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6 The literature review presented in this chapter covers three topic areas. First, a summary of relevant findings from the 2006 NCHRP Synthesis 357: Use of Geophysics for Transporta- tion Projects is presented (Sirles). Second, a literature review of the various surface and bore- hole geophysical methods discussed in this synthesis document is presented. This section also includes a brief description of each method and its typical applications, recent examples of method application, and an updated matrix of geophysical methods and associated geologic investigation objectives. Third, a literature review of training and education resources related to geophysical measurements is presented. Review of Findings from NCHRP Synthesis 357: Use of Geophysics for Transportation Projects NCHRP Synthesis 357: Use of Geophysics for Transportation Projects (Sirles 2006) presented the state of the industry on the use of geophysics in transportation projects. The synthesis information was acquired from a variety of sources, including a literature review, interviews of agency personnel, and a survey questionnaire. The survey questionnaire was sent to 70 agencies, including transportation agencies for all 50 U.S. states, several federal agencies, and most Canadian provinces. The response rate was 90%. A summary of the 2006 report is presented in the following paragraphs. Among the most pertinent findings was that 86% of agencies reported some use of geophysical methods. Percentages are given as a proportion of the agen- cies that reported use of geophysical methods (i.e., agencies that did not use geophysical methods were not included in the denominator). Frequency of Geophysics Use The survey found that among agencies that used geophysical methods, a majority (56%) performed between 1 and 5 geophysical investigations per year, 9% performed less than 1 inves- tigation per year, and 20% performed more than 10 investigations per year (Figure 1). Also, 60% of respondents reported that their agencyâs use of geophysics had been increasing during the past 5 years. In-House versus External Contracting The 2006 survey found the most common source of geotechnical measurements to be external contractors (46%). The second most common source was a combination of both in-house and contract capabilities (41%), and the least common source was only in-house capabilities (13%). C H A P T E R 2 Literature Review
Literature Review 7 Perceived Benefits of Geophysics and Deterrent to Use When agencies were asked what they considered the greatest benefits of geophysics in transportation projects, their top three answers were as follows: data acquisition speed (21%), cost benefit (19%), and better subsurface characterization (17%), as shown in Figure 2. The three greatest deterrents to use were as follows: (1) lack of understanding, (2) non- uniqueness of results, and (3) lack of confidence, as shown in Figure 3. The top three actions to overcome these deterrents were (1) training, (2) experience (and sharing experiences), and (3) implementing standards. Methods and Applications The most commonly used methods reported by the respondents were seismic (26%), ground-penetrating radar (GPR) (22%), and vibration monitoring (22%), as shown in Fig- ure 4. The most common applications for geophysics (excluding nondestructive deflection testing [NDT]) were bedrock mapping (32%), subsidence investigations (22%), and mapping of soil deposits (11%), as shown in Figure 5. Also, 70% of the agencies reported that they use only âproven, state-of-the-practice geophysical methods.â Figure 1. Number of geophysical investigations performed per year, as reported in 2006 synthesis (Sirles 2006). Figure 2. Greatest benefits of geophysics in agency transportation projects, as reported in 2006 synthesis (Sirles 2006).
8 Advancements in Use of Geophysical Methods for Transportation Projects Figure 3. Greatest deterrents to use of geophysics in agency transportation projects as reported in 2006 synthesis (Sirles 2006). Figure 4. Most commonly used geophysical methods, as reported in 2006 synthesis (Sirles 2006). Figure 5. Most common applications of geophysics as reported in 2006 synthesis (Sirles 2006).
