Use of Geophysics for Transportation Projects (2006)

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Much of the following text and information presented in this appendix was provided for the synthesis project by FHWA. The documents were prepared as part of the Geophysics Workshop currently in preparation by FHWA. APPENDIX A Methods and Techniques 43

44 Geophysical Method Selection Matrix FHWA/State DOT Name: Phone: Title: Cell: Agency: Fax: Address: E-mail: City/State/Zip: Web: Geophysical Contractor Name: Phone: Title: Cell: Agency: Fax: Address: E-mail: City/State/Zip: Web: Project Location: □ State □ County □ City □ Street Address: Estimated Budget: □†$2,500 □ $2,500–$10,000 □ $10,000–$50,000 □ $50,000–$100,000 □ $100,000–$500,000 □ >$500,000 □ Specify Specific Project Location: Time Frame: □†2 weeks □ 2 weeks–2 months □ 2 months–6 months □ >12 months □ Specify Mobilization Date: Demobilization Date: Number of Mobilizations: Project Objective: Bridge (chapter two) □ Depth to foundation □ Foundation socketing into bedrock □ Integrity testing of foundation □ Testing other substructure elements □ Rebar quality □ Foundation scour Decks (chapter three) □ Stability analysis □ QA/QC of new decks □ Baseline condition assessment □ Existing deck evaluation □ Presence, pattern, density of rebar □ Rebar condition/corrosion □ Concrete condition/integrity □ Incipient spalling Pavement (chapter four) □ QA/QC of new □ Condition evaluation □ Segregation in hot mix asphalt □ Moisture variation □ Rock pockets □ Voids beneath □ Cracking □ Condition/integrity Roadway Subsidence (chapter five) □ Clay content □ Expansive clay □ Voids, cavities □ Sinkholes □ Abandoned mines Subsurface Characterization (chapter six) □ Depth of bedrock □ Depth of structures □ Depth of fractures Subsurface Characterization (chapter six) □ Identifying weak zones within bedrock □ Mapping lithology □ Locating shallow sands and gravels □ Mapping groundwater surface and flow Imaging Buried Manmade Features (chapter six) □ Utilities □ Unexploded ordnance (UXO) □ Pipeline □ Underground storage tanks □ Contaminant plumes Vibration (chapter seven) □ Monitoring vibration □ Specify:

45 Road Type: □ Country lane □ 2-lane road □ 4-lane road □ Freeway □ Freeway interchange □ Bridge □ No shoulder □ Not applicable □ Specify: Surface Vegetation: □ Trees □ Grass □ Bare □ Shrubs □ Specify: Vegetation Density: □ Sparse □ Light □ Medium □ Heavy Geology: □ Unknown □ Limestone □ Clay □ Sand □ Shale □ Granite □ Specify: Traffic Control: □ Road closed □ Flagger(s) □ provided by: □ Not applicable □ Specify: Target Depth: □ 1–10 cm □ 10–100 cm □ 1 meter □ 10 meters □ 15 meters □ 100 meters □ 1 kilometer □ >1 kilometer □ Specify: Target Size: □ 1–10 cm □ 10–100 cm □ 1 meter □ 10 meters □ 15 meters □ 100 meters □ 1 kilometer □ >1 kilometer □ Specify: Cultural Features Supporting Information: □ Boring logs □ Site history □ Site photographs □ Water table depth □ Specify: Slopes: □ 0°–30° □ 30°–45° □ >45° Above Ground: □ Power lines □ Buildings □ Roadways □ Railroad □ Fences □ Bodies of water □ None □ Specify: Below Ground: □ Utilities □ Abandoned mines □ Landfill □ Pipelines □ UXO □ None □ Specify: Recommended Geophysical Methods OSHA 1910.120: □ None □ Level D □ Level C □ Level B □ Level A □ Radioactive □ Specify Subsurface Characterization: □ Resistivity □ Electromagnetics □ Ground penetrating radar □ Magnetics □ Seismic refraction □ Seismic reflection □ Cross borehole tomography □ MASW □ SASW □ Specify: Engineered Structures Evaluation: □ Crosshole sonic logging □ Crosshole sonic logging tomography □ Gamma–gamma density □ Impact echo □ Ground penetrating radar □ SASW □ Specify: Notes/Action Items/Comments:

