Patent GB-1248431-A (GB)
Title: Improvements in or relating to methods of marine seismic exploration and to systems for use therein
IPC: F42B3/00
Filed: 1968-10-08
Abstract: 1,248,431. Seismic cartridges. HERCULES Inc. 8 Oct., 1968 [9 Oct., 1967 ; 29 April 1968], No. 47697/68. Heading F3A. [Also in Division H4] Seismic energy is generated underwater by feeding a percussion-initiated explosive charge to a submerged firing device, and initiating the charge in the device. The towed firing device of Fig. 1 comprises housing 10 having a muzzle 18 to which explosive cartridges (shown dotted) are fed one at a time from a ship-borne water-pressure-actuated loader via a conduit and inlet 99. The cartridges are detained in muzzle 18 by a ridge or groove on the cartridge abutting a corresponding shoulder 37 or sprung member on the muzzle. An assembly 19 comprising three coaxial pistons 21, 22, 23 is actuated by a ship-borne hydraulic pump and air compressor system operated in synchronism with the cartridge loader such that (a) when a cartridge has been loaded into muzzle 18, piston 22 is advanced hydraulically until its head member 44 (carrying retracted firing pin 85) engages the cartridge to hold it steady in the muzzle, (b) piston 23 is then advanced by air pressure to drive the firing pin 85 forward and so percussion-initiate the cartridge (which incorporates a detonation-delay device), (c) piston 21 advances shortly after piston 23 and drives piston 23 further forward to punch the cartridge through muzzle 18 so that it detonates outside the firing device and (d) the pistons are then retracted. Air pressure may be used to eject any water from housing 10 via valves connected at 107, 108. The firing unit may be maintained at a given depth by paravanes, and a hydrophone streamer may also be depth-controlled. The uniformity of the generated seismic energy pulses permits computer processing of recorded hydrophone signals to eliminate " bubble pulse " effects. Details are given of various alternative firing devices in which (a) a modified cartridgedetaining and ejecting arrangement is used or no such arrangement is used (the cartridge initiation and ejection being concurrent), (b) assembly 19 comprises two pistons only, or a single piston (which may be spring driven), or (c) no pistons are used and the cartridges are fed from the loader down a conduit from which they emerge, to impact a plate, thus initiating the charge, and then fall away from the plate. The cartridge of Fig. 5 comprises a metal shell 112<SP>11</SP> containing main charge 122<SP>11</SP> and closed by a recessed cap 117<SP>1</SP> with a well which supports a primer assembly 127<SP>1</SP>. The main charge is a mixture of an inorganic oxidizer salt, a fuel and a sensitizer. The oxidizer salt is one or more of the alkali metal or alkaline earth metal nitrates or perchlorates, or ammonium nitrate or perchlorate; the fuel is fuel oil, ground coal, granular aluminium or a mixture thereof; the sensitizer is DNT oil, DNT solids, particulate aluminium (partly in flake form) or mixtures thereof and may also include powdered coal, fuel oil, ferrosilicon or ferrophosphorous. The primer assembly 127<SP>1</SP> comprises a percussionignitable charge in head portion 131a<SP>1</SP> (e.g. a rim-fired or centre-fired empty primed rifle cartridge), a delay fuse (optional) 143<SP>1</SP> (giving a ¢ to 1¢ second delay), a primer charge 137a<SP>1</SP>, 137c<SP>1</SP>, and a base charge 128<SP>1</SP>. Flame from the percussion ignitable charge ignites the delay fuse which burns and in turn detonates the primer charge to cause detonation of the base charge. The percussion-ignitable charge comprises potassium perchlorate, lead styphnate, mercury fulminate, antimony sulphide, lead azide or mixtures thereof. The delay fuse 143<SP>1</SP> comprises mixtures of lead oxide/boron, red lead/ boron, barium peroxide/tellurium/selenium, barium peroxide/selenium, or barium peroxide/tellurium. The primer charge comprises lead azide, mercury fulminate or diazodinitrophenol; a preferred primer is a wafer 137a<SP>1</SP> of compressed diazodinitrophenol which is ignited by flame from the delay fuse and which detonates a capsule of loose diazodinitrophenol 137c<SP>1</SP>. The base charge 128<SP>1</SP> comprises PETN, pentolite, cyclonite, tetryl, RDX or cyclotol. Bead 124<SP>1</SP> or a rim on the cap 117<SP>1</SP> serves to detain the cartridge in the firing device. In a modification, cap 117<SP>1</SP> is not recessed. Numerical examples are given for cartridge dimensions and quantities of the contents.