SHALLOW HIGH RESOLUTION SEISMIC REFLECTION ACQUISITION OVER THE BURIED MAHOMET BEDROCK VALLEY: AN ACCURATE GEOPHYSICAL TOOL FOR GLACIAL SEDIMENT MAPPING

A recent shallow seismic reflection survey was conducted over the buried Mahomet bedrock valley in central Illinois to investigate the potential of this geophysical method for mapping glacial sediments. Two perpendicular P wave seismic lines totaling 1.5 km in length and a shorter S wave seismic line were acquired using a sledgehammer as an acoustic source.

Using an in-line borehole for geologic control, the section shows three distinct seismic units and their geometries. From the top to the bottom: (1) A till unit, characterized by complex reflection patterns or very chaotic to blind seismic facies, includes reflections as shallow as 10 meters. A small, very shallow gas zone or pocket, probably drift gas, has been located at the base of the till unit. Shear wave experiments over this site provide strong evidence for the presence of this gas zone. (2) Mid-Pleistocene sand and gravel (Mahomet Sand) is characterized by channels that are 30 to 200 m wide and 5 to 10 m deep. Within the sand and gravel sequence, a strong reflection interpreted as an erosion surface indicates the presence of a 50-m thick buried valley. (3) A sharp, strong, and continuous reflection delineates the top of the limestone bedrock at a depth of about 110 m.

Environmental and hydrogeological models are dependant on the quality of the three-dimensional observations. In the past, major progress has been achieved in this effort using borehole data integrated with geophysical logs. However, 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 glacial sediments.