Near-surface water content estimation with borehole ground penetrating radar using critically refracted waves

Zero-offset profiling (ZOP) with borehole ground penetrating radar (BGPR) is a promising tool for profiling water contents in the subsurface to great depths with high spatial and temporal resolution. The ZOP method relies on determining the velocity of an electromagnetic (EM) wave that follows a direct path from the transmitter to the receiver. However, near the ground surface, critically refracted energy that travels along the ground surface at the velocity of an EM wave in air may arrive before direct waves that travel through the subsurface. If the critically refracted waves are mistakenly interpreted to be direct waves, the water content will be underestimated. As a result, the water content near the ground surface cannot be determined using standard BGPR analysis. We refer to the depth below which direct waves are the first to arrive as the refraction termination depth. An alternative analysis is presented to determine the water content above the refraction termination depth using the slope of the travel time vs. depth profile. Additionally, guidelines are presented to predict the refraction termination depth for known near-surface water contents.