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Natural Resources Conservation Service National Soil Survey Center Box 60, 207 W. Main Street, Room G-08 Phone: (336) 667-7108 x. 118 Wilkesboro, NC 28697 FAX: (336) 667-0384 Subject: Soils – Geophysical Field Assistance Date: 27 Jan, 2009 To: Gayle Norman

Acting State Conservationist USDA-Natural Resources Conservation Service 430 G Street, #4164 Davis, CA 95616-4164

Purpose: To provide training in the use and operations of electromagnetic induction (EMI) techniques and ground-penetrating radar (GPR) while assessing soils and soil properties in Tuolumne and Calaveras counties. Participants: Sue Malone, MLRA Project Leader, USDA-NRCS, Sonora, CA Ben Marshall, Soil Scientist, USDA-NRCS, Sonora, CA Ed Tallyn, MO-02 Soil Data Quality Specialist, USDA-NRCS, Davis, CA Wes Tuttle, Soil Scientist (Geophysical), USDA-NRCS, Wilkesboro, NC Activities: All activities were completed during the period of 20 to 23 October, 2008. Discussion: In continuation of the geophysical initiative supported through the National Soil Survey Center (NSSC), MO-02 and the California NRCS state office have purchased a TerraSIRch SIR (Subsurface Interface Radar) System-3000, manufactured by Geophysical Survey Systems, Inc.1, and accompanied supporting software. From hereafter the system will be referred to as ground penetrating radar (GPR). The MO-02 staff also has acquired a Geonics EM38 conductivity meter through the NSSC geophysical equipment loaner program. This is part of the NSSC’s continued support of the geophysical initiative and to make geophysical tools available to interested cooperators across the country. High priority will be placed on training and support to the California staff involved with operations of EMI and GPR. The NSSC staff will provide guidance, technical support, and instruction to the MO-02 staff and to the California NRCS state office and field office staff as requested. The training and support provided by the NSSC staff is vital to the continuation of the geophysical initiative across the U.S. New and innovative tools such as electromagnetic induction (EMI) and ground penetrating radar (GPR) need to be explored with benefits recognized and utilized.

Summary: 1. Training was provided on the operation of the TerraSIRch SIR (Subsurface Interface Radar) System-3000,

manufactured by Geophysical Survey Systems, Inc. All participants were given the opportunity to operate, evaluate and complete field surveys using GPR and EMI techniques. Participants completed comparative field tests and evaluated the suitability and reliability of these devices.

2. EMI surveys conducted in an area of Saddlecreek-Whiterock complex, 8 to 20 percent slopes revealed an association

with changes in apparent conductivity and differing “degrees” of weathering within the slate bedrock (Mariposa slate). Higher apparent conductivity was thought to be associated with a higher degree of weathering within the underlying bedrock. Higher apparent conductivity was associated with more observable fractures within the underlying bedrock

1 Manufacturer's names are provided for specific information; use does not constitute endorsement.

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even though soil depths to bedrock were similar across the survey area. Higher amounts of exposed bedrock (rock outcropping) was observed in areas of lower apparent conductivity.

3. GPR records collected in an area of Saddlecreek-Whiterock complex, 8 to 20 percent slopes (Mariposa slate) resulted

in poor interpretative quality. The radar signal was highly attenuated and resulted in limited depths of observation. Interpretations could be made in only very limited areas. Soil borings revealed bedrock at depths dominantly less than 40 cm across the survey area. GPR interpretations were limited in the near surface in the dominantly very shallow and shallow soils as a result of a high amplitude signal produced in the near surface portion of the radar record. The high amplitude signal (“surface pulse”) often masks more subtle features associated with changing soil diagnostic features located in the upper soil profile. EMI techniques proved to be a more reliable method for geophysical interpretations at the site.

4. GPR records collected in an area of Gopheridge-Argonaut complex, 15 to 30 percent slopes were generally of good

interpretative quality. Interpretations from the radar record estimate the depth to bedrock at dominantly 50 cm to 1.0 m which was consistent with soil boring/excavations at the site. A total of 33 observation points (2 separate transects) were interpreted for depths to bedrock from GPR records at the site. The results are as follows: 50cm-100cm (moderately deep-70%) - 23 observations 25cm-50cm (shallow-18%) - 6 observations 100cm-150cm (deep-12%) - 4 observations

5. GPR records collected in an area of Softburn-Motherlode complex, 15 to 30 percent slopes, rocky were generally of

marginal to good interpretative quality. Interpretations from the radar record estimate the depth to bedrock occupying dominantly two depths classes (shallow - 25 cm to 50 cm and moderately deep - 50 cm to 100 cm). It was difficult to interpret exact depths at all observation points because of the vagueness of the soil/bedrock contact in many portions of the radar record but depths to bedrock were thought to dominantly range from 25 cm to 100 cm along the traverse line, as interpreted from the GPR record and observed in soil borings. The high amplitude “surface pulse” was thought to mask more subtle soil/bedrock contact features located at depths less than approximately 40 cm, making interpretations difficult in the upper 40 cm of the soil profile.

6. In an area of Auberry-Sierra complex, 8 to 30 percent slopes, a combination of higher clay content (sandy clay loam

and clay loam) in the subsoil and mixed mineralogy is thought to increase signal attenuation rates and reduce the overall effectiveness of GPR. Limited interpretations could be made along portions of the radar record, usually in the more sloping areas where the clay content of the subsoil was thought to decrease, but overall interpretations were limited. Some interpretable portions of the radar record appeared to have a subtle feature occurring at depths of 1.0 m to 1.25 m. The feature was thought to be associated with a coarser textured underlying horizon and/or soft bedrock weathering from the underlying granitoid-diorite parent material. This interpretation was consistent with observations made from the local soil survey staff in this soil mapping unit.

7. The soil survey team has a very good concept of soils and soil patterns in the areas studied during the field

investigations. Sue Malone is very conscientious and is doing an excellent job in a rather complex and difficult survey area. The high degree of variability with respect to depth to bedrock over very short distances complicates data collection and makes it difficult to predict depths to bedrock with a high degree of confidence. Results from GPR and EMI surveys across differing soil map units support and validate concepts developed by the soil survey staff. Additional findings during the investigations will also help “tweek and fine-tune” questionable soil map units. Results of GPR transect data contained in this report have been sent to the soil survey team in Sonora, CA.

8. Geophysical interpretations are considered preliminary estimates of site conditions. The results of all geophysical

investigations are interpretive and do not substitute for direct soil borings. The use of geophysical methods can reduce the number of soil observations, direct their placement, and supplement their interpretations. Interpretations should be verified by ground-truth observations.

It was my pleasure to work in California again with members of your fine staff. With kind regards, Wes Tuttle Soil Scientist (Geophysical) National Soil Survey Center

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cc: J. Doolittle, Research Soil Scientist, USDA-NRCS, Suite 200, 11 Campus Boulevard, Newtown Square, PA 19073D. M. Golden, Director of Soils Survey Division, USDA-NRCS, Room 4250 South Building, 14th & Independence Ave. SW, Washington, DC

20250 K. Hipple, Acting Director, USDA-NRCS, National Soil Survey Center, Federal Building, Room 152,100 Centennial Mall North, Lincoln,

NE 68508-3866 S. Malone, MLRA Project Leader, USDA-NRCS, Sonora Soil Survey Office, 19777 Greenley Road, Sonora, CA 95370 K. Paris, Soil Data Quality Specialist, USDA-NRCS, 430 G Street, Davis, CA 95616-4164 D. Smith, State Soil Scientist/MO-02 Leader, USDA-NRCS, 430 G Street, Davis, CA 95616-4164 E. Tallyn, Soil Data Quality Specialist, USDA-NRCS, 430 G Street, Davis, CA 95616-4164 L. West, National Leader for Soil Survey Research and Laboratory, USDA-NRCS, National Soil Survey Center, Federal Building, Room

