SHALLOW HIGH RESOLUTION SEISMIC REFLECTION ACQUISITION OVER UNLITHIFIED SEDIMENTS: AN GEOPHYSICAL TOOL FOR ACCURATE 2-D AND 3-D SEDIMENT MAPPING

Several shallow seismic reflection surveys using P- and S-wave methods were conducted over unlithified Quaternary sediments in Illinois to investigate the potential of this geophysical method for mapping such materials. The methods we describe successfully imaged (1) older Pleistocene sediments near the bottom of the fill in the Illinois portion of the buried Teays-Mahomet bedrock valley; (2) younger near surface Pleistocene sediments from the Peoria, Illinois region; and (3) Holocene sediments of the Mississippi River flood plain near East St Louis, Illinois. Based on in-line borehole data for geologic control, the seismic profiles show distinctive seismic units and their geometries that correspond with identifiable lithologies. Till units are characterized by complex reflection patterns or very chaotic to blind seismic facies. Channels or buried valleys on meter to kilometer scales were observed in Mid-Pleistocene sand and gravel. Since erosion surfaces within the Quaternary sediments are generally boundaries with high density and velocity contrasts, they are clearly seen with these acoustic methods. These surfaces can be imaged even when they bound otherwise similar lithologies. In each of the sections studied, the Paleozoic bedrock surface forms a sharp, strong, continuous reflection. Successful environmental and hydrogeological models require detailed high quality three-dimensional observations. In the past, major progress has been achieved in this effort using borehole data integrated with geophysical logs. The number of boreholes required to map the channel-like structures observed in these seismic sections is prohibitive except for detailed site investigations. To merely detect the presence of a 500-m wide buried valley, boreholes would need to be spaced 350 m apart and a much closer spacing would be necessary to obtain a good image of the valley shape. By contrast, the shallow high resolution seismic reflection method with a 1.5-m horizontal trace spacing provides a powerful tool for mapping individual structures within Quaternary sediments.