Patent GB-1505664-A (GB)
Title: Seismic wave detectors
IPC: G01V1/16
Filed: 1975-05-19
Abstract: 1505664 Force transducer INSTITUT FRANCAIS DU PETROLE 19 May 1975 [17 May 1974] 21256/75 Heading G1N [Also in Division H4] A pressure detector, Fig. 2, comprises two electrodes 2, 6 located one on each side of a deformable electret 4 and a compressible and electrically insulating element 3a, 3b placed between the electret 4 and at least one of the electrodes 2, 6. Fig. 1 (not shown) shows the electret 4 in contact with the electrode 5, there being no electrically insulating element 3b interposed. The detector is in strip form and is preferably sheathed in insulating material (17), Fig. 3 (not shown). The electret is made by polarizing a strip of polypropylene, polytetrafluorethylene or polyethylene terephthalate of thickness 8 to 28 microns by electronic bombbardment, by the application of an electric field while heating the material to near its softening point by corona discharge or by U.V. radiation. The electrodes 2 and 5 are connected to apparatus for recording variations in voltage via a preamplifier or transformer (not shown). A plurality, for example three, insulated detectors of Fig. 2 may be stacked one upon the other Fig. 4 (not shown), electrodes of adjacent detectors in the stack being of opposite polarity and the detectors being connected in series so that, as compared with a single detector, the sensitivity is increased whilst the capacity is reduced. To overcome disadvantages arising out of bending, two detectors 1 of the type shown in Figs. 2 and 3 may be arranged on each side of a flexible insulating strip (18), Fig. 5 (not shown). The detectors 1 are not fixed to the strip (18). The polarity of the detectors (1) on one side of the strip (18) is opposite to the polarity of the detectors 1 on the other side. In other arrangements where two detectors 1 fixed to each other through a flexible strip (18) are subjected to bending they deliver parasitic charges and their polarities must be arranged so that the parasitic charges nullify each other. In one arrangement Fig. 6 (not shown) the electrodes are in series and in an alternative arrangement Fig. 7 (not shown) the electrodes are in parallel. A cylindrical pressure detector, Fig. 8, has a flexible insulating, plastics core covered by a metal or metal plated electrode 7, a first insulating grid or lattice 8, an electret 9, a second insulating grid or lattice 10a second electrode 11 and an insulating flexible sheath 12. A simpler cylindrical device has the detector 1 of Fig. 1 (not shown) wound in a spiral configuration at a constant pitch (Fig. 9) around a cylindrical conductive core (13) Fig. 9 (not shown). Alternatively Fig. 13 (not shown) the winding pitch is greater at the ends of the detector than at the centre, where the turns are thus more highly concentrated. Another arrangement Fig. 10 (not shown) has two detectors of Figs. 1 or 2 wound in opposite directions over an insulating cylindrical core (14), a strip (16) of insulating material being interposed between the detectors. The electrodes of the two detectors are connected in parallel to a monitoring circuit (not shown). More than two detectors may be wound around the core (15) and connected as in Fig. 4 or Fig. 5 (not shown). The avove embodiments are particularly suited for use under water. For land use, a detector, Fig. 11, of the type shown in Figs. 1, 2, 4 or 5 has lead weights 21 on one face of the detector 1 and disposed within a flexible sheath 12. The weights 12 assist in transmitting pressure variations and facilitate contact with the ground. Another arrangement of a strip detector Fig. 12 (not shown) uses a strip of greater width towards the centre of the detector than at its ends, thereby providing different sensitivities in different regions.