ES339635A1 - Apparatus for seismic prospecting - Google Patents

Abstract

Apparatus for generating seismic energy comprises rigid top and bottom members resiliently secured together in telescopic fashion to define a closed chamber, inlet means for introducing a combustible gas mixture into the chamber, ignition means for the gas mixture and exhaust means including normally closed valve means operable upon a predetermined increase in pressure in the chamber to open and allow the chamber to communicate with the exterior of the apparatus. Figs. 1 and 2 show a first embodiment in which the closed chamber A (Fig. 2), comprises the space between a relatively massive top member 11 and a relatively light bottom member 12 resiliently secured to member 11 by means including springs 32. The combustible gas mixture, comprising a stoichiometric mixture of oxygen and propylene is supplied via valved conduit means 83, 84 (Fig. 1) and fed into chamber A through four conduits 85 threaded into tapped apertures 20 in the top member 11 each conduit 85 including a section carrying two spark plugs 87 electrically connected in series to a supply 89 providing about 70,000 volts. The exhaust means comprises a double walled cylinder 41, 42 (Fig. 2) inserted in a central aperture in top member 11 and provided with a piston member 44 urged by spring controlled knuckles (detailed in Fig. 5, not shown) so as to seal the chamber from apertures 49, which communicate with the exterior of the apparatus via a muffler 15 and pipes 77, in the absence of high pressure (e.g. the pressure produced by ignition of the gas mixture) in chamber A. When the gas mixture is ignited the sudden increase in pressure produces the required seismic impulse by causing lower member 12 to move away from the more massive top member 11 whilst, almost instantaneously, forcing piston 44 to move upwardly to release the products of combustion to the atmosphere. In a second embodiment (Fig. 6) the resilient means coupling the top and bottom members comprises an air cushion formed in space II by means of pressure air supplied via hose 230 the contained air being vented via 254 into space I when the bottom member 120, which is bolted at 224 to the member 210, is forced downwardly by the ignition of the explosive gas mixture in chamber A1. The oxygen and propylene are supplied via conduits 202, 204 in a valve 196 communicating with an annular groove 130 which is connected to chamber A1 by two diametrically opposed bores 136. Two spark plugs (not shown) are arranged in apertures in ring 138 (which seals groove 130) on either side of the valve 196. When the mixture is ignited the increase in pressure lifts piston 152 (which in this embodiment is retained in the normally closed position by helical spring 154) and allows the combustion products to be vented via the valve 114, muffler 115 and pipe 118 in the same manner as described with reference to Figs. 1 and 2. A skirt 250 is attached to bottom member 120 to prevent, when the gas mixture is ignited, member 120 from being bowed upwardly in the centre due to the fact that the water escapes faster at the edges of 120 than at the centre. The bottom edge of the skirt has a wavy configuration to prevent cavitation and holes 252 are provided to allow air to escape when the apparatus is submerged. In a modification an arrangement, termed a bubble cap, comprising an annular member secured to flange 216 and including a member extending outwardy of member 210 and a skirt member extending downwardly forms an air space above member 210 in which the latter can move freely so that cavitation and dissipation of some of the force creating the seismic impulse which would occur if member 210 moves against the water is prevented (Fig. 7, not shown). The apparatus may also be employed for producing seismic impulses on land.