Development of Improved Geophysical Imaging Techniques for Environmental Site Characterization.

The common midpoint CMP processing technique has been shown to be effective in improving the results of ground penetrating radar profiling. When radar data are collected with the CMP multi-offset geometry, stacking increases the signal-to-noise ratio of subsurface radar reflections and the effective penetration depth. An important aspect of CMP processing is normal moveout velocity analysis. Most, if not all, GPR surveys, are very limited in spatial extent and the common perception is that within the survey range, radar velocity in the shallow subsurface has very slow or no lateral variation. Therefore, a single velocity function might be considered adequate to describe the subsurface. In this study we show that, in fact, lateral variation in radar velocity can be quite significant and that the stacked profile improves as the number of velocity analysis locations is increased, up to some practical limit. Interval velocity can be calculated from the normal moveout velocities derived in the CMP velocity analysis. An approximate relationship between interval velocity and water content is derived. By collecting GPR data in the multi-offset CMP geometry, not only is the radar profile improved but it also allows for an interpretation of subsurface variation in water content.