GB1010788A - Improved geophysical prospecting method and apparatus - Google Patents

Abstract

1,010,788. Seismic prospecting. GENERAL DYNAMICS CORPORATION. June 20, 1963 [June 25, 1962], No. 24646/63. Heading H4D. Seismic waves are generated in the earth by applying a time varying force to the interlink between two linked anchors secured in or on the earth. In one embodiment (Figs. 1, 2 and 3) one of the anchors 12 which is positioned in a bore-hole 14, comprises a pair of part cylindrical plates 18, 20 urged apart by a plurality of hydraulic cylinder-piston combinations 24 supplied with fluid under pressure via a supply pipe 36 and inlet valve arrangement 38. The plates 18, 20 which thus grip the bore-hole firmly, are connected via an interlink 10 with a second anchor comprising an hydraulic ram 66 including a cylinder-piston arrangement to which fluid under pressure is supplied via supply pipe 86 from a source including a pump 100, check valve 102, pressure relief valve 104 and accumulator 106 under the control of a solenoid operated four-way two-position valve 88 the unused port of which is blocked by a plug 90. The valve 88 is operated to cause ram 66 to produce, repetitively, a tension in the link 10 followed by a rapid removal of such tension, so that, as the strain in the ground is applied and relieved, a seismic wave is radiated by a plate 44 clamped to the base of the ram and also, although to a lesser extent, by the lower anchor 12. The seismic waves produced by the two anchors are of opposite phase and by making the distance between the anchors equal to one half wavelength the waves may be made to add. The operation of valve 88 may be programmed to generate any desired sequence of waves such as gated continuous wave train having a single frequency or a frequency modulated wave may be produced. To facilitate insertion and removal of the anchor 12 the elements 18, 20 of the latter are coupled by a plurality of light tension springs 42. Alternatively a vacuum may be applied to the hydraulic system to retract the plates or double acting cylinders may be employed and the plates retracted by fluid pressure. Fig. 4 shows a second embodiment in which the upper and lower anchors each comprise an arrangement similar to anchor 12 of Fig. 1, the time-varying force being supplied by a doubleacting hydraulic cylinder 120.