SYSTEM FOR ACQUIRING SEISMIC DATA

Abstract

A distributed acoustic sensing system for acquiring seismic data is presented. The system includes a sensing cable and an instrument float. The sensing cable is for sensing seismic waves and is suitable for use on the seabed. The instrument float includes instrumentation for acquiring seismic data. The instrument float is connectable or connected to the sensing cable via a riser cable.

Claims

1. A distributed acoustic sensing system for acquiring seismic data, the system comprising: a. a fibre optic sensing cable for sensing seismic waves, the fibre optic sensing cable being suitable for use on the seabed; and b. an instrument floating structure comprising at least some instrumentation for use in the acquisition of seismic data, the instrument floating structure being connectable or connected to the fibre optic sensing cable via a riser cable; wherein the fibre optic sensing cable is a continuous unbranched cable.

2. A system as claimed in claim 1, wherein the sensing cable comprises: c. a sensing part; and d. one or more protective layers arranged around the sensing part.

3. A system as claimed in claim 2, wherein the sensing part comprises a glass fibre part, the glass fibre part preferably consisting of a single glass fibre strand.

4. A system as claimed in claim 2, wherein the protective layer has a lower elastic modulus than the sensing part.

5. A system as claimed in claim 2, wherein the protective layer: e. comprises a silicone layer; and/or f. adheres or is adhered to the sensing part.

6. A system as claimed in claim 2, wherein the one or more protective layers comprise an inner protective layer and an outer protective layer, the inner protective layer being arranged between the sensing part and the outer protective layer, and the outer layer: g. has greater tensile strength and/or weight and/or density than the sensing part and/or the inner protective layer; and/or h. is made of high density polypropylene or high density polyethylene.

7. A system as claimed in claim 2, wherein one or more of the one or more protective layers and/or an outer layer is/are: i. biodegradable; and/or j. arranged to prevent water from contacting the sensing part when arranged underwater for at least one day or at least one week; and/or k. arranged to biodegrade or decompose when underwater for longer than one day or one week.

8. A system as claimed in claim 1, wherein the sensing cable has sufficient density that it will sink down to a seabed.

9. A system as claimed in claim 1, wherein the at least some instrumentation for use in the acquisition of seismic data comprises a receiver for receiving a signal to begin a seismic survey.

10. A system as claimed in claim 1, wherein the instrument floating structure is connected to an anchor.

11. A system as claimed in claim 1, wherein the riser cable is suitable for transmitting optical signals and/or comprises a mooring part or cable.

12. A system as claimed in claim 1, the system further comprising one or more buoys, the buoys being connected to the sensing cable via one or more connection means.

13. A distributed acoustic sensing system for acquiring seismic data, the system comprising: l. a single sensing cable for sensing seismic waves, the sensing cable being suitable for use on the seabed; and m. an instrument floating structure comprising at least some instrumentation for use in the acquisition of seismic data, the instrument floating structure being connectable or connected to the sensing cable via a riser cable; wherein the sensing cable comprises: a sensing part comprising a glass fibre part; and one or more protective layers arranged around the sensing part.

14. A method of deploying a distributed acoustic sensing system for acquiring seismic data, the system being according to claim 1, the method comprising: n. deploying the sensing cable from a vessel; and o. connecting the instrument floating structure via the riser cable to the sensing cable.

15. A method as claimed in claim 14, wherein the system is deployed such that the sensing cable is arranged in such a way that signal interference is minimised or avoided.

16. A method as claimed in claim 14, wherein one or more buoys are connected to the sensing cable, preferably as the sensing cable is deployed.

17. A method as claimed in claim 14, further comprising determining the position of the deployed sensing cable.

18. A method of acquiring seismic data related to a subsea geological structure, the method comprising using a distributed acoustic sensing system as defined in claim 1, the method comprising: p. emitting seismic waves and/or pulses from a seismic source; and q. detecting reflected seismic waves and/or pulses with the sensing cable.

19. A method as claimed in claim 18, the method further comprising recording seismic data representing the detected seismic waves with or at the instrument floating structure.

