Important considerations for locating buried agricultural drainage pipe using ground penetrating radar

Enhancing the efficiency of soil water removal on land already containing a subsurface drainage system typically involves installing new drain lines between the old ones. However, the older drainage pipes need to be located before this approach can be attempted. In ongoing research, a near-surface geophysical method, ground penetrating radar (GPR), has been successful in locating on average 72% of the total amount of drainage pipe present at 13 test plots in southwest, central, and northwest Ohio. The effective use of GPR for drainage pipe detection requires careful consideration of computer processing procedures, equipment parameters, site conditions, and field operations, all of which were thoroughly investigated in this study. Application of a signal saturation correction filter along with a spreading and exponential compensation gain function were the computer processing steps most helpful for enhancing the drainage pipe response exhibited within GPR images of the soil profile. GPR amplitude maps that show the overall subsurface drainage pipe system required additional computer processing, which included 2-D migration, signal trace enveloping, and in some cases, a high frequency noise filter and a spatial background subtraction filter. Equipment parameter test results indicate that a 250-MHz antenna frequency worked best, and that data quality is good over a range of spatial sampling intervals and signal trace stacking. In regard to the site conditions present, shallow hydrology, soil texture, and drainage pipe orientation all substantially influence the GPR response. Additionally, drainage pipe that are as small as 5 cm (2 in.) in diameter can be detected. However, the fired clay or plastic material of which the drainage pipe is comprised does not appear to have much of an impact. Finally, with respect to GPR field operations, bidirectional surveys offer the best chance for finding all the buried drainage pipe possible, and for displaying a subsurface drainage system on an amplitude map, the narrower the spacing between GPR measurement lines, the better the result. Although it is important to note that the amplitude maps generated with a wider spacing between GPR measurement lines, still provided plenty of useful data on drainage pipe location. The information supplied by this study can be employed to formulate guidelines that will enhance the potential of success for using ground penetrating radar in locating buried agricultural drainage pipe.