Seismic characterization of glacial sediments in central Illinois

The vertical distribution of compressional wave velocity (Vp) and shear wave velocity (Vs) acquired from fifteen boreholes in central Illinois using the standard surface-source downhole-receiver method was studied. The velocity logs were compared with lithologic logs and gamma-ray logs acquired from the same boreholes to: 1) better understand the Vp and Vs ranges and variations within glacial sediments, 2) determine whether characteristic seismic velocities could be resolved to distinguish among the three major Pleistocene glaciations of Wisconsin (WI), Illinois (IL), and pre-Illinois (PIL), and 3) examine velocity variations corresponding to heterogeneities in the sediments composing these three major units. Results showed that deposits composing these units had highly variable Vp and Vs values. Only the contact between deposits of the WI and IL episodes could be delineated by a corresponding slight decrease in Vp. Other than that, neither Vp nor Vs logs showed significant contrasts at the contacts between these units. Some individual sediment packages, or intraunits, exhibited distinctive velocity patterns in the study area and were identified more clearly from Vs than from Vp logs. These intraunits are Wisconsin tills (T), Vandalia till (GV) and Mahomet sand (BM).