Phantom subsurface targets in ground-penetrating radar data

Ground-penetrating radar (GPR) has been a very effective tool for exploring the subsurface and the nondestructive testing of nonmetallic structures for the past 40-50 years. The traditional GPR data interpretation is built upon the innate bias that all signals emanate from within the ground and most GPR users are normally under the impression that energy mostly travels straight down leading to the perception that "targets" are beneath the measurement location. The response of features at the ground surface and above ground also is present in most data but not always consciously noted as contributing to the measurements. One class of responses from above-ground features is routinely called "airwaves" because they normally exhibit moveout velocities of air. Often, an above-ground source is not the first thing that comes to mind during data interpretation, unless the user is experienced. Even experienced users can occasionally be misled, as above-ground features are expected to reach the GPR receiver with the moveout velocity of air. Recent experience in some of our surveys has created concerns because the targets at or above the ground surface demonstrated ground wave moveout velocity, which eliminates one of the diagnostic tools. This paper explores this issue, identifies GPR signal paths, and suggests key factors to consider in field operations and data interpretation. To demonstrate the concepts described, we have used numerical modeling and field data sets.