GB2473418A

GB2473418A - Re-deploying seismic receiver cables while carrying out a marine seismic survey

Info

Publication number: GB2473418A
Application number: GB0911049A
Authority: GB
Inventors: Paul Meldahl; Trond Stenseng
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2009-06-25
Filing date: 2009-06-25
Publication date: 2011-03-16
Also published as: WO2010149589A3; GB0911049D0; WO2010149589A2

Abstract

A marine seismic survey is conducted by deploying receiver cables 11 in a longitudinal direction over an active receiver area 12. A seismic source 14 is towed behind a vessel 13 to track across the active receiver area in the longitudinal direction along a first line. The source vessel is turned around and the source is towed to track across the receiver area in the reverse direction along a second line; parallel to the first. This is repeated over the whole survey area. The receivers may be picked up and re-deployed while the source 13 is being fired. The distance between the cables in a transverse direction may be greater than or equal to twice the maximum offset between the source and receivers. A seismic source is towed and fired along firing lines parallel to the receiver cables while the receiver cables are re-deployed such that the cable arrangement is indexed forwards in the transverse direction. The receiver cables may be attached to a buoy which floats beneath the surface by a rope, when a signal is sent to a control unit on the buoy, a further length of rope is released, allowing the buoy to float to the surface.

Description

Method of Conducting a Seismic Survey The present invention relates to a method of conducting a seismic survey, in particular, a marine seismic survey.

Currently, marine seismic acquisition systems include towed surface acquisition systems and stationary bottom acquisition systems.

In practice, towed surface acquisition systems are normally preferred to bottom acquisition systems. This is because although the seismic qualities and payback from bottom acquisition systems are superior, the advantages are small and/or uncertain, and the cost of sea floor acquisition is greater than the cost of surface acquisition systems.

However, while the cost of surface acquisition is known to be less than the cost of bottom acquisition, surface seismic acquisition tends to be carried out with less than ideal sampling and apertures in the source and receiver domains.

Currently, in bottom seismic acquisition systems, streamers carrying seismic receivers or nodes are positioned at the sea floor within a relatively small receiver area (generally a rectangle) as a relatively sparse 2-dimensional grid.

One or more sources are fired in a regular grid above the receiver area, covering an area larger than the receiver area. The relatively small receiver area has to be moved several times and each time, the sources must be fired, in order to cover the complete survey area.

One reason therefore for the large costs involved in acquiring seismic data from a bottom system with better geometry is simply that it is necessary to shoot from 2 to 10 times more in order to create a sufficient sampling area and density in the source and receiver domains. In addition, each time the receivers are re-positioned, the sources are fired over and outside the receiver aperture.

The overlap in receiver aperture and the out-of-receiver area shooting for every new receiver aperture move is a major inefficiency which results in wasted effort, both on the source side and the receiver side.

Furthermore, when the source vessel turns, no shooting is carried out and the expensive cables are inactive. Also, moving the bottom receiver array normally also stops seismic acquisition. The presence of surface objects which are connected to the seismic source cable may disturb the shooting in several places. The functions of these objects may include signal transmission, power supply, providing attachment points for capturing the cable and winding it on to a reel for transport.

The object of the present invention is to address these difficulties and drawbacks.

According to a first aspect of the invention, a method of conducting a marine seismic survey of a survey area comprises: deploying a plurality of receiver cables to define an active receiver area the cables extending in a longitudinal direction; towing a seismic source behind a source vessel to track across the active receiver area in the longitudinal direction along a first line, turning the source vessel and its source around and towing the source to track back across the receiver area in the reverse longitudinal direction along a second line, generally parallel to the first line; the second line being spaced from the first line in a transverse direction which is generally perpendicular to the longitudinal direction; repeating the tracking, turning and tracking back steps whereby the source vessel and its source track across the active receiver area along successively advanced lines; continuously firing the source at particular positions and/or time intervals while it is tracking the receiver area, and continuously detecting the seismic responses to the firing using the deployed receiver cables.

In this way, it will be appreciated that continuous acquisition of data is possible. The greater receiver area and greater number of receivers means that vastly greater amounts of data can be gathered, thus improving the quality of the overall survey.

Preferably, the receiver cables are deployed in a pattern and are preferably arranged in parallel. The receivers can be picked up and re-deployed while the source array is being fired. Preferably, the source is fired repeatedly during the entire time it is tracking across the receiver area in the longitudinal direction.

