APPENDIX B
FINAL REPORT GEOPHYSICAL INVESTIGATION WATER TOWER NO. 6 SITE PLANT CITY, FL Prepared for Madrid Engineering Group, Inc. Bartow, FL Prepared by GeoView, Inc. St. Petersburg, FL
February 28, 2010 Mr. Larry Madrid, P.E. Madrid Engineering Group, Inc. P.O. Box 2506 Bartow, FL 33831-2506 Subject: Transmittal of Final Report for Geophysical Investigation Water Tower No. 6 Site Phase 2- Plant City, FL GeoView Project Number 8123 Dear Mr. Madrid, GeoView, Inc. (GeoView) is pleased to submit the final report that summarizes and presents the results of the geophysical investigation conducted at the Tank No. 6 Site. Ground penetrating radar and seismic reflection were used to evaluate near-surface geological conditions. GeoView appreciates the opportunity to have assisted you on this project. If you have any questions or comments about the report, please contact us. GEOVIEW, INC. Michael J. Wightman, P.G. President Florida Professional Geologist Number 1423 Chris Taylor, P.G. Vice President Florida Professional Geologist Number 2256 A Geophysical Services Company 4610 Central Avenue Tel.: (727) 209-2334 St. Petersburg, FL 33711 Fax: (727) 328-2477
Page 1 1.0 Introduction The site was located along National Guard Drive in Plant City, Florida. Prior to the geophysical investigation, two surface depressions had formed north of Water Tank No. 6. The first depression was approximately 40 feet (ft) in diameter, 2 ft deep and located approximately 70 ft north of the water tank. The second depression was approximately 10 ft in diameter, 3 ft deep and located approximately 160 ft north of the water tank. A prior geophysical investigation utilizing ground penetrating radar (GPR) was conducted in January 2010 (GeoView Final Report Number 7837). For this investigation the area for the GPR survey was expanded, but also included an area from the previous survey. In addition, a high-resolution seismic reflection survey was performed. The purpose of the geophysical investigation was to help characterize near-surface geological conditions around the water tower and to identify subsurface features that may be associated with sinkhole activity. The location of the geophysical survey area is provided on Figure 1. The investigation was conducted on February 14 and 15, 2010. A discussion of the field methods used to generate the report figures is provided in Appendix A2.1. 2.0 Description of Geophysical Investigation 2.1 Ground Penetrating Radar Survey A GPR survey was conducted around within the fenced enclosure of the water tower area and to the east and west of the fenced enclosure. The GPR survey was conducted along a series of perpendicular transects spaced approximately 10 ft apart. The GPR data was collected with a Mala radar system. The GPR settings used for the survey are presented in Table 1. Table 1 GPR Equipment Settings Used for Exterior and Interior GPR Surveys Location Antenna Frequency Time Range (nano-seconds) Estimated Depth of GPR Signal Penetration Exterior 250 MHz 1/ 210 20 to 30 ft bls 1/ MHz means mega-hertz and is the mid-range operating frequency of the GPR antenna. 2.1 Seismic Reflection Survey The seismic survey was conducted along four transect lines. The locations of the lines are shown as Line 1, Line 2, Line 3 and Line 4 on Figure 1. Data acquisition was accomplished using a Geometrics Geode, 48-channel seismograph
Page 2 system. Geoview acquired compression- (P-) wave seismic reflection data using a basic 48-geophone spread. The geophones were placed in a line with 5-foot spacing between each of the geophones. Shot points were performed at 5-foot intervals along each of the transects. The energy source used at each shot point consisted of a steel plate being struck by a 12-pound sledgehammer. Each shot point consisted of multiple hammer blows in an attempt to maximize the desired signal while minimizing background noise. A total of 225 shot points were recorded during the survey A description of the geophysical techniques and the methods employed for geological characterization studies is provided in Appendix A2. 3.0 Identification of Possible Sinkhole Features The features observed on that are most commonly associated with sinkhole activity are: A downwarping of reflector sets, that are associated with suspected lithological contacts, toward a common center. Such features typically have with a bowl or funnel shaped configuration and can be associated with a deflection of overlying sediment horizons caused by the migration of sediments into voids in the underlying limestone. If the GPR reflector sets are sharply downwarping and intersect, they can create bow-tie shaped GPR reflection feature, which often designates the apparent center of the GPR anomaly. A localized significant increase in the depth of the penetration and/or amplitude of the GPR signal response. The increase in GPR signal penetration depth or amplitude is often associated with either a localized increase in sand content at depth or decrease in soil density. An apparent discontinuity in reflector sets, that are associated with suspected lithological contacts. The apparent discontinuities and/or disruption of the reflector sets may be associated with the downward migration sediments. The greater the severity of these features or a combination of these features the greater the likelihood that the identified feature is a sinkhole. It is not possible based on the geophysical data alone to determine if an identified feature is a sinkhole or, more importantly, whether that feature is an active sinkhole.
