Patent GB-1519971-A (GB)
Title: Acoustic recieiver for underground strata exploration
IPC: G01V1/16
Filed: 1975-07-10
Abstract: 1519971 Seismic receivers RUHRKOHLE AG 10 July 1975 [13 July 1974] 29135/75 Heading G1G [Also in Division H4] An acoustic receiver, Fig. 1, for investigation of rock strata comprises a plurality of mechanical/ electrical transducers 3 attached to an elongate coupling element 1, arranged one behind the other along the longitudinal axis of the coupling element 1 and connected together both electrically and mechanically. A counter mass 5 is connected to the transducers in as free a manner as possible in relation to the coupling element 1. The transducers 3 may be arranged separately or in groups and be mounted at a central hole or at one side or on opposite sides of the element (1) Fig. 2 (not shown). Received acoustic signals set the element 1 and attached parts of the transducers 3 in vibration but, because of the inertia of the counter mass 5, deformation of the transducers 3 provides an electrical signal. High frequency signals require a relatively low mass 5 directly connected to the transducers 3. For measuring low frequency signals a larger counter mass 5, Fig. 3, is mounted by  elastic strips 7 or by a diaphragm to the coupling element 1. The bridge 6 between the transducers 3 may be a delay line. Element 1 has a length to diameter ratio of at least 15:1. For receiving acoustic signals at an angle to the axis 4 of element 1 the direction of greatest sensitivity of transducers 3 is at a corresponding angle to the axis 4. By design of element 1 and suitable arrangement of transducers 3, reception of shear waves is possible. The transducers 3 may be of Piezo-electric or strain gauge types and are preferably sensitive to a narrow frequency range. The different transducers 3 may be tuned to different frequencies. They may be electrically connected directly to each other or a differential amplifier 8 is associated with each transducer 3, the outputs of the differential amplifiers 8 being combined by a transducer and/or RLC network, a single cable being connected to evaluation apparatus (not shown). The transducers 3, counter mass 5, bridge 6 and amplifier 8 may be symmetrically arranged around the coupling element 1. A fully encapsulated construction Fig. 4 (not shown) gives protection against fire damp and/or is electrically inherently safe and shields against electromagnetic interference. A cylinder (10) acts as the capsule and coupling element. Transducers 3 may be arranged in two groups (15, 16) Fig. 6 (not shown) each with a counter mass (5a, 5b) jointed by a part (17) of a coupling element (1) acting as a mechanical delay line and of length corresponding to #/2 of the frequency used. The transducers (3) are connected so that acoustic signals entering in the axial direction of the coupling element (1) are in phase whilst interference noise from other directions is suppressed. Several receivers may be arranged at distances corresponding to the wavelength of the acoustic signals and the electrical signals delivered by them made to undergo phase comparison, making possible spatial scanning of the strata. Noises emitted by living organisms enclosed in rock may be detected.