Better time picks=better travel times=better velocities: progress in developing public-domain software

Abstract

Recognizing the importance of quantitative estimates of GPR velocities, we have implemented several standard approaches as public domain software. A field observation that a rainfall event of 3.5 cm was associated with a decrease in the velocity of a shallow reflected phase from 0.104 to 0.085 m/ns, along with antecedent literature on the effects of soil water on GPR velocities, underscored the potential hydrological applications of such results. The algorithms we favor are robust in implementation, provide accurate results, yet are based on methodologies readily grasped by non-specialists and beginning students. Some applications involve such housekeeping elements as format conversion; filtering; gain options; and resampling using FFT interpolation. More useful to the general user are analysis packages for research and teaching: NMO/LMO velocity scans; time-base stretching; event picking; windowed scaling; optimal stacking; and cross-correlation analysis. Testing our procedures on an air wave phase, its velocity from zero-break picks was 0.271 m/ns, and from threshold-break picks was 0.284 m/ns. These are inconsistent and biased low. However, a cross-correlation procedure with an optimized native wavelet provided an estimate of 0.304 m/ns, much closer to the expected value. Such routine checks corroborate the accuracy of particular procedures, and identify potential timing problems.