ENVIRONMENT

FIELD OF THE INVENTION

[0001] The present invention relates to a method of deploying seismic sensors in a marine environment. In particular, the present invention relates to a method of deploying seismic sensors in a marine environment using self-propelled nodes. The present invention also relates to a method of conducting a seismic survey in a marine environment, a seismic survey control module for conducting a seismic survey in a marine environment, and a seismic survey apparatus.

BACKGROUND OF THE INVENTION

[0002] Marine ocean bottom seismic (OBS) surveys typically use seismic sensor devices which are deployed to the seabed. Such seismic surveys are typically undertaken in water and so it is necessary to find a method of efficiently deploying the seismic sensor devices from the surface of the ocean to the seabed. The seismic sensor devices are typically stored and deployed from a marine surface vessel, acting as a deployment vessel, and are then placed on the seabed. The seismic sensor devices are later retrieved from the seabed and returned to a marine surface vessel, which may be the deployment vessel or an alternative vessel. The seismic sensor devices each contain at least one seismic sensor and other electronic modules in a sealed arrangement. Devices are configured to record seismic information which is then downloaded from the seismic sensor devices upon retrieval from the seabed.

[0003] During a seismic survey a large number of seismic sensor devices, for example, more than a thousand, may be deployed to the seabed and are arranged on the seabed in a predetermined pattern. One known means of deploying the seismic sensor devices is through the use of a remotely operated vehicle (ROV). Seismic sensor devices are loaded onto the ROV which is lowered into the water and manoeuvred to the seabed. The ROV individually locates the seismic sensor devices in the predetermined pattern by moving along a course and locating each seismic sensor device in a predetermined position in a linear row. Each row is known as a receiver line. Upon forming a first receiver line, the ROV is manoeuvred along a second course to form a second receiver line spaced from and parallel to the first receiver line.

[0004] Individually deploying seismic sensor devices in a line using an ROV is highly time consuming and expensive. However, reducing the density of seismic sensor devices acts to lower the data quality of the survey.

[0005] In an attempt to minimise the time taken to deploy and recover seismic sensor devices it is known to use cabled nodes in which a plurality of seismic sensor nodes or devices are mounted to a rope or cable which is subsequently deployed to the seabed. The nodes may be easily spaced along the cable, and so the cabled node arrangement may be efficiently deployed to the seabed to form a receiver line. In such an arrangement, the deployment vessel is moved along a course or pass in a direction with the cable being deployed in that direction to form a receiving node row. However, vessels are only capable of deploying a single cable at a time, and so the number of receiving node rows is dependent on the number of courses or passes that the deployment vessel must undertake. As such, there is a direct relation between the cost and time taken to deploy and recover the seismic sensor devices from the seabed and the number of node rows.

[0006] Once at least one receiving row of seismic sensor devices is formed, the seismic survey may be performed. The seismic survey is undertaken by providing a source signal, which is typically an acoustical vibrational signal, at predetermined positions relative to the seabed. A source vessel, which is typically a second surface marine vessel, is provided to move along a predetermined path along which a source signal, also known as a shot, is generated along a predetermined pattern relative to the seabed. The source signal or shot produces reflective signals from the seabed which are then detected by the seismic sensors and recorded by the seismic sensor devices. The source vessel is configured to move along a first course, before moving along a second parallel course spaced from the first course. The number of lines of shot that the source vessel is capable of generating is therefore dependent on the number of courses that the source vessel undertakes. SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention, there is provided a method of conducting a seismic survey in a marine environment, the method comprising: deploying a plurality of self-propelled receiver nodes from a deployment vessel; operating the self-propelled receiver nodes to locate on the seabed in a predetermined receiver array; wherein a spacing in a first, inline, direction between self-propelled receiver nodes is less dense than a spacing in a second, crossline, direction, transverse to the first, inline, direction, between adjacent self-propelled receiver nodes; and operating a movable transmitter to transmit a series of shots in a predetermined pattern to be detected by the self-propelled receiver nodes in the receiver array by moving along a path in which the movable transmitter transmits the series of shots along a plurality of successive shot lines, with each shot line extending in the first, inline, direction.

