GB2431237A - Marine seismic data acquisition with short streamers - Google Patents

Abstract

Marine seismic data is acquired using a shooting vessel 24 having one or more seismic sources 25 followed by a towing vessel 21 having a plurality of streamers 23. The two vessels preferably follow the same path and the offset between the source and the nearest receiver on the streamers may be up to 2000 metres. The streamers 23 are significantly shorter than conventional streamers, preferably less than 2000 metres in length. The towing vessel may also tow additional seismic sources 22. The distance between the sources 25 and the streamers 23 compensates for the long offsets that are lost by shortening the streamers. Using shorter streamers reduces cross line errors due to feathering and allows for more repeatable positioning of the receivers in a 4D acquisition.

Description

Marine Seismic Data Acquisition The present invention is concerned with

acquiring 4D marine seismic data.

Acquiring 4D marine seismic data can be very difficult, partly due to poor weather, partly due to technical problems. Surveys with a large number of streamers, short cable distance and relatively long streamers have been operationally challenging - especially in bad weather. There have been incidents related to equipment breakage followed by streamer tangling.

Another problem is the amount of non productive time related to deployment and recovery of equipment. Deployment time and recovery time could alternatively have been data acquisition time. Long streamers give longer line changes.

Since 4D is very time critical, it has to be acquired during as short a time period as possible.

Another problem with existing technology is "feather"- i.e. cross currents moving the streamers sideways with a certain angle. Steering the vessel, source and streamer fronts give the best repeatability in the front of the spread, and the crossline deviation from the preplot (shooting plan) will increase backwards the streamers. For 5000 meters streamers and a feather of 5 degrees, the far trace will be crossline dislocated by 175 meters, unless this is compensated.

Steerable cables are one compensating technology but these are costly and difficult to manage.

According to the present invention, a method of acquiring 4D marine seismic data involves the use of shorter streamers and an additional shooting vessel in order to obtain better repeatability for the seismic receiver groups. Using 50% of the normally required cable length would provide full offset with better repeatability. Operational advantages are also obtained.

Thus, by using half the normally required streamer length (related to the target), operational and repeatability advantages are obtained, while the use of an extra shooting vessel enables the required offset to be obtained for data analysis.

More particularly, this invention solves the problems associated with existing technology as follows: Operational: * Quicker deployment and recovery of towed equipment (streamers) saving operational time * Significantly shorter line-changes obtainable due to shorter run-in, shorter run-out and faster vessel turn-around * Reduced risk for streamer tangling * More robust spread when it comes to operation * More operation time per elapsed time is positive when it comes to 4D acquisition - we want a snapshot of the reservoir not an image acquired over long time.

Positioning repeatability: The industry has tried to obtain better positioning repeatability, i.e. to locate the sources and streamers as close to the base survey as possible, when acquiring a monitor survey.

This has been attempted by utilizing steerable sources, steerable streamers, SOS source (Self Overlapping Source), etc. Employing the short streamer/additional source vessel method will contribute to increasing the seismic receiver groups repeatability because shorter streamers allows for better positioning of the receiver groups related to preplot (shooting plan). This due to less feather.

It also allows for denser streamer towing and thereby denser crossline sampling.

The sources will be just as easy or easier to position relative to the preplot, compared with conventional methods. The source towed by the additional source vessel will be significantly easier to position, due to a shorter layback than normal.

This invention can be utilized both for conventional streamers and steerable streamers.

Further advantages and improvements achieved by this invention include: This is in general a more robust 4D acquisition method. Operationally, there will be more shooting time - shorter deployment and recovery and shorter line changes.

The method will also meet new requirements of 4D acquisition, in that the dataset is acquired during as short a period as possible. This in order to avoid too large changes in the reservoir during the acquisition period, either from production or intervention.

The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows schematically a prior art towing arrangement with no feather; Figure 2 is a view similar to Figure I, with feather; Figure 3 shows schematically a towing arrangement according to the invention; Figure 4 is an illustration of increased receiver positioning repeatability; Figure 5 is a view similar to Figure 3 with additional streamers; and Figure 6 is a view similar to Figure 4 with the arrangement of Figure 5.