Literature Review 9 Budgeting and Cost The 2006 survey results indicated that the fiscal budgets of most transportation agen- cies (67%) do not include geophysics. Of âotherâ sources of funding for geophysics, 63% of respondent agencies reported funds of less than $100,000. Only 10% used funds in excess of $100,000 for geophysical investigations. Despite a lack of independent funds for geophysical investigations, several agencies reported that such work is often funded through large annual budgets for geotechnical investigations. The survey also reported that the decision maker for budget issues related to the use of geophysics was typically either the division or branch manager (32%) or the team leader or project manager (22%). Only 7% of agencies allocated annual funds for geophysical research. The majority of geophysical investigations cost less than $10,000, as shown in Figure 6. Training A common theme in the synthesis was the need for standards and more educational training. At the time of the survey, FHWAâs Application of Geophysical Methods to Highway Related Problems (Wightman et al. 2004) had only recently been developed. Though 69% of the respondents indicated they were aware of the report, only 4% of the agencies reported that they provided training related to geophysics. Training was also identified as one of the best ways to increase staff confidence in the use of geophysical methods. Geophysical Methods and Applications for Transportation Projects This section includes brief descriptions of the basic principles and common applications of the surface and borehole geophysical techniques referenced in the synthesis. Select examples discussed in the literature of noteworthy applications are also presented. For detailed descrip- tions of the methods and their applications, readers should refer to FHWAâs Application of Geophysical Methods to Highway Related Problems (Wightman et al. 2004) or to other resources described in the training and education section of this chapter. In addition, this section includes an updated matrix of appropriate surface and borehole methods and their associated geological investigation objectives. Figure 6. Typical costs of geophysical investigations and number of investigations per year at certain cost level as reported in 2006 synthesis (Sirles 2006).
10 Advancements in Use of Geophysical Methods for Transportation Projects Surface Geophysical Measurements Surface geophysical methods are defined as methods that are performed without the need for penetration into the subsurface through use of a borehole or penetrometer. All sources and receivers used in the measurement are placed on or above the surface of the earth. Categories of surface geophysical methods include seismic methods, electrical methods, electromagnetic methods, magnetic methods, and gravity methods. Each category comprises several types of measurements, which are briefly outlined in the next section. Seismic Methods When soil or rock is mechanically disturbed, body waves consisting of compression waves (i.e., p-waves) and shear waves (i.e., s-waves) propagate outward from the source. If a free surface is present, surface waves (i.e., Rayleigh waves or Love waves) will propagate along the boundary. Seismic methods make use of these propagating mechanical waves to infer condi- tions below the surface. One advantage of using seismic waves for measurement is that the velocity of the waves can be directly related to the small-strain modulus of the material. Therefore, in addition to providing a subsurface velocity profile, the velocity values serve as important parameters for engineering analysis, especially for earthquake applications. Seismic Refraction. Seismic refraction is used to create a model of the thickness, depth, and velocity of subsurface layers by recording the arrival of waves at the surface emanating from critically refracted waves that travel along layer boundaries. It is one of the most commonly used methods in geotechnical near-surface applications. Refraction measurements may use either p-wave or s-wave arrivals and are performed using a source (e.g., a sledge hammer, a weight drop, or explosives) that is excited at multiple locations and a spread of geophones (typically 24 or more) that are placed at equal intervals along the surface. Where soft layers exist beneath stiff layers, refraction will miss the soft layers and lead to errors in depth calculations. Common applications of refraction are mapping bedrock topography, estimating groundwater depth, and obtaining seismic velocities for excavation evaluation (i.e., rippability). Seismic Reflection. Seismic reflection detects the arrival of body waves (p- or s-waves) that travel downward from the source and reflect off of layer boundaries with a contrast in impedance (i.e., a product of velocity and mass density). The measurement is performed with a seismic source (e.g., a sledge hammer, a weight drop, a minivib [seismic vibrator], or explosives) and a spread of receivers that record the wave arrivals. The reflected arrival is often of low amplitude and is never the first on the wave record, so the data are processed in a number of ways to determine the reflected arrivals. Reflection surveys are not as common as refraction measurements are in engineering practice, but they are useful for applications that require mapping of the subsurface stratigraphy or other features such as faults and subsurface anomalies. Figure 7 shows a shear wave reflection profile from a survey performed in Norway using a minivib source. Seismic Refraction Tomography. Seismic refraction tomography is an alternative approach to processing refraction data that produces two-dimensional (2D) profiles of p-wave or s-wave velocities. In the tomographic approach, the subsurface is divided into a number of cells, as opposed to the continuous layers of constant velocity used in conventional processing. The ray paths are traced through the model, and velocity values for each cell are determined through an inversion process. Compared with conventional refraction processing methods, the tomo- graphic inversion approach to processing refraction data can better handle variations in lateral velocity and provides a higher-resolution image of the subsurface. The method is used in many of the same applications as is seismic refraction, but it is especially effective for imaging lateral