[0008] As such, it is possible to efficiently align seismic sensors on the seabed in a predetermined pattern. The predetermined pattern of nodes may be achieved during a single deployment vessel pass. The use of self-propelled receiver nodes helps enable the seismic sensors to be arranged with the nodes being more densely spaced in the second, crossline, direction than in the first, inline, direction, and so an arrangement of the nodes may be provided in which a high resolution seismic survey may be undertaken.

[0009] The method may comprise operating the self-propelled receiver nodes to locate in the receiver array in at least two receiver rows, with each receiver row extending in the first, inline, direction and each receiver row being spaced from an adjacent receiver row in the second, crossline, direction; and wherein the spacing in the second, crossline, direction between the adjacent receiver rows is denser than the spacing in the first, inline, direction between adjacent self-propelled receiver nodes in each receiver row.

[0010] By deploying the receiver nodes in such a configuration it is possible to efficiently deploy the nodes to the seabed causing a minimisation in the time and cost required to undertake seismic surveys. [0011] The method may comprise operating the self-propelled receiver nodes to locate in the receiver array in receiver columns, with each receiver column extending perpendicular to the receiver rows.

[0012] The self-propelled nodes may be driven marine devices. The self-propelled nodes may be glider marine devices. The self-propelled nodes may include one or more of an adjustable buoyancy device, a wing, control surfaces, and a thruster.

[0013] The method may comprise operating at least two of the self-propelled receiver nodes to concurrently descend from the node deployment vessel to the seabed.

[0014] The term“concurrently descend” means that the at least two receiver nodes are descending through the marine environment to the seabed at the same time but do not necessarily locate on the seabed simultaneously.

[0015] As such, it is possible to efficiently manoeuvre and locate more than one self- propelled node on the seabed at the same time. Two or more receiver rows may be formed simultaneously.

[0016] The method may comprise operating the self-propelled receiver nodes to concurrently form at least three receiver rows.

[0017] The self-propelled receiver nodes may be operated to descend to the seabed independently of one another. As such, the configuration of the receiver may be easily altered.

[0018] The method comprises operating a movable transmitter to transmit a series of shots in a predetermined pattern to be detected by the self-propelled receiver nodes in the receiver array by the movable transmitter moving along a path in which the movable transmitter transmits the series of shots along the path to define a plurality of successive shot lines, with each shot line extending in the first, inline, direction. Thus the series of shots are transmitted as the moveable transmitter moves in the first, inline, direction.

[0019] As such, it is possible for the equipment to be efficiently used to minimise the distance of travel required to perform the survey for each apparatus and to minimise the time required to conduct the survey. [0020] The method may comprise operating the deployment vessel to move in the first, inline, direction; wherein the plurality of self-propelled receiver nodes are deployed from the deployment vessel as the deployment vessel moves in the first, inline, direction.

[0021] Moving both the deployment vessel and the transmitter in the same (inline) direction is advantageous since it enables the transmitter to start moving and transmitting the series of shots at the same time as the deployment vessel is deploying the nodes, without crossing each other's paths.

[0022] Typically the deployment vessel moves predominantly in the inline direction as it is deploying the plurality of self-propelled receiver nodes. For instance the deployment vessel may deploy the self-propelled receiver nodes as it moves along a path with a number of courses which each extend in the first, inline, direction, the length of each course being greater than the space between adjacent courses. Thus the deployment vessel moves predominantly in the first, inline, direction, except between courses.

[0023] Typically the path of the movable transmitter is predominantly in the inline direction, rather than the crossline direction. For instance the path may have a number of courses, each course corresponding with a respective shot line, which each extend in the first, inline, direction, the length of each course being greater than the space in the crossline direction between adjacent courses. Thus the movable transmitter moves predominantly in the first, inline, direction, except between courses.

[0024] The method may comprise operating the movable transmitter to move along the path in which a spacing in the second, crossline, direction between successive shot lines is less dense than a spacing in the first, inline, direction between the position of each adjacent pair of the series of shots transmitted in each shot line.

