NO338052B1 - Procedure for seismic survey using autonomous nodes - Google Patents

Description

The invention relates to a method for seismic survey using autonomous nodes on a seabed. The invention also applies to a node setter for underwater towing over a seabed, for deploying ropes with seismic nodes on the seabed.

A marine seismic survey involves firing an array of acoustic sources, such as air guns, at a known position in a so-called shot. Acoustic waves from the air cannons are propagated through the water and down into an underground formation, where they are reflected and refracted in the different formation layers. The reflected and refracted waves are captured and recorded for later analysis to provide information about the structure or formation of the seabed.

To capture as much as possible from the reflected and refracted waves, seismic sensors such as geophones must be placed in close acoustic contact with the seabed, and hydrophones placed directly above the seabed. The seismic sensors are normally placed in seismic nodes, where each node contains one or more sensors.

To carry out such a seismic survey, a matrix of seismic nodes is placed on the seabed. The matrix can be arranged as a rectangular grid with a seismic node at each intersection point. The normal distance between adjacent nodes along a rope is between 25 and 400 meters and there is between 100 and 800 meters between the ropes. After the nodes are located, a source vessel, towing acoustic sources, fires a series of shots at known positions. The seismic response signals from each shot are captured by each node in the array. Each node can perform some signal processing. When the desired shots have been completed, the nodes are picked up and stored for deployment at the next survey.

The nodes can be placed or placed one by one, e.g. by a mini-submarine (ROV), or they can be connected using an undersea cable (OBC), normally at distances of 12.5, 25 or 50 metres. An OBC simplifies retrieval, as individual, small nodes can otherwise be difficult to find and pick up. Several OBCs can be placed parallel to each other so that the nodes form the desired matrix on the seabed.

An OBC can also provide communication lines so that each node can transmit seismic data to a survey vessel in real time. Examples can be found, for example, in US patent numbers 4,942,557 and 4,780,863. A disadvantage of these methods and devices is that the communication line must pass through a chamber from the sensor and controller in the node to a transmitter or receiver on the survey vessel. As the installation depth increases, either the risk of leakage at the penetrations for the communication line or the cost of sealing the chamber increases. Another disadvantage is that cable laying from the seabed to the surface can cause movement, vibration and other noise that interferes with the measurements taken by the seismic sensors inside the nodes.

To overcome these drawbacks, each node can be autonomous, ie remain cut off from the survey vessel for as long as the survey is in progress. An autonomous seismic node contains seismic sensors, a recorder, a memory to store seismic signals and a power source, e.g. a battery pack, to supply power to the node during the survey. After collection, seismic data is transferred to the survey vessel for later analysis.

US patent application 2013/0058192 to Gateman et al, assigned to the applicant of the present application, describes a seabed seismic cable with autonomous node capsules which are inserted into corresponding covers mounted along the OBC cables. In this application, the OBC(s) are deployed from a vessel, and are thus subjected to vertical forces caused by multiple nodes, each having negative buoyancy, suspended from the vessel. As the nodes are spaced out at regular intervals along the OBC, the vertical forces increase with depth to the seabed. In addition, an OBC suspended from the vessel is exposed to horizontal forces from underwater currents. As the depth increases, these forces will probably cause deviations from the desired route on the seabed.

However, in order to estimate the depth of a reflective layer from a time delay between a shot and a response, the distance between the source and the node must be known. Therefore, the node positions must be accurate to give a correct image of the formation. Furthermore, fixed intervals between neighboring nodes will minimize errors when seismic data is interpolated, because an interpolation value between two adjacent nodes that are far apart is less accurate than an interpolation value between two adjacent nodes that are close together. Consequently, nodes should be deployed at regular intervals at known positions to achieve the best possible survey with a predetermined number of nodes. The ability to place the seismic nodes with high accuracy on the seabed is also very important for seismic surveys that are repeated over time in the same survey area, e.g. 4D surveys.

A wire connects the nodes together and determines the distance between the nodes, but this does not ensure that the nodes are placed along a predetermined route, e.g. along a straight line instead of a zigzag pattern. As mentioned, horizontal forces caused by underwater currents make it difficult to control deployment by maneuvering the OBC from the surface.