Literature Review 11 variability. An example of seismic tomography for imaging lateral variability between boreholes using borehole-to-surface tomography is presented in Figure 39 (in Chapter 4). Horizontal-to-Vertical Spectral Ratio. The horizontal-to-vertical (H/V) spectral ratio (HVSR) method (sometimes called single-station passive) is a passive seismic method that uses recordings of ambient noise on a single, three-component sensor collected over a span of several minutes. When the sensor detects a strong impedance contrast (velocity contrast greater than about 2), a peak will be observed in a plot of the ratio of the frequency spectrum from the horizontal sensor divided by the spectrum from the vertical sensor. This frequency of the peak in the HVSR plot is used in earthquake engineering applications to estimate site frequency but can also be used to estimate the depth to bedrock. Recent studies have focused on using this method to detect landslide slip surfaces (Pazzi et al. 2017), and a novel use of this method is described in the case examples in Chapter 4. Full Waveform Inversion. Methods such as seismic refraction tomography use the first arrivals of the waves of interest to create subsurface velocity models. Full waveform inversion (FWI) is a more complex approach that uses the full recorded waveform in the inversion process. The data collection equipment, like that used in other seismic methods, consists of a spread of geophones and multiple impacts of a seismic source (typically a sledge hammer) in and around the geophone spread. The subsurface velocity model is divided into cells, and an inversion approach is used to generate a 2D model of p-wave and s-wave velocities that produces theo- retical waveforms that match the recorded waveforms. The method has been shown to be effec- tive for detecting subsurface anomalies such as abandoned mines and karst features (see Sullivan et al. 2016; Zheng et al. 2016) and has also been used for applications such as foundation reuse (Nguyen et al. 2016). Recently, the capabilities of FWI have expanded to include three- dimensional (3D) modeling, which has successfully detected voids in karst terrain (Tran et al. 2019). An example of the imaging capabilities of 3D FWI is shown in Figure 8. Active Source Surface Wave Methods. Surface wave methods use measurements of surface waves (typically Rayleigh waves) to infer shear wave velocity profiles. Active source methods use an impact source (hammer or drop weight) or a vibratory source (e.g., Vibroseis) to create surface wave energy that is recorded by a spread of vertically oriented geophones positioned on the surface. The phase velocity of the surface wave at different frequencies is determined from Figure 7. Depth migrated section from shear wave reflection measurements using a minivib source in Trondheim, Norway (Krawczyk et al. 2013).
12 Advancements in Use of Geophysical Methods for Transportation Projects the geophone recordings, and a dispersion curve that relates phase velocity to frequency is developed. The shear wave velocity profile that produces a matching dispersion curve is deter- mined through an inversion process. The spectral analysis of surface waves (SASW) method and the multichannel analysis of surface waves (MASW) method are two surface wave methods commonly used in geotechnical applications. Common applications include devel- opment of shear wave velocity profiles for earthquake engineering and imaging of the subsur- face for stratigraphy or anomaly detection. One limitation of the method is that it requires a low-frequency source (heavy drop weight or large vibrator) to generate deep shear wave velocity profiles. Passive Surface Wave Methods. Passive surface wave methods are based on the same prin- ciples as are active surface wave methods, but they use ambient surface wave energy. Because the location of the source is not known, a 2D (e.g., circular) array is used so the source direction can be determined. The abundance of ambient low-frequency energy makes this method especially useful for deep profiling applications (i.e., depths of hundreds of meters). Recently, Deschenes et al. (2018) used passive measurement to generate shear wave velocity profiles to depths of more than 1,000 meters in the Canterbury region of New Zealand, as shown in Figure 9. A variation on the passive method is the refraction microtremor (ReMi) method, which uses a linear array and makes assumptions regarding the characteristics of the ambient wavefield. Electrical Methods Electrical methods measure the electrical characteristics of subsurface materials, such as resis- tivity, voltage decay, or electric fields, to infer subsurface conditions. Although the electrical properties measured are rarely of direct use in engineering analysis, the values can be associated with subsurface conditions. Figure 8. Field experiment results showing distribution of shear wave velocity (Vs) and compression wave velocity (Vp) (m/s) in final inverted model. Low-velocity regions indicating voids were confirmed by standard penetration test (V = velocity) (Tran et al. 2019).