46 SEISMIC REFLECTION Introduction Seismic methods are the most commonly conducted geo- physical surveys for engineering investigations. Seismic data provides engineers and geologists with the most basic information related to the elastic properties (strength) of rocks using well-understood geophysical procedures and common equipment. Seismic waves are created using either an impulsive source (hammer, explosives) or a vibrating source called vi- broseis. In either case these seismic waves travel into the ground and this energy is partitioned. When it reaches a rock layer that has a different impedance (related to velocity and density) some of the energy is reflected back to the ground surface, some is refracted along the interface, and some con- tinues deeper into the ground. As the refracted wave travels along the interfaces, energy is continuously transmitted back to the ground surface. Geophones placed on the ground sur- face detect the reflected and refracted waves. Seismic waves can be divided into two main groups; body waves and sur- face waves that exist only near a boundary. Body Waves These have the highest velocity of all seismic waves and are called compressional or pressure or primary (P-wave). The particle motion of P-waves is extension (dilation) and com- pression along the propagating direction. P-waves travel through all media that support seismic waves, which includes solids, gases, and liquids. Compressional waves in fluids; e.g., water and air, are commonly referred to as acoustic waves. The second wave type is the transverse or shear wave (S- wave). S-waves travel slower than P-waves in solids, usually at about 60% of the speed of P-waves. S-waves have particle motion perpendicular to the propagating direction. These transverse waves can only travel through materials that have shear strength. S-waves do not exist in liquids and gases, as these materials have no shear strength. Surface Waves Two types of waves, which exist only at “surfaces” or inter- faces, are Love and Rayleigh waves. Traveling along a sur- face, these waves attenuate rapidly with distance from the surface. Surface waves travel slower than body waves. Love waves have particle motion similar to S-waves. Rayleigh waves travel in an ellipse similar to ocean waves. Surface waves are produced by surface impacts, explosions, and wave- form changes at boundaries. Love and Rayleigh waves are also portions of the surface wave train in earthquakes. These sur- face waves carry greater energy content than body waves and travel more slowly, thus arriving after body waves. Because of their greater energy content, surface waves may cause more damage than body waves during an earthquake. Data Collection A source, geophone, and seismograph are needed to collect data for a seismic survey. The source can be a hammer strik- ing the ground, aluminum plate or weighted plank, weights of varying sizes that are allowed to drop onto the ground, rifle shot, harmonic oscillator, waterborne mechanisms, or explo- sives. The source is commonly referred to as a shot; however, this does not necessarily imply explosives. The source will vary depending on the objective of the survey, particularly the desired investigated depth and physical properties of the rocks at the sites. The sensor receiving the seismic energy created by the source is called the geophone. The sensors are either ac- celerometers or velocity transducers, and convert ground movement into voltage. Geophones can be placed in a variety of geometric patterns referred to as a line, spread, or string of geophones depending on the objectives of the survey. The seismograph records input geophone voltages in a timed sequence. Seismographs store the signals as digital data at a discrete time. A portion of the seismic energy striking the interface be- tween two differing materials will be reflected from the inter- face. The ratio of the reflected energy to the incident energy is called the reflection coefficient. The reflection coefficient is defined in terms of the densities and seismic velocities of the two materials: where R = reflection coefficient; ρb1, ρb2 = densities of the first and second layers, respectively; and V1, V2 = seismic velocities of the first and second layers, respectively. Data Processing Processing is typically done by geophysicists that specialize in seismic processing using special purpose computers. These techniques are expensive, but technically robust and excellent results can be achieved. A close association of the field geo- physicist, processor, and the consumer is absolutely essential if the results are to be useful. Well logs, known depths, results from ancillary methods, and the expected results should be furnished to the processor. At least one iteration of the results should be used to ensure that the final outcome is successful. R V V V V b b b b = − + ρ ρ ρ ρ 2 2 1 1 2 2 1 1

47 One important conclusion of the processing is the depth sec- tion. The production of depth sections requires conversion of the times of the reflections to depths by derivation of a veloc- ity profile. Well logs and check shots are often necessary to confirm the accuracy of this conversion. Figure A1 shows a schematic of the seismic reflection method illustrating the raypath through successive layers. The unique advantage of seismic reflection is that it per- mits the mapping of many horizons or layers with each shot. Figure A2 shows the raypaths of arrivals recorded on a multi-channel seismograph. Note that the subsurface depth is exactly one-half the distance between geophones on the surface. Figure A3 shows the arrivals on a seismic reflection record. The various arrivals are identified on Figure A3. Advantages The unique advantage of seismic reflection data is that it per- mits mapping many horizons or layers with each shot. Limitations Variations in field techniques are required depending on depth. Containment of the air-blast is essential in shallow re- flection work. Success is greatly increased if shots and phones are near or in the saturated zone. Severe low-cut fil- ters and arrays of a small number (1–5) of geophones are re- quired. Generally, reflections should be visible on the field records after all recording parameters are optimized. Data processing should be guided by the appearance of the field records and extreme care should be used to stack refractions or other unwanted artifacts as reflections. References 1–3, 5, 7, 8, and 10. SEISMIC REFRACTION Introduction The refraction seismic method is used to measure the depths and velocities of subsurface layers. It is particularly useful for mapping the depth and topography of the bedrock surface. It can also be used to find the elastic properties of these layers, which are useful for engineering purposes. Using the velocity of the bedrock, the rippability of the bedrock can be deter- mined along with an estimate of the size of machine required. Basic Principles of the Refraction Seismic Method When seismic waves are created on the ground surface they penetrate the subsurface until they encounter layers with dif- ferent velocities and/or densities. In the case of refraction seis- mic, the subsurface layers must have successively increasing layer velocities with depth. At the layer boundaries, part of the incident wave is reflected back to the ground surface, part is transmitted deeper into the ground, and part is refracted along the surface of the layer boundary. Figure A4 shows the inci- dent, reflected, transmitted, and refracted waves at a layer boundary with different velocities on either side. This drawing also shows the wave that travels along the ground surface, called the direct wave, and the air wave. As the refracted wave travels along the refractor surface, seismic energy is continu- ously refracted back to the ground surface. FIGURE A1 Schematic of seismic reflection method. FIGURE A2 Multi-channel recordings for seismic reflection. FIGURE A3 Simple seismic reflection record.