A similar problem of maneuvering a cable from the surface is encountered in seabed surveys using camera, sidescan radar, etc. NO 326789 B1 shows a submersible survey platform towed by a vessel and maneuvered to maintain a predetermined height above the seabed and a predetermined lateral position with consideration of the intended cable route. The vertical movement is performed by a winch connected to the tow rope, and the horizontal movement necessary to adjust the course of the platform is performed by side propellers.

EP 2,712,802 A1 describes a cable laying system for an OBC. A frame is lowered from a vessel on the surface and contains a coiled cable that is laid out on the seabed.

An object of the invention is to provide an improved method for seismic survey and an improved device for deploying seismic nodes on a seabed while maintaining the advantages of prior art. Further objectives and advantages of the invention appear in the description. The goals are achieved by features in the description and requirements.

The invention thus relates to a method for seismic investigation using autonomous seismic nodes on a seabed. According to the invention, the following steps are performed: - attaching seismic nodes to a rope; - loading the rope with seismic nodes into a node setter; - to lower the node-layer into the sea; - to tow the node setter across the seabed; - laying out the rope with seismic nodes on the seabed; - to collect seismic data using seismic nodes;

- to retrieve the rope with seismic nodes from the seabed, and

- to transmit seismic data from the seismic nodes.

The system is characterized by the fact that loading the rope involves wrapping the rope around parallel rods in a zigzag pattern in a magazine; and to remove the rods and load the magazine into the node setter.

Thereby, a method is achieved for seismic survey with autonomous nodes on a seabed where the above-mentioned problems related to laying out with an OBC from a surface vessel are reduced or removed. The invention is thus particularly suitable for great depths.

According to a preferred embodiment, seismic nodes are attached to the rope at a predetermined distance. This can be done on a vessel. The same vessel can be used to lower the node setter into the sea.

The recovery of the rope with seismic nodes can be done by connecting a recovery line to the rope, and towing the rope on board the vessel using the recovery line. To facilitate this, one or more floating coupling devices can be attached to the rope in advance for connecting the recovery line. The connection can be made using an ROV. In this way, a rational retrieval of the rope is achieved.

After retrieving the rope with seismic nodes from the seabed, preferably to a vessel, the seismic nodes can be separated from the rope, and seismic data transmitted from the seismic nodes.

The invention also applies to a node layer for underwater towing over the seabed to lay out a rope with seismic nodes on the seabed, including a coupling unit for a tow rope, and a storage for the rope with seismic nodes.

According to the invention, the warehouse is a magazine with removable rods to wrap the rope with seismic nodes into the magazine when it is outside the node setter, then to remove the rods and to load the magazine into the node setter. In this way, a rational handling of the rope with seismic nodes is achieved.

Preferably, the magazine is adapted for loose packing of the rope, essentially without a support structure to minimize the packing volume.

The knotter can be adapted to allow the rope to run freely during the laying of the rope. Alternatively, the knotter can be adapted to feed out the rope in a controlled manner during the laying of the rope. The nosediver may include one or more deflectors to control lateral movement, and one or more trim planes to control vertical movement during towing. This allows the rope to be fed out essentially along a line, at a predetermined position, thereby deploying seismic nodes at predetermined positions without dragging seismic nodes along the seabed, and also maintaining the distance between the seismic nodes.

With the invention's method and node setter, seismic nodes can be placed on the seabed with greater accuracy than in known technology, which is important for a correct seismic analysis.

The node logger may include instrumentation to detect obstacles in its path, e.g. a light source and a camera. The node setter can also include a control loop and necessary data services and a control system that guides it away from the obstacles.

Two or more node layers can be towed by the same vessel to simultaneously lay two or more ropes with seismic nodes, for a time- and cost-effective laying of seismic nodes.

The invention will now be explained with reference to the attached drawings, where:

Figure 1 illustrates the towing of a node setter over a seabed, for the deployment of ropes with seismic nodes on the seabed from the node setter, according to the invention; Figure 2 illustrates the connection, of a retrieval line, to the rope with seismic nodes by means of an ROV, according to the invention; Figure 3 illustrates retrieval of the rope with seismic nodes from the seabed, i according to the invention; Figure 4 illustrates packing the rope with seismic nodes for placement in a magazine, according to the invention; Figure 5 illustrates a magazine for the rope with seismic nodes, according to the invention; Figure 6 illustrates towing the node setter over the seabed, for deployment of the rope with seismic nodes on the seabed, in a situation where an obstacle lies in front of the node setter; and Figure 7 illustrates the towing of three node-layers over the seabed, for the deployment of

three ropes with seismic nodes on the seabed, according to the invention.