152, 100 Centennial Mall North, Lincoln, NE 68508-3866 Equipment: The radar unit is the TerraSIRch SIR (Subsurface Interface Radar) System-3000, manufactured by Geophysical Survey Systems, Inc.1 Morey (1974), Doolittle (1987), and Daniels (1996) have discussed the use and operation of GPR. The SIR System-3000 consists of a digital control unit (DC-3000) with keypad, color SVGA video screen, and connector panel. A 10.8-volt Lithium-Ion rechargeable battery powers the system. This unit is backpack portable and, with an antenna, requires two people to operate. The antenna used in this study has a center frequency of 200 MHz. The RADAN for Windows (version 5.0) software program was used to process the radar records (Geophysical Survey Systems, Inc, 2003).1 Processing included color transformation, surface normalization, time-zero adjustment and range gain adjustments. Geonics Limited manufactures the EM38 meter. 1 This meter is portable and requires only one person to operate. No ground contact is required with this meter. McNeill (1980) and Geonics Limited (1998) have described principles of operation for the EM38 meter. Lateral resolution is approximately equal to its intercoil spacing. The EM38 meter has a 1 m intercoil spacing and operates at a frequency of 14,600 Hz. When placed on the soil surface, this instrument has a theoretical penetration depth of about 0.75 and 1.5 m in the horizontal and vertical dipole orientations, respectively (Geonics Limited, 1998). Values of apparent conductivity are expressed in millisiemens per meter (mS/m). The Allegro field computer was used in combination with the EM38 meter to record and store EMI data. The field computer is keypad operated and measurements can either be automatically or manually triggered. EMI data was geo-referenced with a GR-213 receiver manufactured by HOLUX Technology, Inc. 1 To help summarize the results of this study, the SURFER for Windows (version 8.0) developed by Golden Software, Inc. was used to construct two-dimensional simulations. Grids were created using kriging methods with an octant search. Ground-Penetrating Radar: Ground-penetrating radar is a time scaled system. The system measures the time it takes electromagnetic energy to travel from an antenna to an interface (i.e., soil horizon, stratigraphic layer) and back. To convert travel time into a depth scale requires knowledge of the velocity of pulse propagation. Several methods are available to determine the velocity of propagation. These methods include use of table values, common midpoint calibration, and calibration over a target of known depth. The last method is considered the most direct and accurate method to estimate propagation velocity (Conyers and Goodman, 1997). The procedure involves measuring the two-way travel time to a known reflector that appears on a radar record and calculating the propagation velocity by using the following equation (after Morey, 1974):

V = 2D/T [1] Equation [1] describes the relationship between the propagation velocity (V), depth (D), and two-way pulse travel time (T) to a subsurface reflector. During this study, the two-way radar pulse travel time was compared with measured depths to known subsurface interfaces within each study site. Computed propagation velocities were used to scale the radar records. Electromagnetic Induction: Electromagnetic induction is a noninvasive geophysical tool that can be used for soil and site investigations. Advantages of EMI are its portability, speed of operation, flexible observation depths, and moderate resolution of subsurface features. Results of EMI surveys are interpretable in the field. This geophysical method can provide in a relatively short time the large number of observations that are needed to comprehensively cover sites. Maps prepared from correctly interpreted EMI data provide the basis for assessing site conditions, planning further investigations, and locating sampling or monitoring sites. 1 Manufacturer's names are provided for specific information; use does not constitute endorsement.

