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United States Natural Resources 207 W. Main Street Department of Conservation Room G-08 Agriculture Service Wilkesboro, NC 28697 Subject: SOI -- Geophysical Field Assistance and Demonstration Date: 28 September, 2004 To: Roy Vick, MO14 Leader/State Soil Scientist, USDA-NRCS, 4405 Bland Road, Suite 205, Raleigh, North Carolina 27609 Purpose and Discussion: To provide geophysical assistance in the update of Iredell County, NC. New methods in soil survey, including electromagnetic induction (EMI) techniques and the use of ground penetrating radar (GPR) were demonstrated. The applicability and potential future use of geophysical tools were discussed. In conjunction with soil sampling, a Chinese delegation of soil scientists is helping in the sampling process along with soil scientists from the local, state and national office. New techniques in soil survey (geophysical tools) were demonstrated to the Chinese delegation of soil scientists. Participants: Ming Chen, Ph.D., Quality Assurance Officer & Visiting Assistant, Everglades Research and Education Center, University of Florida, Belle Glade, FL Kent Clary, MO-13/Soil Data Quality Specialist, USDA-NRCS, Shelby, NC Bob Engel, Soil Scientist, USDA-NRCS, NSSC, Lincoln, NE Haihua Gan, Ph.D., Associate Professor, Department of Land Resources, South China Agric. Univ., John Kelley, MO-14/Soil Data Quality Specialist, USDA-NRCS, Raleigh, NC Roger Leab, MLRA Project Leader, USDA-NRCS, Greensboro, NC Ying Lu, Ph.D., Associate Professor, South China Agric. Univ., Guangzhou, China Milton Martinez, Project Leader, USDA-NRCS, Statesville, NC Robert Ranson, Soil Scientist, USDA-NRCS, Statesville, NC Wes Tuttle, Soil Scientist (Geophysical), USDA-NRCS, NSSC, Wilkesboro, NC Roy Vick, MO-14 Team Leader/State Soil Scientist, USDA-NRCS, Raleigh, NC Mike Wilson, Soil Scientist, USDA-NRCS, NSSC, Lincoln, NE Xinming Zhang, Ph.D., Associate Professor, South China Agric. Univ., Guangzhou, China Activities: All field activities were completed the 20-22 July and 04-06 August, 2004. Overview: Soil sampling and complete soil characterization is being done to aid in the update soil survey of Iredell County, NC. At least 11 pedon sites were chosen to be sampled. Geophysical tools were utilized to more accurately capture soil features and soil characteristics across the landscape. The information contained in this report has been used for site selection for soil sampling and to better understand the complexities of the soils in the survey area. The information gathered using geophysical tools will aid in soil correlation and facilitate the update of the soil survey in Iredell County. Equipment: Dualem Inc. manufactures the Dualem-2/4 meters.1 Taylor (2002) describes the principles of operation for these meters. The meters consist of one transmitter and two receiver coils. One receiver coil and the transmitter coil provide perpendicular (PRP) geometry. The other receiver coil provides a horizontal co-planar (HCP) geometry with the transmitter coil. This dual system permits two depths to be simultaneously measured without rotating the coils. The depth of penetration is “geometry limited” and is dependent upon the intercoil spacing and coil geometry. The Dualem-2/4 meters operate at a frequency of about 9 kHz. The 1 Trade names are used to provide specific information. Their mention does not constitute endorsement by USDA-NRCS.