Figure 1 illustrates a vessel 6 sailing on a sea surface 8 in a direction 18. The vessel 6 tows a node setter 4 in the sea 5 using a tow rope 11. The node setter 4 is towed over the seabed 2, normally at a low height, approximately a couple of meters . A rope 3 with seismic nodes 1 is fed out from the node setter 4, on the seabed 2. The size of the seismic nodes 1 is exaggerated for the sake of the illustration. The distance between the sea surface 8 and the sea bed 2, i.e. the depth, can be 1000 m or more.

According to the invention, seismic nodes 1 are attached to the rope 3 before the situation illustrated in figure 1. Seismic nodes can be attached to the rope on board the vessel 6, or before the seismic nodes 1 and the rope 3 are brought on board the vessel 6. For the purpose of mounting seismic nodes 1 on the rope 3, each seismic node 1 can have a clamp. The clamp can be opened manually or automatically, then the rope 3 can be passed through the clamp and the clamp closed, thereby attaching the seismic node 1 to the rope 3. The seismic nodes 1 can be attached to the rope at a predetermined location or at a predetermined distance, normally 25 - 400 m.

The rope 3 with the seismic nodes 1 is then loaded into the node setter 4, and the node setter 4 is lowered into the sea 5 to the position shown in figure 1, which can be done by an active heave compensated winch. The rope 3 with the seismic nodes 1 is deployed from the node setter 4, as illustrated in Figure 1.

After the deployment of the rope 3 with seismic nodes 1, the node setter 4 can be removed. A source of seismic signal, e.g. an air cannon, can be towed in the sea 5 by a surface vessel 6, and fired for the collection of seismic data by seismic nodes. This is not illustrated. A source of seismic signal, e.g. an air cannon, can be towed in the sea 5 by a surface vessel 6. The seismic nodes 1 are autonomous, i.e. there is no communication to or from the seismic nodes 1 while seismic data is collected. For the collection of seismic data, seismic nodes 1 will normally contain different types of sensors, e.g. hydrophones, geophones; a power source, e.g. a battery; a watch; an electronic processor for an initial processing of seismic data and control of the seismic node; and a memory for storing seismic data.

Figure 2 illustrates the situation after the collection of seismic data is finished, before the retrieval of the rope 3 with seismic nodes 1. A retrieval line 9 is connected to a floating coupling device 10, which in this embodiment is a floating ring 10. The ring 10 is mounted to the rope 3 with a coupling rope 19. The floating coupling device 10 can be mounted on the rope 3 before laying out the rope 3 or after laying out the rope 3. Alternatively, the recovery line 9 can be mounted directly on the rope 3, i.e. the floating coupling device 10 and the coupling rope 19 can be dispensed with. In the illustrated embodiment, the connection of the retrieval line 9 is performed by an ROV 13. Alternatively, the ROV 13 can grab the rope 3 or a seismic node 1 directly to retrieve the rope 3 from the seabed 2.

Figure 2 illustrates two separate floating coupling devices 10 mounted to the rope 3, thereby enabling connection between the recovery line 9 and the rope 3 in two places.

In Figure 3, the rope 3 with seismic nodes 1 is retrieved from the seabed 2, by

the rope 3 onto a vessel 6 using the recovery line 9. The vessel 6 lies on the sea surface 8, sailing in direction 18. The recovery line 9 and the rope 3 with seismic nodes 1 are normally hauled up onto the vessel 6 with a winch.

After picking up the rope 3 with seismic nodes 1, seismic data is transferred from the seismic nodes 1. This can be done after the seismic nodes 1 are separated from the rope 3. Transmission of seismic data can be done manually or automatically by connecting seismic nodes 3 to a computer system on board the vessel 6 or elsewhere, and then transmit seismic data.

The knotter 4 is a construction made of steel, aluminum or composite, which contains a coupling unit for the tow rope 11. The coupling unit can be a lifting lug that enables tension transfer between the tow rope 11 and the knotter 4. The knotter 4 can also have instrumentation for measuring depth, position, pressure and flow and monitoring of the node setter 4 and its surroundings; it may have a processor with a control system, and a connection to a means of communication with the towing vessel 6. The means of communication with the towing vessel 6 may be an umbilical cord with optical or electrical signal cables for the transmission of information. The umbilical cord may also contain hydraulic pipes or electrical cables for power supply. Alternatively, the node adder 4 can have its own power supply, e.g. batteries. The tow rope 11 can be an integral part of the umbilical cord.

