AN IMPROVED AIR-POWERED IMPULSIVE SHEAR-WAVE SOURCE

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