## 1 Introduction  
A 1985 economic survey showed that the states comprising the Midwest U.S. (Illinois, Indiana, Iowa, Ohio, Minnesota, Michigan, Missouri, and Wisconsin) by that year had approximately 12.5 million ha that contained subsurface drainage systems, with cropland accounting for the large majority of areas possessing these buried drainage pipe networks [1]. In the more than 30 years that have elapsed since the 1985 survey, a very sizeable amount of additional agricultural drainage pipe has been installed throughout the Midwest U.S. In order to improve crop yields, farmers within this region often need to repair drain lines that are not functioning properly or install new drain lines between the old ones to improve soil water removal efficiency. Whether for system repairs or efficiency improvements, locations of the preexisting drain lines are required; however, in most cases, a map of the original subsurface drainage system installation is no longer available. Present methods of locating drain lines involve the use of hand-held tile probes or heavy trenching equipment. Finding drainage pipes with a tile probe (Figure 1a) is both time consuming and extremely tedious. With a tile probe, it is sometimes difficult at a particular spot beneath the ground surface to distinguish a clay tile drainage pipe from the large stones that are commonly present in glacially derived soils of the Midwest U.S. A tile probe also easily penetrates through a corrugated plastic tubing (CPT) drainage pipe, thereby making the presence of the CPT drainage pipes difficult to ascertain, while at the same time potentially causing pipe collapse problems. Locating drainage pipes with heavy trenching equipment (Figure 1b) is effective, but causes costly pipe damage that has to be repaired before the trench is backfilled. Consequently, an effective, efficient, and noninvasive (i.e., nondestructive) method is needed for mapping subsurface drainage systems in agricultural fields. In a preliminary study [2], an unmanned aircraft system (UAS) with a thermal infrared camera exhibited promise in regard to drainage pipe mapping at an agricultural field in central Ohio, USA; however, the technology is now only just beginning to be rigorously evaluated for this particular application. Prior research has demonstrated the feasibility of using ground penetrating radar (GPR) in agricultural settings to locate buried clay tile and CPT drainage pipe over a range of soil types and shallow hydrologic conditions [3; 4; 5]. Additional research has shown that GPR, under certain circumstances, can isolate the location of drainage pipe obstructions [6]. The strength of the GPR drainage pipe response for specific soil moisture conditions was found to depend on the orientation of the GPR antennas relative to the drain line directional trend [7]. All prior GPR drainage pipe detection research focused on testing the feasibility of this geophysical technology on small agricultural experiment plots. The practicality of employing GPR for determining drain line patterns in larger farm fields has not been adequately assessed. Ground penetrating radar integrated with a Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS) may be an effective, efficient, and noninvasive (i.e., nondestructive) means of mapping drain lines within agricultural fields. Consequently, GPR-RTK/GNSS was evaluated at three agricultural fields. The overall process involved mapping the latitude and longitude coordinates of points in the field beneath which drainage pipes were potentially detected and then using this map to provide insight on drain line patterns. The guiding hypothesis for this research can be stated; \"GPR integrated  
Figure 1: Present methods for finding drainage pipes: (**a**) hand-held tile probe; (**b**) heavy trenching equipment with inset photo showing an example of the drainage pipe damage caused during this excavation operation.