The node setter 4 also has a storage for the rope 3 with seismic nodes 1. According to the invention, the storage is a magazine 7, to pack the rope 3 with seismic nodes 1 into the magazine 7 when it is outside the node setter 4, and load the magazine 7 into the node layer 4. Figure 4 illustrates the principle of packing the rope 3 with seismic nodes 1. The rope 3 is wrapped around parallel bars 14 in a zigzag pattern. After winding, the rope 3 and the rods 14 are separated. This gives a loosely packed rope 3 without a space-consuming support structure. The package volume is thus minimized. Figure 5 illustrates a magazine 7 for the rope 3, including end walls 15, side walls 16 and a non-illustrated bottom, which forms an inner space 17 for the rope 3. The rope 3 can be held together by rubber bands or the like to keep it together during handling and stacking in magazine 7.

In one design, the bottom of the magazine 7 has holes for the rods 14. Before the rope 3 is wrapped, the rods 14 are placed in the holes, thereby inserting into the magazine 7. The rope 3 is wrapped around the rods 14 in a zigzag pattern, and after the winding is finished the rods 14 are pulled out of the magazine 7 through the holes in the bottom, leaving the rope 3 loosely packed in the magazine 7. Alternatively, the end walls 15 and side walls 16 of the magazine 7 can be removed during the winding of the rope 3, and put back after the winding is finished.

The knotter 4 can contain a frame for the magazine 7. The magazine 7 can be self-supporting or be supported by the surrounding structure when it is inside the knotter 4.

The loose packing of the rope 3 in a zigzag pattern means that the rope 3 can run freely out of the magazine 7, in the node layer 4, during the laying of the rope 3 with seismic nodes 1 on the seabed 2. Alternatively, the rope 3 can be fed out in a controlled manner during laying . A controlled discharge can be achieved by guiding the rope 3 with seismic nodes 1 between two rollers that accommodate the thickness variation of the rope 3 and the seismic nodes 1, and controlling the speed of the rollers according to the towing speed of the node setter 4. The rollers can be made of soft rubber, and possibly having coincident patterned as transverse ribs, to enable both the rope 3 and the seismic nodes 1 to be gripped.

Figure 6 illustrates towing the node setter 4 over the seabed 2 in a situation where an obstacle 12 lies in front of the node setter 4. The node setter 4 preferably has instrumentation to detect obstacles 12 that lie in the way. Such instrumentation can be one or more light sources and a camera, or sonar. Preferably, the node setter 4 also has a regulation loop which guides it away from the obstacles 12 in its path. The route setter 4 can thus change his route to a new route, and thus avoid the obstacle 12.

In order to achieve lateral movement of the knotter 4, the knotter 4 preferably has one or more deflectors to control lateral movement during towing. The nodedegger 4 can also have propellers for the same purpose. Vertical steering can be achieved by hauling up and releasing the tow rope 11 or umbilical cord. In contrast, for a more accurate vertical control, the node setter 4 preferably has one or more trim planes to control vertical movement during towing. Propellers can also be used for vertical steering. The annotator 4 can have its own navigation system, e.g. an internal navigation system, or it can be fully controlled from the vessel 6.

The instrumentation, control system and control equipment, i.e. deflectors, trim plane and/or propellers, enable the rope 3 to be laid out at a predetermined position, and thus the seismic nodes 1 to be laid out at a predetermined position.

Figure 7 illustrates a vessel 6 seen from above, which tows three node layers 4 for simultaneous laying of three ropes 3 with seismic nodes 1. The node layer 4 is held in position by forces 21 from deflectors or propellers. The normal distance between the ropes 3 is 100 - 400 m. This provides a time-saving and efficient way of laying out seismic nodes 1 on the seabed 2.