[0025] It is therefore possible to maximise the number of shots that the movable transmitter is able to generate in a single course or pass, that is prior to changing direction to undertake another course over a survey area, and to minimise the number of courses undertaken by the moveable transmitter. The spacing in the second, crossline, direction between successive shot lines may be at least twice the distance of the spacing in the first, inline, direction between the position of each adjacent pair of the series of shots taken in each shot line and, optionally, at least four times the distance.

[0026] The method may comprise operating at least two movable transmitters to transmit a series of shots in a predetermined pattern to be detected by the self-propelled receiver nodes in the receiver array by each movable transmitter moving along a path in which each movable transmitter transmits the series of shots along its path to define a plurality of successive shot lines, with each shot line extending in the first, inline, direction. The paths of each movable transmitter may differ. Each movable transmitter may move along a path covering a distinct portion of the array.

[0027] The method may comprise operating a source vessel to move along the path, the source vessel comprising the movable transmitter.

[0028] The method may comprise operating at least two source vessels, with each source vessel comprising a movable transmitter.

[0029] The self-propelled nodes may be autonomous underwater vehicles.

[0030] The method may comprise operating the self-propelled nodes so that at least one seismic sensor of each self-propelled receiver node couples with the seabed.

[0031] The node deployment vessel may comprise a deployment apparatus. The method may comprise moving the node deployment apparatus to move in the first, inline, direction and operating the node deployment apparatus to deploy the self-propelled receiver nodes as the node deployment apparatus moves in the first, inline, direction. The node deployment apparatus may be towed node deployment apparatus which deploys the self-propelled receiver nodes as the towed node deployment apparatus is towed in the first direction. Alternatively the node deployment apparatus may be a conveyor system which deploys the self-propelled receiver nodes into the water from a deck of the node deployment vessel.

[0032] The spacing in the second, crossline, direction between the adjacent self- propelled receiver nodes may be at least half of the distance of the spacing in the first, inline, direction between adjacent self-propelled receiver nodes and, optionally, at least a quarter of the distance. [0033] The method may comprise operating the movable transmitter to move along the path in which a spacing in the second, crossline, direction between successive shot lines is less dense than a spacing in the first, inline, direction between the position of each adjacent pair of the series of shots transmitted in each shot line.

[0034] Self-propelled receiver nodes may continue to deploy to the seabed to form a further row or further rows as the moveable transmitter generates the shot lines, i.e. at the same time as the moveable transmitter generates the shot lines.

[0035] The method may further comprise detecting seismic data from at least some of the shots with the self-propelled receiver nodes on the seabed.

[0036] The step of forming the further row or further rows may comprise: a) retrieving one or more rows of the self-propelled receiver nodes from the seabed after the one or more rows have detected seismic data from at least one of the shot lines; and b) re deploying the one or more rows on the seabed to form the further row or rows. This enables a so-called rolling survey to be conducted.

[0037] Step a) of retrieving may be performed as the moveable transmitter generates shot lines, and step b) of re-deploying may be performed as the moveable transmitter generates shot lines.

[0038] Step a) of retrieving and step b) of re-deploying may each be performed by the deployment vessel, by a re-deployment vessel which is different from the deployment vessel, or by the self-propelled receiver nodes moving away from the seabed and re deploying under their own power.

[0039] The deployment or re-deployment vessel may comprise towed apparatus, and step a) of retrieving may be performed by the towed apparatus as the towed apparatus is being towed.

[0040] The deployment or re-deployment vessel may comprise towed apparatus, and step b) of re-deploying may be performed by the towed apparatus which re-deploys the self-propelled receiver nodes as the towed apparatus is being towed.

[0041] Step a) of retrieving one or more rows of the self-propelled receiver nodes from the seabed may comprise operating the one or more rows of the self-propelled receiver nodes to ascend from the seabed to a deployment or re-deployment vessel, and then retrieving the one or more rows of the self-propelled receiver nodes with the deployment or re-deployment vessel.