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Electromagnetic induction uses electromagnetic energy to measure the apparent conductivity of earthen materials. Apparent conductivity is a weighted, average conductivity measurement for a column of earthen materials to a specific depth (Greenhouse and Slaine, 1983). Variations in apparent conductivity are caused by changes in the electrical conductivity of earthen materials. The electrical conductivity of soils is influenced by the type and concentration of ions in solution, volumetric water content, temperature and phase of the soil water, and amount and type of clays in the soil matrix (McNeill, 1980). The apparent conductivity of soils increases with increases in soluble salts, water, and clay contents (Kachanoski et al., 1988; Rhoades et al., 1976). Electromagnetic induction measures vertical and lateral variations in apparent electrical conductivity. Values of apparent conductivity are seldom diagnostic in themselves. However, relative values and lateral and vertical variations in apparent conductivity can be used to infer changes in soils and soil properties. Interpretations are based on the identification of spatial patterns within data sets. To assist interpretations, computer simulations of EMI data are normally used. To verify interpretations, ground-truth measurements are required. Discussion: The soil survey party has experienced extreme difficulty while conducting soil borings, soil excavations and data collection in the soil survey area. The high degree of variability with respect to depth to bedrock over very short distances complicates data collection. It is difficult to predict depths to bedrock with a high degree of confidence. It is hoped that geophysical techniques can aid in data collection and help with visualization and model/concepts concerning the underlying bedrock. Study Site 1: Mariposa Slate (Saddlecreek-Whiterock complex, 8 to 20 percent slopes) EMI Survey The study site was located 12 miles south-southwest of Sonora, CA in Tuolomne County. The survey area was in pastureland in an area mapped Saddlecreek-Whiterock complex, 8 to 20 percent slopes, (USDA-NRCS, Soil Survey in progress). The shallow, somewhat excessively drained Saddlecreek soils formed in residuum and colluvium from slate rocks on footslopes, backslopes and summits of hillslopes on Sierra Nevada foothills. The very shallow and shallow, somewhat excessively drained Whiterock soils formed in material weathered from metasedimentary rocks on foothills. Saddlecreek is a member of the loamy-skeletal, mixed, superactive, thermic Lithic Haploxerepts family. Whiterock is a member of the loamy, mixed, superactive, nonacid, thermic Lithic Xerorthents family. Survey Design: An EMI survey was conducted across the site (Figure 1). Survey procedures were simplified to expedite fieldwork. The survey area was semi-rectangular in shape. Two parallel lines defined the upper and lower boundaries of the survey site. Survey lines were irregular in length to accommodate the survey area and ranged from 100 m to 150 m. Survey traverse lines were spaced approximately 10 m apart. Traverse line end points were marked and provided ground control. An EMI survey was completed by walking at a fairly uniform pace between similarly numbered survey lines on the opposing set of parallel lines in a back and forth pattern. The EM38 meter was carried at a height of approximately 3 inches above the surface and was operated in the continuous mode with measurements recorded at a 1-sec interval. The meter was carried in the vertical dipole orientation. Measurements of apparent conductivity were geo-referenced. Results: A total of 1592 measurements were recorded with the EM38 meter. In the vertical dipole orientation apparent conductivity averaged 5.7 mS/m and ranged from -7.5 to 14.5 mS/m. One-half of the observations had an apparent conductivity between 4.3 and 6.8 mS/m. Soil borings and minor excavations at the site revealed an association with changes in apparent conductivity and differing “degrees” of weathering within the slate bedrock (refer to Figure 1). Higher apparent conductivity was thought to be associated with a higher degree of weathering with more fractures being observed in the underlying bedrock, even though soil depths to bedrock were similar across the survey area. Soil borings at point “B” (area of higher apparent conductivity) revealed 25 cm of soil material underlain with bedrock containing more fractures (and appeared to be more weathered) as compared to soil borings observed at point “D”. Soil borings at “D” (area of lower apparent conductivity) revealed approximately 25 cm of soil material underlain by less fractured and “harder” bedrock (the term “harder” is used very loosely). More exposed bedrock (rock outcropping) was also apparent near point “D”. Depths to bedrock were consistently less than 40 cm throughout the survey area as revealed in soil borings and excavations. Slightly deeper soil profiles occurred in drainageways but were very localized. Soil excavations at “A” (in a drainageway) revealed densic material at 58 cm. The densic material was too difficult to excavate with hand tools. The field staff thought that the bedrock contact was located just beneath the densic contact. Additional soil borings within the drainageway resulted in bedrock at depths less than 50 cm. An anomalous feature (spike in apparent conductivity) observed at point “C” was attributed to the presence of metal debris located just beneath the soil surface.