20. A method as claimed in claim 18, the method further comprising receiving a signal at the instrument floating structure, the signal comprising instructions to start a seismic survey.

21. A method of recovering a distributed acoustic sensing system as defined in claim 1, the method comprising gathering or retrieving the instrument floating structure and/or one or more buoys connected to the sensing cable via one or more connection means.

Description

[0118] Preferred embodiments of the invention will now be described with reference to the accompanying figures, in which:

[0119] FIG. 1 is a schematic plan view of a distributed acoustic sensing system according to an embodiment;

[0120] FIG. 2 is a schematic side view of the distributed acoustic sensing system of FIG. 1;

[0121] FIG. 3 is the schematic plan view of the distributed acoustic sensing system of FIG. 1 illustrating a method of recovery;

[0122] FIG. 4 is a further schematic plan view illustrating a method of recovery of the distributed acoustic sensing system of FIG. 1; and

[0123] FIG. 5 is a schematic cross-sectional view of a cable for use in the distributed acoustic sensing system of FIG. 1.

[0124] FIGS. 1 and 2 illustrate a distributed acoustic sensing (DAS) system 1 for acquiring seismic data and for use on a seabed 22.

[0125] The DAS system 1 comprises a sensing cable 2, an instrument float 3 with associated main anchor 6, and buoys 4 with corresponding anchors 5.

[0126] The instrument float 3 is attached to the sensing cable 2 and main anchor 6 via a riser cable 8. The buoys 4 are attached to the sensing cable 2 and anchors 5 via ropes or cords 7.

[0127] FIGS. 1 and 2 illustrate the system 1 deployed and ready for use. In such a situation, the sensing cable 2 and anchors 5 and 6 are located on the seabed 22. The instrument float 3 and the buoys 4 are located on the sea surface 21. The sensing cable 2 is arranged over a geological structure 20 (the outline of which is indicated with the dashed line) to be surveyed.

[0128] FIG. 5 is a cross-sectional schematic illustration of the sensing cable 2. This figure is not to scale and merely illustrates the relative positions of the different layers or parts of the cable. However, the different layers or parts of the sensing cable 2 may have different (e.g. different relative) thicknesses than those shown. As illustrated in FIG. 5, the sensing cable 2 is formed of a glass fibre 2a, an intermediate layer 2b and an outer layer 2c.

[0129] The glass fibre 2a has a diameter of around 125 μm and is formed of a single glass fibre strand. The optical signal is guided in a core of the fibre 2a. which typically has a diameter of around 10 μm.

[0130] The intermediate and outer layers 2b, 2c protect the glass fibre 2a from water ingress and mechanical damage.

[0131] The intermediate layer 2b is formed of silicone. This has good adhesion to the glass fibre 2a and a low elastic modulus. The low elastic modulus helps to transfer any compressive forces acting on the sensing cable 2 into a linear strain along the fibre length.

[0132] The intermediate layer 2b has a thickness of around 600 μm.

[0133] In some embodiments, a biodegradable intermediate layer 2b is used.

[0134] The outer layer 2c is made of high density polypropylene or high density polyethylene. This forms a hard, water-resistant outer coating of the sensing cable 2. It also adds weight to the sensing cable 2 and provides an increased tensile strength.

[0135] The outer layer 2c has a thickness of around 1.5 mm-4 mm.

[0136] In some embodiments (not shown), two or more outer layers, e.g. of high density polypropylene or high density polyethylene, are used.

[0137] In some embodiments, fibres, such as natural rubber or cellulose fibres, are included in the outer layer 2c to provide an increased tensile strength.

[0138] In some embodiments, a biodegradable outer layer 2c is used, such as described above.

[0139] In any embodiment, the outer layer 2c (and intermediate layer 2b) should delay water penetration to the glass fibre 2a by around one week. This should provide sufficient time for a seismic survey to be performed.

[0140] The total diameter of the sensing cable 2 is around 1.5 mm-4 mm. The sensing cable 2 should have sufficient weight or density to be deployable (i.e. to sink down to the seabed 22) and the intermediate and/or outer layer(s) 2b, 2c must provide water protection for the glass fibre 2a. However, the sensing cable 2 should also ideally have the minimum thickness of intermediate and/or outer layer(s) 2b, 2c needed to achieve these objectives, in order to reduce the about of material to be disposed of after use.