Page 3 4.0 Survey Results 4.1 Discussion of GPR Survey Results Results of the additional GPR survey indicated the presence of two welldefined, relatively continuous sets of GPR reflectors at depth ranges of 5 to 8 ft bls and 12 to 25 ft bls. The upper GPR reflector set likely correlates to the interface between the unsaturated and saturated soils (water table). The lower GPR reflector set is likely associated with some change in lithological conditions at that depth range. One GPR anomaly area was identified at the site during this phase of the investigation. The anomaly area is designated as GPR Anomaly 3 on Figure 1. GPR Anomaly 3 is elliptical in shape with total area of approximately 60 square ft. The anomaly is located southwest of the water tower. The apparent vertical relief of the upper portion of the anomaly area is 1 to 3 ft as characterized by the observed minor downwarping of the GPR reflector set. A localized increase in the depth of penetration of the GPR signal was also observed within the anomaly area. The apparent center of the feature is characterized as the area of maximum penetration of the GPR signal. The previously identified GPR anomalies (GPR Anomaly 1 and 2) are also shown on Figure 1. The anomalies are numbered in order of significance with GPR Anomaly 1 being the most significant and GPR Anomaly 3 being the least significant. 4.2 Discussion of Seismic Reflection Survey Results The final processed high-resolution seismic reflection sections are presented in Appendix 1 on Figures 2, 3, 4 and 5. Seismic reflection sections illustrate geologic layers as continuous ( coherent ) horizons that can be traced across the full section. Disturbances such as faults, sinkholes, and cavities are observed as corresponding offsets or loss of continuity in these reflecting horizons. The horizontal scale on these sections is merely distance along the particular transect. The vertical scale is the reflection time. That is, it is the time taken from the moment of the hammer blow for the seismic wave to travel down to the geologic horizon, reflect/echo from it, and travel back up to the ground surface. To convert these times to actual depth, we need to know or approximate wave speed in the subsurface or have a deep borehole for calibration. Since such borehole velocity information was not available at the site, approximate velocity values for the type of soils encountered at the site were used. The velocities used to calculate the depths were 1,000 feet per second for unsaturated soils, 5,000 feet per second for saturated soils, and 6,000 feet per second for weathered limestone. If we estimate
Page 4 the depth to the bottom of the cap rock as being approximately 75 feet bls, then the reflection (two-way time) from the bottom of rock should be observed at a time of approximately 40 to 50 milliseconds. The seismic data was of acceptable high quality. On the seismic cross sections, there is a strong, mostly consistent reflection (highlighted in yellow) that occurs near the 45 millisecond mark. It is suspected that this reflection is from the top of rock. The four seismic lines tie together very well (i.e. they are imaging the same reflection character at the same times), which indicates that the data is consistent and of high quality across the site. Line 1 (Figure 2) shows the reflector slightly dipping down to the west. However, there are no obvious disruptions or discontinuites on the seismic reflections of Line 1. We do not see indications on Line 1 that would be consistent with potential karst development. Line 2 (Figure 3) shows a minor deflection of the reflector. The center of this area is located at a horizontal distance of 135 to 150 ft along Line 2. This area is shown as Seismic Anomaly 2 on Figure 1, 3 and 4. It is noted that there are no apparent disruptions to this reflector and therefore, it is likely that this minor anomaly is not indicative of karst activity. Line 3 (Figure 4) shows the same minor deflection of the reflector that was identified on Line 2. The center of this area is located at a horizontal distance of 70 to 90 ft along Line 3. This area is shown as Seismic Anomaly 2 on Figure 1, 3 and 4. It is again noted that there are no apparent disruptions to this reflector and therefore, it is likely that this minor anomaly is not indicative of karst activity. However, due to the fact that this feature was visible on two crossing lines, it was identified as a possible anomalous feature. Line 