Orientation of a downhole triaxial geophone

Abstract

The bearing and elevation of a geophone’s axes are unknown when a geophone package is placed in a deep exploration borehole. Auxiliary instruments, such as inclinometers, compasses or gyros, may be incorporated into the downhole package to give some indication of local attitude relative to some external reference field, but these solutions may encounter difficulties because small gyros drift, the geomagnetic field may be distorted locally or screened by casing, and inclinometers add to the cost and weight of the downhole receiver. An alternate approach to determining the orientation of a remote tool is to compare its triaxial response to that expected for a known elastic wavefield. The accuracy of the measurement depends on the quality of the geophone‐to‐formation coupling and on the extent of one’s knowledge of the externally impressed wavefield.