The Coastal Plainer: Fall 2011

Ground Penetrating Radar in a Big Wetland

By Howard Yamataki, MLRA Leader, MLRA Soil Survey Office 15–8, Fort Myers, Florida, and Martin Figueroa, MLRA Senior Soil Scientist, Fort Myers, Florida

Ground penetrating radar (GPR) has been used for soils work in Florida for over 30 years. Our sandy soils and drained Histosols have been good media for this technology. GPR has been used to help determine composition of map units and to help understand the effects of drainage (especially subsidence) over time. James Doolittle, now a national leader, was one of the pioneers of GPR in Florida.

Despite our successes with GPR, we have always found it challenging to map soils and use GPR in areas with surface water. When we started fieldwork in the Water Conservation Area (WCA) in 2009, we did not use GPR because of these challenges. The WCA is a large area of Histosols underlain by limestone bedrock in the southeastern part of Florida. It is used as an urban water supply and a conservation area. Portions of this large wetland are owned by the State of Florida and by Native American tribes. My office has been charged with mapping the northern part of the area. Our previous bad experiences dragging the GRP over grassy vegetation gave us some doubts about using it in the WCA. But we also realized the technology had advanced over a 30-year period, so in September 2011 we decided to give it a try.

Our first task was to figure out how to affix the GPR to an airboat, which is our main mode of transportation within the WCA. Senior Soil Scientist Martin Figueroa designed the method in figure A. He also set up a series of flags that corresponded to marked points made during the “run.” Figure B is an image of the GPR output from our airboat run.

The GPR image shows depth to limestone for the first 35 meters of the transect. Transect lengh was 1,600 meters and was marked at 30-meter intervals. Based on the GPR data, the average depth to limestone was less than 81.4 centimeters (Lauderhill soils) for 75 percent of the transect and more than 81.4 centimeters (Pahokee soils) for the remaining 25 percent.

In addition to the method shown in figure A, we also hand towed the GPR aerial. Figure C is an image of the GPR output from the hand-towed run. The length of the GPR transect in figure C is 15 meters. The image shows two contrasting layers. At about 15 centimeters below the water surface, there is periphyton deposition over a muck layer. The second layer, at an average depth of 21 centimeters, is limestone. Mark 1 to mark 3 defines the Dania soil series by depth of rock.

Because our experience using the GRP in the WCA environment was very limited, both methods seemed inconclusive to us. The following week, however, Martin attended a national training session that gave him the opportunity to ask James Doolittle to assist us with interpreting the WCA transects. Jim immediately deemed our experiment a success and pointed out the part of our readout that indicated depth to limestone bedrock. He enhanced and refined our GPR images, making it clear our experiment had indeed turned out well. The improvements to the imagery were done using Radan 6.6 software, which is used for processing GPR data.

There are thousands of acres to be mapped in south Florida. Our field tools will be bucket augers, special muck samplers, probes, and... GPR.

Figure A.—Ground penetrating radar attached to the side of an airboat.

Figure B.—Output of the GPR from an airboat run.

Figure C.—Output of the GPR from a hand-towed run.

Documenting the depth to limestone at reference points, which is a part of the task for a GPR transect. (Martin Figueroa, senior soil scientist, and Rick Robbins, Florida State office soil scientist)

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