Introduction

There has been a tremendous amount of literature written on the utilization of geophysical techniques for agricultural research in the last 20 or more years. These geophysical techniques include ground-penetrating radar (GPR), electromagnetic methods (EMs), metal detection, magnetometry, and resistivity. But much of what has been published is associated either directly or indirectly with the use of GPR for agricultural analysis.

The use of GPR as a geophysical technique to study its performance in the agricultural discipline was applied to study the variability of soils in Florida in the late 1970s. The success of GPR to investigate ranges in soil properties and facilitate the mapping of soils in Florida led to the incorporation of GPR as a routine field instrument in the Florida Cooperative Soil Survey Program. From this early achievement, the use of GPR in the agricultural arena blossomed to monitor direct and indirect applications of GPR to agricultural situations. Direct applications include those investigations that have an immediate impact on agricultural production and management. These applications may include such examples as determining limited soil depth restricted by bedrock, hardpan, or a shallow water table; surveying root biomass in a loblolly pine forest; assessing stress in citrus trees; and mapping shallow underground soil features affecting agricultural production. Indirect applications may consist of, but are not limited to, investigating hydrocarbons in soil, locating buried drainage pipes, estimating moisture contents in the vadose zone, detecting coarse layers in a sandy soil, evaluating subsurface pathways for nitrogen loss, and identifying offsite movement of agrochemicals. As you may imagine from this list, GPR has been used more to investigate soils than agricultural plants. Even though several geophysical techniques have been used for agricultural reasons, this chapter will restrict its major discussion to the use of GPR as a geophysical tool in the agricultural environment.

3.2 SOIL SURVEYING USING GPR

The application of GPR technology to study soils was begun in Florida by Benson and Glaccum (1979) and was reported by Johnson et al. (1980). The basic objective of their studies was to determine if GPR could be used to accurately identify soil features and their depths for soil survey purposes. How could this new geophysical tool be integrated into day-to-day soil mapping? They determined that the radar could accurately locate soil features such as spodic and argillic horizons as well as depth to the water table. Probably the most surprising result of the study was the brief time it took to obtain the information as compared to traditional soil mapping. This was also documented by Doolittle (1987) when he reported a decrease in cost (70 percent) and an increase in productivity (210 percent) when he compared doing transects using GPR versus conventional methods

Thus, GPR was incorporated into the Florida Cooperative Soil Survey Program in 1981, as well as in other cooperative soil survey programs, as a routine field tool to investigate subsurface features. Much has been published in this respect. Here are some of the early publications on this subject:

• Study soil microvariability (Collins and Doolittle, 1987)

• Increase quality and efficiency of soil surveys (Collins et al., 1986; Doolittle, 1982, 1987; Doolittle and Collins, 1995; Puckett et al., 1990; Schellentrager and Doolittle, 1991; Schel-lentrager et al., 1988)

• Determine thickness and characterize the depths of organic soil materials (Collins et al., 1986; Doolittle, 1983; Doolittle et al., 1990; Shih and Doolittle, 1984)

• Chart the depths to relatively shallow (<2 m) water tables in predominantly coarse-textured soils (Shih et al., 1985)

• Estimate depths to argillic (Asmussen et al., 1986; Collins and Doolittle, 1987; Hubbard et al., 1990; Truman et al., 1988a, 1988b) and spodic horizons (Collins and Doolittle, 1987; Doolittle, 1987)

• Improve soil-landscape modeling (Doolittle et al., 1988)

Most of the work cited above was done by the U.S. Department of Agriculture (USDA). In fact, Doolittle and Asmussen (1992) published a review of the previous ten years (1981 to 1991) on how the USDA (specifically the Soil Conservation Service, now known as the Natural Resources Conservation Service, and the Agricultural Research Service) used GPR to investigate agricultural soils. They reported the successful use of GPR to "map soils; chart the lateral extent and estimate the depth to soil horizons; and delineate hard pans, water tables, bedrock, and unsaturated flow in the vadose zone," as well as to "assess soil compaction and plow pan development; variations in soil texture, organic matter content, humification, and cementation; thickness of soil horizons, geologic layers, and peat; and movement of water and contaminants in soils." (p. 139)

Some state agricultural experiment stations were also using radar, and many were doing this in collaboration with the USDA. At the same time, other countries were getting more involved with the application of GPR to agricultural circumstances. An outcome was the establishment of the GPR international radar conferences held every even year and alternating between being hosted in the United States and other nations. At the same time, other commercial companies were developing and marketing GPR equipment which subsequently dropped the price of a standard unit. The private sector has been slower in accepting this technology.

GPR has been used by archaeological, engineering, and environmental consulting companies, but not to a great extent by agricultural businesses. One reason could be the soil conditions in which GPR performs well, as discussed next.

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