48 Refraction seismic surveys can be conducted using two kinds of seismic energy, called compression and shear waves. Compression waves (P-waves) are the most common energy source and are created by impacts such as a hammer hitting a plate placed on the ground surface. In compression waves, the particle motion is parallel to the direction of travel of the wave, propagating as a series of contractions and dila- tions of the rock particles. Shear waves (S-waves), in which the particles oscillate orthogonal to the direction of travel, are used along with P-waves to obtain the elastic properties of the rocks and to locate fracture zones. Creating shear waves is more diffi- cult than creating compression waves. This requires hitting a plank of wood held firmly on the ground surface, or a specially made device, with a side impact. The velocity of S-waves is usually about three-fifths of the velocity of P-waves in consolidated rock. In the seismic refraction method, the seismic waves that result from a ground impact are recorded, as illustrated in Figure A5. Normally, a minimum of five shots are used for each seismic spread. Only four shots are shown in Figure A5 for clarity. Normally, a shot in the center of the spread is also recorded. Additional shots may be recorded, mostly depend- ing on the expected overburden velocity changes. If these changes are expected to be significant then more shots may be needed to better define the overburden velocity, thus im- proving the accuracy of the overall interpretation. Data recording seismic wave detectors, called geophones, are planted in the ground along a straight line, usually with equal spacings. The number of geophones depends on the sur- vey and the seismic recorder being used, but usually varies between 24 and 96. Sometimes the geophones near the ends of the spread have smaller separations to obtain the velocity of thin overburden layers. Shots are positioned as described previously and the data are recorded on the seismograph. The source of the seismic energy can be a hammer hitting a metal plate placed on the ground surface, a small black powder charge placed in a hole about 2 ft deep, a weight drop system, or explosives. Processing and Interpretation The data are interpreted using one of several methods. Proba- bly the most commonly used method is called the Generalized Reciprocal Method (GRM). This method provides depths and velocities under each geophone and usually produces reliable results providing the refractor dips at less than 20 degrees rel- ative to the ground surface. The first step is to pick the first arrival times of the seis- mic waves for each geophone and each shot. These arrival times are then plotted as a function of distance from the shot location (time–distance plot, Figure A5). The time–distance data are then input into the interpretation program and the data are then interpreted to give overburden and refractor depths and velocities. The method can be used to determine the depth and velocities of up to about four refractors under ideal conditions. Method Limitations The main sources of error in computing depth to bedrock from seismic refraction surveys are: • Low signal-to-noise ratios owing to insufficient source energy, cultural noise, wind noise, rain, or other local sources of vibrations. FIGURE A5 Schematic showing a seismic refraction time–distance curve and the refracted waves.FIGURE A4 Main seismic waves and wave partitioning that occurs at a seismic interface.

49 • Lateral variations in the overburden velocity. • Potential “hidden layers” resulting from a low-velocity layer overlain by a higher-velocity layer (velocity rever- sals), layers too thin to support refracted wave energy, or thin layers with a low velocity contrast with the layers on either side. References 1–3, 5, 7, 8, and 10. CROSSHOLE SEISMIC TESTING Introduction Crosshole seismic testing is conducted to determine the prop- erties of soils not rock using P- and S-waves in boreholes. The information obtained from these tests can be used to compute shear modulus, Young’s modulus, and Poisson’s ratio for sta- tic/dynamic analysis. Basic Principles of Crosshole Seismic Testing The crosshole seismic testing method is similar to seismic methods; however, it provides information on soil properties rather than rock properties. Two or more boreholes are used in this method. One borehole is instrumented with an energy source and the additional boreholes are instrumented with re- ceivers (geophones or hydrophones). Figure A6 illustrates the field setup for a crosshole seismic test. Field Data Recording Crosshole seismic testing surveys are conducted using two or more (three are recommended for optimum results) bore- holes. These boreholes are drilled to approximately 15 m and are spaced in a straight line approximately 3.0 m apart. The spacing can be increased to 4.5 m if the S-wave velocities will exceed 450 m/s, a common occurrence in alluvial mate- rials. It is recommended that the boreholes be cased with PVC and grouted. This provides a smooth uniform surface that minimizes sidewall disturbance during testing. A bore- hole deviation survey must be conducted to ensure that the true vertical depth and horizontal position of any point in the borehole can be calculated. A calibration test for the P- and S-wave must be per- formed in the hole. It is best to perform separate tests for the P- and S-waves for optimum results. The energy source is lowered into the borehole approximately 1.5 m, the same distance as the receivers in the remaining boreholes, and the source is activated. The signal amplitude and duration of the wave is adjusted so that they are both displayed in their entirety. Once the signal amplitude and duration have been adjusted, repeat the test at subsequent 1.5-m intervals for the source and receivers until the bottom of the borehole is reached (Table A1). Interpretation determines the true vertical depth and horizontal position of any point in the borehole using the information from the deviation survey. Identify the arrival of the P-wave train followed by the S-wave train. The data are tabulated with three separate travel times, source to the first receiver, source to the second receiver, and the time difference between the first and second receiver. A com- puter program for crosshole seismic data interpretation is used to facilitate the number of calculations required for these data. These computer programs should be capable of solving the corrected distances, true velocities using Snell’s law, and the interface depth. Limitations • Poor borehole construction affects the data quality. • Refraction events from high-velocity layers (either above or below a low-velocity layer) may be misinterpreted. • Shear velocity is azimuthally anisotropic (velocity changes in direction). FIGURE A6 Field setup for crosshole seismic testing. TABLE A1 RANGE OF P-WAVE VELOCITIES FOR COMMON ROCK TYPES Rock/Fluid Type Velocity Range (ft/s) Freshwater 4,600 Sand (saturated) 4,900–6,600 Clay 3,280–8,200 Sandstone 6,500–18,000 Shale 3,100–16,700 Limestone 9,800–18,000 Dolomite 8,200–21,300 Granite 18,000–20,300 Gabbro 21,000–23,000 Glacial till 5,000–8,500

