickel can be used to detect and some- times trace an iron or steel pipe. There are two general types of magnetic surveys applicable to utility detection: total field and gradient. A total field survey measures magnetic-field sources at the ground surface. Because the magnetic field of a pipe is typi- cally small and hard to interpret, total field magnetic surveys are rarely used as a utility location method. Other stronger magnetic-field sources include the earthâs internal magnetic field (caused by convection fields in the earthâs rotating liq- uid outer core), its external fields (caused by electric currents in the earthâs ionosphere), and small magnetized materials inthe earthâs crust (such as rocks, soil, and man-made, placed objects). In general, total field surveys are usually used for environmental surveys. Occasionally, an anomalous reading is caused by a pipe. The gradient survey method, which uses gradiometers, has the best application to find buried utility objects. With the gra- dient method, a single instrument is used to cancel the effects of internal and external magnetic fields through the placement of two total-field sensors within about 20 in. of each other. In the absence of a nearby source of iron, these sensors are in bal- ance. As the detector moves closer to a magnetic object, the shape and intensity of the magnetic field causes an imbalance in the sensors. This imbalance creates a reading that the equip- ment operator can interpret. The buried pipeâs magnetic field is very weak compared to the total field. The buried pipeâs field contribution decreases rapidly as the distance between the sensors and the pipe increases. Pipes that are more than several feet below the sur- face will be difficult to detect unless they have a very high ini- tial magnetic strength. Initial magnetic strength is related to object shape, internal structure, purity of material, and the objectâs location on the earth during manufacturing. The field technician has no control over these factors. As a result, some utility-related iron-bearing structures, such as valve boxes, manhole covers, septic systems, magnetic âmarkerâ tanks, wells, Parker-Kalon survey nails, and iron casings, can be found more easily than others. It is generally easier to detect a vertical linear structure than a flat round horizontal one. Depth estimation is not possible with magnetic methods. The current state of the art for the gradient survey method uses gradiometers extensively in grid searches to discover iron-bearing structures. Total field methods are rarely used as a primary utility detection tool. Elastic Waves (Acoustics/Sound/Mechanical) When pipes are nonmetallic and a metallic conductor cannot be inserted into it, elastic waves may be used to detect and trace the pipes. An elastic wave requires the creation of an initial energy input, after which the wave travels through the medium until all the energy has been transferred. The ground, the pipe, or the pipeâs product may act as the medium. In general, the more noncompressible (rigid) the material, the less the wave attenuates over distance. As with EM methods, there are three aspects of the elastic wave propagation and detection. The elastic wave must be introduced into the propagating medium, travel through it, and be detected after its travel, including any reflections or refractions that occur due to buried structures. Several factors can be controlled when the wave is introduced, including the waveâs rough frequency range and the method of coupling the wave generator to the surface to be vibrated. Once a frequency
30is selected, wave propagation through the earth or the utility is beyond the technicianâs control. Receiving the elastic wave is also largely out of the technicianâs control, although various types of ground surfaces can enhance or decrease the signal. For instance, detecting the signal over a concrete surface is usually easier than over a soft dirt surface. There are three basic techniques for using elastic waves to image utilities. They are seismic reflection, seismic refraction, and acoustic emission. Many studies have been performed to assess the applicability of seismic reflection and refraction as utility-imaging techniques. So far, these techniques have only been useful under specialized conditions and with rigorous procedures, because most utilities are too small for detection by the large wavelength of seismic (acoustic) waves. There are no commercial applications of these two techniques for pipe and cable location at this time, although acoustic pipe location equipment developed by the Gas Technology Institute is near- ing the commercialization stage, as discussed in chapter 6. The discussion that follows is limited to acoustic emission. This method is fairly standard for tracing nonmetallic water lines, but it is relatively useless as a search technique. Acoustic emis- sion has no depth-estimation capabilities. Acoustic Emission A pipe under rapidly varying mechanical stress may deform and generate noise. Various transducers, which are linear acceler- ometers that translate motion into electrical signals, can mon- itor this noise, or acoustic emission. The premise is that the noise will be loudest directly over the pipe, because the elastic wave will have traveled the shortest distance at this point and less signal attenuation will have occurred. How- ever, the type of surface (for example, soil or concrete), fill (rock or clay), compaction, ground moisture, and so forth may distort the noise distribution. There are three methods for using acoustic emission techniques, all of which are sus- ceptible to interference from noise, such as that produced by aircraft, automobiles, trains, and electrical transformers. It is a good trace technique, but it is not an effective search tech- nique, and access to the utility system must be available at one point. Active Sonics This method involves inducing a sound onto or into a pipe, which can be accomplished by striking the pipe at an exposed point or by introducing a