[0141] The sensing cable 2 typically has a total length of around 10-30 or 40 km. However, in some embodiments, the sensing cable 2 could have a length of more than 40 km, e.g. up to 50 km or more. The actual physical length can be even longer, but the active sensing length of the sensing cable 2, with existing technology, is typically limited to the order of 40 km or less, dependent on the type of interrogator technology used. In the future, developments in the interrogator and/or sensing cable technology may allow even longer lengths of (active) sensing cable 2 to be used, such as up to 50 km or more.

[0142] The sensing cable 2 has a first, free end 2a and a second end 2b. The second end 2b is connected, via riser cable 8 to the instrument float 3.

[0143] The instrument float 3 comprises the instrumentation for performing a seismic survey. It comprises an interrogation unit, a GPS antenna, a battery and a DAS control system with a hard drive or memory.

[0144] The interrogation unit contains a distributed acoustic sensing interrogator which is arranged to send out optical pulses and decode the phase of received Rayleigh backscatter, converting it to a distribution of instantaneous strain rate along the fibre, which in turn is sensitive to acoustic pressure changes (or hydrostatic pressure changes).

[0145] The GPS antenna provides a clock reference signal for the DAS control system.

[0146] The battery provides sufficient power for the DAS control system for one seismic survey or about one day.

[0147] The DAS control system controls the seismic survey and records the data received from the sensing cable 2 in the memory.

[0148] The instrument float 3 comprises a radio signal receiver which can receive a radio signal from a vessel instructing the DAS control system to begin a seismic survey.

[0149] The instrument float 3 is designed such that it will float on the sea surface 21.

[0150] The instrument float 3 is connected to a main anchor 6 via the riser cable 8. The riser cable 8 also connects to the sensing cable 2 and allows seismic (optical) signals to be passed from the sensing cable 2 to the instrument float 3.

[0151] The riser cable 8, as well as being able and arranged to transmit optical signals from the sensing cable 2 to the instrument float 3, also provides a mooring means for the instrument float 3. As such, the riser cable 8 comprises a signal-transmitting fibre-optic cable with a waterproof coating and a mooring cable such as a rope or chain. Thus, a signal-transmitting cable and a mooring cable are provided in (or in the form of) a single riser cable 8.

[0152] In one embodiment, the signal-transmitting cable is threaded through the mooring cable (e.g. inside an outer casing). In another embodiment, the signal-transmitting cable is threaded through apertures provided on the mooring cable at a plurality of locations along the length of the mooring cable.

[0153] In an alternative embodiment, a mooring cable for the instrument float is provided separately to (i.e. not attached or connected to) the signal-transmitting cable.

[0154] In either case, the mooring cable is stronger than the signal-transmitting cable.

[0155] In use, the mooring cable is arranged such that it experiences a greater load or strain than the signal-transmitting cable, i.e. it is arranged to prevent the signal-transmitting cable from experiencing any potentially damaging loads or strains.

[0156] The riser cable 8 is stronger than the sensing cable 2. This is because, when in use, it has to withstand greater forces, e.g. tensile forces, than the sensing cable, which is arranged on the seabed.

[0157] The sensing cable 2 is connected to the riser cable 8 by splicing it to the signal-transmitting cable of the riser cable 8 at (or near) the bottom of the riser cable 8.

[0158] The main anchor 6 helps to keep both the instrument float 3 and the sensing cable 2 in a relatively fixed position (there may still of course be some movement due to currents, for example). Any suitable anchor 6 can be used. Both the riser cable 8 and the sensing cable 2 are connected to the main anchor 6.

[0159] In an embodiment, the main anchor 6 is a patent anchor and the sensing cable 2 and the riser cable 8 are connected to the main anchor 6 in close proximity to each other and in a location on an upper surface of the main anchor 6. Connecting the cables 2 and 8 to the main anchor 6 on an upper surface thereof can help to avoid shearing of either cable (particularly the less robust sensing cable 2) from the main anchor 6 if they rub, for example, on the sea bed. A connector is provided on the main anchor 6 for connecting the riser cable 8 (e.g. its mooring cable) to the main anchor 6. A further connector is also provided to connect the sensing cable 2 to the main anchor 6.