This invention is a method for 4D seismic data acquisition where approx. 50% of the normally required streamer length is utilized. The method involves at least two vessels and can in its simplest variant be used with one dedicated source vessel and one vessel towing both source and streamers.

Figure 1 shows a towing vessel 11 in ideal conditions, with a series of sources 12 and a series often long streamers 13, each 4000m long.

Figure 2 shows the situation in practice where there is a 50 feather, caused by a cross current 14. This gives rise to large cross-line errors at mid and far offsets.

Figure 3 shows a system according to the invention, in which a towing vessel 21 tows a series of sources 22 and a series often short streamers 23, each 2000m long. In addition, there is a source vessel 24 located about 2000m in front of the towing vessel 21, which tows a further series of sources 25.

The two series 22,25 sources are alternating; however, for a number of target depths it should be possible to record shots for both sources on one record; this is in order to avoid losing fold as otherwise would occur in a normal flip flop mode.

As can be seen, the feather angle is the same as in Figure 2 (i.e. 5 ) but the crossline deviation is reduced by 50% for the last receiver groups. However, the additional sources 25 provide the long offsets that are lost by shortening the streamers 23.

Figure 4 shows that the additional sources 25 will effectively provide observations as if the streamers 23 were arranged as shown, with a "reset" as shown at 26.

Figure 5 shows a short streamer dual vessel high density operation with one seismic vessel 31 towing one series of source 32 and 20 streamers 33 each 2000m long. It is the shorter streamers which operationally allow for denser towing. There is then an additional source vessel towing one source approx 2000m in front of the streamer fronts. The two sources are alternating; however, for a number of target depths it should be possible to record shots for both sources on one record, this is in order to avoid loosing fold as in a normal flip flop mode.

Claims (12)

1. Claims 1. A method of acquiring 4D marine seismic data which comprises:

   deploying a shooting vessel having a seismic source along a predetermined path; deploying behind the shooting vessel a towing vessel along a predetermined path; the shooting vessel having a plurality of streamers, each streamer carrying a series of seismic receivers; the streamers being significantly shorter than streamers conventionally employed; generating a seismic event at predetermined firing positions using the shooting vessel; monitoring the response to the seismic event using the receivers; then repeating the process at a later date along the same paths, and at the same firing positions.
2. 2. A method as claimed in Claim 1, in which the streamers are up to 50% of the length of conventional streamers. ..: *
3. 3. A method as claimed in any preceding Claim, in which the two vessels *. S.

   are steered along the same predetermined path. S.. * S... S...
4. 4. A method as claimed in any preceding Claim, in which the towing.. : vessel also carries a seismic source and the method includes generating seismic events with this source also.
5. 5. A method as claimed in any preceding Claim, in which both vessels tow a plurality of seismic sources, and the method includes generating seismic events with some or all of the sources.
6. 6. A method as claimed in any preceding Claim, in which the streamers are up to 2000m in length. a
7. 7. A method as claimed in any preceding Claim, in which the two vessels are deployed so that the offset between the source on the shooting vessel and the nearest receiver on the streamers is up to 2000m.
8. 8. Apparatus for acquiring 4D marine seismic data which comprises a shooting vessel and a towing vessel; the shooting vessel having at least one seismic source; the towing vessel having a plurality of streamers; the streamers each carrying a series of seismic receivers; the receivers being significantly shorter than streamers conventionally employed.
9. 9. Apparatus as claimed in Claim 8, in which the streamers are up to 50% of the length of conventional streamers.
10. 10. Apparatus as claimed in Claim 8 or Claim 9, in which the towing vessel * also carries a seismic source. #
11. 11. Apparatus as claimed in Claim 10 in which both vessels two a plurality of seismic sources. ::::
12. 12. Apparatus as claimed in any of Claims 8 to 11, in which the streamers are up to 2000m in length.