SH-WAVE SEISMIC REFLECTION TO MAP SHALLOW AQUIFERS, A GEORADAR-LIKE ACOUSTIC METHOD?

Georadar, which detects reflections from electromagnetic pulses to image subsurface features, is often cited as the geophysical tool of choice for very shallow mapping projects. However, this technique has limited depth of penetration in areas with a shallow water table and when fine-grained sediments are present. At depths greater than about 30 meters, P-wave high resolution seismic reflection has proven to be a very good aquifer mapping tool. This technique provides useful results when the water table is near the surface or when fine-grained sediments are present at the surface of the ground. In general, the method gives poor results when the near surface has low velocity characteristics. Further, P-wave propagation is adversely affected by the gas content in the sediment pores. Acoustic techniques utilizing shear-wave reflections have been more successful in Illinois than either georadar or P-wave reflections.

Since 2001 we have been developing acquisition and processing techniques for shear-wave seismic data acquisition that give outstanding results in most of the surface conditions in Illinois, including in the places where the P-wave reflection technique does not produce good results e.g. river flood plains and thick loess deposits. With geophone arrays mounted on sleds pulled along roads, we have achieved a data acquisition rate of up to 2 km/day.

Through several examples we demonstrate that SH-wave seismic reflection has the potential to provide crucial structural modeling information for hydrogeology in places where neither the P-wave seismic reflection nor the Georadar could give good results. This technique has given information from the surface down to 100 meters in various terrains such as glacial terrains including subglacial tunnel-channels, coarse fluvial sediment with a deep water table, near-surface faulting and cave collapses buried under loess.