noise source into the pipe. The noise source may be pulled through metallic, nonmetallic, empty, or filled pipes, or it may be carried by a tractor device, thereby getting the sound closer to the detection point. By marking or measuring the loudest points at the ground surface, the utility may be traced. A linear accelerometer, which is basically anamplified stethoscope, may also be used to detect sound at the surface. Active sonics is employed when a manhole cover is struck with a hammer to introduce sound waves into both the air inside the manhole cover and the pipe itself. If there are multi- ple manhole covers at a remote location, it may be possible to tell which cover is relatedâor directly attached via the pipe network or the air inside the pipeâto the one being struck by listening to the sound. Similarly, in an area where there is an exposed pipe and it is desired to know if it is the same pipe entering an adjacent basement, the pipe can be struck with a hammer and the sound that is carried along the pipe can be detected in the basement. Direct access to these utility struc- tures makes the job easy, but detecting the resulting sound through the ground becomes more difficult, so sound amplifi- cation devices are used. Most of these devices were developed for the water leak detection industry but are now used by the utility detection business as well. Passive Sonics A second sonic method relies on the ability of the pipeâs product to escape. This method is sometimes known as pas- sive sonics. For instance, water escaping a pipe at a hydrant or service petcock (or at a leak) will vibrate the pipe. This vibration will carry along the pipe for some distance before attenuation. Factors such as product pressure, shape and size of orifice, and type of pipe material will affect the initial sound generation. Pipe material, surrounding material, compaction, and product will affect the distance the sound travels along the pipe. Factors already mentioned affect the sound detection between the receiver and the pipe. There are several commercial manufacturers for devices using these principles. Detection over the pipe is again made with an amplified device that typically provides a sound and visual reading of signal intensity. In ideal conditions of low ambi- ent noise, shallow pipes, smooth rigid ground surface, and high fluid pressure, detection of the pipe can occur for per- haps a thousand feet, at best. Under normal site conditions, several hundred feet is more typical. Resonant Sonics A third sonic method relies on the pipeâs product being a noncompressible fluid (water in most cases). Interfacing the fluid surface (at a hydrant, for example) and generating a pres- sure wave in the fluid will create detectable vibrations in the pipe. It is possible to tune the oscillatorâs frequency to one (or more) of the resonant frequencies of the pipe, usually result- ing in more tracing distance. A disadvantage is the need for many different types of fluid/oscillator interfaces. Utility-joint damage is possible, so wave intensities are generally small,
31decreasing tracing distance. There is at least one commercial manufacturer of equipment using this technique. Summary of Elastic Waves All three methods of elastic waves may be used when necessary, but the methods are rarely used in normal circumstances. Borehole Geophysics Bringing the signal generator or signal detection device closer to the utility can enhance the aforementioned methods. One way to do this is to bore holes in the ground that allow for sig- nal generation and detection closer to the utility of interest. Boreholes offer great promise not only for radio waves, mag- netics, and elastic waves but also for X-rays. Boreholes make it possible to get a better signal for propagation onto the utility and to use the âshadowâ of the utility as a detection method. Radiation is an issue, and X-rays are not viable unless a detec- tor is located on the other side of the structure from the gener- ator. The ground may be used as a natural shield, and the shadow of a utility between two boreholes, or nonmetallic sewer pipes, may provide a point of focus. Remaining issues include damage caused by insects and small animals living in the ground, potential state licensing for X-ray technicians, and safety or regulatory challenges. Nuclear soil-density gauges are commonly used in geotechnical practice, but as regulation of these gauges increases, alternative methods of inferring soil density and compaction are being developed. There is no known commercial X-ray research under way for imaging buried utilities. There are many commercial applications of borehole geo- physics for environmental engineering. Some of these seem to be adaptable for utility detection. This method is a better search technique than a trace technique. Boreholes raise the issue of potential damage to existing util- ities. Obviously, the smaller the hole, the less chance for dam- age. Air- and water-vacuum devices and microdirectional boring devices may be useful as compared to traditional hole- boring machines. Horizontal boreholes from right-of-way line to right-of-way line might be useful for horizontal imaging. Vertical boreholes might give a better utility-depth estimate. At a state-of-the-art level, there are occasional uses of air and vacuum devices for creating boreholes to determine the exis- tence and horizontal location of a utility, but these techniques are not used in general practice. Microgravitational Techniques In theory, microgravitational techniques may be used to detect extremely large, predominantly empty utilities or tunnels. The concept is that the expected gravitational force at a givenpoint on the earth can be calculated. This gravitational force is directly related to the effects of mass. If a large utility or tunnel is empty, the empty space has much less mass than if it is filled with product. The survey must be precise because of the small values being measured. Nearby sources of above-grade mass must be addressed, as well as regional