Seismic streamer

Abstract

There is provided a solid seismic streamer cable for use in seismic surveying in marine environments. The streamer is characterised by a buffer layer 2 which is provided with a cut-out 50 and a sensor element arranged in the cut-out 50. There is also provided an associated hydrophone for integration into the seismic streamer cable. The hydrophone is characteristic in a split-element sensor base 10, 11 being suited for efficient mounting into the cut-outs 50 of the seismic cable. There is also provided an associated accelerometer for integration into the seismic streamer cable. The accelerometer is characteristic by a split-element sensor base 30, 35 for being efficiently arranged into the cut-outs 50 of the seismic cable. A method of producing a seismic streamer cable according to the invention incorporating a hydrophone or accelerometer according to the invention is also provided.

1 . A solid seismic streamer cable:
 characterized by a buffer layer provided with a cut-out and a sensor element arranged in the cut-out.   
     
     
         2 . The solid seismic streamer cable of  claim 1 , wherein the sensor element comprises a split-element sensor including an optical fibre sensor coil. 
     
     
         3 . The solid seismic streamer cable of  claim 2 , comprising an optical fibre including the sensor coil, the optical fibre including fibre Bragg gratings (FBGs) forming optical fibre interferometers. 
     
     
         4 . The solid seismic streamer cable of  claim 3 , wherein the optical fibre comprises multiple fibre Bragg gratings FBG 1 -FBG n  arranged so that for each coil there is a corresponding pair of FBGs, one FBG on each side of the hydrophone coil. 
     
     
         5 . The solid seismic streamer cable of  claim 3 , wherein the optical fibre comprises an intermediate FBG between two neighbouring sensor coils, the intermediate FBG being common to FBG pairs, each pair associated with one of the sensor coils. 
     
     
         6 . The solid seismic streamer cable of  claim 3 , wherein a plurality of hydrophone coils H 1 -H n  are arranged on the cable between two fibre Bragg gratings FBG 1 -FBG 2 . 
     
     
         7 . The solid seismic streamer cable of  claim 3 , wherein FBGs on two facing sides of two neighbouring hydrophones have the same wavelength. 
     
     
         8 . The solid seismic streamer cable of  claim 3 , wherein a pair of FBGs, one FBG on each side of a hydrophone coil has the same wavelength, while another pair of FBGs on a neighbouring hydrophone coil, with one FBG on each side of the neighbouring hydrophone coil, has a different wavelength. 
     
     
         9 . The seismic streamer cable of  claim 1 , comprising:
 a core part incorporating the buffer layer; and   a protective layer for protecting the core part and the sensor element.   
     
     
         10 . The seismic streamer cable of  claim 1 , wherein the at least one sensor element comprises at least one of an optical fibre hydrophone and an optical fibre accelerometer. 
     
     
         11 . The seismic streamer cable of  claim 1 , comprising at least one reference element arranged in one of the slots. 
     
     
         12 . The seismic streamer cable of  claim 9 , wherein the core part comprises a multilayer structure. 
     
     
         13 . The seismic streamer cable of  claim 9 , wherein the core part, the sensor element and the protective layer are co-axially arranged. 
     
     
         14 . The seismic streamer cable of  claim 1 , comprising optical fibre arranged loosely with an excess length in at least one groove of the buffer layer to effectively decouple the optical fibre and fibre Bragg gratings (FBGs) from strain in the seismic cable. 
     
     
         15 . The seismic streamer cable of  claim 14 , wherein the groove is filled with a grease, wax or another material which can be hardened by cooling prior to coiling, such that at operating temperatures the material is softened thereby allowing the optical fibre to sink into the material, resulting in an excess fibre length. 
     
     
         16 . The seismic streamer cable of  claim 14 , wherein the groove is filled with an initially hard and chemically dissolvable material which can be removed by a chemical process after fibre winding. 
     
     
         17 . The seismic streamer cable of  claim 14 , wherein the optical fibre is arranged onto a shrinkable element arranged in the buffer layer groove. 
     
     
         18 . The seismic streamer cable of  claim 1 , wherein optical fibre wound on the buffer layer between sensors acts as a distributed strain sensor, the strain sensor being formed by the wound optical fibre with two spaced FBGs arranged one on each side of two neighbouring sensors and produced to have the same wavelengths on facing sides. 
     
     
         19 . An accelerometer for integration into a seismic streamer cable, the accelerometer characterized by:
 a split-element sensor base arranged into cut-outs in a buffer layer of a seismic cable, the sensor base including a first and a second part;   the first part being provided with first attachment means for fixing to a streamer cable core part;   second attachment means being arranged to movably couple the second part to the first part; and   an optical fibre coiled around outer surfaces of the sensor base so as to form an accelerometer, the fibre being strained when the accelerometer is subjected to acceleration causing the second part to move in relation to the first part.   
     