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Figure 1 – Apparent conductivity measured with the EM38 meter in an area of Saddlecreek-Whiterock complex, 8 to 20 percent slopes. Apparent conductivity values were measured in mS/m (millisiemens/meter). An additional EMI survey (refer to Figure 2) was completed approximately 100 m west of the EMI survey completed in Figure 1. A closer survey line interval spacing (5 m) was used to try and identify any differences that may occur in spatial patterns and associated changes in soil characteristics using different intensities of data acquisition (survey line interval spacing of 10 m (Figure 1) compared to 5 m (Figure 2)). Results: (Figure 2) A total of 452 measurements were recorded with the EM38 meter. In the vertical dipole orientation apparent conductivity averaged 8.0 mS/m and ranged from 3.5 to 14.4 mS/m. One-half of the observations had an apparent conductivity between 6.5 and 9.1 mS/m. The EMI survey in Figure 2 resulted in similar interpretations when compared to results from Figure 1. Higher apparent conductivity appeared to be associated with a higher degree of weathering within the underlying bedrock. Bedrock observed in areas of higher apparent conductivity appeared to contain more observable fractures. Soil borings at points “A” and “D”

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revealed soil material of approximately 30 cm overlying slate bedrock. The underlying bedrock at “A” was more fractured and presumed to be more weathered as compared to the underlying bedrock at “D”. No soil borings were taken at “C” but this area of lower apparent conductivity contained much more exposed bedrock (rock outcropping) thought to be associated with “harder”, less weathered and less fractured underlying bedrock.

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Figure 2 – Apparent conductivity measured with the EM38 meter in an area of Saddlecreek-Whiterock complex, 8 to 20 percent slopes. Apparent conductivity values were measured in mS/m (millisiemens/meter).

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Photo 1. An area of Saddlecreek-Whiterock complex, 8 to 20 percent slopes. Nearly “vertically tilted” exposed bedrock can be observed in the center of the photo. Lower apparent conductivity was observed in and around this area. The underlying bedrock in this area also appeared to be less fractured and less weathered. GPR GPR records collected at site 1in an area of Mariposa slate resulted in poor interpretative quality. The radar signal was highly attenuated and resulted in limited depths of observation. Interpretations could be made in only very limited areas. Soil borings revealed bedrock at depths dominantly less than 40 cm across the survey area. GPR interpretations were limited in the near surface in the dominantly very shallow and shallow soils as a result of a high amplitude signal produced in the near surface portion of the radar record. The high amplitude signal (“surface pulse”) often masks more subtle features associated with changing soil diagnostic features located in the upper soil profile. A combination of a high amplitude signal produced in the near surface portion of the radar record and high signal attenuations rates resulted in very limited interpretations with GPR at the site. EMI techniques proved to be a more reliable method for geophysical interpretations at the site. Site 2:Gopheridge Geologic Formation (Gopheridge-Argonaut complex, 15 to 30 percent slopes) The study area was located in Calaveras County approximately 6 miles west-southwest of community of Copperopolis, CA. The site was in grassland and intermixed timber. Representative GPR surveys (Figure 3 and Figure 4) were completed in areas of Gopheridge-Argonaut complex, 15 to 30 percent slopes (USDA-NRCS, Soil Survey in progress). The moderately deep, well drained Gopheridge soils formed in material weathered from metabasic volcanic rock, dominantly greenstone on backslopes of hillslopes on Sierra Nevada foothills. The moderately deep, well drained Argonaut soils formed in materials weathered from metavolcanic rock sources on foothills. Gopheridge is a member of the loamy-skeletal, mixed, superactive, thermic ultic Haploxeralfs family. Argonaut is a member of the fine, mixed, superactive, thermic Mollic Haploxeralfs family. Results: An 80 m traverse line was established across an area of this map unit. Survey flags were inserted in the ground at intervals of 5 m and served as reference points. A survey was completed with the 200 MHz antenna at a scanning time of 45 ns. Based on a measured depth (76 cm) to a subsurface reflector, the velocity of propagation through relatively dry loam was estimated to be about 0.14 m/ns. The dielectric permittivity was 4.36. Radar records obtained with the 200 MHz antenna were generally of good interpretative quality. In the radar record (Figure 3) a black line highlights the approximate interface/contact between soil material and the underlying bedrock/parent material. Interpretations from the radar record estimate the depth to bedrock at dominantly 50 cm to 1.0 m along this portion of the GPR traverse line. A soil excavation at point “A” determined bedrock to be at a depth of 76 cm. This was consistent with interpretations from the radar record at this location.

