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An improved air-powered impulsive shear-wave source

U.S. Geological Survey, Menlo Park, CA 94025

Abstract

We have previously constructed an air-powered, impulsive, SH-wave source for shear-wave velocity measurements using a downhole configuration. One shortcoming of that source is the existence of a forerunning signal generated by the reaction to the rapid acceleration of an inertial mass when a poppet value is opened. By changing the poppet valve to a ball valve, a cleaner signal is generated. The improvement comes about because when a ball valve is opened, its flow capacity increases gradually, resulting in a smooth and gradual acceleration of the inertial mass. Tests involving ground coupling and driving air pressure were conducted to investigate physical parameters of shear-wave generation. We illustrate as an application the measurement of shear-wave velocity in a loess layer.

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