effects and the move- ments of celestial bodies. Elevations must be determined to millimeter accuracies. Obviously, this procedure is time- consuming and expensive, but useful results might be obtained in certain favorable cases (2). The method is theoretically pos- sible, but there are few practical applications. This is not used except in rare cases. Isotopic (Radiometric) Techniques A utility or the area immediately surrounding it may be detected through scintillation or Geiger counters if either is carrying or has been contaminated by uranium, thorium, or other radioactive compounds. Isotopic techniques would be very effective, if not for the health, safety, and permitting issues that preclude its use in generally uncontrolled environ- ments. This could work as a search or trace technique. This method is theoretically possible, but it is not used in practice. Chemical Techniques Chemical detection may be employed as a search technique, but it is rarely employed as a trace technique. The concept is that products conveyed in pipes, left near pipes following construction techniques, or outgassed from the plastic pipe product may exhibit a chemical signature that can be detected. For example, natural gas that is leaking through pipe joints or other breaks in the pipe can be detected with flame ion- ization or photo ionization techniques. Natural gas leaks also affect vegetation and soil in observable ways by displac- ing oxygen. Trained personnel may be able to use this vege- tation damage, as well as detection of an introduced odorant, as an indication of natural gas piping in the area. The state of the art of this approach remains theoretical for general utility detection practice, but the approach is typically only used by natural gas companies as part of their leak detection operations. Data-Processing Techniques Many of the methods that have been mentioned can be com- bined with data-processing techniques and mathematical algo- rithms to enhance results. Data processing can range from traditional practices, such as manual data graphing, to complex algorithmic formulas linked to graphical outputs. However, caution must be exercised because data interpretation in the office rather than during field investigations does not allow
32interpretations to be immediately cross-checked through other field measurements. Geophysical diffraction tomography (GDT) is a specific data-processing technique based on the principles of optical holography. It can be used with sound waves and electromag- netic waves in a variety of data collection geometries and techniques. Surface and borehole methods are used with ground-penetrating radar, seismic reflection, and offset ver- tical seismic profiling. This technique requires a large amount of data collection and data manipulation with, generally, pro- prietary algorithms. In the current state of the art, data is collected from mul- tiple antennas at different frequencies, integrated with GPS locational information, and processed with algorithms to pro- duce 3-D displays of underground utilities. The approach is being used by a few firms on specific projects. Its use is expected to grow. Marker Methods The most common type of utility marker is that of a paint or chalk mark on the ground. The mark is usually temporary and serves an immediate damage-prevention, repair, or attach- ment purpose or else it marks a location for a subsequent sur- vey. In some limited instances these marks are made using more permanent means, but concerns over aesthetics, secu- rity, and mark maintenance usually preclude this practice. These temporary marks are well understood and will not be discussed further. The concept and practice of emplacing permanent visible markers, tracer wires or tape, buried magnets, or other buried detectable devices has been around for decades. As the tech- nology has advanced, so has the sophistication of the marker devices. Currently, a variety of methods are in use. All these methods have a distinct drawback: a marker is not part of the utility structure; therefore, over time, the markerâs location may no longer be indicative of the utilityâs location. Visual Markers Utility Signs/Pipeline Markers Some utility owners place aboveground signs near their facil- ities. Owners of high-pressure gas transmission lines, major water pipelines, and fiber optic lines have been marking their facilities in this way for years. These methods have the dis- advantage of indicating the utility only at a single point. If multiple markers are visible, a general trend of the line may be apparent. Markers are generally installed for warning pur- poses; therefore, they may be placed in a location most visible to others rather than directly over the utility. Markers may be removed and replaced in another location by vandals, mow- ing crews, and others. These markers serve as a good detectionmethod but not a trace method. Examples include surface- marking posts and horizontal surface decals or curb markers, marking posts with readily accessible connections to locating wires, permanent magnet markers, and marker panels. Flush Markers The traditional means of identifying the location of buried airport facilities by the Federal Aviation Administration has been to place 2-ft Ã 2-ft Ã 6-in. concrete markers flush with the ground, immediately above marked features. These heavy markers cost about $100 each and require painting as well as ongoing attention to remove grass and repair soil erosion. Mowing equipment can easily displace the markers, which can compromise facility records and excavation accuracy (3). Surface-to-Structure Markers Utility owners in the San Antonio District of the Texas DOT use a marker that is embedded in the soil from the ground sur- face down to the utility. They emplace this marking system on a case-by-case basis, usually when data on the depth of a util- ity structure is requested by the DOT. Excavation is done by hand to expose the utility, and a 2-in. PVC pipe is then placed in the excavation, and dirt is backfilled around