GB1399735A - Acoustic well logging sondes - Google Patents

Abstract

1399735 Acoustic well-logging SCHLUMBER INLAND SERVICES Inc 24 May 1972 [24 May 1971 (3)] 24370/72 Heading H4D An acoustic well-logging sonde having a body of generally cylindrical form is characterized in that at least one transmitting and/or receiving transducer comprises one or more transducing elements the active face or faces of which lies or lie on a convex surface so that the overall sensitivity of the transducer is substantially uniform over a range of angles in a plane containing the sonde longitudinal axis. In the Fig. 2 embodiment the sonde includes two transmitting (TX) transducers 36, 37 in the sonde body 40 and two receiving (RX) transducers 66, 68 on a pivoted, wheeled measuring pad 42 which is spring-urged towards the borehole wall. The sonde body is urged towards the wall by sprung arms 78, 80 but is held slightly away from the wall by sprung wheels 94, 96. The pad, arms and wheels can be retracted by hydraulic actuators when the sonde is being lowered. The transducers may be piezoelectric or magnetostrictive cylinders with their axes horizontal, spherical, of disc or torpid sector form, or comprise a group of strip-form elements. Their directivity patterns are such that their sensitivity, over a range of incidence angle α (Fig. 3) for which refracted waves will be detected, is substantially constant and such that the waves are received at an RX transducer substantially in phase irrespective of the exact point of emission on the TX transducer surface or exact point of incidence on the RX transducer surface. To prevent acoustic vibrations other than desired refractions from reaching the RX transducers, or at least to attenuate or retard undesired vibrations, the sonde body 40 and measuring pad 42 are each constructed as a mechanical low-pass filter comprising flexibly linked masses (details given, Figs. 4, 6, 7, 8, none shown) and include apertures which lengthen the vibration travel path. In addition the wheels 94, 96 and pad wheels 62, 64 are provided with shock-absorbing tyres, and the transducers are arranged in recesses and spaced from the recess walls by greater than a half wavelength of desired acoustic waves in fluid contained by the borehole. The RX transducer outputs are electrically filtered to suppress undesired signals. The sonde may be used to obtain a differential measurement of the wave propagation times from one TX to two RX transducers. All the TX and RX transducers may be mounted on a common pad instead of as shown.