The GPR refraction method

Abstract

Refracted phases have been a recognized feature in wide‐angle common midpoint (CMP) and common shotpoint (CSP) ground‐penetrating radar (GPR) surveys for many years, but historically their use has been limited and overshadowed by conventional reflection profiling techniques. In truth, the GPR refraction method holds great promise, as we show using four field examples from three sites in the northeastern United States. Typical refractors are shallow bedrock or coarse‐grained unconsolidated sediments (where the dielectric constant K ≈ 6–7) beneath partially saturated fine‐grained sand or silt (K ≈ 15–16). Under appropriate conditions, refracted phases can be detected readily at transmitter–receiver (Tx–Rx) offsets of >15 m and from depths of >2 m. Adapting procedures from refraction seismology, we interpret GPR data from reversed CSP soundings using dipping planar interface models and delay time analyses. Results from the two approaches are quite consistent, further corroborated at one of the sites by a GPR reflection profile and directly verified by a series of hammer‐driven probes. In addition to refractions from higher velocity layers in the subsurface, the air refracted phase (the ubiquitous phase refracted from the air–earth interface) can provide important constraints on interpreting the subsurface—particularly useful when attempting to resolve a shallow interface (depth ≤ 0.5 m) whose GPR signature might be masked by interacting phases at early traveltime and small transmitter–receiver offsets. Finally, a cautionary note for interpreting conventional reflection profile data when delineating a higher velocity interface at a depth that might be on the order of the antenna separation or less. Since the refracted phase arrives earlier than the reflected phase for Tx–Rx offsets beyond the critical distance, in certain cases the first break on a reflection radargram may actually be a refraction. CMP or CSP soundings in support of conventional GPR profiles can resolve such concerns.