it. This gives future personnel direct access to the utility. They can measure the depth and record an elevation. They typically place a cap over the open end of the PVC pipe. This method does raise issues of future utility integrity and security. Parker-Kalon Nails/Survey Markers Since 1981, subsurface utility engineers have been placing PK nails, hubs, and lathes or other semipermanent markers directly over a utility after a test hole has been excavated. These markersâ locations are recorded and referenced to the utility beneath it. Some engineers use unique markers that display other information, such as company name, type of utility, test- hole number, and depth. Lately, security issues are being raised over display of this additional information. Continuous Buried Markers Tracer tapes and wires are sometimes placed in the backfill near newly constructed nonmetallic water and gas lines. The tapes and markers are generally metallic and exposed at a meter or service riser. They can serve two purposes: warning an excavator during construction that a utility is nearby and providing a means to use a pipe and cable locator to trace the marker, since the utility itself is difficult to detect. Tracer tapes sometimes are color coded and may have other information written on them, such as the utility owner, utility
33type, or just a cautionary message. A disadvantage of a tracer tape is that once it is broken, it is difficult to splice the tape back into a continuous conductor, rendering future detection by geophysical means highly improbable. Tracer wires have the advantage of being easily spliced if broken. In actuality, excavators rarely perform this practice, and its problems are the same as those of tracer tape. Because they are small (usually #12 coated wire), tracer wires do not have other sources of information imprinted on them. Tracer wires or tapes are not necessarily placed directly over the utility. Utility construction methods often involve a large backhoe-excavated trench. Utilities can be on one side of the trench and the tracer on the other. Also, if the utility is exposed during a future excavation, it is not guaranteed that the tracer will be replaced in its original position. In some cases, the tracer can even end up below the utility it is intended to mark. These are some of many reasons why, despite the value of a tracer wire or tape, the signal received and inter- preted at the surface may not be accurate as to its representa- tion of the actual utility location. Initial attempts to solve this problem involved wrapping the wire around the pipe. This was found to cause plastic pipe to melt or to introduce an explosion hazard because of nearby lightning strikes, so this practice is no longer used by the gas industry. Fiber optic conduits may have trace wires emplaced directly within the conduit, along with the fiber optic line. New methods for implanting magnetic markers into pipe- lines or warning tapes are being investigated. Although a patent was granted for plastic gas pipes (U.S. Patent 5,173,139), such pipes are not yet in commercial use. Single-Point Buried Markers Magnets have long been used to indicate utility structures. Gas distribution companies may place a small magnet in its plas- tic curb box structures so that maintenance crews can find the curb box if it gets covered with soil or vegetation. For several decades, small magnets have been placed in the roadway ma- terial directly over a utility when exposed by subsurface utility engineers, allowing a standard gradiometer to be used in the future to find the magnet. In the 1970s, a high-gauss magnet was developed that could be placed 6 ft into a utility excava- tion for future detection. Magnetic orientation was crucial to receive a maximum signal at the surface. Passive Electromagnetic Balls These 4-in. diameter plastic balls with an embedded sonde were developed in the 1980s. The concept was that these balls would be placed at strategic locations during the installation of new utilities, such as at future sewer line connections and at stubs of gas and water services. The balls required a matching receiver and their battery life was limited, but the intent wasfor these balls to only last several years until utility service was connected. These devices were soon replaced by passive markers, which act as passive antennas, reflecting the query signal from the sig- nal source without need for an internal power source. They are not affected by moisture, minerals, chemicals, and temperature extremes. Internal components are self-leveling, ensuring that they will always be in a horizontal orientation for best signal strength, regardless of how the device is placed in the ground. Some manufacturers have developed marker balls with specific frequencies so that a signal reception would indicate a partic- ular utility. This requires different receiving antennas, but it provides utility-owner specificity. These are generally in the high frequency (HF) or ultrahigh frequency (UHF) range. Radio-Frequency Identification Tag or Balls New radio-frequency identification (RFID) technology is rap- idly developing as a means to both locate utilities at a specific point and characterize utilities. Active markers can be given unique preset identification numbers. The placement crew can also program them with utility information. Color cod- ing the markers also provides visual differentiation. Using this method, it is possible to quickly find a buried marker at a later date using a surface scan, verifying the utility details contained in the RFID tag, and then using a localized excavation process to physically expose the utility, if necessary. Summary of Marker Methods Physical utility marking can be an important tool to reduce damage in future excavation activities. Either during construc- tion or in the future when a utility is exposed, a physical marker can be installed. Physical markers can indicate the proximity of a utility line by using stakes or buried marker tapes. Electro- magnetic markers or identification systems can employ passive marker balls or RFID markers under development, or they can be conductive wires or tapes that allow electric connections to provide an EM locating signal from the wire or tape. The wire or tape provides an EM signal path for nonconductive pipes or telecommunication fiber-optic cables, and they can be installed either within the conduit or immediately above the pipe. RFID methods are still in development, although they have been in field use for several years. They show strong potential for application to utility locating and characteriza- tion problems. Summary of Geophysical Methods Research by United Kingdom Water Industry Research (UKWIR) in 2000 and 2001 concluded that, at best, existing technologies had no better than a 50% success rate in identi-
