SEISMIC NODE, METHOD AND USE THEREOF FOR OCEAN BOTTOM SEISMIC SURVEYING

Abstract

A seismic node (1) for an ocean bottom seismic survey comprising: At least one seismic sensor capsule (2), a seafloor casing (6) comprising a lower surface configured to make contact with a seabed. The seismic sensor capsule (2) comprises first engagement means; the seafloor casing (6) comprises second engagement means (10). The first and second (10) engagement means are adapted to releasable engage with each other whereby the seismic sensor capsule (2) is releasably fastened to the seafloor casing (6). The seismic sensor capsule (2) is adapted to be removed from the seafloor casing (6) after a certain time T. The seafloor casing (6) is configured to be left permanently on the seabed.

Claims

1-13. (canceled)

14. A seismic node for an ocean bottom seismic survey, the seismic node comprising: at least one seismic sensor capsule comprising a first capsule surface and an opposite second capsule surface; and a seafloor casing comprising an upper surface and an opposite lower surface configured to make contact with a seabed, wherein the seismic sensor capsule comprises first engagement means, and that the seafloor casing comprises second engagement means, said first and second engagement means are adapted to fit with each other whereby the seismic sensor capsule is releasably fastened to the seafloor casing, said seismic sensor capsule is adapted to be removed from the seafloor casing after a certain time T and for being transported by a vehicle to a surface of the ocean, while the seafloor casing is configured to be left permanently on the seabed.

15. The seismic node according to claim 14, wherein the seismic sensor capsule is a water-tight pressure housing containing a seismic sensor pack and accessories such as electronics, seismic sensors, batteries, control units, memory cards.

16. The seismic node according to claim 14, wherein the seafloor casing comprises seafloor-casing friction-means adapted to provide a friction force between the seabed and the seafloor casing said friction force is larger than the hydrodynamic forces provided by the ocean current at the seabed.

17. The seismic node according to claim 14, wherein the seismic sensor capsule is adapted to be calibrated before deployment on the seabed, and said seismic sensor capsule is advantageously adapted to be calibrated after having been removed from the seafloor casing after the time T has passed.3

18. The seismic node according to claim 14, that the seafloor casing left at the seabed after the seismic sensor capsule has been removed is adapted to engage with a new seismic sensor capsule, said new seismic sensor capsule is the same as has been removed but is advantageously in a calibrated state, or the seismic sensor capsule is different from the removed seismic sensor capsule and advantageously in a calibrated state.

19. The seismic node according to claim 14, wherein the seafloor casing comprises passive acoustic reflectors or similar means.

20. The seismic node according to claim 14, wherein the first and second engagement means comprises a tight fit between the seismic sensor capsule and the seafloor casing.

21. A method for performing an ocean bottom seismic survey the method comprising: placing a seismic node comprising a seafloor casing and at least one seismic sensor capsule on a seafloor, wherein the seismic sensor capsule(s) is/are releasably attached to the seafloor casing, and that the seismic sensor capsule(s) is/are removed from the seafloor casing by a vehicle, after a certain time T has passed, said seismic sensor capsule(s) is/are transported to a surface vessel where data registered by the seismic sensor capsule(s) are extracted from the seismic sensor capsule(s), or the data are extracted while the seismic sensor capsule(s) is/are still in the ocean, while the seafloor casing is left permanently at the same place of the seabed, said seafloor casing is a stationary and immovable unit.

22. The method according to claim 21, wherein a seismic sensor capsule is calibrated and that the calibrated seismic sensor capsule is transported by the vehicle to any stationary and immovable seafloor casing placed at the seabed for performing the seismic survey.

23. The method according to claim 21, wherein at least one seismic sensor capsule is installed in the seafloor casing left at the seabed, said seismic sensor capsule(s) is/are recording passive data until a next planned survey is performed.

24. The method according to claim 21, wherein the vehicle is carrying at least one dummy seismic sensor capsule, which is installed in the seafloor casing after the seismic sensor capsule(s) has/have been removed.

25. The method according to claim 21, wherein the vehicle is a Remotely Operated Vehicle (ROV) or an Autonomous Underwater Vehicle (AUV).

26. Use of the seismic node comprising: at least one seismic sensor capsule comprising a first capsule surface and an opposite second capsule surface; and a seafloor casing comprising an upper surface and an opposite lower surface configured to make contact with a seabed, wherein the seismic sensor capsule comprises first engagement means, and that the seafloor casing comprises second engagement means, said first and second engagement means are adapted to fit with each other whereby the seismic sensor capsule is releasably fastened to the seafloor casing, said seismic sensor capsule is adapted to be removed from the seafloor casing after a certain time T and for being transported by a vehicle to a surface of the ocean, while the seafloor casing is configured to be left permanently on the seabed, for performing the method according to claim 21.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] FIG. 1A is a perspective view of a seismic node according to the invention, comprising a sensor capsule and a seafloor casing.