[0042] Optionally the self-propelled receiver nodes have batteries which are recharged between step a) of retrieving and step b) of re-deploying. The batteries may be recharged on-board the deployment or re-deployment vessel, or recharged by the towed apparatus.

[0043] Optionally the seismic data is downloaded from the self-propelled receiver nodes between step a) of retrieving and step b) of re-deploying. The seismic data may be downloaded on-board the deployment or re-deployment vessel, or downloaded by the towed apparatus.

[0044] According to an aspect of the present invention, there is provided a computer program product comprising instructions which, when the program is executed by a processor, cause the processor to carry out at least one of the methods described above.

[0045] According to an aspect of the present invention, there is provided a computer readable medium having stored therein the computer program product as set out above.

[0046] According to an aspect of the present invention, there is provided a non- transitory computer readable medium comprising computer readable instructions that, when executed by a processor, cause performance of the method as set out above.

[0047] According to an aspect of the present invention, there is provided a seismic survey control module for conducting a seismic survey in a marine environment, the seismic survey control module comprising: a controller configured to: output a control signal to deploy a plurality of self-propelled receiver nodes from a deployment vessel, as the deployment vessel moves in a first, inline, direction; and output a control signal to operate the self-propelled nodes to locate on the seabed in a predetermined receiver array; in which a spacing in a second, crossline, direction, transverse to the first, inline, direction, between the receiver nodes in the receiver array is denser than a spacing in the first, inline, direction between the receiver nodes in the receiver array. [0048] According to an aspect of the present invention, there is provided a seismic survey control module for conducting a seismic survey in a marine environment, the seismic survey control module comprising: a controller configured to: output a control signal to deploy a plurality of self-propelled receiver nodes configured to act as seismic sensors from a deployment vessel; output a control signal to operate the self-propelled nodes to locate on the seabed in a predetermined receiver array, wherein a spacing in a first, inline, direction between self-propelled receiver nodes is denser than a spacing in a second, crossline, direction, transverse to the first, inline, direction, between adjacent self-propelled receiver nodes; and output a control signal to operate a movable transmitter to transmit a series of shots in a predetermined pattern to be detected by the self-propelled receiver nodes in the receiver array by moving along a path in which the movable transmitter transmits the series of shots along a plurality of successive shot lines, with each shot line extending in the first, inline, direction.

[0049] The controller may be configured to output a control signal to operate the node deployment vessel carrying the plurality of self-propelled receiver nodes to move in the first, inline, direction.

[0050] According to an aspect of the present invention, there is provided a seismic survey apparatus comprising a plurality of self-propelled nodes and a seismic survey control module as set out above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0052] Figure 1 is a schematic side view showing a seismic survey apparatus with a deployment of self-propelled receiver nodes from a deployment vessel to a seabed;

[0053] Figure 2 shows a schematic view of the seismic survey apparatus of Figure 1 ;

[0054] Figure 3 is a schematic view of an arrangement of the self-propelled receiver nodes of Figure 1 located on the seabed in a receiver array;

[0055] Figure 4 is a schematic plan view showing the self-propelled nodes being deployed to the seabed and locating in the receiver array of Figure 3 on the seabed; [0056] Figure 5 is a schematic plan view showing further self-propelled nodes being deployed from the deployment vessel to the seabed following Figure 4, and a source vessel generating shots in a predetermined pattern to be detected by the self-propelled receiver nodes;

[0057] Figure 6 shows a schematic plan view showing the further deployment of self- propelled receiver nodes to the seabed following Figure 5 and further shots being generated by the source vessel;

[0058] Figure 7 shows a schematic plan view showing the further deployment of self- propelled receiver nodes to the seabed following Figure 6 and further shots being generated by the source vessel;

[0059] Figure 8 shows a rolling survey method;

[0060] Figure 9 shows a pattern of node locations and shot locations relative to the seabed;

[0061] Figure 10 is a flowchart showing a method of conducting a seismic survey in a marine environment;

[0062] Figure 11 is a flowchart showing a method of conducting a seismic survey in a marine environment; and

