Abstract: 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.