4 (Figure 5) shows an area of significant downwarping of the suspected top of rock reflector and discontinuities in the underlying and overlying reflectors. This area is identified as Seismic Anomaly 1 on Figures 1 and 5. The location of this feature corresponds well to the location of the surface depression and to the location of the previously identified GPR Anomaly 1. The anomalous conditions are located at a horizontal distance of 45 to 95 ft along Line 4. It noted that the reflectors from Line 4 appear to return to an undisturbed state at a horizontal distance of 100 ft, which is approximately 20 ft north of the fence and 75 ft north of the center of the water tower. Accordingly, based on the results of the combined GPR and seismic reflection studies the sinkhole feature does not appear to extend underneath the water tower.
APPENDIX 1 FIGURES AND EXAMPLE OF GPR ANOMALY 3
Water Table Soil Layer GPR Anomaly 3 GPR Transect 1 Showing Example of GPR Anomaly 3
A2-1 APPENDIX 2 DESCRIPTION OF GEOPHYSICAL METHODS, SURVEY METHODOLOGIES AND LIMITATIONS A2.1 On Site Measurements The measurements that were collected and used to create the site map were made using a fiberglass measuring tape. The degree of accuracy of such an approach is typically +/- 5% for lengths and +/- 2.5 degrees for angles. A2.2 Ground Penetrating Radar Ground Penetrating Radar (GPR) consists of a set of integrated electronic components that transmits high frequency (200 to 1500 megahertz [MHz]) electromagnetic waves into the ground and records the energy reflected back to the ground surface. The GPR system consists of an antenna, which serves as both a transmitter and receiver, and a profiling recorder that both processes the incoming signal and provides a graphic display of the data. The GPR data can be reviewed as both printed hard copy output or recorded on the profiling recorder s hard drive for later review. GeoView uses a Mala GPR system. Geological characterization studies are typically conducted using a 250 MHz antenna. A GPR survey provides a graphic cross-sectional view of subsurface conditions. This cross-sectional view is created from the reflections of repetitive short-duration electromagnetic (EM) waves that are generated as the antenna is pulled across the ground surface. The reflections occur at the subsurface contacts between materials with differing electrical properties. The electrical property contrast that causes the reflections is the dielectric permittivity that is directly related to conductivity of a material. The GPR method is commonly used to identify such targets as underground utilities, underground storage tanks or drums, buried debris, voids or geological features. The greater the electrical contrast between the surrounding earth materials and target of interest, the greater the amplitude of the reflected return signal. Unless the buried object is metal, only part of the signal energy will be reflected back to the antenna with the remaining portion of the signal continuing to propagate downward to be reflected by deeper features. If there is little or no electrical contrast between the target interest and surrounding earth materials it will be very difficult if not impossible to identify the object using GPR. The depth of penetration of the GPR signal is very site specific and is controlled by two primary factors: subsurface soil conditions and selected antenna frequency. The GPR signal is attenuated (absorbed) as is passes through earth
A2-2 materials. As the energy of the GPR signal is diminished due to attenuation, the energy of the reflected waves is reduced, eventually to the level that the reflections can no longer be detected. As the conductivity of the earth materials increases, the attenuation of the GPR signal increases thereby reducing the signal penetration depth. In Florida, the typical soil conditions that severely limit GPR signal penetration are near-surface clays and/or organic materials. The depth of penetration of the GPR signal is also reduced as the antenna frequency is increased. However, as antenna frequency is increased the resolution of the GPR data is improved. Therefore, when designing a GPR survey a tradeoff is made between the required depth of penetration and desired resolution of the data. As a rule, the highest frequency antenna that will still provide the desired maximum depth of penetration should be used. A GPR survey is conducted along survey lines (transects) that are measured paths