Acoustic wave transmission system and method for transmitting an acoustic wave to a drilling metal tubular member

Abstract

An acoustic wave transmission system comprises an acoustic wave generating metal tubular member for converting information about the bottom of a borehole, which is obtained by a bottom hole sensor, into an acoustic wave. The acoustic wave generating metal tubular member includes a acoustic wave generating mechanism having at least a magnetostrictive oscillator which is mounted in a recess formed in an outer wall of the acoustic wave generating metal tubular member, and on which a compressive load is imposed by means of a pre-load mechanism using a vise. The magnetostrictive oscillator is constructed of a stack of thin plates each made of a metal magnetostrictive material having a property of increasing its dimensions when magnetized, the thin plates being bonded together by a heat-resistant adhesive. The magnetostrictive oscillator can thus have a buckling strength large enough to resist the compressive load imposed thereon by the pre-load mechanism and a stress due to a strain caused in itself. The acoustic wave generating metal tubular member further includes an excitation current supplier for supplying either a rectangular, sinusoidal, or triangular alternating excitation current modulated with the information about the bottom of the borehole and having a frequency that is half the carrier frequency of the acoustic wave, or a series of excitation pulses modulated with the information about the bottom of the borehole and having a pulse repetition rate that is equal to the carrier frequency of the acoustic wave, to an excitation winding wound around the magnetostrictive oscillator.