Three-Dimensional Imaging of Glacial Sediments Using Shallow High-Resolution Seismic Reflection Techniques in Support of Geologic Mapping in Illinois

Project funded by the Illinois Board of Higher Education

Andre Pugin, Timothy Larson & Steven Sargent
Illinois State Geological Survey

Introduction
During the last 15 years, shallow seismic reflection using P (compressive) wave properties has been developed as a geophysical tool to provide exceptional views of the subsurface geology in two or three dimensions on land as well as beneath water. Usually, this method has been applied in projects covering fairly small areas of investigation. However, over the last decade, Canadian researchers have shown that this technique can be applied on a regional scale and that it is an appropriate tool for mapping glacial sediments in two dimensions. A three-dimensional volume can be investigated by combining intersecting or closely spaced 2-dimensional profiles. Because high-resolution reflection data can be acquired relatively quickly and inexpensively, the method can be used to provide detailed high quality three-dimensional observations for environmental, engineering and hydrogeological models. More recently, the use of S (shear) wave reflection has regained interest in the scientific community. Under development at the Illinois State Geological Survey (ISGS), this technique can be used to improve images of the shallower subsurface. As applied in Illinois, S wave reflection focuses on the first 30 m (100 ft) of the subsurface, while P wave reflection is optimum at depths below 25 m (83 feet). This project has both educational and research components.

Educational Component
The need for trained scientists to conduct reflection surveys will drastically increase in the near future as the benefits of land- and water-based seismic reflection surveys are recognized by the geologic and engineering communities. Teaching programs with access to state-of-the-art equipment and real-world research projects are needed to equip students to fill that future demand.

Initial Results
Figure 1 show results obtained in year 2000 in the Fisher area. This initial experiment demonstrates the high resolution power of seismic reflection in observing a major erosion surface within the Mahomet aquifer. Initial studies have shown that there is a direct connection from upper to deep aquifers. From previous work, the existence of hydraulic windows have been suspected in the Allerton Park (west of Monticello, see figure 2). Figure 3 displays a seismic section that runs through the modern Sangamon valley. The section clearly shows an erosional surface that characterizes the presence of a buried valley playing the role of an hydraulic window connecting upper and lower aquifers. New borehole data, which will be drilled this summer, will help to calibrate this new geophysical observation.

Figure 1. Seismic cross-section showing Mahomet aquifer (sand & gravel above the bedrock) near Fisher IL (Champaign Co.)

Figure 2. Site map showing work being conducted near Allerton Park (Piatt Co.)

Figure 3. Seismic cross-section showing the Mahomet aquifer west of Monticello (Piatt Co.). The tills that confine the aquifer appear to have been eroded and filled with sand & gravel.

Last Updated: May 7, 2002
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