Literature Review 13 Resistivity. Electrical resistivity (ER) measurements are among the most common in geo- technical applications. This method measures the apparent resistivity of a volume of soil and rock by injecting current through a pair of electrodes, then measuring the potential difference between those electrodes and a pair of electrodes on the surface. Resistivity soundings to determine varia- tion in depth can be performed by increasing the spacing between electrodes to increase the depth of penetration. Traverses to determine lateral variation can be performed by maintaining fixed spacing between electrodes and placing them at fixed intervals along the surface. Electrical resis- tivity tomography (ERT), or electrical resistivity imaging (ERI), involves creating 2D images of the subsurface resistivity values from the apparent resistivity values collected by a multielectrode configuration at the surface. The resulting image shows both the depth and the lateral variation in resistivity and can be useful for detecting features with different electrical properties, such as changes in soil, voids and fractures in rock, depth of bedrock, variations in saturation, and changes in groundwater conditions. Field conditions, such as buried metal, overhead lines, or metal fences, can negatively influence the quality of the data. The use of ERI to determine bedrock depth and detect the disturbed area of a shallow landslide is illustrated in Figure 28 and Figure 33, respectively, in Chapter 4. Induced Polarization. Induced polarization (IP) measurements are often performed in conjunction with ER measurements. Like ER, the IP measurement is performed by injecting current into the ground through two electrodes and monitoring the potential using two other electrodes. In this case, the decay in voltage over time is monitored. In geotechnical applications, the ER/IP method has been used to detect open and clay-filled cracks (Schmutz et al. 2011) and to map landfills (Gazoty et al. 2012), and has recently proved useful for estimating the unknown depths of bridge foundations (Tucker et al. 2015), as shown in Figure 10. Self-Potential. Self-potential or spontaneous potential (SP) is a passive electrical technique that infers subsurface conditions by measuring small, naturally occurring electrical fields. The survey is performed by measuring the potential difference between two electrodesâone base Figure 9. Deep shear wave velocity profile developed from inversion of passive surface wave measurements (Deschenes et al. 2018).
14 Advancements in Use of Geophysical Methods for Transportation Projects electrode and one mobile electrode that is moved around the survey location. In engineering applications, the SP method is used primarily to map the flow of groundwater, including flow conditions in karst terrain (Bumpus and Kruse 2014). Electromagnetic Methods Several geophysical methods use the principles of electromagnetism (EM) to infer subsurface conditions. These methods are used to image the subsurface in an effort to detect features of interest. Induction methods (time-domain EM and frequency-domain EM) use a time-varying magnetic field as a source to generate electrical currents in the ground and are effective for locating changes in conductivity (the reciprocal of resistivity) in the subsurface. Another EM method, GPR, uses high-frequency EM radiation to propagate radar pulses into the ground and detect subsurface features from reflections of the radar pulse. Ground-Penetrating Radar. GPR is one of the most commonly used methods in near- surface geophysical investigations. The method propagates high-frequency EM waves into the subsurface to record reflections from subsurface interfaces with contrasts in the dielectric constant. The travel time and amplitude of the received waves can be used to infer the location of subsurface structures or features. For geophysical applications, antennae with frequencies in the range of 25 to 1,500 megahertz are typically used, while higher-frequency systems are used for nondestructive testing applications such as pavement evaluations. Lower-frequency systems will penetrate deeper below the subsurface at the expense of target resolution. GPR can be used to detect a wide range of features for geotechnical engineering applications. Such features might include subsurface stratigraphic changes, bedrock depth, the water table, or underground storage tanks. One common application is the detection of cavities and voids in karst terrain (see Estrada-Medina et al. 2010), as illustrated in Figure 11. A major limitation of GPR is its limited depth penetration in conditions of clayey and saturated soils. Time-Domain EM. Time-domain EM (TDEM, also known as transient EM) is an active source method to determine the resistivity distribution of the subsurface. TDEM is based on the induction of electrical currents in the subsurface by an EM transmitter loop. After the transmitting signal is turned off, a receiver coil is used to record the secondary currents that are generated in the subsurface by the EM source. A sounding curve that relates the measured resistivity as a func- tion of time after the signal is turned off is developed. A model of subsurface resistivity is created by matching the theoretical sounding curve to the measured curve. The depth of investigation is controlled by the size of the transmitter loop and can range from tens of feet to thousands of feet. TDEM is effective for identifying conductive layers and is commonly used in engineering applica- tions to detect metallic objects (such as utilities, underground tanks, or unexploded ordnance), to map the groundwater surface, and to map changes in soil and rock (Hicken and Best 2013). Figure 10. Inverted IP section showing interpreted foundation element (rectangular) in a medium-dense fine sand (Tucker et al. 2015 with permission from ASCE).