It will be understood that as the source vessel and source array advance to the midpoint of the active receiver area in the lateral direction, there will be some receiver cables which are still beyond the maximum source/receiver offset in the forward lateral direction, and therefore not yet utilised, and some receiver cables which are beyond the maximum source/receiver offset in the rearward lateral direction. These latter cables are therefore redundant and these are the cables that are redeployed. The number of cables picked-up for redeployment by a receiver vessel at one time may be from 1 to 4.

Preferably, the method includes towing a second seismic source behind a second source vessel to track across the active receiver area along lines which are intermediate the lines taken by the first vessel and its source; and continuously firing the second source at particular positions and/or time intervals while it is tracking across the receiver area. This has the advantage of enabling a more sparse density of both source arrays and receiver arrays to be adopted because the iterative approach enables any "gaps" in the original information to be "filled-in" by the iteration. Indeed, the iteration can be tuned to the actual specific requirements and, of course, there can be more then one iteration. When the survey procedure is repeated, it would be possible to deploy and redeploy the cables at positions which are spaced in the lateral direction from their respective positions in a previous survey procedure.

Naturally, the survey procedure can be repeated more than once.

There may be a plurality of source and/or receiver vessels. Preferably the seismic responses are continuously detected by the deployed receiver cables.

Preferably, detecting the responses is carried out in separate intervals, equal or slightly less than the time interval between two subsequent shots. The method may include the further step of defining a maximum offset M between a seismic source and a seismic receiver, and deploying the receiver cables so that the distance between the outermost cables in the active receiver area, in the transverse direction is 2 2M. Preferably the lines along which the source is towed are spaced from the outermost receiver cables in the active receiver area by a distance that is 2 M, measured in the transverse direction.

The source vessels may be arranged so that one vessel is tracking and shooting while one (or more) is turning around at the end of a track and advancing. In this way, shooting can be continuous throughout the survey, allowing for continuous detection. The vessels may be manned or unmanned.

According to a further aspect of the invention, there is provided a method of conducting a marine seismic survey, comprising: a) defining a maximum desired offset M between a seismic source and a seismic receiver; b) deploying an arrangement of generally parallel seismic receiver cables at the seabed, the cables extending in a longitudinal direction, the distance between the outermost cables in a transverse direction generally perpendicular to the longitudinal direction being ? 2M; c) picking up receiver cables from a rear transverse end of the receiver cable arrangement; d) redeploying the picked-up cables at a forward transverse end of the receiver cable arrangement, whereby the receiver cable arrangement is indexed forwards in the transverse direction; the redeployed receiver cables at the forward end being arranged substantially parallel to the adjacent receiver cables; e) towing and firing a seismic source along firing lines generally parallel to the receiver cables using a source vessel while the receiver cables are being picked up and redeployed, the firing lines being spaced from the outermost cables in the receiver cable arrangement by a distance that is equal to or greater than M, measured in the transverse direction; and 0 detecting responses from the seismic source using the deployed receiver cables.

The method may include the additional steps: g) towing and firing a second seismic source along second firing lines generally parallel to the receiver cables using a second source vessel while the receiver cables are being picked up and redeployed, the firing lines of the second source being intermediate the firing lines of the first source; and h) detecting responses from the second seismic source array using the deployed receiver cables. Preferably, this procedure is repeated more than once. A plurality of source and/or receiver vessels are employed. Again, the vessels may be manned or unmanned.

Preferably, the receiver and source cables are in the range 3000 to 6000m in length e.g. 4000m, and are spaced apart by a distance in the range 50 to 200m, e.g. lOOm. Individual receivers and sources may be spaced apart along their cables by a spacing in the range 1.25 to 12.5m, preferably about 5m.

Preferably, individual source array cables are spaced apart by a distance in the range 25m to lOOm. Preferably, a single source array is towed by a single source vessel. Preferably, source vessels each carrying an individual source array are spaced apart by 200-600 meters in the longitudinal direction.

The firing intervals may be at regular positional spacings andlor may be at regular time intervals. Preferably, the shots are all synchronised with the receivers, for example, using relative or absolute time. Preferably, the distance between two adjacent shot points in the longitudinal direction is in the range 25 -100 meters.

Preferably, the maximum offset between a source and a receiver is in the range 2000m to 6000m. Preferably, the extent of the receiver area in the transverse direction is twice the maximum offset between a source and a receiver.

Preferably, the source array is towed a distance equal to the maximum offset beyond the receiver area in the transverse direction.

Preferably, time interval between the shots for an individual source array is in the range 1 to 60 seconds for an individual source preferably in the range 5 to 60 seconds. Preferably, the timing of all of the firing and all of the detection is synchronised, e.g. by using absolute time information collected from satellites.

Preferably, the sampling in the source and receiver domains is symmetrical.

