Precision GPR measurements: assessing and compensating for instrument drift

Recognizing the contributions that ground penetrating radar (GPR) is making to a broad range of near surface geophysical applications, including geotechnical, agricultural and watershed management activities, our group has undertaken a series of studies to identify the precision and accuracy with which GPR traveltimes, velocities and interval properties can be estimated under controlled conditions. We have recently reported on the analysis of a sequence of high quality common mid-point (CMP) soundings collected on eight different days at a single location on "clean" (low attenuating) stratified glacial drift in Southern New England. The results indicated that the two-way traveltime and velocity may be optimally estimated with precisions of ± 0.7 ns and ± 0.001 m/ns at the 95% confidence level. The high degree of precision in that dataset made it possible to observe that there may be non-random patterns in the differences between estimates of the same parameter, suggesting possible systematic biases in the data. Since one source of such a bias might be due to timing errors in the instrumentation, this report addresses the issue of time base drift; namely, ways to identify its presence, to assess its influence on estimating GPR parameters, and to compensate for its effect. Drift effects can be readily identified and compensated for. An invariant condition in a GPR survey is that the true velocity of the direct air phase should be the velocity of an electromagnetic pulse in free space. Thus, upon carefully determining the observed velocity of the air phase, and using the ratio between it and 0.29979 m/ns , one can correct the entire time base of a radargram. We illustrate this compensation procedure using two CMPs from the same site that exhibit a 14.7% difference in direct air phase velocity, and the depth estimates differ by 0.5 m. After correcting the time base, the difference between the depth estimates improves to 0.05 m.