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Figure 3. A representative portion of a radar record (transect 1) from an area of Gopheridge-Argonaut complex, 15 to 30 percent slopes. The depth scale is expressed in meters. A total of 17 observation points (refer to Appendix for point data) along transect 1 were interpreted for depths to bedrock. The results are as follows: 25cm-50cm (shallow) - 4 observations (24%) 50cm-100cm (moderately deep) - 13 observations (76%) A second GPR survey was completed at the site. A 150 m traverse line was established across an area of this map unit. Survey flags were inserted in the ground at intervals of 10 meters and served as reference points. A survey was completed with the 200 MHz antenna at a scanning time of 45 ns. Figure 4 is a representative portion of the radar record collected during the second GPR survey at the site. A black line highlights the approximate interface/contact between soil material and the underlying bedrock/parent material. Interpretations from the radar record estimate the depth to bedrock at dominantly 60 cm to 95 cm along this portion of the GPR traverse line. A soil excavation at point “A” determined bedrock to be at a depth of 83 cm. This was consistent with interpretations from the radar record at this location.

Figure 4. A representative portion of a radar record (transect 2) from an area of Gopheridge-Argonaut complex, 15 to 30 percent slopes. The depth scale is expressed in meters. A total of 16 observation points (refer to Appendix for point data) along the transect 2 were interpreted for depths to bedrock. The results are as follows: 25cm-50cm (shallow) - 2 observations (12%) 50cm-100cm (moderately deep) - 10 observations (63%) 100cm-150cm (deep) - 4 observations (25%) Sum of transects 1 and 2 (33 observation points) from an area of Gopheridge-Argonaut complex, 15 to 30 percent slopes: 25cm-50cm (shallow) - 6 observations (18%) 50cm-100cm (moderately deep) - 23 observations (70%) 100cm-150cm (deep) - 4 observations (12%)

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Site 3: Softburn-Motherlode complex, 15 to 30 percent slopes, rocky (median between the highway) The study area was located 13 miles southwest of Sonora, CA in Tuolomne County. The site was in an area mapped Softburn-Motherlode complex, 15 to 30 percent slopes, rocky (USDA-NRCS, Soil Survey in progress). The shallow, well drained Softburn soils formed in colluvium and residuum derived from metavolcanic rock, dominantly greenschist. Softburn soils are on backslopes on Sierra Nevada Foothills. The deep, well drained Motherlode soils formed in colluvium over residuum weathered from greenschist on interfluves of hills on Sierra Nevada foothills. Softburn is a member of the loamy, mixed, superactive, thermic, shallow Typic Haploxerepts family. Motherlode is a member of the fine-loamy, mixed, active, thermic Typic Haploxeralfs family.

