Rod-type multipole source for acoustic well logging

Abstract

Method and apparatus for acoustic wave generation and transmission into a subsurface earth formation, particularly adapted for use in borehole shear wave logging. A logging sonde adapted to be suspended in a borehole traversing a formation houses a source of acoustic waves. The source simultaneously generates a plurality of acoustic waves propagating generally radially outward from locations within the sonde housing adjacent the vertical central axis thereof and will interfere to produce multipole acoustic waves in the formation. Detector means within the housing spaced longitudinally from the source detect acoustic energy in the formation resulting from the generated waves. In a preferred embodiment, the source includes a cross-shaped rod assembly comprised of two rods perpendicular to each other and joined at their mid-points through which the central axis passes. The rods lie in a plane perpendicular to the central axis and are preferably each comprised of a different magnetostrictive material such that the strain constant of the rods differ in sign. Ends of each rod have a respective acoustic radiator attached thereto facing radially outward. Upon electrical excitation of the rods, they may be caused to vibrate along their respective longitudinal axes substantially 180.degree. out of phase with respect to each other. Vibration of the reflectors of the first and second rods, in response thereto, set up respective first and second pairs of acoustic waves also out of phase with respect to each other which propagate from the radiators into the formation, where they interfere to produce the desired quadrupole acoustic wave.