The shear velocity of a region of an earth formation may be determined by a logging tool insertable in a wellbore penetrating the formation which includes plural transducers for generating compressional waves for transmission through the formation as shear wave between the signal generating transducers and an array of signal receiving transducers spaced from the signal generating transducers. The transducers may be plural in number and arranged to effect impingement of the compressional wave on the borehole wall at an angle of incidence preferably in the range of 35.degree. to 75.degree. to a normal to the wall surface. The transducer array is controlled so that a significant portion of the compressional wave energy transmitted through liquid in the wellbore is converted to shear waves at the wellbore wall, which shear waves constructively interfere to develop a torsional wave propagating along the wellbore between the transducers and the receiver elements. Strong shear waves are thus generated in formation materials where the shear velocity may not be otherwise measurable.
A low frequency torsional transducer for the generation and/or reception of shear waves in a substantially solid medium. The transducer includes a spool like radiating structure to which are fixedly attached spaced piezoelectric elements. The elements undergo shear movement upon being electrically excited or upon receiving shear waves. This shear movement causes the opposite ends of the radiating structure to move torsionally. The torsional movement causes radiation of shear waves which can be detected by a sensor and used to determine the degree of support for shear wave propagation in the media.
