The length and width of fissures in a bore hole is determined by alternately flowing liquid into and out of the formation from an isolated zone of a bore hole, at different frequencies, and measuring the dynamic pressure of the flowing fluid. The length of fissures in the isolated zone is determined from frequency peaks at high pressure to flow ratios, and the thickness of the fissures is determined from variations of pressure-flow ratios with frequency by comparison with known data. Sonde apparatus for practicing the method is also disclosed.
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