along which the GPR antenna is moved. Electronic marks are placed in the data by the operator at designated points along the GPR transects. These marks allow for a correlation between the GPR data and the position of the GPR antenna on the ground. For geological characterization surveys, the GPR survey is conducted along a set of perpendicularly orientated transects. The survey is conducted in two directions because subsurface features such as sinkholes are often asymmetric. Spacing between the transects typically ranges from 10 to 50 feet. Closely spaced grids are used when the objective of the GPR survey is to identify all sinkhole features within a project site. Coarser grids are used when the objective is to provide a general overview of site conditions. After completion of a survey using a given grid spacing, additional more-closely spaced GPR transects are often performed to better characterize sinkhole features identified by the initial survey. This information can be used to provide recommended locations for geotechnical borings. Depth estimates to the top of lithological contacts or sinkhole features are determined by dividing the time of travel of the GPR signal from the ground surface to the top of the feature by the velocity of the GPR signal. The velocity of the GPR signal is usually obtained from published tables of velocities for the type and condition (saturated vs. unsaturated) of soils underlying the site. The accuracy of GPR-derived depths typically ranges from 20 to 40 percent of the total depth.
A2-3 A2.3 Seismic Reflection Seismic reflection profiling involves the measurement of the two-way travel time of seismic waves transmitted from surface and reflected back to the surface at the interfaces between contrasting geological layers. Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these layers. The strength of the contrast in the acoustic impedance of the two layers determines the amplitude of the reflected signal. The reflected signal is detected on surface using an array of high frequency geophones (typically 24-96). As with seismic refraction, the seismic energy is provided by a 'shot' on surface. For shallow applications this will normally comprise a hammer and plate, weight drop or explosive charge. In most reflection surveys shots are deployed at a number of different positions in relation to the geophone array in order to obtain reflections from the same point on the interface at different geophones in the array. Each common point of reflection is termed a common mid-point (CMP) and the number of times each one is sampled determines the 'fold coverage' for the survey. Traces relating to the same CMP are stacked together to increase the signal-to-noise ratio of the survey before being combined with other CMP's stacked traces to produce a reflection profile. In order to stack related CMP traces a stacking velocity is applied to each trace. This accounts for the difference in two-way travel time between the normal incidence reflection (vertical travel path below the shot) and those at increasing offsets from the shot (known as the normal moveout or NMO). The stacking velocity will vary down the trace to take account of the increase in velocity with depth for each reflection event. Interpretation and Limitations of Geophysical Data The analysis and collection of geophysical data is both a technical and interpretative skill. The technical aspects of the work are learned from both training and experience. Having the opportunity to compare data collected in numerous settings to the results from geotechnical studies performed at the same locations develops interpretative skills for geological characterization studies. The ability to collect interpretable information at a project site is limited by the attenuation (absorption) of the signal by underlying soils. Once the signal has been attenuated at a particular depth, information regarding deeper geological conditions will not be obtained. GPR and seismic data can only resolve subsurface features that have a sufficient electrical or density contrast between the feature in question and surrounding earth materials. If an insufficient contrast is present, the
A2-4 subsurface feature will not be identified. GeoView can make no warranties or representations of geological conditions that may be present beyond the depth of investigation or resolving capability of the equipment or in areas that were not accessible to the geophysical investigation.
