| The research presented here consists of two case studies: the first from a study site in Illinois and the second from a site in Arkansas. In both instances, geophysical investigations were conducted to characterize the subsurface. At the Illinois site, borehole control, downhole seismic (DHS), seismic refraction tomography (SRT) and multichannel analysis of surface waves (MASW) data were acquired for the purpose of seismic site characterization. Shear wave and compressional wave velocities were used to estimate depth to bedrock and to generate 1-D plots depicting variations in Poisson's Ratio, elastic moduli and density. The average shear wave velocity in the upper 100 ft was calculated and the national earthquake hazards reduction program (NEHRP) class D was assigned to the site based on MASW and DHS data results. At the Arkansas site, borehole control, electrical resistivity tomography (ERT), seismic refraction tomography (SRT), and multichannel analysis of surface waves (MASW) data were acquired with the objective of verifying and mapping a postulated fault. A comparative evaluation of the overall usefulness of the ERT, SRT and MASW techniques was also performed. The comparison showed that ERT and SRT tools generated remarkably similar images of the fault. The MASW tool generated a slightly different image of the fault. The research demonstrates that integrated use of seismic (seismic refraction tomography, multichannel analysis of surface waves and downhole seismic) and electrical (electrical resistivity tomography) methods is an effective approach in terms of assessing soil and rock in the New Madrid Seismic Zone. |