[0063] Figure 12 is a flowchart showing a method of conducting a rolling seismic survey in a marine environment.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0064] A seismic survey apparatus 1 comprising a plurality of autonomous underwater vehicles (AUVs) 10 is shown in Figure 1. A deployment of the AUVs 10 to a seabed 30 is shown. The AUVs 10 act as self-propelled receiver nodes. The receiver nodes are deployed to form a receiver array 80 (shown in Figure 3) on the seabed 30 as part of a seismic survey. The AUVs 10 each act as a receiver to record seismic information. The AUVs 10 each include a sensing module 12 comprising a seismic sensor 13, shown in Figure 2. [0065] Although the deployment of seismic sensors is described herein by reference to AUVs, the self-propelled receiver node may be any device that is capable of manoeuvring autonomously or semi-autonomously in a marine environment. The self- propelled node includes, for example, an autonomous or a remote-controlled device. In embodiments, the self-propelled node is a glider or driven propulsive device. Each self- propelled node is a discrete device.

[0066] A surface vessel acts as a deployment vessel 20. The surface vessel may also act as a retrieval vessel, or in one configuration an alternative vessel may be used to retrieve the AUVs 10. The deployment vessel 20 comprises a deployment/retrieval apparatus 21. The deployment/retrieval apparatus 21 is a submersible device. The deployment/retrieval apparatus 21 is loaded with AUVs 10 on the deck of the deployment vessel 20. The deployment/retrieval apparatus 21 carrying the AUVs 10 is then lowered into the water 31 by a tether 22, as shown in Figure 1, until it is at a required depth. At this point the deployment vessel 20 may be stationary or it may be moving. The deployment/retrieval apparatus 21 is manoeuvred together with the surface part of the deployment vessel 20.

[0067] After the deployment/retrieval apparatus 21 containing the AUVs 10 has been submerged as in Figure 1, the deployment vessel 20 is driven to the left in a first direction (an inline direction) as shown in Figure 1 by arrow 23 so that the deployment/retrieval apparatus 21 containing the AUVs 10 moves in the first direction. The AUVs 10 are then deployed from the deployment/retrieval apparatus 21 as the deployment/retrieval apparatus 21 is towed in the first direction.

[0068] During the deployment process, the AUVs 10 are forced out of the deployment/retrieval apparatus 21 by the action of the water flowing through the deployment/retrieval apparatus 21. In addition, or alternatively, alternative motive forces such as the AUVs 10 operating a propulsion means 11 is used to provide or assist its ejection from the deployment/retrieval apparatus 21.

[0069] The propulsion means 11 of each AUV 10 is operated to manoeuvre the AUVs 10. The propulsion means 11 used to manoeuvre the AUV 10 may include one or more of a thruster, a buoyancy unit, a wing arrangement, control surfaces, and glider profiles. Following deployment, the AUVs descend autonomously to the seabed 30, and land at precisely controlled locations where they acquire seismic data during a seismic survey.

[0070] When the seismic survey is complete, the AUVs 10 return to the deployment/retrieval apparatus 21 where they are retrieved by essentially the reverse process to deployment. The deployment/retrieval apparatus 21 is towed through a retrieval zone by the deployment vessel 20 acting as a retrieval vessel moving in the first, inline, direction, and the AUVs are loaded into the deployment/retrieval apparatus 21 as it is towed through the retrieval zone. After the AUVs 10 have been loaded into the deployment/retrieval apparatus 21, the deployment/retrieval apparatus 21 containing a full payload of the AUVs 10 is lifted out of the water 31.

[0071] The deployment vessel 20 comprising the deploy ment/retrieval apparatus 21 comprises part of the seismic survey apparatus 1.

[0072] Referring to Figure 2, the seismic survey apparatus 1 for completing the seismic survey is shown schematically. The seismic survey apparatus 1 includes the plurality of AUVs 10. One of the plurality of AUVs 10 is shown in Figure 2.

