New Development Enhances Geophysical Imaging Quality and Helps in Seismic Hazards Assessment

The Illinois State Geological Survey Geophysics Section continues to broaden the spectrum of near-surface geophysical applications, including those utilizing seismic reflection, ground-penetrating radar, and electrical, magnetic, and other geophysical methods for stratigraphic imaging. New developments in data acquisition, processing, interpretation, and modeling have been made. Improved data acquisition systems and methods have led to larger surveys with higher temporal and spatial resolution.

Geophysics section members prepare to gather data.New techniques for processing and visualization of large amounts of data have enabled more sophisticated stratigraphic interpretation. For example, the seismic group, which is a part of the Geophysics Section, acquires an average of 35 miles of seismic data along secondary roads in Illinois every year. Data are acquired using a multi-channel compressional wave, shear wave, and surface wave land streamers built by the seismic group. The acquired data are processed, and the results are complemented by shallow drill holes, sample collection, and an understanding of actual materials to provide the best possible image of the upper 150m of the shallow subsurface.

A new development to the downhole seismic program was recently made to enhance the quality and efficiency of the acquired seismic data. An advanced self-oriented downhole geophone and new downhole seismic analysis software were adopted. These new developments have made both the acquisition and processing of downhole seismic data almost three times faster than before. With this advance, the seismic group is acquiring data from more boreholes in Illinois.

The measured downhole seismic data (seismic P-wave and S-wave velocity vs. depth) will be used initially to calibrate the seismic reflection data. As a further step in the data analysis, the group intends to integrate the downhole seismic data acquired from each surveyed site and use them to describe the velocity-structure of the major glacial units at the site. This process will result in a new characterization of the glacial sediments based on their seismic velocity, which is a key parameter for other studies such as seismic hazards assessments.