FINAL REPORT GEOPHYSICAL INVESTIGATION TANK NO. 6 SITE PLANT CITY, FL Prepared for Madrid Engineering Group, Inc. Bartow, FL Prepared by GeoView, Inc. St. Petersburg, FL
January 17, 2010 Mr. Larry Madrid, P.E. Madrid Engineering Group, Inc. P.O. Box 2506 Bartow, FL 33831-2506 Subject: Transmittal of Final Report for Geophysical Investigation Tank No. 6 Site - Plant City, FL GeoView Project Number 7837 Dear Mr. Madrid, GeoView, Inc. (GeoView) is pleased to submit the final report that summarizes and presents the results of the geophysical investigation conducted at the Tank No. 6 Site. Ground penetrating radar was used to evaluate near-surface geological conditions. GeoView appreciates the opportunity to have assisted you on this project. If you have any questions or comments about the report, please contact us. GEOVIEW, INC. Michael J. Wightman, P.G. President Florida Professional Geologist Number 1423 Chris Taylor, P.G. Vice President Florida Professional Geologist Number 2256 A Geophysical Services Company 4610 Central Avenue Tel.: (727) 209-2334 St. Petersburg, FL 33711 Fax: (727) 328-2477
Page 1 1.0 Introduction The site was located along National Guard Drive in Plant City, Florida. Prior to the geophysical investigation, two surface depressions had formed north of Water Tank No. 6. The first depression was approximately 40 feet (ft) in diameter, 2 ft deep and located approximately 70 ft north of the water tank. The second depression was approximately 10 ft in diameter, 3 ft deep and located approximately 160 ft north of the water tank. The purpose of the geophysical investigation was to help characterize nearsurface geological conditions in the area of the depressions and to identify subsurface features that may be associated with sinkhole activity. The survey area included the areas surrounding the depressions and the water tank. The location of the geophysical survey area is provided on Figure 1. The investigation was conducted on January 13, 2010. A discussion of the field methods used to generate the report figures is provided in Appendix A2.1. 2.0 Description of Geophysical Investigation 2.1 Ground Penetrating Radar Survey A GPR survey was conducted around the depressions and the water tank. The GPR survey was conducted along a series of perpendicular transects spaced approximately 10 ft apart. The GPR data was collected with a Mala radar system. The GPR settings used for the survey are presented in Table 1. Table 1 GPR Equipment Settings Used for Exterior and Interior GPR Surveys Location Antenna Frequency Time Range (nano-seconds) Estimated Depth of GPR Signal Penetration Exterior 250 MHz 1/ 210-290 20 to 35 ft bls 1/ MHz means mega-hertz and is the mid-range operating frequency of the GPR antenna. A description of the GPR technique and the methods employed for geological characterization studies is provided in Appendix A2.2. 3.0 Identification of Possible Sinkhole Features Using GPR The features observed on GPR data that are most commonly associated with sinkhole activity are: A downwarping of GPR reflector sets, that are associated with suspected lithological contacts, toward a common center. Such features
Page 2 typically have with a bowl or funnel shaped configuration and can be associated with a deflection of overlying sediment horizons caused by the migration of sediments into voids in the underlying limestone. If the GPR reflector sets are sharply downwarping and intersect, they can create bow-tie shaped GPR reflection feature, which often designates the apparent center of the GPR anomaly. A localized significant increase in the depth of the penetration and/or amplitude of the GPR signal response. The increase in GPR signal penetration depth or amplitude is often associated with either a localized increase in sand content at depth or decrease in soil density. An apparent discontinuity in GPR reflector sets, that are associated with suspected lithological contacts. The apparent discontinuities and/or disruption of the GPR reflector sets may be associated with the downward migration sediments. The greater the severity of these features or a combination of these features the greater the likelihood that the identified feature is a sinkhole. It is not possible based on the GPR data alone to determine if an identified feature is a sinkhole or, more importantly, whether that feature is an active sinkhole. 4.0 Survey Results 4.1 Discussion of GPR Survey Results Results of the GPR survey indicated the presence of two well-defined, relatively continuous sets of GPR reflectors at depth ranges of 5 to 7 ft bls and 12 to 25 ft bls. The upper GPR reflector set likely correlates to the interface between the unsaturated and saturated soils (water table). The lower GPR reflector set is likely associated with some change in lithological conditions at that depth range. Description of GPR Anomalies Two GPR anomaly areas were identified at the site. The anomaly areas are designated as GPR Anomalies 1 and 2 on Figure 1. The anomalies are numbered in order of significance with GPR Anomaly 1 being the most significant and GPR Anomaly 2 being the least significant. A description of each of the anomalies is as follows: GPR Anomaly 1 GPR Anomaly 1 is semi-elliptical in shape with total area of approximately 1,300 square ft. The anomaly is located north of the water tank and the boundaries of the anomaly are almost identical to the outer boundary of the larger surface
