Abstract

Reliable evaluation of shear (S) wave characteristics in boreholes may be facilitated by the system proposed here, in a wide variety of geologic conditions and depths. The measurements can be done with a sonde, suspended freely in water contained in a borehole. The main part of the sonde consists of a source, a filter tube, and receivers.In this system, the wave field is treated approximately as that in an infinite homogeneous solid medium, because the wavelength is sufficiently longer than the borehole diameter. The source behaves as a single point force. The direct S-wave is detected on a line (borehole axis) perpendicular to the force axis, in which predominant radiation of shear wave is expected. This fact is completely different from some modified systems of sonic log, in which 'shear wave' is the converted-refracted wave propagating as the shear wave along a borehole wall.The proposed source system is the 'indirect-excitation type,' wherein the force is applied to a borehole wall indirectly through a pressure distribution of doublet-type excited in the water. Based on its principle, this source system eliminates generation of dilatational noise waves and also assures operation at greater depths because no work is done against the external pressure, as a whole, at the source.The proposed receiver system is the suspension type, wherein horizontal motion of the borehole wall (ground motion of S-wave) is detected through corresponding water motion by a detector of neutral buoyancy.The fundamental applicability of this logging system was confirmed by experiments at shallow depths.

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