Abstract

A subsurface point body force can be realized using a body force on the axis of a small-radius borehole drilled perpendicular to the force. This observation, originally made by Kitsunezaki (1980), is having a significant impact in shear-wave logging, and may potentially affect downhole seismic sources. Applying the principle of reciprocity to this finding, a horizontal vibration sensor on the axis of a vertical hole will be unaffected by the existence of the hole at low frequencies. Numerical methods determine both the frequency below which, and the offset beyond which, borehole-related effects are negligible. If the shear wavelength is greater than ten times the diameter of the hole and if the measurement is made at least one shear wavelength from the point force, then the borehole effects will be minimal.

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