34fying buried assets. The report concluded, âThese trials show the need for substantial improvements in equipment performanceâ (4). The numerous locations where utilities are exposed and where detection signals or aids can be introduced means that a better performance can be achieved when using all the avail- able commercially developed tools that have been mentioned. In other words, with the appropriate time, money, equipment, and training, a majority of existing utilities within our trans- portation rights-of-way can be detected and traced. The tech- nological challenges are (1) to more cost-effectively locate, characterize, and manage utility information for transportation projects and (2) to improve detection in difficult circumstances. Ongoing private and public research efforts are incremen- tally improving our abilities in detection, tracing, and data interpretation, and much of our existing technology can be made more user friendly. Examples of potential improve- ments may include more variety in pipe and cable locator antennas, such as making them detachable and interchange- able; better and more adaptable direct connection devices for the wide variety of sizes, shapes, and limited space access of todayâs utilities; better ways of introducing conductors into nonconductive conduits or pipes; and easier methods of gen- erating sound, such as through internal pipe/conduit travel and advanced sound-pickup devices with better ambient noise filtering. Single-platform multitool devices, better sig- nal processing algorithms, target recognition patterns/artificial intelligence, signal symmetry indicators, and automatic and user-controlled signal gain would also improve abilities, as would better ways to excavate a borehole and development of miniature or shaped transmitting antennas for signal gener- ation and reception in a borehole. Excavation Methods of Locating Utilities Individual geophysical methods are insufficient to accurately determine the three-dimensional location of a utility. As has been discussed, some methods can give a depth estimate; when conditions are ideal, those estimations may be some- what accurate. Although horizontal estimations are generally more reliable because most measurements are in the horizon- tal plane, they too can be inaccurate due to the congestion of the underground site, as well as other factors. Conduit encase- ment limits are not detectable through most EM methods. This leads to excavation as the most reliable method to accu- rately locate an underground structure horizontally and ver- tically. Once exposed, a utility can be measured and referenced to other features or survey control. Excavation methods vary, but they all bear some risk of damage to the utility being sought or to an unknown utility. The two best-known methods for limiting the potential fordamage are air/vacuum excavation and water/vacuum exca- vation. Both methods use a powerful vacuum to remove soil from an excavation. The difference lies in the method of loos- ening the soil before removal. Both methods require pavement or concrete at the ground surface, or deeper if old roadways are submerged, to be removed through traditional means, such as jackhammers, rock drills, or concrete saws. For extremely shallow utilities or utilities embedded in the roadway, there is always a higher risk of damage. Water excavation methods offer a much greater force to break up the existing soil, and excavation is usually faster than with air. The disadvantage is that a greater force can more readily damage the wrappings and coatings on cathod- ically protected gas lines. A less-understood disadvantage is that of subsequent soil compaction and paving integrity. Water-saturated soil is typically not suited for backfill. Even if the saturated soil is not used as backfill, the soil surround- ing the test hole may be disturbed by the water saturation, which may in turn lead to ground settlement. A third dis- advantage is that of cathodic cells in the soil. Introducing moisture around a pipe where moisture may not typically exist can change the cathodic currents in the ground, leading to an increased risk of corrosion. A fourth disadvantage to water excavation is the operational difficulty that arises when the air or ground temperature is below freezing. Air excavation methods are more labor intensive and time- consuming than water excavation methods, and coatings and wrappings can still be damaged. But one advantage of the air method is that the material removed from the test hole can typ- ically be used to backfill the test hole. Site cleanup is usually easier to accomplish. All excavation methods are encountering a relatively new problem. Controlled density fill (CDF, or flowable fill) is a material some municipalities and utility owners use as an eas- ier way to backfill trenches and other excavations. CDF serves the secondary purpose of getting rid of select waste material, such as fly ash, that is produced by power and sewage plants. When mixed and applied correctly, it is easily fragmented and removed. When mixed improperly, it can become as hard as concrete. Increasingly, utilities are becoming encased in this CDF. It is unclear how much of this CDF is mixed improperly, but anecdotal evidence indicates that the amount is signifi- cant. Exposing utilities encased in improperly mixed CDF is dangerous, since destructive tools are necessary to break the CDF apart. CDF emplacements can be massive, and in the future, exposing significant portions of utility systems may be more difficult and less safe. One advantage of CDF is that it is relatively uniform, making a contrast with a utility more obvi- ous, so long as the utility is not made of concrete. This would imply that surface geophysical methods may work better over utilities encased in CDF than in a less homogeneous backfill. In the future, methods, techniques, and equipment developed to