Figure 5. A representative portion of a radar record from an area of Softburn-Motherlode complex, 15 to 30 percent slopes, rocky. The depth scale is expressed in meters. Results: A 210 m traverse line was established across an area of this map unit. Survey flags were inserted in the ground at intervals of 10 meters and served as reference points. A survey was completed with the 200 MHz antenna at a scanning time of 45 ns. Based on a measured depth (65 cm) to a subsurface reflector, the velocity of propagation through relatively dry loam and clay loam was estimated to be about 0.097 m/ns. The dielectric permittivity was 9.5. Radar records obtained with the 200 MHz antenna were generally of marginal to good interpretative quality, depending on location along the transect line. In the right-hand portion of the radar record (Figure 5) a black line highlights the approximate interface/contact between soil material and the underlying bedrock/parent material. A soil excavation at point “A” revealed bedrock at 65 cm which is consistent with interpretations from the radar at this location. The soil/bedrock contact in the middle and left-hand portions of the Figure 5 was highly variable and an exact depth to bedrock was difficult to interpret. Depths to bedrock are thought to dominantly range between 30 cm and 75 cm in the middle and left-hand portions of Figure 5. Interpretations from the radar record estimate the depth to bedrock occupying dominantly two depths classes (shallow - 25 cm to 50 cm and moderately deep - 50 cm to 100 cm). It was difficult to interpret exact depths at all observation points along the traverse line because of the vagueness of the soil/bedrock contact, but depths to bedrock were thought to dominantly range from 25 cm to 100 cm. The high amplitude “surface pulse” was thought to mask more subtle soil/bedrock contact features located at depths less than approximately 40 cm, making interpretations difficult in the upper 40 cm of the soil profile. Excavations revealed bedrock at depths as shallow as 25 cm along the traverse line. Also, additional excavations revealed a gradational boundary between soft bedrock and hard bedrock in profiles along other portions of the traverse line. In one excavation, soft bedrock occurred at 25 cm with hard bedrock occurring at 43 cm. Gradational boundaries are often difficult to detect with GPR. More investigative work with the GPR in this map unit accompanied with ground truthing needs to be completed to achieve a more accurate assessment of soil components. Depth to bedrock is thought to vary significantly over very short distances in the Softburn-Motherlode complex, 15 to 30 percent slopes, rocky. Exposed bedrock was observed along the traverse line and was oriented almost vertical (refer to Photos 2 and 3). The variability associated with the orientation and shape of the exposed bedrock is thought to be indicative of the underlying soil/bedrock contact in many portions of this map unit and contribute to the variability of bedrock depths. This was evident in the radar record collected at the site.

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Photo 2. Ben Marshall, Soil Scientist, USDA/NRCS (left-towing the 200MHz antenna) and Ed Tallyn, Soil Data Quality Specialist (right-reviewing the radar record) conduct a GPR survey in a area of Softburn-Motherlode complex, 15 to 30 percent slopes, rocky.

Photo 3. Exposed bedrock in an area of Softburn-Motherlode complex, 15 to 30 percent slopes, rocky. The vertical orientation of rock outcrops is characteristically common across the landscapes. The variability in depth of the underlying soil/bedrock interface is thought to be highly influenced by this orientation of the bedrock.

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Site 4: Granitoid – Diorite Geologic Area The study area is located 13 miles southwest of Sonora, CA in Tuolomne County. The site is dominantly in pasture with some areas of timber. The deep, well drained Auberry soils formed in material weathered from intrusive, acid igneous rocks on foothills and mountainous uplands. The very deep, well drained Sierra soils formed in residuum from granitic rocks on western footslopes of the Sierra Nevada Mountains. Auberry is a member of the fine-loamy, mixed, semiactive, thermic Ultic family. Sierra is a member of the fine-loamy, mixed, active, thermic Ultic Haploxeralfs family.

