Introduction To The Gpr Refraction Method

Wide-angle common mid-point (CMP) and common shot-point (CSP) ground penetrating (GPR)<br>“soundings” often show refracted phases. Most frequently these are refractions at the ground-to-air<br>interface (“air”-refractions); less frequently, but not exceptional, are refractions from a higher velocity<br>medium in the subsurface (“ground” refractions), such as coarser-grained unconsolidated sediments or<br>bedrock. Although early work by Annan, Davis, Arcone, Delaney, Fisher and McMechan (c.f. Annan et<br>al., 1976; Arcone, 1984; Delaney et al., 1990; and Fisher et al., 1992) set the stage for interpreting GPR<br>refractions, the vast majority of current GPR investigations employ reflection techniques. Adapting<br>standard refraction procedures from seismology to GPR CMP and CSP data, we use field examples from<br>the Northeast U.S. to briefly review the interpretation of GPR refractions using planar dipping interface<br>models. We then introduce the GPR delay time method whereby, using reversed CSP data, each field<br>station receiving or transmitting a refracted phase provides an estimate of the local depth to the refractor.<br>We next describe an inverse procedure for migrating these depth estimates to construct a smooth<br>(splined) refractor using a computer application that we are planning to place in the public domain. We<br>conclude by comparing our migrated results to conventional procedures, emphasizing the synergy of<br>using refracted phases in conjunction with conventional GPR reflection profiling.