50 • ASTM requirement for three boreholes is costly. • Good shear wave energy can be difficult to generate and record. References 7, 8, 10, 11, and ASTM D4428. GROUND PENETRATING RADAR Introduction Ground penetrating radar (GPR) surveys are commonly used for relatively shallow investigations, usually less than 10 m, although deeper targets can be detected under ideal condi- tions. The method has many applications, including locating underground storage tanks (UST), utilities, bedrock topogra- phy, and cavities in the bedrock. It is also used to evaluate roadbed integrity and to locate structural features in build- ings such as post-tension cables. The method relies on obtaining reflections of electro- magnetic (EM) energy from objects beneath the ground or other surface, much like the seismic reflection method. Re- flections occur when the object provides a contrast in its relative permittivity (also called dielectric constant) com- pared with that of the host material. The relative permittiv- ity for most geologic materials is dominated by that of wa- ter, which has a relative permittivity of 80. The velocity (V ) of an EM wave in a medium with a relative permittivity of ε is given by: where c is the velocity of light (3  108 m/s). V c= ε The wavelength of an EM wave in a medium can be found using the equation where is the frequency of the wave. Table A2 provides values of the relative permittivity, ve- locity, and electrical conductivity of EM waves for some com- mon materials. These properties often vary with frequency; the table is for frequencies of approximately 100 MHz. The success of a GPR survey depends mostly on three factors: the dielectric properties of the target and host, the electrical conductivity and clay content of the ground, and the size of the target. As mentioned earlier, the target has to provide a contrast in dielectric properties with the host to be observed. The electrical conductivity is important because EM waves become increasingly attenuated with increasing conductivity, which will limit the depth of penetration. In general, a target whose dimensions are smaller than approx- imately one-third of the wavelength of the GPR signal will probably not be imaged. Figure A7 illustrates two modes for taking GPR data; re- flection and common mid-point. The most commonly used mode is reflection. On some GPR systems the transmitter and receiver are housed in one unit and therefore may be restricted to recording data using the reflection mode. Several companies manufacture GPR equipment. Usually a wide range of antennae are available from each manufac- turer, providing frequencies ranging from less than 25 MHz to more than 1 GHz. Some of these instruments house both f λ = Vf TABLE A2 RELATIVE PERMITTIVITY, VELOCITY, AND CONDUCTIVITY OF SOME COMMON MATERIALS Material Relative Permittivity (ε) Velocity (m/ns) Conductivity (mS/m) Ice 3–4 0.16 0.01 Freshwater 80 0.033 0.5 Seawater 80 0.01 3,000 Sand saturated with freshwater 20–30 0.06 0.1–1.0 Dry sand 3–5 0.15 0.01 Limestone 4–8 0.12 0.5–2 Granite 4–6 0.13 0.01 Silt 5–30 0.07 1–100 Clay 5–40 0.06 2–1,000 Shale 5–15 0.09 1–100

51 the transmitter and receiver in one box, whereas others pro- vide separate transmitter and receiver assemblies. High fre- quencies provide better resolution than lower frequencies but have less depth penetration. Conversely, lower frequencies provide better depth penetration but provide lower resolution. Survey Procedures Two general types of antenna are available. With the first type the antenna is designed to be in direct contact with the ground or surface containing the target. With the second type, called a horn antenna, the antenna is placed 2 or 3 ft above the ground. Data recorded with the horn antenna are also sampled at a much higher frequency than with the ground coupled antenna and can therefore be mounted be- hind a vehicle, allowing surveys to be conducted at up to 30 mph. Surveys conducted with the horn antenna are generally designed for investigations to depths less than approxi- mately 6 in., such as roadbed surface analysis. When using a unit that houses both the transmitter and re- ceiver, the system is pulled along the ground at a slow speed. The data can usually be viewed on a monitor at the time of the survey, thereby allowing the operator to check that the target is being identified. If it is desired to calculate the depth to an imaged target for a GPR survey, it is advisable to locate a suitable target at a known depth to calibrate the system and conduct a traverse across the feature. Such targets may include culverts and other features that should provide a clear response. If no calibration target is available, then the relative permittivity of the ground will need to be estimated. Velocity information can also be found from CMP surveys, where they can be obtained from the variation in reflection time with offset. If such a survey is not feasible, then an estimate of the relative permittivity will need to be made using geologic knowledge and the informa- tion presented in Table A2. Method Limitations The main limitation of the GPR method is usually insuffi- cient depth of penetration. This is often owing to conductive soils or overburden, usually because of high salt or clay con- tent. Using lower-frequency antennae can sometimes mini- mize this limitation. Because of the dependence of depth penetration on the local ground conditions, such as electrical conductivity and clay content, the success of GPR surveys is site-specific, which sometimes cannot be accurately pre- dicted ahead of the survey. GPR data can be subject to interference from a number of sources. Local radio transmitters can saturate the electronics. Metal objects can interfere with the reflections. Above ground features, such as utility lines, can produce reflections that might interfere with the reflections from the target. References 8–11 and 15. DC RESISTIVITY MEASUREMENTS Introduction Resistivity measurements can be used to find the vertical and lateral variations in the resistivity of the subsurface. These measurements have many uses, including measuring the depth to the top of the water table, determining the depths and resis- tivities of geologic layers, mapping voids, fractures, and other geologic features. The method is most appropriate for depths generally less than approximately 200 ft, although deeper in- vestigations can be performed. Resistivity measurements can be divided into two main groups, soundings and traverses (profiling). Soundings are used to find the depths and resistivities of the geologic lay- ers under the sounding site. Traverses are used to map the lateral variations in resistivity. It is also possible to effi- ciently conduct surveys combining both soundings and tra- verses, although the instruments generally used to perform these measurements employ transmitters with limited power and therefore provide limited depth penetration. Figure A8 shows the resistivity ranges of some common rock types. Resistivity measurements can be recorded using several instruments. The most commonly used instruments are the Sting/Swift (from Advanced Geosciences Inc.), Iris (from Iris Instruments, France), and a Swedish company called ABEM. These companies produce instruments that can be used to measure the resistivity of the ground using a simple four elec- trode array or they can be used as an automated system where FIGURE A7 Two modes used to record GPR data.

52 an array of electrodes are positioned before recording the data. Once this is done, and the electrodes connected to the recording instrument, the data are recorded with the instru- ment automatically switching between the required electrodes so as to acquire the data along the whole line. Basic Principles of Resistivity Measurements There are numerous electrode arrays that can be used to mea- sure resistivity. These include Dipole–Dipole (used either in line or equatorially), Schlumberger, Wenner, Pole–Pole, Pole–Dipole, and the Square array. Each has particular ad- vantages and disadvantages. Each array has four electrodes, two for injecting electrical current into the ground (current electrodes) and two different electrodes for measuring the re- sulting voltage (potential electrodes). The most commonly used electrode arrays are illustrated in Figure A9. Unless the ground is homogeneous, the measured resis- tivity does not represent that of any particular layer until the data have been interpreted. Therefore, the measured resis- tivity is called Apparent Resistivity, can be thought of as a composite resistivity that includes contributions from all of the layers under the sounding site to the depth of investiga- tion of the measurement. The equations for converting the measured resistance (V/I) into Apparent Resistivity are given below for the dif- ferent arrays. Dipole–Dipole array Pole–Dipole array Wenner array and Pole–Pole array Schlumberger array The Square array is shown in Figure A10. Square array; configuration A In configuration B, no voltage is measured if the ground is electrically homogeneous. ρ π= − 2 2 2 a V I ρ π= L l V I 2 2 ρ π= 2 aV I/ ρ π= +2 1n n a V I ( ) ρ π= + +n n n a V I ( )( )1 2 FIGURE A8 Resistivity ranges of some common rock types. FIGURE A9 Common electrode arrays. FIGURE A10 Square electrode array.

