The effect of a dipping layer on the first-arrival traveltime from zero-offset and fixed-offset borehole radar

The effect of a dipping layer on the first-arrival traveltime from single-offset profiling with borehole radar is investigated, including both zero-offset and fixed-offset profiling. In our study, offset refers to the vertical distance between bistatic radar antennae. Using forward modelling of electromagnetic wave travel, the traveltime of zero-offset and fixed-offset profiles through a dipping layer is compared to those of flat-lying layers. The model considers three distinct raypaths: direct, critically refracted and cross-dip refracted. Whereas critical refraction only occurs in a layer of low propagation velocity relative to an adjacent high-velocity layer, cross-dip refraction can occur in any velocity structure. The forward model demonstrated that the slope of the traveltime through the cross-dip portion of the profile is approximately half of that in the critically refracted portion. To obtain the electromagnetic wave propagation velocities above and below the dip, only one profile is necessary. However, to invert for the dip angle and position, two profiles with different offsets must be considered together.