PROFILING QUATERNARY SEDIMENTS USING SUBSURFACE GEOPHYSICAL TECHNIQUES

In northeastern Illinois, borehole logging and seismic surveying are playing a critical role in mapping projects designed to improve the understanding of the regional geology and hydrogeological framework. Geophysics is particularly important because the study area is covered by glacial drift from 150 to 350 feet in thickness, the stratigraphy is complex and there are limited high-quality geologic data or exposures.

Well logging measures the physical properties of the sediments surrounding a borehole. Many types of glacial sediment exhibit a log pattern indicative of the depositional processes or environments of deposition. These log patterns can be used to characterize sediment sequences. Natural gamma-ray logging is routinely used because the method can be applied above or below the water table, in cased or uncased holes, and in boreholes with diameters ranging from 2 to 10 inches.

High-resolution seismic reflection can link borehole geophysical data for analyses in the 2nd and 3rd dimension. The arrival travel times of shear or compressive waves reflected at interfaces between layers of different acoustic impedance can trace in a profile the lateral changes in facies and lithologies and thereby define the internal structures of landforms (e.g., kettle holes) and ice deformation features that are otherwise not evident, except in outcrop.

Borehole and seismic data are combined with other lithologic information to generate cross-sections and 3-D stratigraphic models. Defining the type, thickness and extent of Quaternary sediments is important for characterizing the subsurface geology, locating sand and gravel aquifers, and developing natural resources and land-use strategies.