Preferably, the entire area to be surveyed is effectively made up of a multiplicity of active receiver areas.

By means of this invention, higher quality P&S data can be acquired in a less costly way than the lower quality P-waves currently detected by surface seismic techniques.

Preferably, each receiver cable has a housing containing a battery, a clock a recording device and a control unit. Each receiver cable also preferably has a floatation device such as a buoy attached to it by a rope and arranged to float beneath the water surface when the cable is deployed. Conveniently, the floatation device is a buoy and it may be arranged to float about 20m beneath the surface. It may be allowed to float to the surface by allowing the rope connecting it to the cable to be extended or wound out by about 25m. Thus, preferably, when a receiver cable is to be picked up, a signal is sent to the control unit, and the control unit releases a further length of rope to allow the floatation device to float to the surface and thereby enable the cable to be picked up. Preferably, when a receiver cable is picked up, the battery is recharged, the clock is reset to satellite time and seismic data is downloaded from the recording device.

According to another aspect of the invention, there is provided a method of conducting a marine seismic survey of a survey area which comprises: deploying a plurality of receiver cables to define an active receiver area the cables extending in a transverse direction; towing a seismic source array behind a source vessel to track across the active receiver area at a first position; turning the source vessel and source array of source cables around and advancing to a second position different from the first position, then towing the source array to track across the receiver area at the second position; repeating the tracking, turning and advancing steps whereby the source vessel and source array track back and forth across the active receiver area at different positions; shooting the source array at regular intervals while it is tracking back and forth across the receiver area, and detecting the seismic responses using the deployed receiver cables; and synchronising the firing of all the source shooting and all the receiver direction.

Preferably, the timing of the firing and the detection is synchronised using relative or absolute time. Preferably, absolute time derived from satellite information is used. Preferably, the optimum position for any shot is defined as being within a suitable positional range. Preferably, the optimum time for any shot is defined as being within a suitable time window.

The invention also extends to apparatus for conducting a marine survey of a survey area which comprises: source cables; a source vessel arranged to tow the source cables; receiver cables; and a receiver vessel arranged to pick up and redeploy the receiver cables; and in which each receiver cables has a housing containing a battery, a clock a recording device and a control unit, and a floatation device such as a buoy attached to it by a rope and arranged to float beneath the water surface when the cable is deployed, the control unit being arranged, upon receipt of a signal, to release the floatation device to the surface while still maintaining its connection to the cable.

The invention also extends to a seismic survey report produced using the above method, optionally following any data processing, and to the processing of seismic data obtained using the above methods.

The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of the receiver cables defining an active receiver area; Figure 2 us a similar view showing source vessels tracking across the active receiver area; Figure 3 shows a source vessel and its source array in more detail; Figure 4 is a schematic representation showing the re-deployment of receiver cables; Figure 5 is a schematic diagram of the survey operational area; and Figure 6 us a schematic shooting plan.

As shown in Figure 1, receiver cables 11 are deployed to define an active receiver area 12. In the active receiver area, there are 70 receiver cables 11, each 6000m long and spaced in parallel, 25m apart. Individual receivers (not shown) are spaced along the cables 11 at 5m intervals.

A shown in Figures 2 and 3, a source vessel 13 tows two overlapping source arrays 14, each consisting of three source cables 15.

In performing a survey, t\vo source vessels 13 travel across the receiver area 12 in a direction parallel to the receiver cables 11, shooting continuously. The timing of the shooting for each source array is arranged so that each fires at a predetermined optimum position. The sources are synchronised with themselves and with the receivers. The two source vessels are spaced 400m apart and are staggered, as shown. The resulting "shooting point" grid is 25 x 5m. This is achieved by each source vessel following a path which is between previously completed paths.

When a source vessel 13 reaches the edge 17 of the active survey area 12, it turns through 1800 as shown by the arrow 18 and tracks back across the active survey area 12 at an advanced position. In this way, the entire active survey area 12 is covered.

While the source vessels 13 are tracking back and forth across the active survey area 12, the receiver cables 11 are redeployed, using from 2 to 4 receiver vessels 19. As shown in Figure 4, a receiver vessel 19 picks up a cable at the trailing end 21 of the active receiver area 12, and redeploys it is the leading end 22 of the active receiver area 12. This operation is illustrated by two receiver vessels 18 which are sequentially picking up and re-deploying two receiver cables ha and 1 lb. In this way, the active receiver area 12 is also advanced.

