This method is commonly used to determine compressional and shear wave velocity versus depth. In a downhole seismic survey, a seismic source is placed on the surface near a borehole, and two geophones are placed at selected depths in the borehole. The raw data obtained from a downhole survey are the travel times for compressional and shear waves from the source to the geophones and the distance between the source and geophones. Compressional waves are generated by striking a steel plate with a sledge hammer. The steel plate is located two to five feet from the boring.
Shear waves travel slower than compressional waves.
Therefore, compressional waves often interfere with shear waves. This interference sometimes makes identification of the first shear wave arrival difficult. To improve the resolution of the shear wave arrival, the seismic source is designed to produce a signal which contains a large shear wave component and a signal enhancement seismograph is used to process the received signals from the geophone. The beam is coupled to the ground by weighing it down with the front tires of the field recording truck. The beam is offset a distance of five to ten feet from the borehole to minimize direct coupling of the seismic energy to the casing.