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Dualem-2 meter has a 2-m intercoil spacing and provides penetration depths of 1.3 and 3.0 m in the PRP and HCP geometries, respectively. The Dualem-4 meter has a 4-m intercoil spacing and provides penetration depths of 2.5 and 6.0 m in the PRP and HCP geometries, respectively. The meter is keypad operated and measurements can either be automatically or manually triggered. No ground contact is required with this device. Lateral resolution is approximately equal to the intercoil spacing. The device is portable and requires only one person to operate. Values of apparent conductivity are expressed in millisiemens per meter (mS/m). Where geo-referencing was desired using the Dualem 2/4 meters, an Allegro field computer was used to collect EMI data with a GM-210 GPS receiver manufactured by HOLUX Technology, Inc. to collect the coordinates of observation points. 1 To help summarize the results of this study, SURFER for Windows (version 8.0) program, developed by Golden Software, Inc., 1 was used to construct two-dimensional simulations. Grids were created using kriging methods with an octant search. The radar unit is the TerraSIRch Subsurface Interface Radar (SIR) System-3000 (here after referred to as the SIR 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, SVGA video screen, and connector panel. A 10.8-volt lithium-ion rechargeable battery powers the system. The SIR System-3000 weighs about 9 lbs (4.1 kg) and is backpack portable. With an antenna, this system requires two people to operate. A 200 MHz antenna was used in this study. The RADAN for Windows (version 5.0) software program was used to process the radar records (Geophysical Survey Systems, Inc, 2003). 1 EMI Survey The study site was located on a rural farm in northern Iredell County approximately 5 miles west of the community of Union Grove, NC. The site was located in a hay field. The site was in an area that has been mapped Cecil fine sandy loam, 2 to 6 percent slopes, eroded and Cecil sandy loam, 2 to 6 percent slopes, eroded (Cawthorn and Jenkins, 1964, field sheet #1). The very deep, well drained, Cecil soils formed in residuum weathered from felsic, igneous and high-grade metamorphic rocks of the Piedmont uplands. Cecil is a member of the fine, kaolinitic, thermic Typic Kanhapludults family.

A wildcat survey was completed at the site. A grid system survey was not feasible due to the irregular shape of the area to be surveyed. The survey was conducted by traversing the survey area while providing representative areal coverage to accurately capture apparent conductivity and spatial distribution patterns. Walking at a fairly uniform pace across the grid area with the Dualem-2 meter completed the survey. The Dualem-2 meter was operated in the continuous mode with measurements recorded at a 2-sec interval. The Dualem-2 meter was held at hip height with measurements of apparent conductivity geo-referenced and measured in the HCP and PRP geometries. A total of 969 measurements were recorded with the Dualem-2 meter in the shallower sensing PRP (0 to 1.3 m) geometry. Apparent conductivity averaged about 3.5 mS/m with a range of 2.5 to 5.2 mS/m. One-half the observations had values of apparent conductivity between about 3.2 and 3.8 mS/m. A total of 969 measurements were recorded with the Dualem-2 meter in the deeper sensing HCP (0 to 3.0 m) geometry. Apparent conductivity averaged about 3.3 mS/m with a range of -2.1 to 7.8 mS/m. One-half the observations had values of apparent conductivity between about 2.5 and 4.1 mS/m. Results Lower apparent conductivity at point “A” in Figure 1 was attributed to a thinner soil profile containing less clay and less moisture, comparatively speaking. Soil borings at point “A” revealed a shallower soil component. Soft bedrock was at 21 inches (53cm). Soil borings at point “B” revealed a soil profile containing a thicker clayey subsoil with no bedrock within 60 inches. Soil borings were not taken at point “C” but higher apparent conductivity at this point is thought to be attributed to a thicker soil profile containing more clay and moisture, as compared to point “A”. 1 Trade names are used to provide specific information. Their mention does not constitute endorsement by USDA-NRCS.

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Figure 1. Apparent conductivity measured with the Dualem-2 meter in the perpendicular (PRP) geometry (shallower sensing mode) in an area of Cecil fine sandy loam, 2 to 6 percent slopes, eroded and Cecil sandy loam, 2 to 6 percent slopes, eroded.

Ground Penetrating Radar (GPR) 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.

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GPR Survey A radar survey was conducted in an area of Madison gravelly fine sandy loam, 2 to 6 percent slopes, eroded (Cawthorn and Jenkins, 1964, field sheet #6). The study site was located approximately 4.5 miles west of the community of Union Grove, NC. The site was located in an area of recently harvested timber. The study area chosen was thought to contain soils with soft bedrock at depths less than 60 inches (1.5 meters). These shallower soils are in contrast with the previously mapped very deep Madison soils. Could GPR be used effectively in soils containing a clayey (fine) subsoil (1:1 kaolinite clay). Could GPR be used to help show the composition of shallower soils within the soil mapping units? Results: A 200 m traverse line was established across the site. Survey flags were placed at a 10 m interval. The survey was completed with the 200 MHz antenna at a scanning time of 50 ns. Based on a measured depth (0.48m) to metal reflector, the velocity of propagation through relatively dry very gravelly sandy loam (0 to 3cm), very gravelly sandy clay loam (3 to 12cm), and clay (12 to 48cm) was estimated to be 0.11 m/ns. The dielectric permittivity was 7.58. The radar record (Figure 2) provided satisfactory penetration depths and satisfactory resolution of subsurface features. The brighter colors are reflections of higher amplitudes and represent contrasting soil features. One of the major contrasting soil features observed in Figure 2 is the interface (approximate) between earthy soil material and soft bedrock which is indicated by the black line. Depths to soft bedrock were verified with soil borings and excavated pits and closely matched the depths observed in the radar record. Moderately deep and deep to soft bedrock soil components were the dominant soil components. The map unit will be recorrelated to more accurately capture the varying depths to soft bedrock, as observed in the radar record.

