Shear wave profiles of surficial deposits in Ohio using multichannel surface wave methods
Sheehan, Jacob, R., Green, Douglas, H.1, Hansen. Michael, C.2
(1) Ohio University, (2) Ohio Geological Survey
Presented April 4, 2002, Geological Society of America
Sectional Meeting, Lexington Kentucky
The Multi-channel Analysis of Surface Waves (MASW) method is used to generate two-dimensional shear-wave profiles in three areas of Ohio. The areas of study are located near Cleveland, Dayton, and Athens. The Athens sites were used for developing data-collection methods and the sites near Dayton were used to compare MASW to other shear-wave methods. The primary study sites are in Cleveland and have been selected in cooperation with the Ohio Geological Survey in support of a seismic-hazard mapping project. An important factor in seismic-hazard studies is the average shear-wave velocity of the uppermost 30 meters of the subsurface. A hammer source was used with a 7.5-centimeter thick by 22-centimeter diameter nylon strike plate in conjunction with a 24-channel seismograph. Optimal results were obtained using a 10 meter offset, 1 meter station spacing and 12 active channels in a roll-along mode. Using 4.5 Hz geophones, Rayleigh waves were recorded with frequencies ranging from 2-80 Hz.
At most study sites this methodology gave results with high signal-to-noise ratio (>0.8) to a depth significantly below the 30 meters required for seismic-hazard characterization in a fraction of the time and cost required for conventional shear-wave studies. For example, a two-hour survey was conducted in the Cuyahoga River valley, where surficial geology consists of 3 meters of sand and gravel over silt and clay. The inverted shear-wave velocity ranges from 200 m/s at the surface to just over 600 m/s at a depth of 30 meters, with a harmonic mean of 360 m/s. Low-velocity layers complicate the surfave-wave inversion and require more iterations for the inversion to converge.
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