35apply CDFâs properties to the advantage of surface imaging could be further explored. For the time being, knowing whether the CDF (or concrete) has a utility embedded in it and how deep that utility may be embedded is important to safely identify encasements that lack additional and reliable informa- tion about the utility itself. Data Management Maintaining accurate and complete records for existing utili- ties is without question the best way to locate those utilities in the future. In the past, there were many reasons for failing to create and update records, some of which were discussed in chapter 3. However, CADD and GIS databases and GPS survey technology are approaching a point at which record genera- tion, management, and updating can be done inexpensively, easily, and more reliably than ever before. Transferring location data from the geophysical device to the end data user is just as important a process as using the geophysical device in the first place. Subsurface utility engi- neers place a premium on this aspect, as does ASCE 38-02 (5), and they use licensed professionals to certify the survey process and final mapping deliverables. Some pieces of surface geophysical equipment are now incorporating GPS equipment directly into the original data- gathering process. Geophysical data and position data are downloaded to the office in a single process and a map of the results is generated. This practice has advantages and disadvantages. The advantages include no markings on the ground and little additional survey time. The disadvantages include the technicianâs inability to make on-site decisions about the perceived accuracy of the received signal. Quality assurance checks of the survey data correlated to the surface geophysical data are nonexistent, unless other topographic or reference features are included in the data set. This danger is alleviated somewhat when laser theodolites are used in con- junction with camera recorders. Data users must be informed of the process and make their own decisions as to the reliabil- ity of the data versus its generation cost. Because this is a rela- tively new technology, error statistics are not yet available. Record Generation During Utility Installation Who Makes the Record Traditional practice was that utility owners generated records of their own utilities. Survey procedures until recently were expensive and time-consuming, and utility owners frequently did not employ in-house surveyors. As a result, utility own- ers mostly used existing topographic features as a reference for where their facilities were installed. There were varying standards for how to measure distances from these features,depending on the utility company and sometimes even the construction supervisor. Records were made for the general purpose of knowing on what side of the street the backhoe should start excavating for maintenance activities or new service hookups. Vertical data was rarely recorded. Although general depths were mostly desired and adhered to, variations due to discovered obstacles during excavation may have changed the depth of installations. Even now, it is rare to gen- erate reliable, accurate, and recoverable utility-location data during installation. In some cases, developers or design-build project construc- tors may construct utilities as part of their development efforts, and they can then deed them back to the utility owners. Record generation in this instance may be the responsibility of the developer. Discussion is under way in certain states on whether util- ity owners should be the entities to generate and maintain the records of their facilities in the public right-of-way. Many states have statutes requiring utilities to do so and to furnish this information to other public entities when requested. Some state DOTs and municipalities are beginning to keep or generate utility records themselves, as a right-of-way man- agement tool. Records from an Exposed Utility When a utility is constructed in an open-cut trench, tradi- tional survey methods are available to record its location in three dimensions. Referencing this survey data to a recover- able survey control is crucial to retrieving this location in the future. Topographic references or nonpermanent markers are insufficient. State plane coordinates, latitude/longitude based on GPS techniques, or county or municipal controls can be used. Survey accuracy could be specified and standardized for utility data surveys that are intended for record generation. Many state DOTs require third-order accuracy equivalents; this is probably sufficient for most record purposes. A common misconception is that surveying exposed utilities automatically results in utility quality level A data. However, in order for data to be referenced as such, it must be certified by a registered professional. The proliferation of handheld GPS devices for survey and the ease of their use have resulted in a lot of data that appears definitive but whose accuracy depends on the type of GPS and collection method used. Records from Trenchless-Emplaced Utilities When a utility is constructed through trenchless means, record generation becomes more complex. There are three methods available. None is accomplished through direct measurement; therefore, any mapping data resulting from indirect location measurements should not be portrayed as quality level A data.
