Summary: Enhancing the efficiency of soil water removal, and in turn crop productivity, on land already containing an agricultural subsurface drainage system, typically involves installing new drain lines between the old ones. However, before this approach can be attempted, the older drainage pipes need to be located. A research investigation has found that ground penetrating radar (GPR) was successful in locating on average 74% of the total amount of pipe present at fourteen test plots in southwest, central, and northwest Ohio. This method worked well in finding clay tile and corrugated plastic tubing drainage pipe down to depths of around 1 meter (3 feet) within a variety of different soil materials. Additional research has focused on various factors that can be important for achieving success using GPR to find agricultural drainage pipes. Results with respect to equipment parameters indicate that choosing the proper antenna frequency is crucial, and antennas with a center frequency of around 250 MHz seem to work best. Also, data quality is similar over a sampling interval range of 2.5 to 10 cm (1 to 4 inches) between points along a measurement line and for averaging 4 to 32 signal traces at a measurement point. In regard to site conditions, shallow hydrology, soil texture, and drainage pipe orientation all influence the GPR response. Moist soils with pipes at least partially filled with air provide good GPR field conditions, assuming the radar signal penetrates to the drain line depth. Sandy soils allow greater radar signal penetration than do clayey soils. Field operations (spacing distance between measurement lines and bidirectional vs. unidirectional surveys) along with the algorithms used in the computer processing sequence to generate GPR profiles and GPR maps are all very important considerations. Furthermore, GPR has also proven capable of providing information on drainage pipe condition with respect to the presence of flow obstructions. The information obtained in this study can hopefully be used to develop guidelines that will enhance the potential for success of using ground penetrating radar to locate buried agricultural drainage pipe and evaluate their condition.
This schematic illustrates how ground penetrating radar (GPR) equipment is used to locate buried agricultural drainage pipes. The principle of the GPR method involves directing an electromagnetic radio energy (radar) pulse into the subsurface, followed by measurement of the elapsed time taken by the signal as it is travels downwards from the transmitting antenna, partially reflects off a buried feature, and is returned to the surface, where it is picked up by a receiving antenna.
The preceding figures depict some typical ground penetrating radar (GPR) results with regard to locating buried agricultural drainage pipe. Figure a and Figure b are GPR generated images of the soil profile. As shown in these profiles, the GPR response to different buried drainage pipe orientations range from upside-down U-shaped to distinct linear banded features. Figure c is a map depicting the relative radar energy reflected back to the surface from a depth interval of 0.9 to 1.4 meters. Lighter shades on the map indicate greater reflected radar energy. Lightly shaded or white linear features often represent drainage pipes. Figure d is an interpreted map based on the GPR data showing the drainage pipe system with dashed lines. The blue line on Figure d indicates the line position along which the Figure a GPR profile data were collected. The red line on Figure d indicates the line position along which the Figure b GPR profile data were collected.
This GPR profile from data collected along trend directly over top of a drainage pipe in saturated soil shows the abrupt transition from a water-filled pipe (west side) to an a air-filled (east side). This abrupt transition in the GPR response is due to a pipe obstruction that completely blocks the flow of water.
Barry J. Allred
USDA/ARS – Soil Drainage Research Unit
590 Woody Hayes Drive, RM. 234
Columbus, Ohio 43210