53 Resistivity Soundings Resistivity soundings are conducted by measuring the Ap- parent Resistivity starting with a small electrode spacing and continuing to take measurements while increasing the elec- trode spacing until the required investigation depth has been reached. As the electrode spacings are increased the depth of investigation also increases. This is sometimes called a geo- metric sounding. Generally the Schlumberger electrode array is best for soundings. This is because the voltage measuring (potential) electrodes are only moved when necessary to increase the measured voltage, thus minimizing offsets in the apparent re- sistivity sounding curve caused by local features near the po- tential electrodes; that is, boulders, shallow bedrock changes, soil resistivity changes, or other features that might cause significant lateral changes in the measured voltage. A sounding curve can be plotted showing the Apparent Resistivity versus electrode spacing as illustrated in the up- per drawing in Figure A11. Computer software is used to in- terpret the sounding curve producing a model showing the depths to the top, thickness, and resistivities of the layers under the sounding site. For this interpretation it is assumed that the layers under the sounding site are horizontal with no lateral changes in resistivity. Resistivity Traverses/Soundings Resistivity traverses are used to locate lateral variations in the resistivity of the subsurface. With automated resistivity systems, where the instrument automatically switches to the relevant electrodes, many different electrode spacings and locations can easily be recorded, thus producing an Apparent Resistivity section covering the line of electrodes. These data now combine both vertical (sounding) and lateral (traverse) resistivity information. Such sections are often called Pseudo Sections, because they show Apparent Resistivity plotted against electrode spacing along a traverse. These sections are interpreted using software that produces a section showing the modeled resistivity against depth. Azimuthal Resistivity Measurements Resistivity surveys can also be used to locate the occurrence of fractures and their orientation, using a technique called Azimuthal Resistivity. With this method, resistivity readings are taken while the array is rotated about its center. If the fracture zone is saturated with water and/or contains clay or soil then it may have a lower resistivity than that of the host rock and this may be observed as lower resistivities when the line of electrodes is parallel to the fracture. With the square array, using diagonal electrodes for the voltage and current (see Figure A10) can be used to assess the occurrence of fractures. If no lateral resistivity changes are present within the area of influence of the array then the measured voltage will be zero. This method assumes that horizontal layering is present at the sounding site. Capacitively Coupled Instruments One of the more time consuming aspects of resistivity sur- veys is the time needed to insert electrodes into the ground. To overcome this problem, capacitively coupled electrode systems are available. In these systems, an array of capaci- tively coupled electrodes is connected by a cable and can be dragged along the ground by an operator walking at a slow speed. Data are then recorded as the system moves. How- ever, these systems inject only very small currents into the ground and are usually limited to resistive ground conditions and depths of 10 to 20 ms. In addition, the surface conditions must be smooth and flat with little or no vegetation. Method Limitations The conventional resistivity method requires that electrodes be inserted into the ground, making it quite labor intensive. If the ground is hard then this may be difficult. In addition, FIGURE A11 Plot of measured resistivity (Apparent Resistivity) against AB/2 (electrode spacing) and a diagram showing the Schlumberger electrode array and lines of current flow in the ground.

54 in dry ground conditions, the electrodes may need to have saline water poured on them to lower the electrical resistance between the electrode and ground. The interpretation of re- sistivity soundings necessarily assumes that the subsurface is horizontally layered with no lateral variations in resistivity. References 1–3, 6, 7, and 10. TIME DOMAIN ELECTROMAGNETIC SOUNDINGS Introduction Time Domain Electromagnetic (TDEM) soundings are done to obtain the vertical resistivity distribution of the subsurface. By performing several soundings along a line both the lateral and vertical variations in the resistivity of the subsurface can also be observed. Some of the uses of resistivity measurements include find- ing the depth to the top of the water table, mapping geologic structure, and providing resistivity maps to aid in aquifer dis- covery and evaluation. Several instruments are available for conducting TDEM soundings, with the most commonly used being the EM37, EM47 Protem, and EM57 systems manufactured by Geonics of Toronto, Canada. The depth of investigation varies depending on the instru- ment used and the geologic conditions, but varies from about 20 ft to more than 1,000 ft. Figure A12 shows the resistivity ranges of some common rock types. Basic Principles of TDEM Soundings The TDEM method uses EM waves to image the subsurface. A square loop of wire is laid on the ground through which is passed electrical current having a positive on time followed by an off time. This is then followed by a negative on time and then another off time. This process is repeated while the data are being recorded. This current produces an EM field that penetrates the ground. When the current turns off, a rapidly changing EM field is created that generates secondary cur- rents in the ground. The magnitude of the secondary currents depends on the conductivity of the ground. The secondary currents, which are also time varying, produce their own time varying EM fields, which are detected by a receiver coil placed on the surface of the ground. The receiver coil records the signal after the transmitter current has turned off. Because the transmitter produces a square wave current, as described above, repeated at a predefined frequency, the received signal is stacked so as to improve the signal-to-noise ratio. The depth of investigation is related to the length of time after the trans- mitter loop current has turned off. Figure A13 illustrates the layout of the system and the re- ceived signal when the transmitter current turns off. The in- strument then converts this signal to Apparent Resistivity values for a series of times (time gates) after the current has turned off. These values are shown plotted as a measured re- sistivity (Apparent Resistivity) versus time plot, also illus- trated in Figure A13, called a sounding curve. In this case, the sounding curve is that which would be observed over a low resistivity layer lying between more resistive layers. FIGURE A13 Schematic showing the field layout of the TDEM equipment, the transmitter current and received signal, and the resulting sounding curve. FIGURE A12 Resistivity ranges of some common rock types.

