Abstract

This experimental study of the generation of shear waves by explosive sources stemmed from Heelan's theoretical result that pressure acting on the wall of a cylindrical hole in a solid should radiate shear waves quite as effectively as compressional waves. The measurements confirm this expectation, but good overall agreement was not achieved until expressions were derived which take into account radiation from strong water-pulse waves in the shothole.Our results show that the ratio of shear-to-compressional amplitudes generated by an explosive source increases as the charge size decreases. At an angle of 45 degrees, the ratio is approximately unity for a charge consisting of 10 ft of Primacord. We found that the shot-generated water pulse (tube wave) is a strong shear source, continuously generating shear energy in the formation as it travels in the borehole. This drastically affects the directivity of SV waves and in Pierre shale gives a pattern whose maximum is near-vertical. This suggests the possibility of prospecting with shear waves, using a distributed charge detonated at shear velocity to generate substantial downward-direction shear energy in the earth. However, the substantially larger attenuation of shear waves compared to compressional waves has discouraged us from pursuing this further.

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