36Tracking Heads (Sondes) During Installation One system for controlling an underground boring tool involves the use of gyroscopes, accelerometers, magne- tometers, or all of these to track the movements of the boring tool. The location of the boring tool is essentially in real time. Gyroscopes and accelerometers track the motion of the boring head with respect to a reference starting point and the earthâs gravitational field. The instruments are subject to error drift with time, and their absolute position needs to be regularly cal- ibrated to provide an accurate output. Magnetometers track the motion of the boring head with respect to the earthâs mag- netic field. Measurements with magnetometers can suffer from magnetic interference, so sometimes an auxiliary system is used to create a localized magnetic field in the area of the bore that overrides the natural magnetic and electrical fields in areas where interference is suspected. For the borehole mapping to have a global reference, GPS or other survey methods must be used to provide the actual location of the boring head at the start and end of the bore. Some inference about the accuracy of measurements along the bore can be made from the error between the surveyed location at the end of the bore and the location provided by the bore-tracking device. A second method, which is also the more popular method when conditions allow and surface access above the bore is possible, uses a walkover system that requires a crewmember to walk along the drill path with a receiver that detects the sig- nal from a radio sonde mounted on the drill head. Magnets may also be used as a confirming location source. The princi- ples that apply to this method of tracing the drill head are the same as those discussed in the âGeophysical Methodsâ section. Depths and locations are subject to the same sources of inter- ference. Some tracking instruments are linked to a GPS system. Some systems automatically create a drawing of the installation based on the GPS location of the tracking head and the EM sig- nals received from the sonde at the tracking device. Surface Geophysical Tracing After Installation This method is no different than that of using a surface geo- physical method for tracing an older utility. Once the trench is backfilled, traditional surface geophysical means can be used at any time to designate the utility. If this action is performed under the direction of a registered professional, the data may be depicted as quality level B data, assuming all other necessary actions are followed, as per the ASCE guidelines (5). Gyroscopic Referencing Gyroscopic-based pipeline or conduit mapping systems are designed to determine the XYZ location of utilities, but to use a gyroscopic referencing system, there must be an entrance and exit point. In general, these systems are capable of mapping pipes with internal diameters of 2 in. to 48 in. Systems can betethered to a control unit that stores the data or else the logging data can be stored inside the system with no power or data cable tether (such as required for a robotic camera). One advantage of such a system is that it is independent of the pipe material. EM sondes may not be effective in metal pipe because a signal may not be broadcast beyond the metal pipe barrier. Depth of pipe emplacement can also be a detectability issue with sondes. The most common way to move the gyroscope through the pipe is a pulling wire. A winch is typically used on runs up to 4,000 ft. On runs over 4,000 ft, other methods of propulsion are often used, such as robots, compressed air, or pumping. The quantity and radii of bends in the pipe or conduit may preclude the gyroscope passing through the pipe. There is no definitive run length. The achievable length is largely dependent on the shape of the pipe (for example, sharp bends reduce the achievable length) and the mode of propulsion. It is necessary to have available intermediate coordinates for very long runs (similar to the requirements of intelligent pigging applications). Such coordinates might be available at a utility structure to correlate the horizontal positioning. The elevation may be more difficult to access at an intermediate point. Standard tolerances in X, Y, and Z of 0.25% of distance between known waypoints can be observed. (For example, the tolerance for a 400-ft run is 1 ft, for a 2,000-ft run it is 5 ft, and so forth). Most pipelines can be mapped with a high degree of accuracy by establishing reference points with known geo- graphic coordinates at the start and end of the run and on very long runs at known intervals between the two (http://www. geospatialcorporation.com/technical.html). Performing this work under the direction of a licensed professional may lead to depiction of this data at quality level A at the end points and quality level B for all points between. The judgment of the pro- fessional is needed to decide when the X and Y data should not be certified at quality level B but dropped to quality level C due to differences in precision and accuracy values between the beginning and end of the run. Summary of Record Generation The state of the art of record generation during utility instal- lation uses a registered professional to survey and certify open-trenched utilities as quality level A data at actual instrument-reading points and described as to accuracy and precision for all interpolated points. Geophysical tracing of a trenchless-emplaced utility and subsequent survey is certi- fied by a registered professional at quality level B. Gyroscop- ically surveyed data is certified at its end points as quality level A data and described as to accuracy and precision for all interpolated points. However, constructed utilities are rarely referenced to recoverable survey control, described as to accuracy and precision parameters, or certified by a registered professional.
