HIGH-RESOLUTION SEISMIC IMAGING OF GLACIAL TERRAINS IN ILLINOIS, HYDROGEOLOGIC RESOURCE MANAGEMENT APPLICATIONS

In the Great Lakes region, near surface glacial sediments are characterized by coarse and fine unlithified deposits in both large- and moderate-scale features. Fine, lake and glacial till sediments are deposited in wide sheets. Coarse sediments form large fluvial sandurs or fill pro- and subglacial meltwater channels. In Illinois, large coarse sediment bodies often are found in 20-km wide and 100-m deep buried Tertiary bedrock valleys, e.g. Mahomet valley. On a smaller scale, shallow and narrow, very complex channel structures are 1 to 2 km wide, and inter-stratified with an erosive base that mainly cuts down into the till sheets. Since the grain-size distribution is directly related to the permeability, sedimentary bodies can be classified as hydrogeological aquitards (fine-grain sediments) and aquifers (coarse-grain sediments). For many decades, hydrogeologists, as well as geophysicists, have targeted the coarse sediments in these buried channels and valleys for ground water resources. The aquitards are important safeguards for underlying aquifers. In order to model the ground water flow, a reliable 3-D geologic image is necessary. From a geophysical perspective, the best method for obtaining these images depends on both the sediment type and the scale of the target feature. High-resolution seismic reflection surveys using the pressure wave (P-wave) technique have provided outstanding images of the shape of the deeper bedrock valleys and their sedimentary fill. However, when the channel structures are less than 50-m deep and have a small base impedance contrast, they are sometimes very difficult to observe using P-wave seismic reflection techniques. The electrical method is an adequate tool to map such aquifers when the resistivity contrast between sediments is sufficient. Similarly, radar methods are only successful in certain, limited conditions. Horizontal shear wave reflection methods (SH-wave) have high resolution and provide accurate images of shallow features. We have developed a new land-streamer to improve the efficiency of SH-wave data acquisition. This SH-wave technique offers a good alternative to radar methods in clay-rich environments. We will present several case histories illustrating the use of high resolution P- and SH-wave seismic reflection techniques in imaging aquifers and aquitards in Illinois.