GB752296A - Mechanically coupled acoustic well logging system - Google Patents

Abstract

752,296. Supporting transmitters and receivers. SOCONY MOBIL OIL CO., Inc., [formerly SOCONY-VACUUM OIL CO., Inc.]. Aug. 5, 1954, No. 22816/54. Class 40 (4). [Also in Group XXXVIII] A well logging unit for acoustic investigation of bore holes (see Group XXXVIII) comprises a first transducer for the generation of compressional waves, a second transducer for generation of an output signal upon detection of said waves, rigid coupling means extending between and secured at the ends thereof to the transducers to maintain them in spaced relationship, and a plurality of spaced weights secured to said coupling means at intervals which are short compared to the wavelength generated by the first transducer, and of such a mass as to form with said coupling means a structure of low transmissivity to said waves. In the embodiment of Fig. 2, end members 30 and 31 adapted threadedly to engage the transmitting and receiving transducers respectively are coupled by two steel rods 33, 34 carrying uniformly spaced lead weights 36, 37, 38, 39 . . . 40. The structure is encased in rubber which fills the spaces between weights and rods and also provides a relatively thin coating over the periphery of the weights to form a streamlined unit corresponding to the size of the top and bottom transducers such that the whole is unlikely to lodge or be caught on protuberances when lowered or raised in a bore hole (Fig. 1, not shown). A modification (Fig. 3, not shown) utilizes six steel tubes instead of rods 33 and 34, and another modification (Fig. 6) comprises a thin-walled stainless steel cylinder 70 welded to end coupling members as 71 and having weights 72, 73 . . . secured at spaced points internally. In order to withstand high hydrostatic heads encountered at great depths in bore holes the interior of the cylinder 70 is filled with liquid and for this purpose channels 72a, 73a . . . are provided. Electrical conductors may be threaded through the tubes of the former modification or channels 72a, 73a ... in the latter. In a further construction the transducers are not externally attached to the coupling unit but are built-in, effectively replacing the two weights 36 and 40 of Fig. 2. A magneto-strictive transmitter is provided by washer-like laminations 87a (Fig. 8) of magnetic material stacked to form an elongated cylinder and carrying a toroidal winding (not shown) the ends of which are connected to terminals 107 and 108. Weights 90 are attached to the coupling rods, such as 80, by solder poured in molten form into the annular grooves 90a. The rubber casing extends as a sheath 91. A receiver insensitive to the magnetic field of the transmitter comprises piezo-electric crystals 123 arranged one on each face of a hexagonal mass 120 shown in sectional plan and elevation in Figs. 9 and 10 respectively. Rods 80 pass through holes 121 in the mass 120, rubber tubes 121a serving to support the mass whilst insulating it acoustically from the rods. An insulating layer of rubber 125 is formed over the mass 120 and crystals 123. A cylindrical metal shield 126 encircles this and is electrically connected to the mass 120. A second rubber sheath 127 covers the shield 126 and insulates it from an outer conductive shield 128. The moulded rubber sheath 91 then encases the entire structure. An amplifier may be carried in the end housing 88. A formula for the cutoff frequency of the coupling in terms of the physical dimensions is given.