The survey operational area is shown schematically in Figure 5. At any one time during the survey, the recording area or active receiver area 51 is defined as the area over which active receivers are deployed. In addition, at the rearward end (bottom in Figure 5) of the active receiver area 51, there is an area designated the pick-up area 52 where there are receivers which are beyond the desired maximum source/receiver offset and are therefore ready to be picked up and re-deployed. They are re-deployed to the deploy area 53 at the forward end (top in Figure 5) of the active receiver area 51. At each end of the active receiver area 51 there is noise area 54.

The active receiving area 51 extends 6000m in the transverse direction and 6000m between the noise areas 54. The noise areas 54 are about 1200m in width in the end to end or lateral direction.

During the survey two source vessels are following the active lines 55 across the recording area. The vessels travel a distance of 3000m beyond the transverse edges of the active receiver area 51, passing through a further noise area 56 at each edge, again about 1200m in width (in the transverse direction).

The source vessels turn within the 3000m beyond the transverse edges.

The lines along which a source vessel has travelled and shot are designated shot lines 57 and the lines still to be "shot" are designated lines to be shot 58.

A typical shooting pattern is shown in Figure 6. The transverse (horizontal) rows 61 of dots indicate individual receivers on cables and the lateral (vertical) rows 62 of dots indicate optimum shooting positions. In some cases, some of which are indicated by squares 63 with included dots, a shooting position and receiver will coincide.

The two source vessels 64, 67 each have two respective source arrays, 65, 66 and 68, 69. The vessels 64, 67 are spaced apart by a distance of 6000m.

The shooting sequence would be array 65 first with the vessels 64,67 in their positions as shown in Figure 6 and with the array 65 in the position shown.

Then, as the vessels 64,67 track from right to left, the next source array to shoot would be array 68 when it reaches the position indicated 71. The next array to shoot would be array 69 when it reaches the position indicated at 72, then array 66 when it reaches position 73. The sequence would then be repeated.