Page 3 depression. The apparent vertical relief of the upper portion of the anomaly area is 10 to 20 ft as characterized by the observed downwarping of the GPR reflector set. A localized increase in the depth of penetration of the GPR signal was also observed within the anomaly area. The apparent center of the feature is characterized as the area of maximum downwarping and/or discontinuity of the previously referenced GPR reflectors. GPR Anomaly 2 GPR Anomaly 2 is elliptical in shape with total area of approximately 130 square ft. The anomaly is located northwest of the water tank and west of the larger surface depression. The apparent vertical relief of the upper portion of the anomaly area is 3 to 5 ft as characterized by the observed downwarping of the GPR reflector set. A localized increase in the depth of penetration of the GPR signal was also observed within the anomaly area. The apparent center of the feature is characterized as the area of maximum downwarping of the previously referenced GPR reflectors. In addition to the two GPR anomalies, there was one large area that was characterized by a minor downwarping of the GPR reflectors. This feature covered much of the central portion of the survey area and was centered about GPR Anomaly 1. With the exception of the area within Anomaly 1, there were no breaks or other apparent discontinuities of the GPR reflector within this feature. It is suspected that this feature may be indicative of a relic paleo-sink which is currently stabilized. It is suspected that the only the unstable portions of this feature are within GPR anomalies 1 and 2. The GPR did not identify any anomalous areas near the northern, smaller depression. However, for safety reasons, GPR data was not collected directly across this depression. It is suspected that the karst feature associated with this depression area has near-vertical walls and does not extend much beyond the boundaries of the depression (within the depth range of the GPR signal).
APPENDIX 1 FIGURE, PHOTOGRAPHS, AND EXAMPLES OF GPR ANOMALIES
Photograph 1 Facing south toward the larger depression Photograph 2 Facing south toward the smaller depression with the larger depression and water tank in the background Photograph 3 Facing north toward the larger depression
Water Table Soil Layer GPR Anomaly 1 GPR Transect 8 Showing Example of GPR Anomaly 1 GPR Anomaly 2 GPR Anomaly 1 GPR Transect 24 Showing Example of GPR Anomaly 2
A2-1 APPENDIX 2 DESCRIPTION OF GEOPHYSICAL METHODS, SURVEY METHODOLOGIES AND LIMITATIONS A2.1 On Site Measurements The measurements that were collected and used to create the site map were made using a fiberglass measuring tape. Right angles were estimated using the exterior walls of the residence. The degree of accuracy of such an approach is typically +/- 5% for lengths and +/- 2.5 degrees for angles. A2.2 Ground Penetrating Radar Ground Penetrating Radar (GPR) consists of a set of integrated electronic components that transmits high frequency (200 to 1500 megahertz [MHz]) electromagnetic waves into the ground and records the energy reflected back to the ground surface. The GPR system consists of an antenna, which serves as both a transmitter and receiver, and a profiling recorder that both processes the incoming signal and provides a graphic display of the data. The GPR data can be reviewed as both printed hard copy output or recorded on the profiling recorder s hard drive for later review. GeoView uses a Mala GPR system. Geological characterization studies are typically conducted using a 250 MHz antenna. A GPR survey provides a graphic cross-sectional view of subsurface conditions. This cross-sectional view is created from the reflections of repetitive short-duration electromagnetic (EM) waves that are generated as the antenna is pulled across the ground surface. The reflections occur at the subsurface contacts between materials with differing electrical properties. The electrical property contrast that causes the reflections is the dielectric permittivity that is