     
         20 . The accelerometer of  claim 19 , wherein the first attachment means comprises a glue joint. 
     
     
         21 . The accelerometer of  claim 20 , wherein the second attachment means comprises a flexible element comprising a blade shaped metal spring which is attached into a first longitudinal slit in the first part and into a corresponding second slit in the second part, whereby the second part is movably suspended by the flexible element. 
     
     
         22 . The accelerometer of  claim 20 , wherein the second part is mounted so as to define a surrounding space whereby the second part is able to move about an axis defined by the second attachment means. 
     
     
         23 . The accelerometer of  claim 20 , comprising two end sections, arranged at first and second ends of the accelerometer, the end sections serving to close the said surrounding space when the accelerometer is arranged as part of a streamer cable. 
     
     
         24 . The accelerometer of  claim 23 , where the end sections are formed as semicircular sections of an annulus. 
     
     
         25 . The accelerometer of  claim 20 , comprising covers which serve to protect the optical fibre of the accelerometer and the movable second cylindrical part from pressure waves and other external mechanical influences. 
     
     
         26 . The accelerometer of  claim 20 , wherein the first cylindrical part is provided with a channel that extends from a space containing the fibre coil of the accelerometer and to oil or gel filled voids between the cable core part and an outer protective layer or cladding of the seismic cable so as to equalize the pressure between said space and voids. 
     
     
         27 . The accelerometer of  claim 20 , wherein the first and second parts are generally shaped as halves of a cylinder in order to fit smoothly around the cut-out core part when assembled. 
     
     
         28 . The accelerometer of  claim 20 , wherein at least one of the first and second parts are provided with helical grooves for allocating and guiding the fibre over a part of the accelerometer. 
     
     
         29 . A method of producing a seismic streamer cable comprising:
 providing a cable core part;
 providing a buffer layer of the core part with cut-outs at selected locations of the cable; and 
 arranging a split-element sensor base element in one or more of the cut-outs. 
   
     
     
         30 . The method of producing a seismic streamer cable according to  claim 29 , wherein at least a part of an optical fibre is arranged in grooves of the buffer layer by a method which provides a larger effective coiling diameter during coiling than when the cable is completed to obtain an optical fibre excess length. 
     
     
         31 . The method of producing a seismic streamer cable according to  claim 30 , wherein the depth of the groove is increased by removing material after the optical fibre is arranged in the groove. 
     
     
         32 . The method of producing a seismic streamer cable according to  claim 30 , comprising filling the groove with grease, wax or another material which may be hardened by cooling, and allowing the fibre to be coiled onto the material when hard and allowing the fibre to sink into the material as it softens when brought back to an operating temperature, typically the temperature of the sea. 
     
     
         33 . The method of producing a streamer cable according to  claim 30 , wherein the optical fibre is coiled onto an initially strained central structure of the cable which is subsequently relaxed, thereby providing an excess optical fibre in the grooves. 
     
     
         34 . The method of producing a seismic streamer cable according to  claim 30 , wherein the effective depth of the groove is increased by softening a material under the optical fibre after it has been arranged in the groove. 
     
     
         35 . The method according to  claim 29 , comprising:
 providing a protective cladding covering the core part and sensor element of the cable.   
     
     
         36 . The method according to  claim 29 , wherein the arranging of a sensor element comprises:
 arranging an optical fibre on the cable core part and on the split-element sensor base so as to form the sensor element.   
     
     
         37 . The method of  claim 36 , wherein the step of arranging the optical fibre forms at least one of a hydrophone and an accelerometer. 
     
     
         38 . The method of  claim 35 , wherein the protective cladding is arranged so as to provide the finished cable with a longitudinally uniform outer profile. 
     
     
         39 . The method of  claim 29 , wherein the cut-outs in the buffer layer are provided in a machining process which removes parts of the buffer layer. 
     
     
         40 . The method of  claim 29 , wherein the cut-outs are provided in a periodic arrangement or a regularly spaced manner along the streamer cable. 
     
     
         41 . The method of  claim 29 , comprising arranging a sensor reference in one of the cut-outs. 
     
     
         42 . The method of  claim 41 , wherein the arranging of a sensor reference comprises combining at least two inner reference mandrel parts, at least two reference cover parts and an optical fibre reference coil between the inner reference mandrel parts and the reference cover parts. 
     
     
         43 . The method of  claim 29 , comprising providing the streamer cable with an outer jacket, for example, by an extrusion process. 
     
     
         44 . The method of  claim 29 , comprising providing grooves in the buffer layer for allocating, in a loose manner, at least sections of the optical fibre.