Effect of the overlying sedimentary sequence on the seismic imaging of the bedrock surfaces: McHenry County, Illinois

High-resolution shear-wave seismic reflection has been used routinely for almost a decade by the Illinois State Geological Survey to image shallow bedrock surfaces (<40 m) and overlying, often water-saturated, Quaternary sediments. Data are acquired using a 24-channel land streamer with 14-Hz geophones spaced at 0.75-m intervals. The energy source is a 1-kg hammer striking the horizontal axle of a rolling cylinder. The close geophone spacing and low-energy source were designed to increase the resolution potential by enhancing the high frequency and broadening the spectral bandwidth of the seismic signals. This system successfully imaged the shallow bedrock surface at some locations, but not others. In McHenry County, for instance, the bedrock surface was not resolvable from 10 of 25 km of shear wave data obtained over the past 2 years. Where the sedimentary sequence above the bedrock was relatively uniform, the shear-wave energy penetrated more deeply through the sediment and resolved the bedrock surface at depths of 40 to 60 m. At other locations, where the shear-wave energy encountered interfaces with great seismic impedance contrast (e.g., sand and gravel over dense till), a significant fraction of the energy was reflected at these interfaces while the transmitted fraction became much weaker to detect a bedrock surface at <10 m depth. Reflections from the bedrock surface deteriorate as depth to bedrock increases. The results indicate that accurate seismic imaging of the bedrock surface is highly dependent on the nature of the sedimentary sequence overlying the bedrock as well as the depth to bedrock. Understanding this relationship gives insights into the appropriate applications for seismic reflection surveys using a low-energy source.