directly related to conductivity of a material. The GPR method is commonly used to identify such targets as underground utilities, underground storage tanks or drums, buried debris, voids or geological features. The greater the electrical contrast between the surrounding earth materials and target of interest, the greater the amplitude of the reflected return signal. Unless the buried object is metal, only part of the signal energy will be reflected back to the antenna with the remaining portion of the signal continuing to propagate downward to be reflected by deeper features. If there is little or no electrical contrast between the target interest and surrounding earth materials it will be very difficult if not impossible to identify the object using GPR. The depth of penetration of the GPR signal is very site specific and is controlled by two primary factors: subsurface soil conditions and selected antenna
A2-2 frequency. The GPR signal is attenuated (absorbed) as is passes through earth materials. As the energy of the GPR signal is diminished due to attenuation, the energy of the reflected waves is reduced, eventually to the level that the reflections can no longer be detected. As the conductivity of the earth materials increases, the attenuation of the GPR signal increases thereby reducing the signal penetration depth. In Florida, the typical soil conditions that severely limit GPR signal penetration are near-surface clays and/or organic materials. The depth of penetration of the GPR signal is also reduced as the antenna frequency is increased. However, as antenna frequency is increased the resolution of the GPR data is improved. Therefore, when designing a GPR survey a tradeoff is made between the required depth of penetration and desired resolution of the data. As a rule, the highest frequency antenna that will still provide the desired maximum depth of penetration should be used. A GPR survey is conducted along survey lines (transects) that are measured paths along which the GPR antenna is moved. Electronic marks are placed in the data by the operator at designated points along the GPR transects. These marks allow for a correlation between the GPR data and the position of the GPR antenna on the ground. For geological characterization surveys, the GPR survey is conducted along a set of perpendicularly orientated transects. The survey is conducted in two directions because subsurface features such as sinkholes are often asymmetric. Spacing between the transects typically ranges from 10 to 50 feet. Closely spaced grids are used when the objective of the GPR survey is to identify all sinkhole features within a project site. Coarser grids are used when the objective is to provide a general overview of site conditions. After completion of a survey using a given grid spacing, additional more-closely spaced GPR transects are often performed to better characterize sinkhole features identified by the initial survey. This information can be used to provide recommended locations for geotechnical borings. Depth estimates to the top of lithological contacts or sinkhole features are determined by dividing the time of travel of the GPR signal from the ground surface to the top of the feature by the velocity of the GPR signal. The velocity of the GPR signal is usually obtained from published tables of velocities for the type and condition (saturated vs. unsaturated) of soils underlying the site. The accuracy of GPR-derived depths typically ranges from 20 to 40 percent of the total depth. Interpretation and Limitations of GPR data
A2-3 The analysis and collection of GPR data is both a technical and interpretative skill. The technical aspects of the work are learned from both training and experience. Having the opportunity to compare GPR data collected in numerous settings to the results from geotechnical studies performed at the same locations develops interpretative skills for geological characterization studies. The ability of GPR to collect interpretable information at a project site is limited by the attenuation (absorption) of the GPR signal by underlying soils. Once the GPR signal has been attenuated at a particular depth, information regarding deeper geological conditions will not be obtained. GPR data can only resolve subsurface features that have a sufficient electrical contrast between the feature in question and surrounding earth materials. If an insufficient contrast is present, the subsurface feature will not be identified. GeoView can make no warranties or representations of geological conditions that may be present beyond the depth of investigation or resolving capability of the GPR equipment or in areas that were not accessible to the geophysical investigation.