35 Near Surface Geophysics

Thursday, 4 April 2002
1:00 PM - 5:00 PM

SHEAR WAVE PROFILES OF SURFICIAL DEPOSITS IN OHIO USING MULTICHANNEL SURFACE WAVE METHODS

Paper number: 35-9
Program: THEME SESSIONS
Time: 3:40 PM – 4:00 PM

Authors

Abstract

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 30m of the subsurface. A hammer source was used with a 7.5-cm thick by 22-cm diameter nylon strike plate in conjunction with a 24-channel seismograph. Optimal results were obtained using a 10m offset, 1m 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 (>.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 surface- wave inversion and require more iterations for the inversion to converge.