55 Unless the ground is homogeneous the measured resistiv- ity does not represent that of any particular layer until the data have been interpreted. Therefore, the measured resistiv- ity is called Apparent Resistivity, which can be thought of as a composite resistivity that includes contributions from all of the layers under the sounding site, to the depth of investiga- tion of the measurements. The sounding curve is interpreted using software that iter- atively modifies a proposed resistivity model (layer thickness, depths, and resistivities) until the calculated sounding curve matches the field curve. Survey Procedures TDEM soundings require a transmitter loop to be laid out along with the receiver coil, transmitter, generator, and receiver. For small depths of investigation where small transmitter loops are used, it is sometimes advantageous to place the receiver coil ex- ternal to the transmitter loop. The depth of investigation is related to the transmitter loop size, and can range from about one and one half to three times the side length of the transmitter loop. If both near surface and deeper interpretations are required, then two soundings may be performed with different transmitter loop sizes. References 3 and 7–10. Method Limitations Because this is an EM method with a transmitter loop gener- ating EM fields, surface and subsurface metal will influence the data and should be avoided. Moreover, this metal does not necessarily have to be grounded, as in the case with resistiv- ity methods that use grounded electrodes. Metallic items such as metal fences, buildings with steel reinforcement, concrete with reinforcing bar, buried pipelines, and other metal fea- tures can influence the data. Power lines can also influence the data because they create electrical noise. TDEM soundings are ideal for locating conductive layers, but are less effective at locating resistive layers. The method responds to the conductivity-thickness product (conductance) of the layer and for thin layers it may be difficult to determine either the conductivity or the thickness of the layer accurately. CONDUCTIVITY MEASUREMENTS USING FREQUENCY DOMAIN ELECTROMAGNETIC (FDEM) INSTRUMENTS Introduction Measuring the electrical conductivity of the subsurface can be done relatively quickly. Several instruments are com- monly used providing investigation depths from less than a meter to approximately 60 m. These measurements are used for many purposes, including mapping soil/rock thickness, mapping the topography of subsurface layers, and locating fracture zones, clay beds, contaminant plumes, and prior ex- cavations such as burial pits and buried metallic objects. They are also used in agriculture to estimate the salinity of the soil. This note provides a brief description of the method along with some examples of how the method can be used. The conductivity of common materials varies over a wide range, as shown in Table A3. The most commonly used FDEM instruments are man- ufactured by Geonics Ltd. of Canada and are the EM31, EM31-MK2, EM31-3, EM34-3, EM34-XL, and EM38. The EM31-MK2 is similar to the EM31 but includes a data logger incorporated into the central console. With the stan- dard EM31, the data logger is separate. Another EM31, called the EM31-3, is also available that has three receiver coils at distances of 1, 2, and 3.66 m from the transmitter coil, providing three investigation depths recorded simul- taneously. A high-powered EM34-3 is also available, called the EM34-XL. This improves the signal-to-noise ra- tio by a factor of 10 at the 40-m coil separation and 4 at the 10- and 20-m coil separations. The system is useful in ar- eas where increased cultural and/or atmospheric noise is expected. All of the Geonics instruments listed previously convert the measured instrument response into “Apparent” conductivity before logging the data. The terms Apparent conductivity or “Terrain” conductivity are commonly used to describe the units of measurement recorded when using these instruments. This is because the measurement will only provide the true conductivity of the subsurface if it is homogeneous. In other cases where the ground is comprised of layers or other features having different conductivities, each measurement is the com- posite of all the contributions from each of these layers, or TABLE A3 CONDUCTIVITY RANGES OF COMMON MATERIALS Material Conductivity (mS/m) Air 0 Distilled water 0.01 Freshwater 0.5 Seawater 3,000 Dry sand 0.01 Wet sand 0.1–1 Limestone 0.5–2 Shales 1–100 Silts 1–100 Clays 2–1,000 Granite 0.01–1 Dry salt 0.01–1 Ice 0.01 Metals infinite

56 volumes, with different conductivities within the depth of in- vestigation of the instrument. Basic Principles of Conductivity Measurements Figure A14 illustrates the concept behind measuring the electrical conductivity of the ground using EM induction techniques. For simplicity, a buried metal tank is presented in the drawing. The transmitter consists of a coil through which oscillating electrical current is passed. This current generates an oscillating EM field that penetrates the ground, illustrated by the red lines, called the primary EM field. This oscillating EM field then induces secondary oscillating currents in conductive material in the ground. The greater the conductivity of the ground the stronger will be the sec- ondary currents. These oscillating secondary currents also generate sec- ondary oscillating EM fields that are detected by the receiver coil of the instrument. The instrument then compares the sec- ondary signals with the signal from the transmitter and pro- duces two components, one called the in-phase signal and the other called the out-of-phase (quadrature) signal. The out-of- phase signal is used to calculate the apparent conductivity of the ground and the in-phase signal is used when searching for highly conductive objects, such as buried metal tanks and pipes. Survey Procedures The EM31 and EM38 require only one operator. The EM34-3 and EM34-XL require two people. The EM31-3 is cumber- some and heavy, requiring a trailer and tow vehicle. Each of these instruments can be used in two different modes, one called the vertical dipole mode and the other called the horizontal dipole mode. When planning a survey it is important to understand the differences between the two modes. Figure A15 illustrates the relative contributions of a thin layer at depth (z = depth/coil separation) to the appar- ent conductivity indicated by the instrument. The horizontal dipole data are shown as H(z) and that of the vertical dipole as V(z). In the vertical dipole mode (where the plane of the coils is parallel to the ground surface), the depth of penetration is maximized and the influence of changes in the near surface conductivity are minimized. In the horizontal dipole mode, the measurements are more sensitive to changes in the near surface conductivity, although depth of penetration is less. There are other important differences between the two modes, in particular the shape of the anomaly that is observed over a vertical conductive feature, such as may be found in a fracture zone. This is illustrated under the heading Terrain Conductivity Surveys. The approximate depths of investigation for the more com- monly used instruments, in each of the two modes, is presented Table A4. With the EM31, EM34, and EM38 instruments, the data are usually plotted at the mid-point between the transmitter and receiver coils. FIGURE A14 Schematic showing mechanics of EM induction method for measuring electrical conductivity of the ground. FIGURE A15 Relative responses from vertical and horizontal dipole modes when measuring electrical conductivity of the ground.