Literature Review 15 Frequency-Domain EM. In the frequency-domain EM (FDEM) method, the transmitter loop generates a sinusoidally varying current at a specific frequency. The measurement is based on analysis of the in-phase and out-of-phase components of the secondary field recorded at the surface. Terrain conductivity meter and very low frequency EM are two types of FDEM. Terrain conductivity meter measurements can be performed without ground contact and are effective for detecting a range of subsurface features associated with changes in conductivity. Magnetic Methods Magnetic methods are passive methods used to measure localized distortions in the Earthâs magnetic field caused by the presence of ferromagnetic materials. Data can be collected either through a single sensor that measures the strength of the magnetic field or through two sensors placed at different heights above the ground surface that measure the gradient of the magnetic field. Magnetic methods are commonly used to locate buried metallic objects. Gravity Methods Gravity methods passively measure changes in the force of gravityâattributable to local variations in densityâto infer subsurface conditions. Engineering applications of gravity methods are often termed âmicrogravity measurements.â Measurements are taken with a gravimeter, and the data are corrected to account for other factors, such as elevation, surrounding topography, and tidal effects. Precise values of ground elevation (within 2 centimeters) at the measurement points must be determined. The gravity method is used in engineering applications to detect subsurface voids or cavities and to determine overburden thickness. Paine et al. (2012) describe the use of microgravity measurements and radar interferometry to assess the risk of collapse in sinkhole-prone areas. Borehole Geophysical Measurements Many of the surface geophysical measurements just described can also be performed in a borehole. The objective of borehole geophysical methods is typically to infer properties of the surrounding soil and rock. In addition, many of these methods provide a continuous record of depth that can be correlated with data from other boreholes or data from soil samples taken at specific depths. The borehole methods mentioned in the survey are briefly described in the following paragraphs, along with the soil properties that can be obtained from the measurements. Figure 11. Soil pockets observed in a quarry wall (left), and GPR record showing soil pockets (SP = soil pocket) (Estrada-Medina et al. 2010).