[0073] Each AUV 10 has a sensing module 12 configured to acquire information, in particular to acquire data when the vehicle is grounded on the seabed 30. The sensing module 12 is configured to abut against, and be positioned in a stationary position on the seabed 30. The sensing module 12 is configured to be coupled with the seabed 30. When coupled with the seabed 30, the sensing module 12 is in engagement with the seabed 30 such that it is in contact with and held in a stationary condition.

[0074] The sensing module 12 comprises a seismic sensor 13. The seismic sensor 13 is in the AUV 10. The seismic sensor 13 is configured to detect seismic data indicating one or more characteristics at the seabed 30 when the AUV 10 is located on the seabed. These may be characteristics of the seabed 30 itself, or near the seabed 30, but which are taken from the seabed 30. In embodiments, the sensing module 12 comprises two or more seismic sensors 13 which detect the same or different characteristics of the seabed 30. The seismic data is stored by the AUV 10.

[0075] A second marine surface vessel acts as a source vessel 41. The source vessel 41 comprises a moveable transmitter 40. The moveable transmitter 40 is configured to produce a source signal to be detected by the AUVs 10 acting as receiver nodes. The source signal comprises a series of shots 60, forming the source signal. The shot 60 is an acoustical or vibrational signal. The production of the series of shots provides for the seismic survey to be undertaken. The source vessel 41 is moveable along a predefined path to perform a series of shots 60 to provide a pre-determined pattern, as will be described below. The shot 60 may be produced at or proximal to the surface of the water 31.

[0076] The seismic survey apparatus 1 comprises a seismic survey control module 50. The seismic survey control module 50 is configured to operate the seismic survey apparatus 1 to perform at least one of the method of deploying seismic sensors in the marine environment and the method of conducting a seismic survey in the marine environment as described herein. The seismic survey control module 50 communicates with the sensing module 12 and propulsion means 11 of each AUV 10 via communication path 51. The seismic survey control module 50 is configured to output a control signal to control the deployment/retrieval apparatus 21 and each AUV 10. The seismic survey control module 50 comprises a controller 52 including a processor 53 and a memory 54. The control signal is provided to the AUV 10 in at least one of the prior to loading onto the deployment/retrieval apparatus 21, following loading onto the deployment/retrieval apparatus 21, during deployment to the seabed 30, on the seabed 30, and during recovery from the seabed 30.

[0077] The seismic survey control module 50 is configured to control the deployment vessel 20 and the source vessel 41. The seismic survey control module 50 is also configured to operate the moveable transmitter 40. The seismic survey control module 50 may be formed of two or more sub-modules, at least one of which may be on one of the AUV 10, the deployment vessel and the source vessel 41.

[0078] The moveable transmitter 40 is configured to receive signals from the seismic survey control module 50 via communication path 56. The deployment vessel 20 is configured to receive signals from the seismic survey control module 50 via communication path 57. [0079] When the seismic survey apparatus 1 is operated, the AUVs 10 acting as receiver nodes are deployed to the seabed 30. An arrangement of AUVs 10 on the seabed in the predetermined receiver array 80 is schematically shown in Figure 3. Each AUV 10 is indicated by a circular-shaped icon. The AUVs 10 are shown arranged in receiver rows, Rl, R2, R3, etc., and receiver columns, Cl, C2, C3, etc. The receiver rows and columns of AUVs 10 are indicated by dashed lines. The rows extend in the first, inline, direction (the left-right direction in Figure 3), and are spaced from each other in a second, crossline, direction (the up-down direction in Figure 3). The columns extend in the second, crossline, direction (the up-down direction in Figure 3), and are spaced from each other in the first, inline, direction (the left-right direction in Figure 3).

[0080] In the arrangement of the receiver array 80 shown in Figure 3, the AUVs 10 are shown distributed on the seabed in an orthogonal array. The columns extend perpendicular to the rows. In alternative array arrangements, AUVs 10 in adjacent rows and/or columns may be offset from each other. In such an arrangement, the columns may extend transversely, but not perpendicular, to the rows.