37Record Updates Utilities are exposed for reasons other than initial installation. Typical reasons may include ongoing maintenance activities such as repairs, new service connections, and anode emplace- ment. Other construction activities in the area (by parties other than the utility owner) may also expose the utilities. Ongoing highway construction and paving repair are two such reasons. Exposed utilities can be surveyed and documented in the same fashion as those undergoing installation. The current state of the art has a registered professional survey and certify exposed utilities as quality level A data at actual instrument-reading points and described as to accu- racy and precision for all interpolated points. However, exposed utilities are rarely documented at all, let alone ref- erenced to recoverable survey control, described as to accu- racy and precision parameters, or certified by a registered professional. Record Maintenance The maintenance of records is an area of diverse utility and agency practices. Utility owners are usually thought of as the entities required to develop and keep records, but there is no clear mandate on the generation, accuracy, or completeness of those records except in the case of select owners, such as interstate pipeline companies. Other issues include record format, scale, standardization, and centralization. GIS systems are beginning to address these issues. While early GIS systems had poor positional reference data, an increasing number of systems have positional control that is first- or second-order survey accuracy. Airports, munic- ipalities, and industrial complexes were early proponents of GIS systems that include utility data. Some of these systems are quite robust, with many utility information attributes. Although utility quality levels in accordance with CI/ASCE 38-02 are lacking in most of these systems, notable exceptions include the levels at the Port of Seattle, Raleigh-Durham Air- port, and some districts within the Texas DOT. Permitting systems are one way in which those that control the land can gather and maintain utility information, but per- mit systems rarely require accurate location information for the utilities. For instance, some state DOTs reference utility permits for new installations by highway mile-marker posts. While this may give an indication that there is a utility in the general area, it does not serve adequately for design purposes. Another problem is that of blanket permits, whereby utility owners are given flexibility in adding, changing, or maintain- ing their facilities as far as required specific documentation. Other problems include permit retrieval. Several states are combining GIS and utility-permitting applications. Texas DOT, in conjunction with the Texas Transportation Institute, has developed a GIS-based systemto automate the utility-permitting process. This permitting system enables engineering drawings and other supporting documentation, which can include utility-quality-level depic- tion data, to be uploaded, and the uploaded documents to be converted into PDF files. GIS-based visualization of permit- ted sites, such as a map, a system to track permits through the approval process, and notification and reporting capabilities are also enabled. The systemâs goal is to eventually provide a comprehensive inventory of utilities within the Texas DOT right-of-way. Con- trolling data on new utilities is the first step. Adding data on existing utilities can occur on a project-by-project basis. The Pennsylvania one-call system has begun collecting util- ity data in addition to providing excavation notices to utility owners. The intent is to provide a secure repository for, and a point of access to, subsurface utility engineering data, received from project owners, to the affected facility owner. The system requires that each owner of a project valued at more than $400,000 provide to the one-call system utility information at the appropriate quality level. It is yet to be determined how utility relocations and new utilities constructed during or after the project are to be added to this repository. To date, no data has been furnished while data compatibility, formatting, and other issues are under discussion. State DOTs generally control the utilities in their rights-of- way. This control could include requiring utility owners to furnish accurate and comprehensive information regarding the location and character of their utilities to the DOT in exchange for the privilege of occupying that right-of-way. The permit process must include standardized data attributes and formatting. The state of the art allows public agencies use of GIS to inventory utilities within the right-of-way. Project maps are appended or utility reference files are directly added to GIS layers. Permits require utility owners to provide record draw- ings in specific formats. In practice, however, there is typically no centralized utility records database, and existing utility information from project plans is not readily retrievable. References 1. National Research Council. Seeing into the Earth: Noninvasive Char- acterization of the Shallow Subsurface for Environmental and Engi- neering Applications. National Research Council, Washington, D.C., 2000, 148 pp. 2. Butler, D. K. Potential Fields Methods for Location of Unex- ploded Ordnance. The Leading Edge, Vol. 20, No. 8, Aug. 2001, pp. 890â895. 3. Airport Business Magazine, June 2006. 4. Ashdown, C. Mains Location Equipment: A State of the Art Review and Future Research NeedsâFinal Report. UKWIR 01/WM/06/1. United Kingdom Water Industry Research, 2001, p. 39. 5. American Society of Civil Engineers. Standard Guidelines for the Col- lection and Depiction of Existing Subsurface Utility Data. ASCE Stan- dard No. CI/ASCE 38-02, ASCE, Reston, Va., 2002, 20 pp.