GB1193381A - Acoustic Borehole Logging Technique - Google Patents

Abstract

1,193,381. Well-logging. MARATHON OIL CO. 23 Jan., 1968, No. 3568/68. Heading H4D. An acoustic velocity log of a borehole is obtained by applying a plurality of directional acoustic signals simultaneously to the borehole wall at points spaced round the wall and lying in a first plane perpendicular to the borehole axis, and sensing the propagated signals at spaced points lying in a second plane perpendicular to the borehole axis and spaced a known distance from the first plane. The Fig: 1 logging tool 12 carries a ring of equally-spaced transmitter transducer units 20 below two rings of equallyspaced receiver transducer units 20. The transducer units are sprung against the borehole wall or casing. Each transducer unit comprises (Fig. 2) three cylindrical barium titanate cylinders 26, 28, 30 (or magnetic coil and core assemblies) on a tetrahedral mounting block 32 having a contact button 24 through which acoustic energy is coupled to the borehole. Inertial masses 34 are fixed to the transducers. For transmission of acoustic energy in a selected direction the three transducers of a unit 20 are energized simultaneously by signals of appropriate amplitude and phase; for reception from a selected direction the outputs of the three transducers in a receiver transducer unit are combined and weighted appropriately. Pulsed or continuous wave energy may be used in determining propagation velocity, through formations traversed by the borehole, by comparing arrival times or phases of signals received at one receiver ring with those of signals received at the second receiver ring or with the transmitted signals. In one propagation mode the energization of the transmitter units 20 is such that all transmitted beams are coplanar and each beam is tangential to the borehole wall in the same rotary direction, thereby tending to twist the borehole in a shear mode. In a second mode the beams are coplanar and directed radially outward relative to the wall, tending to move the wall radially in both shear and compression modes. In a third mode the beams are all in the same direction, parallel to the borehole axis, tending to vibrate the wall axially in a compressional mode. In a fourth mode the beams are coplanar and all in the same direction, tending to flex the wall in a shear mode. The direction characteristics of the receiver units 20 are matched to those of the particular propagation mode, whereby reception of extraneous signals is largely eliminated. By switching rapidly between the various propagation modes, an effectively continuous velocity-log for all modes may be obtained.