Claims (29)

1. Method for seismic survey using autonomous seismic nodes (1) on a seabed (2), comprising: - attaching the seismic nodes (1) to a rope (3); - loading the rope (3) with seismic nodes (1) into a node setter (4); - to lower the node setter (4) into the sea (5); - to tow the node setter (4) over the seabed (2); - laying out the rope (3) with seismic nodes (1) on the seabed (2); - to collect seismic data using seismic nodes (1); - to retrieve the rope (3) with seismic nodes (1) from the seabed (2), and - to transmit seismic data from seismic nodes (1). characterized in that loading the rope (3) involves wrapping the rope (3) around parallel bars (14) in a zigzag pattern in a magazine (7), removing the bars (14) and loading the magazine (7) into the knotter (4) . 2. Method according to claim 1, where mounting seismic nodes (1) on the rope (3) is done on board a vessel (6). 3. Method according to claim 1 or 2, where loading the rope (3) with seismic nodes (1) into the node setter (4) is done by packing the rope (3) with seismic nodes (1) in a magazine (7) and then load the magazine (7) into the node setter (4). 4. Method according to claim 3, comprising packing the rope (3) loosely, essentially without a support structure, to minimize the packing volume. 5. Method according to claim 3 or 4, where the rope (3) is held together by rubber bands or similar to keep it together during handling and stacking in the magazine (7). 6. Method according to any of the preceding claims, where the node setter (4) is lowered into the sea (5) by an active heave-compensated winch. 7. Method according to any of the preceding claims, where laying out the rope (3) with seismic nodes (1) on the seabed (2) is done by letting the rope (3) run freely. 8. Method according to any one of claims 1 - 6, where laying out the rope (3) with seismic nodes (1) on the seabed (2) is done by feeding out the rope (3) in a controlled manner. 9. Method according to any of the preceding claims, where seismic nodes (1) are laid out in predetermined positions. 10. Method according to any of the preceding claims, where the retrieval of the rope (3) with seismic nodes (1) from the seabed (2) is done by hoisting the rope (3) onto the vessel (6) 11. Method according to claim 10, where the vessel (6) lies on the sea surface (8). 12. Method according to claim 10 or 11, comprising connecting a recovery line (9) to the rope (3), and hoisting the rope (3) onto the vessel (6) using the recovery line (9). 13. Method according to claim 12, comprising connecting a floating coupling device (10) to the rope (3), and connecting the recovery rope (9) to the floating coupling device (10). 14. Method according to claim 13, comprising connecting several separate floating coupling devices (10) to the rope (3), to enable connection of the recovery rope (9) to the rope (3) in several places. 15. Method according to claim 1 or any of claims 9-14, comprising the use of an ROV (13). 16. Method according to claim 15, where the ROV (13) grabs the rope (3) or a seismic node (1). 17. Method according to any one of the preceding claims, comprising separating the seismic nodes (1) from the rope (3) after retrieving the rope (3) with seismic nodes (1) from the seabed (2). 18. Method according to any of the preceding claims, where two or more node layers (4) are towed by a vessel (6), to simultaneously lay out two or more ropes (3) with seismic nodes (1). 19. A node layer (4) for underwater towing over the seabed (2), for laying out a rope (3) with seismic nodes (1) on the seabed (2), comprising a coupling unit for a tow rope (11), and a storage for the rope (3) with seismic nodes (1), characterized in that the warehouse is a magazine (7), with removable rods (14) to wrap the rope (3) with seismic nodes (1) into the magazine (7) when it is outside the node setter (4), then to remove the rods (14) ) and to load the magazine (7) into the node setter (4). 20. Footer (4) according to claim 19, where the magazine (7) is adapted for loose packing of the rope (3), essentially without a support structure, in order to minimize the packing volume. 21. Node maker (4) according to claim 19 or 20, where the node maker (4) comprises a frame for the magazine (7). 22. Footer (4) according to any of claims 19-21, where the magazine (7) is self-supporting. 23. Node setter (4) according to any of claims 19-21, where the magazine (7) is supported by surrounding structure when it is inside the node setter 4. 24. Node layer (4) according to any one of claims 19 - 23, where the node layer (4) is adapted to allow the rope (3) to run freely during the laying of the rope (3). 25. Node layer (4) according to any one of claims 19 - 23, where the node layer (4) is adapted to feed out the rope (3) in a controlled manner during the laying of the rope (3). 26. Footer (4) according to any one of claims 19 - 25, comprising one or more deflectors to control lateral movement during towing. 27. Footer (4) according to any one of claims 19 - 26, comprising one or more trim planes to control vertical movement during towing. 28. A footer (4) according to any one of claims 19 - 27, comprising instrumentation for detecting obstacles (12) in its path. 29. Footer (4) according to any one of claims 19 - 28, comprising a control loop which guides it away from the obstacles (12) in its path.