Claims

Clai nis 1. A method of conducting a marine seismic survey of a survey area which comprises: deploying a plurality of receiver cables to define an active receiver area the cables extending in a longitudinal direction; towing a seismic source behind a source vessel to track across the active receiver area in the longitudinal direction along a first line, turning the source vessel and its source around and towing the source to track back across the receiver area in the reverse longitudinal direction along a second line, generally parallel to the first line; the second line being spaced from the first line in a transverse direction which is generally perpendicular to the longitudinal direction; repeating the tracking, turning and tracking back steps whereby the source vessel and its source track across the active receiver area along successively advanced lines; continuously firing the source at particular positions and/or time intervals while it is tracking the receiver area, and continuously detecting the seismic responses to the firing using the deployed receiver cables.
2. A method as claimed in Claim 1, in which receivers are picked up while the source is being fired.
3. A method as claimed in Claim 1 or Claim 2, in which the source is fired repeatedly during the entire time it is tracking across the receiver area in the longitudinal direction.
4. A method as claimed in any preceding Claim, which further comprises: towing a second seismic source behind a second source vessel to track across the active receiver area along lines which are intermediate the lines taken by the first vessel and its source; and continuously firing the second source at particular positions and/or time intervals while it is tracking across the receiver area.
5. A method as claimed in any preceding Claim, in which the seismic responses are continuously detected by the deployed receiver cables.
6. A method as claimed in any of Claims 1 to 3, in which detecting the responses is carried out in separate intervals, equal or slightly less than the time interval between two subsequent shots.
7. A method as claimed in any preceding claim, further including the step of defining a maximum offset M between a seismic source and a seismic receiver, and deploying the receiver cables so that the distance between the outermost cables in the active receiver area, in the transverse direction is ? 2M.
8. A method as claimed in Claim 7, in which the lines along which the source is towed are spaced from the outermost receiver cables in the active receiver area by a distance that is ? M, measured in the transverse direction.
9. A method of conducting a marine seismic survey, comprising: a) defining a maximum desired offset M between a seismic source and a seismic receiver; b) deploying an arrangement of generally parallel seismic receiver cables at the seabed, the cables extending in a longitudinal direction, the distance between the outermost cables in a transverse direction generally perpendicular to the longitudinal direction being �= 2M; c) picking up receiver cables from a rear transverse end of the receiver cable arrangement; d) redeploying the picked-up cables at a forward transverse end of the receiver cable arrangement, whereby the receiver cable arrangement is indexed forwards in the transverse direction; the redeployed receiver cables at the forward end being arranged substantially parallel to the adjacent receiver cables; e) towing and firing a seismic source along firing lines generally parallel to the receiver cables using a source vessel while the receiver cables are being picked up and redeployed, the firing lines being spaced from the outermost cables in the receiver cable arrangement by a distance that is equal to or greater than M, measured in the transverse direction; and 0 detecting responses from the seismic source using the deployed receiver cables.
10. A method as claimed in Claim 9, including the additional steps: g) towing and firing a second seismic source along second firing lines generally parallel to the receiver cables using a second source vessel while the receiver cables are being picked up and redeployed, the firing lines of the second source being intermediate the firing lines of the first source; and h) detecting responses from the second seismic source array using the deployed receiver cables.
11. A method as claimed in any preceding claim, in which the survey procedure is repeated more than once.
12. A method as claimed in any preceding claim, in which a plurality of source and/or receiver vessels are employed.
13. A method as claimed in any preceding claim, in which the source and/or receiver vessels are unmanned.
14. A method as claimed in any preceding claim, in which the receiver cables are in the range 3000m to 6000m in length.
15. A method as claimed in any preceding claim, in which the receiver cables are spaced apart by a distance in the range 50m to 200m.
16. A method as claimed in any preceding claim, in which individual receivers are spaced apart along the receiver cables by a spacing in the range 1.25mto 12.5m.
17. A method as claimed in any preceding claim, in which a seismic source comprises individual source array cables which are spaced apart by a distance in the range 25m to lOOm.
18. A method as claimed in any of Claims ito 16, in which a seismic source comprises a single source array towed by a single source vessel.
19. A method as claimed in any preceding claim, in which a plurality of source vessels each carrying an individual source are spaced apart by 200 to 600 meters in the longitudinal direction.
20. A method as claimed in any preceding claim, in which the distance between two adjacent firing points in the longitudinal direction is in the range to 100 meters.
21. A method as claimed in any preceding claim, in which the maximum offset between a source and a receiver is in the range 2000m to 6000m.
22. A method as claimed in any preceding claim, in which a source is towed a distance equal to the maximum offset beyond the receiver area in the longitudinal direction.
23. A method as claimed in any preceding claim, in which the time interval between the shots for an individual source is in the range 1 to 60 seconds.
24. A method as claimed in any preceding claim, in which timing of all of the firing and all of the detection is synchronised.
25. A method as claimed in Claim 24, in which the synchronisation is achieved using a satellite timing system.
26. A method as claimed in any preceding claim, in which the sampling intervals in the source and receiver domains is symmetrical.
27. A method as claimed in any preceding claim, in which the entire area to be surveyed is made up of a multiplicity of active receiver areas.
28. A method as claimed in any preceding claim, in which each receiver cable has a housing containing a battery, a clock, a recording device and a control unit.
29. A method as claimed in Claim 28, in which each receiver cable has a floatation device such as a buoy attached to it by a rope and arranged to float beneath the water surface when the cable is deployed.
30. A method as claimed in Claim 29, in which, when a receiver cable is to be picked up, a signal is sent to the control unit, the control unit releases a further length of rope to allow the floatation device to float to the surface and thereby enable the cable to be picked up.
31. A method as claimed in Claim 30, in which, when a receiver cable is picked up, the battery is recharged, the clock is reset to satellite time and seismic data is downloaded from the recording device.
32. A method of conducting a marine seismic survey of a survey area which comprises: deploying a plurality of receiver cables to define an active receiver area, the cables extending in a longitudinal direction; towing a seismic source behind a source vessel to track across the active receiver area at a first position; turning the source vessel and source around and advancing to a second position different from the first position, then towing the source to track across the receiver area at the second position; repeating the tracking, turning and advancing steps whereby the source vessel and source track back and forth across the active receiver area at different positions; firing the source at regular intervals while it is tracking back and forth across the receiver area, and detecting the seismic responses using the deployed receiver cables; and synchronising the firing of all the source firing and all the receiver detection.
33. A method as claimed in Claim 32, in which the timing of the firing and the detection is synchronised using relative or absolute time.
34. A method as claimed in Claim 31, in which absolute time derived from satellite information is used.
35. A method as claimed in any of Claims 32 to 34, in which the optimum position for any shot is defined as being within a suitable positional range.
36. A method as claimed in any of Claims 32 to 35, in which the optimum time for any shot is defined as being within a suitable time window.
37. Apparatus for conducting a marine survey of a survey area which comprises: source cables; a source vessel arranged to tow the source cables; receiver cables; and a vessel arranged to pick up and redeploy the receiver cables; and in which each receiver cable has a housing containing a battery, a clock, a recording device and a control unit, and a floatation device such as a buoy attached to it by a rope and arranged to float beneath the water surface when the cable is deployed, the control unit being arranged, upon receipt of a signal, to release the floatation device to the surface while still maintaining its connection to the cable.