[0051] FIG. 1B is a perspective view of a seafloor casing according to the invention.

[0052] FIG. 1C is a view of the seafloor casing shown in FIG. 1B along the longitudinal side of the seafloor casing.

[0053] FIG. 1D is a view of the seafloor casing shown in FIG. 1B along the short side of the seafloor casing.

[0054] FIG. 1E is a view of the seafloor casing shown in FIG. 1B shown from the bottom side and disclosing seafloor-casing friction-means.

[0055] The invention will be explained with reference to FIG. 1A-E, FIG. 1A showing a perspective view of a seismic node 1 used for an ocean bottom seismic survey. It comprises a seafloor casing 6 and a seismic sensor capsule 2. The seismic sensor capsule 2 is attached to the upper surface 7 of the seafloor casing 6. The sensor capsule 2 comprises a first capsule surface 4 comprising features exposed to the water when the device 1 is placed at a seabed during a survey. The sensor capsule 2 is a watertight pressure housing 14 inside which different components are placed and protected by the housing 14. The components may be electronics, seismic sensors, batteries, memory card etc. and may include one or more hydrophones, one or more geophones or accelerometers, and a data recorder.

[0056] The sensor capsule 2 is attached to the seafloor casing 6; the seafloor casing 6 is shown in detail in FIG. 1B-1E.

[0057] The seafloor casing 6 comprises the upper surface 7 and an opposite lower surface 8. The lower surface 8 is configured to make contact with the seabed. The upper surface 7 comprises in this embodiment a cavity. The cavity is shown with dotted lines 18 in FIG. 1C and FIG. 1D. The cavity forms second engagement means 10 engaging with first engagement means of the seismic sensor capsule. The cavity simply encloses the bottom capsule surface—the surface turning towards the seafloor casing 6—and a portion of sidewalls 19 of the seismic sensor capsule 2. The bottom capsule surface and the portion of the sidewalls 19 form first engagement means.

[0058] The first and second 10 engagement means is thereby working by press fit. The seafloor casing 6 in this case has a region of the upper surface 7 formed as the cavity/recess which is shaped like the sensor capsule 2, so it is well coupled to the seafloor casing 6.

[0059] The first and second 10 engagement means could also comprise mechanical means such as projections in one part engaging recesses in the other part.

[0060] The outside of the seismic sensor capsule might also comprise recesses or projections such that a vehicle—an ROV or an AUV—is able to easily grab the seismic sensor capsule 2 when it has to be removed from the seafloor casing 6.

[0061] The lower surface 8 is configured to make contact with the seabed and comprises seafloor-casing friction-means 15 in order to optimize the contact between the seafloor casing 6 and the seabed in such a way that the seafloor casing 6 does not move during its stay on the seabed.

[0062] The seafloor-casing friction-means 15 is in this embodiment formed as a circumferential edge/long ridges extending from the bottom 8 of the seafloor casing 6. The circumferential edge has through-going openings 20 placed in each corner of the bottom 8 of the seafloor casing 6. When placing the seafloor casing 6 on the seabed, the water is not trapped by the circumferential edge. The seafloor-casing friction means 15 could be constructed in other ways such as small half-spheres.

[0063] When a survey has been conducted and the seismic sensor capsule is to be removed from the seafloor casing an ROV or an AUV is directed to the seismic node. The ROV or AUV must be able to carry the sensor capsule and have tools to deploy and recover them. The seafloor casing is equipped with devices that makes it possible for the vehicle to detect its position. These devices are for instance passive acoustic reflectors that reflects acoustics waves send from the vehicle.

[0064] The passive acoustic reflectors are advantageously placed on or adjacent to the seafloor casing 6, so the vehicle is able to detect the seafloor casing 6 when a new survey has to take place, and a seismic sensor capsule 2 therefor must be attached to the seafloor casing 6. An ROV pilot may also use the ROV's navigation system and cameras to navigate to the position of the seafloor casing. However, when the ROV is equipped with an echosounder, it is possible to locate the positioning device by the passive acoustic reflector if the visibility is poor.

[0065] The sensor capsules 2 are recovered from the seafloor casing 6 by the ROV or AUV and brought to a surface vessel. There, the sensor capsules 2 are handled in the same way as cable-based sensor capsules: the control unit, which contains the memory card, is removed from the sensor capsule and mated in a docking cabinet where it connects to the central data network and the data is downloaded.