SEISMIC CHARACTERIZATION OF GLACIAL SEDIMENTS IN CENTRAL ILLINOIS BASED ON DOWNHOLE SEISMIC MEASUREMENTS

Characterization of glacial sediments is a challenge for high resolution seismic methods. The complex nature of glacial sediments and paucity of regional seismic velocity models makes it difficult to select the proper seismic method to apply and later to make the correct seismic interpretations. Ideally, downhole seismic logs should be acquired along each seismic transect to assist in making reliable seismic interpretations. Unfortunately, logs are not always available along every seismic transect. Therefore, we propose constructing generalized seismic velocity models for larger areas using downhole seismic logs collected from available boreholes in these areas. In an attempt to implement this idea, we collected compressional and shear wave velocity profiles (Vp and Vs) from 15 boreholes covering a 10 x 10 km² area in central Illinois. The boreholes penetrated through glacial sediment consisting of multiple units of till, sandy silt, sand and gravel, and silty clay. The data were acquired in PVC-cased holes drilled to depths ranging from 30 to 100 m using a three-component downhole geophone and a surface source. Travel times of the first-arrival seismic waves were analyzed using the direct and interval methods to generate 1-D seismic velocity profiles. These profiles were later compared to corresponding natural gamma and geologic logs. The Vs profiles showed relatively consistent velocity variations with depth. Three subsurface distinct sequences can be identified from significant velocity contrasts, diamictons of the Wisconsinan glaciation, Vandalia Till Member of the Glasford Formation (Illinoian glaciation), and till and glacial lake sediments of the Pre-Illinoian age. On the contrary, the Vp profiles showed no consistent velocity variation pattern, which is most likely due to response of the Vp to variations in soil-water and gas contents. Inspection of the Vp and the Vs profiles suggests that for this area, S-wave reflection surveys will be more successful in characterizing the subsurface than P-wave surveys.