57 EM31 Surveys The EM31 measures the electrical conductivity (Apparent Conductivity or Terrain Conductivity) of the upper 3 or 6 m of the ground, depending on the mode of use, as shown in the table presented above. Readings can be obtained either discretely, by pressing a button, or using a timed mode, tak- ing readings up to twice per second with the EM31 or up to 10 readings per second with the EM31-MK2. Readings will normally be taken along lines crossing the area of interest. The beginning and end of these lines will need to be marked or surveyed prior to data recording, unless a GPS system is carried during the survey. The spatial coordinates of the data points can be obtained by interpolation, if necessary, using software, providing the spatial coordinates of the beginning and end marks have been surveyed. EM34 Surveys Two people are needed to operate the EM34; one for the trans- mitter coil and the other for the receiver coil. When used in the vertical dipole mode, ideally the two coils should be coplanar when a reading is taken. Usually, a flag is placed in the center of the cable joining the two coils. The flag is used to align the position at which a reading is taken. A data logger is used to record the data. Readings are taken along lines crossing the area of interest. The spacing between the readings and the lines depends on the target size and depth. Conventional surveying or GPS can determine the location of the data stations. If only the ends of the lines are surveyed, software can be used to pro- vide interpolated spatial positioning between the beginning and end of each line. EM38 Surveys The EM38 is designed for shallow surveys down to depths of about 1.5 m. As such, it is used to measure the conductivity of the upper soil layers, which is then used to predict the degree of salinity (mostly for agricultural purposes). The system is also used in archeology, where the in-phase component can be used to provide information about soil magnetic susceptibility. Susceptibility is a measure of the amount of magnetic miner- als in the soil. Readings can be taken in a timed mode or at dis- crete stations. As with the other EM systems, lines will need to be laid out before data recording, and spatial coordinates will need to be obtained, either by using differential global po- sitional systems (DGPSs) as the survey is being recorded or by surveying the ends of the lines and interpolating to obtain the spatial coordinates of the data along each line. Terrain Conductivity Surveys To explain the value of the reconnaissance level terrain con- ductivity surveys, two examples are shown. The first example (Figure A16) typifies what results can be expected if the lay- ered earth has relatively homogeneous lateral extending mate- rials and the conductive bedrock (e.g., shale or claystone) has a paleo-channel or dip in its surface. This bedrock feature will manifest itself in the conductivity readings as shown in the up- per graph because of the thicker resistive overburden. The sec- ond example (Figure A17) shows conductivity readings taken in the vertical dipole mode over a vertical electrically conduc- tive feature, such as a fracture zone. TABLE A4 DEPTH OF INVESTIGATION Instrument Coil Separation (meters) Horizontal Dipole (meters) Vertical Dipole (meters) EM31 3.66 3 6 EM34 10 7.5 15 EM34 20 15 30 EM34 40 30 60 EM38 1 0.75 1.5 FIGURE A16 Conductivity readings taken over a conductive shale layer. FIGURE A17 Example of the measured conductivity over a vertical conductive feature when taking readings in the vertical dipole mode.

58 In this mode, the anomaly shape over a vertical or subver- tical, an electrically conductive feature such as a fracture zone is very distinctive and can be used to locate such zones. The characteristic shape of the anomaly expected over such a fea- ture is shown in Figure A17. Conductivity measurements taken over this feature in the horizontal dipole mode would not show these diagnostic features. When using this method to locate vertical conductive fea- tures, it is important to provide sufficient spatial data density such that the anomaly shape is well defined, otherwise the anomaly may be difficult to recognize. Method Limitations If the ground has a low electrical conductivity, then the transmitter can induce only very small electrical currents into the ground. This means that only small secondary elec- tromagnetic fields will be generated, resulting in small voltages being measured by the receiver coil. Thus, the in- ductive method of measuring conductivity is not particularly suited to low conductivity (resistive) areas. Different inves- tigation depths can be achieved by using different modes and coil separations. However, these investigation depths are only approximate. Although soundings can be conducted by taking readings at different orientations and with several instruments, layer depth and conductivity (or resistivity) in- terpretations are only approximate. Other methods, such as resistivity soundings, need to be conducted if layer resistiv- ities and their depths are needed. References 3 and 7–10. MAGNETIC SURVEYS Introduction Magnetic surveys are conducted to evaluate geology, locate lava tubes in igneous rocks, find buried metal objects such as underground storage tanks (USTs) and pipelines, and locate unexploded ordnance (UXO). The depth of investigation varies widely, depending on the target. Geologic structure can be determined to depths of many thousands of feet. USTs, pipelines, and UXO targets are usually shallow. The method will probably only locate shallow lava tubes. Basic Principles of the Magnetic Method The Earth’s magnetic field is a vector quantity and has there- fore a direction and a magnitude. The shape of this field is that which would be produced if a large magnet were placed inside the Earth. Superimposed on this field are time varying fluctuations resulting from electrical activity in the iono- sphere, usually caused by solar flares. The Earth’s magnetic field induces a secondary magnetic field in ferromagnetic objects or geological structures that contains magnetite or other minerals that are magnetizable. This secondary magnetic field then “disturbs” the magnetic field of the earth creating an anomaly that can be detected with a magnetometer. Most magnetometers measure the magnitude of the magnetic field and can do so several times per second. Figure A18 presents a schematic illustrating the magnetic field from a cylindrical ferromagnetic object. In this figure the Earth’s field magnitude has been removed leaving only the magnitude of the field owing to the ferromagnetic cylin- der, often called the magnitude of the anomalous field. Because the magnitude of Earth’s magnetic field changes with time, generally with daily cycles called Diurnal changes, these changes have to be removed from the field data. To do this, a base station is usually set up at a site near the survey area where magnetic anomalies are minimal. This instrument then records the magnitude of the magnetic field at regular in- tervals, say every minute. This allows the oscillations in the magnetic field to be removed from the survey data during pro- cessing. In addition to induced magnetization, remnant magneti- zation can also produce anomalies. Remnant magnetization occurs in geologic materials, usually volcanic and igneous rocks, which originate as hot fluid lava and then cool before eventually solidifying. When the lava, or igneous material, cools below a temperature called the Curie point, the mag- netic domains in the rock (usually magnetite) are oriented in the direction of the existing magnetic field at that time. Be- cause the direction of the Earth’s magnetic field changes over FIGURE A18 Magnitude of anomalous magnetic field created by a ferromagnetic cylinder in presence of the Earth’s field.