[0160] The connectors are arranged such that the riser cable 8 and the sensing cable 2 can be connected to the main anchor 6 in a movable/slidable manner. In other words, the riser cable 8 and the sensing cable 2 can still move, e.g. slide longitudinally, with respect to the main anchor 6 whilst being held at or close to the main anchor 6 by the connectors.

[0161] In order to achieve this, the connectors each comprise a guide in the form of one or more loops, channels or apertures through which the riser cable 8 or the sensing cable 2 can be threaded, thereby allowing the riser cable 8 or the sensing cable 2 to move longitudinally with respect to the main anchor 6 whilst still being connected to it.

[0162] The connectors (especially the connector for the sensing cable 2) comprise rounded and smooth edges, i.e. with no sharp edges, such that they will not cause damage to the riser cable or (particularly) the sensing cable 2 when connected to it. For similar reasons, the connectors are also formed of a relatively soft material such as rubber.

[0163] Two or more buoys 4 are also provided. These are attached to the sensing cable 2 via ropes or cords 7. The ropes or cords 7 are also attached to small anchors 5. Any suitable anchors 5 could be used. The ropes or cords provide simple mechanical attachment between the buoys 4 and the sensing cable 2 and anchors 5. They do not need to transmit any signals so any kind or rope or cord suitable for such connection can be used.

[0164] In alternative embodiments (not shown), for example where the geological structure is particularly large and cannot be covered by a single sensing cable 2, two or more sensing cables may be used. In some cases, these will be provided in a system corresponding to the system 1 described above, so that each sensing cable 2 is connected to a separate instrument float 3. In other cases, multiple sensing cables 2 could be attached to a common or shared instrument float 3.

[0165] In order to deploy the system 1, a vessel (not shown) brings the system 1 to the area in which it is intended to be installed (e.g. above the geological structure 20). When the vessel is located above (or close to above) the geological structure 20, the sensing cable 2 is spooled out, starting with its first, free end 2a. As the sensing cable 2 is spooled out, the sensing cable 2 sinks down to the seabed 22. The cable 2 is spooled out such that it lies over the geological structure 20 in a predetermined pattern or arrangement, curving around such that the geological structure 22 is covered substantially evenly with the sensing cable 2. The positioning of the sensing cable 2 does not have to be particularly accurate but it is simply important that good overall (even) coverage of the geological structure 22 should be provided.

[0166] The first end 2a of the sensing cable 2 is spooled out such that it lies just outside of the area of interest (over the geological structure 22). This bit of excess length of sensing cable 2 allows the sensing cable 2 to be oriented in the correct direction for the rest of the spooling operation.

[0167] The second end 2b of the sensing cable 2 is spooled out such that it is located in a relatively central location over the geological structure 22, or over a most important area to survey. This can provide the best signal-to-noise ratio for the most important area.

[0168] The buoys 4 and instrument float 3 (with their associated anchors 5, 6) are connected to the sensing cable 2 via the cords 7 and riser cable 8 as the sensing cable 2 is spooled out.

[0169] When the sensing cable 2 has been spooled out and is connected to the buoys 4 and instrument float 3, the vessel is no longer connected to the system 1 and is free to move around the sea surface.

[0170] The purpose of the additional buoys 4 and associated small anchors 5 is twofold.

[0171] First, the frictional force between the sensing cable 2 and the seabed increases exponentially with the continuous length of the sensing cable 2 being dragged along the seabed. The smaller buoys 4 effectively divide the sensing cable 2 into segments and allow the sensing cable 2 to be lifted off the seabed in these segments, thereby substantially reducing the peak tension in the sensing cable 2 during retrieval.

[0172] Secondly, in order to stabilise the sensing cable 2, a small anchor 5 located upstream can stabilise the lay, thereby reducing the need for compensating for a moving sensing cable 2 if currents grab hold of or act on the sensing cable 2.

