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Determination of Vp, Vs, glacial drift thickness and Poisson's ratio at a site in Jay County, Indiana, using seismic refraction and multichannel analysis of surface wave (MASW) analysis on a common data set

Abstract

In July 2013 an industry-scale seismic reflection survey was conducted at a site in northern Jay County, Indiana, by geophysics students and faculty of Wright State University. In addition, a separate near-surface seismic dataset was collected locally to examine the Vp, Vs, and Poisson's Ratio of the glacial drift and upper bedrock. This near-surface study successfully used a common dataset that was separately analyzed for both Vp (seismic refraction) and Vs (MASW) to calculate the Poisson's Ratio of the glacial drift and underlying bedrock.

The driller's log for a water well near the east end of this near-surface survey indicates glacial drift (unconsolidated clay and sand) over limestone bedrock at a depth of 110 feet. Water wells in the broader area indicate bedrock depth varying from 110 to 122 feet, but locally as much as 140 feet.

The near-surface seismic data were acquired using a Bison EWG (Elastic Wave Generator) assisted weight drop source that shot every station through a stationary spread of 48 channels using a pair of 24-channel Geode seismographs. Each channel recorded a single vertical 4.5 Hz geophone at a station spacing of 10 feet. Four weight drop records at each source point were summed to enhance the S/N ratio.

The same data volume was processed both for Vs using SurfSeis3 MASW (Multichannel Analysis of Surface Wave) software and for Vp using IXRefrax3 refraction software. The MASW results suggest that the depth to bedrock at the survey location ranges from 115–120 feet (~35 m) with Vs of 1200–2000 feet/sec (366–610 m/s) for glacial drift and 2400–2700 feet/sec (730–823 m/s) for bedrock. The P-wave refraction results suggest the depth to bedrock ranges from 118–122 feet (36–37 m) with average Vp of ~5000 feet/sec (1524 m/s) for a glacial drift and ~17,000 feet/sec (5180 m/s) for limestone bedrock. The Poisson's Ratio for the glacial drift calculated using the Vp and Vs at common location in this study is 0.470–0.473, which is consistent with published results elsewhere.

This study suggests that Poisson's Ratio can be determined using velocities from different analysis methods on the same data set with good results.