59 geologic time, the remnant magnetic field can have a direc- tion that is different from that produced by induction with the present Earth’s magnetic field. Field Data Recording Magnetic surveys are conducted by first setting up a base station, as described previously. The survey is then con- ducted by walking across the area of interest while the mag- netometer records data, usually at several times per second. The data are stored in solid state memory in the instrument. To position the data some magnetometers can be assembled with DGPSs, allowing the spatial coordinates to be acquired simultaneously with the magnetic data. Conventional or GPS surveying of the ends of the lines may be required if DGPS data are not acquired with the magnetometer data. Linear interpolation methods can then be used to assign spa- tial coordinates to the data. Interpretation Magnetic data can be interpreted using computer software to model the anomalies. Generally, an initial model is devel- oped for the source of the anomaly and the program then cal- culates the anomaly resulting from this source. The program then modifies the depth and geometry of the source and re- calculates the anomaly. It does this until a reasonable fit is obtained between the field and model data. This process is called inversion. Another interpretation method is to calculate a function called the Analytic Signal from the field data. Figure A19 il- lustrates this function for a cylindrical source along with the magnitude of the field (Anomaly Magnitude). Because the Analytic Signal peaks over the top of the source, the location of the source is easier to position than it is from the anomaly magnitude data. In addition, the amplitude of the Analytic Signal is related to the susceptibility of the source and the width is related to the depth to the top of the source. Method Limitations The magnetic method only detects objects composed of ferro- magnetic materials, and not metals such as copper, stainless steel, or aluminum. In interpreting magnetic data for geologic targets, there are generally several different solutions that can provide a theoretical fit of the field and model data, therefore each interpreted source is not necessarily unique. Such an inter- pretation is often called a “permissive” interpretation. This means that it is a valid theoretical interpretation but may be one of several possibilities. Nonunique interpretations are much less of a problem when searching for buried ferromagnetic objects. During severe magnetic storms, when the time varying magnetic field changes are significant, it may not be feasible to record field data. References 1–3, 8, and 10. SPECTRAL ANALYSIS OF SURFACE WAVES (SASW) Introduction The SASW method provides bulk estimates of shear wave ve- locities of the subsurface. By taking measurements with an expanding geophone array a vertical profile can be developed showing the variation in shear wave velocity with depth. Basic Principles of the SASW Method The basis of the SASW method is the phenomenon that Rayleigh waves have phase velocities that depend on their wavelength, called dispersion, when traveling through a lay- ered medium. Rayleigh wave velocity depends on the mate- rial properties of the subsurface to a depth of approximately 1 wavelength. These properties are primarily the shear wave velocity, but also the compression wave velocity and the material density. Figure A20 shows the variation of particle motion with depth and illustrates that longer wavelengths penetrate to greater depths. Survey Procedures SASW testing consists of measuring the surface wave dis- persion curve and interpreting it to obtain the corresponding shear wave one dimensional vertical velocity profile. The dispersion curve is the variation of phase velocity of the fun- damental mode Rayleigh wave with frequency. There are FIGURE A19 Anomaly magnitude and analytic signal over a ferromagnetic object.

60 two main methods used in surface wave exploration. The most common is called SASW testing, which uses two geo- phones. The other method, which uses a linear array of geo- phones, is generally called array methods, or Multichannel Analysis of Surface Waves (MASW). The field setup for the SASW method is shown in Figure A21. A dynamic source is used to generate surface waves of dif- ferent wavelengths (frequencies). This can be done using small sources such as a hammer or large sources such as a dozer. These waves are monitored using two or more receivers, as il- lustrated in Figure A21. An expanding receiver array is used to avoid near-field effects associated with Rayleigh waves and source–receiver geometry is optimized to minimize body wave signal. Microtremor surface wave techniques are also becom- ing more widely used. Passive sources typically can see deeper than active sources. MASW used in addition to Mi- crotremor can be used to obtain both shallow and deeper interpretations. Method Limitations The depth of penetration is determined by the longest wave- lengths in the data. Generally, heavier sources generate longer wavelengths. Also, the depth of penetration and resolution are heavily site dependent. Cultural noise at a site may limit the signal/noise ratio at low frequencies. The field setup requires a distance between the source and most distant receiver of two to three times the maximum penetration depth. References 7, 8, 10, and 11. λ λ λ λ FIGURE A20 Schematic showing the variation of Rayleigh wave particle motion with depth. FIGURE A21 Field setup for the SASW method.