[0173] Once the system 1 has been deployed, as described above, before a seismic survey can be performed, the position of the sensing cable 3 must be determined. This can be done using a standard technique of emitting a seismic wave from a seismic source located on the vessel and measuring the first direct arrival. Synchronising the seismic source and varying the instrument float 3 repetition frequency can allow for “tricks” to be performed when only timing the first arrival, thereby effectively allowing the upper frequency limit of the instrumentation with a long cable to be circumvented for positioning purposes. If a spurious signal of a direct arrival from an area of little interest arrives it can essentially be subtracted from a total response to leave the main or wanted response after being identified at a lower pulse repetition frequency of the laser. Shifting the timing essentially shifts the position of the spurious signals, so e.g. a timing can be done so that the high frequency shallow seismic survey can be performed at a higher upper frequency than the total length of the cable would otherwise dictate. The rule of thumb in DAS surveys is to only use a pulse repetition rate as low as the total roundtrip time for the optical signal in the fibre. This would be a trick to not have to reposition the seabed array to achieve also a high frequency survey close to the seismic source, particularly in shallower water.

[0174] Such a method as described above can allow the position of the sensing cable 2 to be determined with a resolution of approximately 1-2 m of cable length, which is substantially better than most existing systems.

[0175] A seismic survey can be initiated by sending a radio signal from the vessel to the instrument float 3, signalling to start a seismic survey. On receipt of this signal, the battery on the instrument float 3 powers the instrumentation on the float to record the signals sensed in the sensing cable 2 and transmitted via the riser cable 8 to the instrument float 3.

[0176] To perform the seismic survey, the vessel travels around, e.g. criss-crossing, over the geological structure 20 and sensing cable 2, emitting seismic waves from a seismic source (e.g. in a standard way as is known in the art).

[0177] The seismic source and the instrument float 3 electronics repetition frequency are synchronised using a recorded GPS clock signal and are time shifted in the seismic processing.

[0178] Seismic data collected during the survey is stored in the memory on the instrument float 3. After the survey has been performed, the/a vessel collects the memory from the float 3 and takes it for further storage, processing and/or analysis.

[0179] During the survey, a gauge length of the order of 5-10 m is typically a good compromise for improving signal-to-noise ratio for weaker signals. In effect the gauge length can be compared to a conventional hydrophone group, only the whole cable over this length is contributing not only discrete hydrophones.

[0180] Once a survey has been performed, the system 1 can be recovered as will now be described.

[0181] In one embodiment, the whole sensing cable 2 is disconnected and left in the sea. Use of biodegradable intermediate and outer layers 2b and 2c can be useful in such cases. In such cases, the instrument float and buoys 4 could still be collected, optionally with the anchors 5, 6, cord 7 and/or riser cable 8.

[0182] In another embodiment, the sensing cable 2 could be spooled back in (in the opposite way to which was deployed initially, and possibly using the same spooling means. The spooled-in sensing cable 2 could then be taken away for appropriate disposal. If the sensing cable 2 snapped or broke during such a spooling-in operation, then any broken-off part of the sensing cable 2, which it was then not possible to spool in, could be left in the sea (e.g. to biodegrade, if possible).

[0183] In a further embodiment, the instrument float 3 and buoys 4 could be used to retrieve the sensing cable 2 by gathering up the instrument float 3 and buoys 4 (which are connected to the sensing cable 2 via the riser cable 8 and cords 7), e.g. in a similar way to which crab pods on a line are retrieved. The large arrow 30 in FIG. 3 indicates the direction in which a vessel could travel to retrieve first the instrument float 3 and then the buoys 4, thereby gathering up the sensing cable 2 with them.

[0184] Alternatively, the buoys 4 could be retrieved (and hence the sensing cable 2) with a trawl gate system 40 such as illustrated in FIG. 4. In this system 40, a trawl gate 42 connected to a vessel 43 with cables 44 via a grapping device 41, and an underwater slack line 45, allows the instrument float 3 and buoys 4 (and the sensing cable 2 to which they are connected) to be captured in the grappling device 41.