This invention relates to marine seismic surveying.
In order to perform a 3D marine seismic survey, a plurality of seismic streamers, each typically several thousand meters long and containing arrays of hydrophones and associated electronic equipment distributed along its length, are towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitised and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
The Applicant""s current method of towing arrays of such streamers involves towing each streamer by means of its lead-in, ie by means of the armoured electrical or electro-optical cable that supplies control signals and electrical power to, and receives the aforementioned digitised signals from, the streamer, as described in U.S. Pat. No. 4,798,156, the armoured outer sheath of the lead-in, typically made of steel wires or high strength synthetic fibers such as Kevlar, serving to bear the towing forces. Using this method, the Applicant can typically tow a 700 meter wide array of eight streamers, each 4,000 meters long.
The drag produced by such an array at the typical towing speed of 5 knots is about 40-45 tonnes, a high proportion of which is cross-line drag due to the transversely extending lead-ins rather than the in-line drag of the streamers themselves. This drag is a very significant contributory factor to the operating costs, primarily fuel costs, of the towing vessel, and therefore to the overall cost of the survey.
In order to increase the efficiency and reduce the cost of marine seismic surveys, it is desirable to use even wider arrays containing even more streamers. However, the Applicant has calculated that using its current towing technique, a 1440 meter wide array of ten streamers would produce a drag of over 70 tonnes, which is excessive, and therefore uneconomic.
It is an object of the present invention in its principal aspects to alleviate this problem.
The lead-ins, which because of their combined signal and power transmission and towing functions are rather expensive, are sometimes damaged in use, eg by shark bites or by impacts with debris. Currently, it is difficult if not almost impossible to repair a damaged lead-in, especially while at sea. It is an object of the present invention in its final aspect to provide solution to this problem.
According to a first aspect of the present invention, there is provided a method of performing a marine seismic survey with an array of seismic streamers towed behind a survey vessel, wherein two or more of the streamers are towed from a single lead-in which comprises a load-bearing outer sheath to bear the towing forces, an electrical or electro-optical core via which control signals and electrical power are supplied to, and data signals are received from, the streamers being towed by the lead-in, and a connector device series connected in the lead-in, the connector device comprising a body member mechanically coupled between the respective portions of the load-bearing outer sheath of the lead-in on either side thereof, to transmit towing forces therebetween, and a towing attachment for mechanical connection to one of the two or more streamers.
According to a second aspect of the invention, there is provided an array of seismic streamers adapted to be towed behind a survey vessel, the array comprising at least one lead-in which has two or more of the streamers connected to be towed thereby, and which comprises a load-bearing outer sheath to bear the towing forces, an electrical or electro-optical core via which control signals and electrical power are supplied to, and data signals are received from, the streamers being towed by the lead-in, and a connector device series connected in the lead-in, the connector device comprising a body member mechanically coupled between the respective portions of the load-bearing outer sheath of the lead-in on either side thereof, to transmit towing forces therebetween, and a towing attachment for mechanical connection to one of the two or more streamers.
In a preferred implementation of the invention, the array comprises 2N streamers, where N is at least 2, the streamers are substantially uniformly spaced apart and symmetrically distributed on each side of the centre line of the vessel, and at least two streamers on each side of the centre line of the vessel are towed from a respective such lead-in.
The Applicant has found that by use of selected implementations of the method of the invention, the drag produced by the aforementioned 1440 meter wide ten streamer array can be reduced from over 70 tonnes to a level in the region of 50 tonnes.
The load-bearing outer sheath of the or each such lead-in is preferably made from high strength synthetic fibers, such as Kevlar fibres.
Preferably, the inner core of the lead-in at each end of the or each connector device is terminated in a respective watertight termination, and the connector device includes elongate flexible electrical or electro-optical connector means for establishing electrical or electro-optical connection between said watertight terminations and between one of said terminations and the streamer connected to the towing attachment.
Conveniently, the inner core of the part of the lead-in on the vessel side of each connector includes separate electrical conductors or optical fibres for the control and data signals of each streamer connected directly or indirectly to that part via that connector.
According to a third aspect of the present invention, there is provided a connector device adapted for series connection in a lead-in to permit the lead-in to tow at least two seismic streamers, the lead-in comprising a load-bearing outer sheath, to bear the towing forces, and an electrical or electro-optical core via which control signals and electrical power are supplied to, and data signals are received from, the streamers, the connector device comprising a body member adapted to be mechanically coupled between the respective portions of the load-bearing outer sheath of the lead-in on either side of the body member, to transmit towing forces therebetween, and a towing attachment for mechanical connection to one of the two or more streamers, said towing attachment being secured to the body member.
The or each body member may advantageously comprise at least two generally tubular portions interconnected by a universal joint, the towing attachment being secured to one of the tubular body portions.
In a preferred implementation of the invention, the or each body member comprises three tubular body portions interconnected by two universal joints, the towing attachment being secured to the middle one of tubular body portions.
The or each towing attachment preferably includes clamping means for detachably clamping it to the tubular body portion to which it is secured.
Advantageously, the or each towing attachment ha s a towing bracket pivotally connected to it, said towing bracket being pivotal about an axis generally perpendicular to the axis of the body member.
According to a final aspect of the present invention, there is provided a connector device adapted for series connection in a lead-in used for towing a streamer, the lead-in comprising a load bearing outer sheath to bear the towing forces, and an electrical or electro-optical core via which control signals and electrical power are supplied to, and data signals are received from, the streamer, the connector device comprising a tubular body member which is adapted to be mechanically coupled between the respective portions of the load-bearing outer sheath of the lead-in on either side of the body member, to transmit towing forces therebetween, and which preferably comprises at least two generally tubular portions interconnected by a universal joint.
Preferably, the or each universal joint of the or each body member is a Cardan-type universal joint.
In all aspects of the invention, at least part of the or each body member is preferably made from titanium.
The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: