Shear wave observation using a down-hole hydrophone array

Abstract

We observed Shear waves using a down-hole hydrophone array to a depth of 453 m. The Shear(S-) waves were clear and a polarity reversal was observed when the source was struck from the opposite direction, in both the transverse case and radial case. Even though the tube wave was strong while the sensors were in the cased section, it was very weak in the non-cased part in contrast to the clear S-wave. We generated the S-wave by striking horizontally on one side of a wooden plank with a 4 kg wooden hammer. The PVDF (Poly Vinylidene Fluoride) hydrophone array was used for recording. The hydrophone has not its own directionality. In this observation, the phase of the waves are consistent and they are easily defined. This fact raises a question why hydrophone can detect polarity reversal. One possibility is the presence of anisotropy. The hodograms created from two directional source for the same depth of the hydrophone suggest the presence of the anisotropy. S-waves change the polarization direction due to anisotropy and split. Radial component of the split wave is converted to P-wave at the borehole and detected by hydrophones. A hydrophone array is easy to handle compared to the geophones which needs to be fixed to a borehole wall, and its use speeds up the S-wave measurement in a borehole. Further research is required to clarify these observations.