Figure 2. A portion of a representative radar record from an area of Madison gravelly fine sandy loam, 2 to 6 percent slopes, eroded. Scale is in meters. Upon further examination of the entire radar record, varying depths to the soft bedrock interface could be observed. The radar survey transect summary is as follows: Depths to Soft Bedrock Components Observed (21 total) Component Percentage 0 to 20 in. (0 to 50 cm) 1 5% 20 to 40 in. (50 to 100 cm) 7 33% 40 to 60 in. (100 to 150 cm) 8 38% > 60 in. (>150 cm) 5 24%

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Figure 3. National Soil Survey Center (NSSC) staff demonstrates the use of the Dualem-2 conductivity meter to the Chinese delegation of soil scientists.

Figure 4. NSSC staff demonstrates and explains the use of ground penetrating radar. GPR surveys have resulted in radar records with good interpretive value for depth to bedrock determinations in the Iredell County, NC soil survey update.

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Figure 5. Ground truthing is an integral part of any geophysical investigation. In the photo, soil features in the excavated pit were compared to the GPR record. Summary: 1. A demonstration on the use, operation and applicability of electromagnetic induction (EMI) and

ground penetrating radar (GPR) was provided. The operation and use of EMI as an ancillary tool in project soil survey and update mapping was discussed. EMI is the tool of choice in conductive soil mediums while ground penetrating radar (GPR) is the usually the tool of choice in resistive soil mediums.

2. Apparent conductivity values did appear to reflect the changes in depth to bedrock at the EMI survey site (Figure 1). Higher apparent conductivity was attributed to an increase in moisture, clay and depth of soil profile, as observed from soil borings.

3. The radar record (Figure 2) provided satisfactory penetration depths and satisfactory resolution of subsurface features in the clayey (fine), kaolinitic soils. Depths to soft bedrock were verified with soil borings and excavated pits and closely matched the depths observed in the radar record. Moderately deep and deep to soft bedrock soil components were the dominant soil components. The map unit will be recorrelated to more accurately capture the varying depths to soft bedrock, as observed in the radar record.

4. Geophysical interpretations are considered preliminary estimates of site conditions. The results of

geophysical site investigations are interpretive and do not substitute for direct ground-truth observations (soil borings and pits). The use of geophysical methods can reduce the number of coring observations, direct their placement, and supplement their interpretations. Interpretations contained in this report should be verified by ground-truth observations whenever possible.

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It was my pleasure to work again in North Carolina with members of your fine staff. Sincerely, Wes Tuttle Soil Scientist (Geophysical) National Soil Survey Center Lincoln, NE cc: B. Ahrens, Director, National Soil Survey Center, USDA-NRCS, Federal Building, Room 152, 100 Centennial Mall North, Lincoln, NE 68508-3866 J. Doolittle, Research Soil Scientist, USDA-NRCS, Suite 200, 11 Campus Boulevard, Newtown Square, PA 19073 M. Golden, Director of Soils Survey Division, USDA-NRCS, Room 4250 South Building, 14th & Independence Ave. SW, Washington, DC 20250 J. Kelley, MO14 Soils Data Quality Specialist, USDA-NRCS, 4405 Bland Road, Suite 205, Raleigh, North Carolina 27609 R. Leab, MLRA Project Leader, USDA-NRCS, 3309 Burlington Rd. Greensboro, NC 27405 J. Kimble, Acting National Leader for Soil Investigations, USDA-NRCS, National Soil Survey Center, Federal Building, Room 152, 100 Centennial Mall North, Lincoln, NE 68508-3866 R. Ranson, Soil Scientist, USDA-NRCS, P. O. Box 5713 Statesville, NC 28687 References: Cawthorn, J. W. and Jenkins, V. S. 1964. Soil Survey of Iredell County, North Carolina. USDA-Soil Conservation Service. US Government Printing Office, Washington DC 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. 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. 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. Taylor, R. S. 2002. Manual for DUALEM Serial Number 8 manufactured for the United States Department of Agriculture. Dualem Inc., Milton Ontario.