Patent GB-975479-A (GB)
Title: Seismic surveying system for water-covered areas
IPC: H01B7/04
Filed: 1962-12-20
Abstract: 975,479. Seismic surveying. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. Dec. 20, 1962 [Dec. 22, 1961], No. 48245/62. Heading H4D. In an underwater seismic surveying system the seismic waves are generated by an elongated source 18 (Fig. 2), comprising a flexible body member 22 (Fig. 4), supplied with a combustible gas mixture via mixing chamber 24 and gas conduits 27, 28 and provided with a plurality of spaced discharge ports having normally closed spring- loaded valves 35 which are set to open at the same pressure and permit gas bubbles to flow outwardly into the water. The bubbles may be detonated by means of a spark discharge between electrodes 42, 43 carried by a probe 36 and energized over conductors 40, 41. When the bubbles consist of a mixture of hydrogen and chlorine detonation may be effected by irradiating them with light of wavelength between 2,800 and 5,000‡ from a source on top of probe 36 or one towed independently by a separate cable. At shallow depths the detonation may be produced by sunlight. However, to ensure that all the bubbles are detonated simultaneously the change in the current flow through a plurality of resistance wires 46, Fig. 5, due to the resistance change caused by increased thermal insulation between the water and the wires when the latter are surrounded by the gas bubbles is utilized via equipment (Fig. 6, not shown), carried by the towing vessel to produce a capacitor discharge through the wires which heats them to a temperature sufficiently high to detonate the bubbles. In an alternative arrangement the resistance wires are replaced by a rubber covered two-conductor cable 63 (Figs. 8, 9 and 10), formed with a plurality of spark gap electrodes consisting of metal rings 60, 61 shunted by a high resistance composed of ceramic or conducting rubber rings 64. The gaps are connected in series with the conductors 58, 59 through which a current flows and in operation, when a gap is enclosed in a gas bubble the resistance across the gap increases and the ensuring reduction in current is utilized as in the embodiment of Fig. 5 to produce a capacitor discharge through conductors 58, 59 so that such gaps break-down and initiate the detonation of the enclosing bubbles. In all of the arrangements useful seismic signals having frequencies between 30 and 60 c.p.s. may be produced by selecting the detonation time so that the bubbles are of a size not yielding high frequencies. In actual surveying in which a record of geological structures under the water is produced on the towing vessel the linear sound source 18 may be towed above the cable 13 which is provided with a plurality of hydrophones as shown in Fig. 2 or, as shown in Fig. 3, a single composite arrangement of sound source 18 and two hydrophone cables 13, 13a may be employed. Instead of being linear the sound source may have other configurations such as circular, rectangular &c. and instead of towing it the sound source may be stationary at a predetermined depth. Depth sensing means may be incorporated at a number of positions on the sound source.