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.