METHOD AND APPARATUS FOR LWD SHEAR VELOCITY MEASUREMENT

摘要

The logging tool 55 of this invention includes a transmitter conveyed on a drilling collar 90 for exciting a quadruple signal in a borehole 26 being drilled by a drill bit and a receiver for receiving the signal. The transmitter 105 is operated at a frequency below the cut-off frequency of the quadruple collar mode. The received signal consists primarily of the formation quadruple mode which, at low frequencies, has a velocity that approaches the formation shear velocity. The transmitter, in one embodiment, consists of eight equal sectors 101 et al of a piezoelectric cylinder mounted on the rim of the drilling collar. The value of the cut-off frequency is primarily dependent on the thickness of the drilling collar. Alternatively, the transmitter may be operated to produce both the collar mode and the formation mode and a processor may be used to filter out the collar mode.