16 Advancements in Use of Geophysical Methods for Transportation Projects Seismic Methods Seismic borehole methods involve exciting body wave (p-wave and s-wave) energy and detect- ing the wavesâ arrival to determine the propagation velocity in the soil or rock. The product derived from these measurements is a depth profile of p- and s-wave velocities, which can be used to determine a materialâs small-strain stiffness properties. Common seismic borehole methods include the downhole seismic, crosshole seismic, and P-S suspension logger methods. Downhole Seismic. Downhole measurements require a single borehole with the source at the surface and a downhole geophone sensor that can be moved to different depths in the bore- hole. P-wave and s-wave energy is excited at the surface, and wave arrival times are recorded by the downhole sensor. The sensor is incrementally moved down the hole, where the measurement is repeated. Downhole measurements can also be taken with a seismic cone penetrometer. Crosshole Seismic. Crosshole seismic measurements require two or three boreholes with an in-hole source and receivers in adjacent boreholes. Measurements are taken by exciting p- and s-waves from the source borehole and detecting the wave arrivals in the receiver boreholes. After a measurement is completed at a given depth, the source and receivers are typically moved together to the next depth. When measurements are performed at several combinations of source and receiver depths, tomographic images of the velocity profile between boreholes can be developed. P-S Suspension Logger. The P-S suspension logger is a single 7-meter downhole probe that contains a source and two receivers. The P-S logger requires only a single fluid-filled borehole and is often used to measure profiles in deep uncased boreholes of several hundred feet. A common engineering application of P-S logging measurements is the generation of deep velocity profiles for use in earthquake site response calculations. Electrical and EM Methods Several of the electrical and EM methods described earlier have also been adopted for use in boreholes, as described in the following paragraphs. Borehole Radar. Borehole radar, like GPR, uses transmitting and receiving antennae to send radar pulses and detect the reflected arrivals. The system must be used in uncased or PVC (polyvinyl chloride)âcased boreholes. Measurements are usually taken in a single borehole but may be used in a crosshole fashion to generate tomographic images. Typical applications of borehole radar include mapping voids, determining fracture orientation and density, imaging the soil profile, and imaging embedded manufactured structures. An example of borehole radar used for fracture mapping (Liu et al. 2006) is presented in Figure 12. Borehole Electrical Resistivity. Measurements of resistivity and spontaneous potential can also be performed in boreholes. Normal resistivity logs use electrodes variably spaced along the borehole probe to record the resistivity of surrounding soil and rock. Borehole measurements of spontaneous potential are performed in fluid-filled holes and record potentials developed between the borehole fluid and the surrounding soil and rock. These measurements are often used to determine lithology. Borehole EM Induction. Borehole EM induction methods are used to measure conductivity or resistivity of the soil and rock surrounding the borehole. This method can be used in both PVC-cased boreholes and dry boreholes. The conductivity and resistivity values can be related to the porosity, permeability, and clay content of the surrounding materials. Nuclear Borehole nuclear methods are passive or active measurement techniques that rely on the detection of unstable isotopes near the borehole. Natural gamma logging measures the
Literature Review 17 background levels of radiation and can be used for lithology, for correlation of strata, and to infer permeability. Active methods such as gamma-gamma logging and neutron logging intro- duce small levels of radiation and measure backscatter from the soil and rock. Gamma-gamma logging can be used to determine density of subsurface strata, and neutron logging can be used to determine moisture content and porosity. Televiewers Borehole televiewers provide oriented images (either optical or acoustic) of the borehole wall. Optical televiewers use a high-resolution digital camera to obtain an optical image of the bore- hole wall. Acoustic televiewers use a sonar transducer to obtain an image of acoustic waves reflected from the borehole wall. The resulting high-resolution images are useful for detecting lithologic contacts, cavities, fractures, and joints, as well as for characterizing the strike, dip, and spacing of these features. An example of an acoustic televiewer image is shown in Figure 36. Table 1 presents a matrix of the applicability of the methods described here to specific geologic investigation objectives. Explanations of the specific objectives listed in Table 1 are as follows: â¢ Determine rock depth. Determine depth to top of rock below a single point. â¢ Determine rock topography. Create 2D model of top of rock. â¢ Detect fracture/fault zones. Detect and locate areas of faulting and fracturing of rock. â¢ Characterize fractures/faults. Determine characteristics of faults and fractures such as orientation, spacing, and in-fill material. â¢ Determine rippability. Determine ease with which rock can be mechanically excavated. â¢ Map weak zones in rock. Locate regions of weakness attributable to factors such as weathering and fracturing. Figure 12. Example of (a) borehole radar section, (b) interpreted fracture reflections, and (c) spatial distribution of fractures (Liu et al. 2006).
