SHEAR WAVE LOGGING USING ACOUSTIC MULTIPOLE DEVICES

摘要

The multipole shear wave logging device of this invention includes a logging sonde, means for generating a 2n-pole shear wave in an earth formation surrounding a borehole containing liquid where n is an integer greater than 2, and means for detecting in the liquid the refraction of the 2n-pole shear wave. In the preferred embodiment the generating means comprises six similar sectors of a hollow piezoelectric cylinder. The six sectors are polarized radially and are so connected to the sonde that they are in the form of a split cylinder coaxial with the sonde axis. Electrical pulses of similar wave forms are applied across the inner and outer cylindrical surfaces of each sector. The electrical pulses are of such polarities that adjacent sectors vibrate radially in substantially opposite phase. Circumferentially polarized sectors may also be used in the place of radially polarized sectors. The vibrations of the six sectors will generate compressional waves in the liquid which will interfere to produce an octopole shear wave in the formation. The compressional wave in the liquid caused by refraction of such octopole shear wave is detected by the detecting means. The detecting means comprises two detectors in the liquid spaced apart longitudinally from each other and from the generating means. The shear wave velocity of the formation is determined from the time interval between detections of the refraction of the octopole shear wave by the two detectors.