Figure 6. A representative portion of a radar record from an area of Auberry-Sierra complex, 8 to 30 percent slopes. The depth scale is expressed in meters. Results: A 200 m traverse line was established across an area of this map unit. A survey was completed with the 200 MHz antenna at a scanning time of 45 ns. Based on a measured depth (65 cm) to a subsurface reflector the velocity of propagation through relatively dry loam and clay loam was estimated to be about 0.097 m/ns. The dielectric permittivity was 9.5. Radar records obtained with the 200 MHz antenna were of marginal interpretative quality (Figure 6). A combination of higher clay content (sandy clay loam and clay loam) in the subsoil and mixed mineralogy is thought to increase signal attenuation rates and reduce the overall effectiveness of GPR in these soils. Limited interpretations could be made along some portions of the radar record, usually in the more sloping areas where the clay content of the subsoil was thought to decrease, but overall interpretations were limited. Some interpretable portions of the radar record appeared to have a subtle feature occurring at depths of 1.0 m to 1.25 m. The feature was thought to be associated with a coarser textured underlying horizon and/or soft bedrock weathering from the underlying granitic parent material. This interpretation was consistent with observations made by the local soil survey staff in this soil mapping unit. Contrasting features associated with large subsurface rock fragments were observed along portions of the radar record. More subtle features associated with stratigraphic features were often difficult to identify. More study needs to be completed with GPR in this soil mapping unit and similar soils to more fully determine applicability.

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Photo 4. A GPR survey is being conducted in an area of Auberry-Sierra complex, 8 to 30 percent slopes. It was difficult to make a meaningful interpretation in portions of the radar record where higher amounts of clay in the subsoil were present resulting in a “washed out” appearance in the radar record. References: Conyers, L. B., and D. Goodman. 1997. Ground-penetrating Radar; an introduction for archaeologists. AltaMira Press, Walnut Creek, CA. 232 pp. Daniels, D. J. 1996. Surface-Penetrating Radar. The Institute of Electrical Engineers, London, United Kingdom. 300 p. Doolittle, J. A. 1987. Using ground-penetrating radar to increase the quality and efficiency of soil surveys. 11-32 pp. In: Reybold, W. U. and G. W. Peterson (eds.) Soil Survey Techniques, Soil Science Society of America. Special Publication No. 20. 98 p. Geonics Limited. 1998. EM38 ground conductivity meter operating manual. Geonics Ltd., Mississauga, Ontario. Geophysical Survey Systems, Inc, 2003. RADAN for Windows Version 5.0; User’s Manual. Manual MN43-162 Rev A. Geophysical Survey Systems, Inc., North Salem, New Hampshire. Greenhouse, J. P., and D. D. Slaine. 1983. The use of reconnaissance electromagnetic methods to map contaminant migration. Ground Water Monitoring Review 3(2): 47-59. Kachanoski, R. G., E. G. Gregorich, and I. J. van Wesenbeeck. 1988. Estimating spatial variations of soil water content using noncontacting electromagnetic inductive methods. Can. J. Soil Sci. 68:715-722. McNeill, J. D. 1980. Electromagnetic terrain conductivity measurements at low induction numbers. Technical Note TN-6. Geonics Ltd., Mississauga, Ontario. 15 p. Morey, R. M. 1974. Continuous subsurface profiling by impulse radar. p. 212-232. IN: Proceedings, ASCE Engineering Foundation Conference on Subsurface Exploration for Underground Excavations and Heavy Construction, held at Henniker, New Hampshire. Aug. 11-16, 1974. Rhoades, J. D., P. A. Raats, and R. J. Prather. 1976. Effects of liquid-phase electrical conductivity water content and surface conductivity on bulk soil electrical conductivity. Soil Sci. Soc. Am. J. 40:651-655.

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Appendix Gopheridge-Argonaut complex, 15 to 30 percent slopes (Transect 1) Station Depth (cm)

1 62 2 80 3 52 25cm-50cm (shallow) 4 84 4 observations (24%) 5 53 6 73 50cm-100cm (moderately deep) 7 94 13 observations (76%) 8 79 9 46

10 33 11 35 12 39 13 80 14 94 15 82 16 96 17 56

AVG 66.9 Gopheridge-Argonaut complex, 15 to 30 percent slopes (Transect 2) Station Depth (cm)

1 81 2 93 3 74 25cm-50cm (shallow) 4 76 2 observations (12%) 5 58 6 61 50cm-100cm (moderately deep) 7 103 10 observations (63%) 8 75 9 42 100cm-150cm (deep)

10 52 4 observations (25%) 11 105 12 96 13 101 14 101 15 79 16 45

AVG 77.6