18 Advancements in Use of Geophysical Methods for Transportation Projects Application Surface Methods Borehole Methods R ef ra ct io n R ef le ct io n R ef ra ct io n T om og ra p h y H V S R FW I S u rf ac e W av e R es is ti vi ty IP S P G P R FD E M T D E M M ag n et ic G ra vi ty D ow n h ol e C ro ss h ol e P S L og ge r R ad ar In d u ct io n N u cl ea r R es is ti vi ty /S P O p ti ca l T el e. A co u st ic T el e. Determine rock depth 1 1 1 2 1 1 1 2 1 2 2 2 Determine rock topography 1 1 1 2 1 2 1 2 1 2 2 2 Detect fracture/fault zones 2 1 1 1 2 1 1 2 1 2 2 2 2 2 2 1 2 2 2 1 1 Characterize fractures/faults 1 2 1 2 1 1 1 Determine rippability 1 1 2 2 1 1 1 Map weak zones in rock 1 2 1 1 2 1 2 2 2 2 2 2 2 2 2 2 2 1 1 Map lithology in rock 2 2 2 2 2 1 2 2 2 2 2 2 2 2 1 1 1 1 1 1 Determine rock mass stiffness 1 1 1 1 1 1 1 Determine rock mass density 2 2 1 Map sands, clays, gravels 2 2 2 2 1 1 2 1 1 2 2 2 1 1 1 Map organic materials 2 2 2 2 2 1 1 2 2 2 2 2 2 Map landfills 2 2 1 2 1 1 2 1 1 1 Determine soil stiffness 1 1 2 1 1 1 1 1 Determine soil density 2 2 2 1 Map groundwater table 1 2 2 1 1 1 1 1 1 1 1 1 Map groundwater flow 1 2 Map landslide extent 1 2 1 1 2 1 2 2 2 2 2 2 2 Identify landslide slip surface 2 2 2 2 2 2 Detect voids, cavities 2 1 1 1 2 1 2 1 2 2 1 1 1 1 Image scour features 1 1 Estimate clay content 1 1 2 2 2 2 1 1 Notes: 1 = primary application (method is effective and commonly used for this application); 2 = secondary application (method can provide valuable direct or indirect information for application); blank spaces indicate that method is not typically used for application. Table 1. Matrix of commonly used geophysical methods in relation to geologic investigation objectives. â¢ Map lithology in rock. Map changes in rock type (most effective in conjunction with borehole control). â¢ Determine rock mass stiffness. Determine quantitative value of small-strain rock stiffness from velocity measurements. â¢ Determine rock mass density. Correlate measurements with rock density or rock density variations. â¢ Map sands, clays, gravels. Map soil type (most effective in conjunction with borehole control). â¢ Map organic materials. Map the lateral extent and depth of organic deposits. â¢ Map landfills. Map the lateral extent and depth of landfill materials. â¢ Determine soil stiffness. Determine quantitative value of small-strain soil stiffness from velocity measurements. â¢ Determine soil density. Correlate measurements with soil density or soil density variations. â¢ Map groundwater table. Locate the depth to the groundwater table or depth to full satura- tion of soils. â¢ Map groundwater flow. Map locations of groundwater movement and flow paths. â¢ Map landslide extent. Map the lateral extent and depth of disturbed soil from a landslide event. â¢ Identify landslide slip surface. Locate the depth of the landslide slip surface. â¢ Detect voids, cavities. Detect the presence of voids and cavities such as karst features or abandoned mines. â¢ Image scour features. Determine depth and extent of existing scour features, as well as previ- ously in-filled scour holes and surfaces. â¢ Estimate clay content. Estimate percentage of clay in soil.
