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Beyond Rippability – A Case History Integrating Seismic Refraction, Electrical Resistivity Imaging and Geotechnical Boring Logs
Missouri University of Science and Technology, Proceedings: 2006 Conference on Applied Geophysics
Jonathan B. Shawver*, William Dalrymple, GEOVision Geophysical Services, Inc., Corona, California;
Robert M. Orr, Wilbur Smith Associates, Kansas City, Missouri
Paper – [pdf] INFR_2006 Shawver Orr Dalrymple Beyond Rippability
Geophysical investigations are best utilized to plan borehole locations and/or correlate geology between borehole locations. A geophysical survey was conducted along a portion of the southbound retaining wall of Missouri Highway 152 in Kansas City, Missouri. The purpose of the geophysical survey was to map bedrock geology, structure, and engineering properties of the shallow (upper 50 ft) subsurface. Geophysical methods used during this investigation included the seismic refraction and 2-D resistivity imaging techniques. Borehole data alone is not an adequate measure of subsurface condition in karstic terrain. Geophysical techniques are not without fault. Seismic data at this site was affected by high ambient noise levels and steep topographic terrain prohibited the placement of far-offend shots, limiting depth of penetration. Interpretation of resistivity data is much more difficult than seismic interpretation independent of any correlating data. Resistivity imaging data are smoothed more than seismic refraction data. Sharp, easily definable geologic contacts are rarely observed in electrical resistivity data.
However, an integrated approach combining all three methods was a successful (and cost-effective) means of characterizing the subsurface geology as opposed to any singular technique.
Results from the geophysical investigation were used to better locate geotechnical borings. Preliminary boring information was used to refine both seismic and resistivity models. Geophysical data indicate that bedrock units strike in a southeasterly direction and dip to the southwest. Alternating shale and limestone sequences were observed in the refined resistivity models. Based on resistivity data models we were able to identify highly fractured zones within the bedrock and other potential areas of poor rock quality. The seismic refraction survey provided different and complimentary data. While not capable of differentiating between the different rock units, the seismic refraction survey provided was able to more accurately map sediment thickness, bedrock velocity, and degree of weathering.