Literature Review 19 Educational and Training Resources A variety of educational and training resources are available to geotechnical engineers and other users of geophysical methods. This portion of the literature review provides a summary of current educational and training resources regarding (1) applications of geophysical methods, (2) fundamentals of geophysical methods, and (3) performing geophysical measurements. Applications of Geophysical Methods Several training and educational resources are available to help geotechnical engineers under- stand the capabilities, limitations, and appropriate applications of the various geophysical methods. This basic level of training does not require users to know how to perform the measurements or even how the methods work, but it instead provides information on how the methods can be used for engineering applications. One of the best sources on appropriate applications of geophysical methods for transportation projects is FHWAâs Application of Geophysical Methods to Highway Related Problems (Wightman et al. 2004). Although this manual is now 16 years old, its information is still relevant and helps users easily identify appropriate methods for different investigation objectives. Another older resource is the U.S. Army Corps of Engineersâ document titled Geotechnical Investigations (U.S. Army Corps of Engineers 2001), which provides in Chapter 4 a 1â4 scale for rating the applicability of methods for various objectives. A more recent resource is Transportation Research Circular No. E-C130: Geophysical Methods Commonly Employed for Geotechnical Site Characterization (Anderson et al. 2008), which includes tables of potential applications as well as information on selecting appropriate methods and geophysical contractors. In addition, a recent California Department of Transportation (Caltrans) document titled Geophysical Methods for Determining Geotechnical Engineering Properties of Earth Materials (Coe et al. 2018) presented a comprehensive overview of the various engineering properties that can be measured or inferred from geophysical methods. Several organizations sponsor webinars or short courses on geophysics-related topics. In 2018, FHWA offered an Every Day Counts (EDC)-5 webinar to transportation agencies. The webinar, Advanced Geotechnical Methods in Exploration (A-GaME), presented examples of the successful use of geophysical methods in transportation projects. Other organizations that offer webinars and short courses include ASCEâs Geo-Institute, the Environmental and Engineer- ing Geophysical Society (EEGS), and the Society of Exploration Geophysicists (SEG). EEGS has even offered the short course âNear Surface Characterization Using the HVSR Passive Seismic Methodâ during its Annual Meeting/Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP). The organization also maintains a âWhat Is Geophysics?â web page that covers many of the basics of geophysical methods and their common applications. Other sources of information on practical applications of geophysics are conference proceedings and journal publications. The ASCE Geo-Institute annual GeoCongress confer- ence, Transportation Research Board Annual Meeting, Society of Exploration Geophysicists Near-Surface Geophysics Technical Section Annual Meeting, and EEGS Annual Meeting/ SAGEEP are all sources of papers on practical use of near-surface geophysics in engineering applications. Relevant journals include Geophysics, Leading Edge, Near Surface Geophysics, Journal of Environmental and Engineering Geophysics, and the Transportation Research Record, to name a few. In addition, most of the major suppliers of geophysical services maintain web pages or down- loadable documents with summary information on the various geophysical methods and their appropriate applications.
20 Advancements in Use of Geophysical Methods for Transportation Projects Fundamentals of Geophysical Measurements In some cases, practicing engineers either need or want a deeper understanding of geophysical methods. Training and education for this audience help users grasp the principles of geophysical measurements and understand the offerings of geophysical contractors. FHWAâs Application of Geophysical Methods to Highway Related Problems (Wightman et al. 2004) describes the techniques, data collection, and data analysis involved in the various methods. So many books and textbooks are devoted to near-surface geophysical methods that it is impos- sible to provide a comprehensive list. A few recent examples include Field Geophysics (Milsom and Eriksen 2011), a small handbook that covers the fundamentals of various geophysical methods; An Introduction to Applied and Environmental Geophysics (Reynolds 2011), which is more comprehensive; and the textbook Surface Wave Methods for Near-Surface Site Characterization (Foti et al. 2017), which is devoted solely to seismic surface wave methods. Performing Geophysical Measurements As more transportation agencies choose to perform geophysical measurements in-house, those agencies are experiencing a growing need for training. Some methods commonly per- formed in-house by transportation agencies include GPR, resistivity, refraction, MASW, and ReMi. Because these methods are often carried out by personnel with limited backgrounds in geophysics, effective training is critical. Equipment vendors are the most frequent suppliers of training on geophysical methods. Many vendors offer courses on how to collect, process, and interpret geophysical data, and some even assist with data processing for difficult cases. In addition to vendors, most of the major conferences mentioned earlier routinely offer short courses on how to perform geophysical measurements. The 2019 SEG meeting offered a 2-day course titled âPractical Seismic Surface Wave Methods,â which covered MASW measurements. In 2018, EEGS offered âPassive Surface Wave Methods: Theory and Practiceâ and âGPR Principles, Practices, and Processing.â