[0081] Figures 4 to 8 show steps in the operation of the seismic survey. Figure 10 shows a process flow SI 00 of a method of conducting a seismic survey in a marine environment, as carried out by a module such as the seismic survey control module 50. The operation of the seismic survey includes a method of deploying seismic sensors in the marine environment. The method of conducting the seismic survey in the marine environment process flow SI 00 begins at step SI 02. The process flow SI 00 is initiated at the seismic survey control module 50.

[0082] The AUVs 10 are loaded onto the deployment/retrieval apparatus 21. Once loaded, the deployment/retrieval apparatus 21 carrying the AUVs 10 is lowered into the water 31 into a position below the surface of the water 31. The deployment/retrieval apparatus 21 carrying the AUVs 10 is tethered to the deployment vessel 20 by the tether 22. At step SI 04, the deployment vessel 20 is moved in the first, inline, direction as indicated by arrow 23 in Figure 1. This first, inline, direction may be a compass heading or any other linear or predetermined direction. The deployment vessel 20 is operated to move at a predetermined speed. The deployment/retrieval apparatus 21 moves along with the surface vessel of the deployment vessel 20. The deployment vessel 20 may be operated to account for drift resulting from one or more of water current, wind and wave action.

[0083] The process flow then moves to step SI 06, at which AUVs 10 are deployed. The deployment/retrieval apparatus 21 is configured to receive an instruction to deploy the AUVs 10. The deployment/retrieval apparatus 21 deploys a plurality of the AUVs 10.

[0084] As the AUVs 10 are deployed, the process flow moves to step S108 at which each of the AUVs 10 descends autonomously to locate on the seabed 30. As AUVs are deployed, the AUVs concurrently descend through the water 31 to the seabed 30. The term“concurrently descend” relates to an arrangement in which at least two of the AUVs descend through the marine environment to the seabed at the same time but do not necessarily locate on the seabed simultaneously.

[0085] The AUVs 10 are operated to locate on the seabed 30 to form the receiver array 80 of receiver nodes on the seabed 30. A limited number of AUVs 10 are shown in the Figures, however it will be understood that the number of AUVs within the receiver array 80 may vary in number and includes any number of receiver nodes. Similarly, a limited number of receiver rows and columns of AUVs 10 are shown in the Figures, however it will be understood that the number of rows and columns may vary. In Figures 4 to 8, the AUVs 10 are shown deployed on the seabed 30 and in the process of being deployed to the seabed. Those AUVs 10 that are in the process of descending to the seabed 30 are shown with arrows providing an indication of their direction of travel.

[0086] As shown in Figure 4, when AUVs 10 are deployed from the deployment vessel 20 by use of the deployment/retrieval apparatus 21, the AUVs 10 concurrently descend to the seabed 30. The deployment vessel 20 is operated to move along a predetermined path 70. The predetermined path 70 includes a number of passes or courses through a survey area which each extend in the first, inline, direction. The passes are spaced from each other in the second, crossline, direction. The AUVs 10 are configured to align on the seabed 30 in a spaced relationship relative to each other. The AUVs 10 align in a predetermined number of receiver rows, for example four rows (R1-R4) as shown in Figure 4, on the seabed 30 as the deployment vessel 20 moves along a first pass 71. The AUVs 10 are operated to concurrently form a plurality of rows on the seabed 30 extending in the first, inline, direction.

[0087] In Figure 4, a plurality of AUVs 10 identified as AUVs 101-104 are operated to locate on the seabed to form row Rl . AUVs 201-204 are operated to form row R2; AUVs 301-304 are operated to form row R3, and AUVs 401-404 are operated to form row R4. Multiple rows and columns or receiver nodes may be formed concurrently through concurrent deployment of the AUVs.

[0088] As deployment of AUVs 10 continues, the deployment vessel 20 continues to move along its path. Upon completion of the first pass 71, the deployment vessel reverses direction, either by turning 180 degrees or by travelling backwards, and moves in the first, inline, direction in the opposing direction to form a second pass 72, as shown in Figure 5. The second pass 72 is parallel to, and spaced from, the first pass 71 in the second, crossline, direction. Further deployed AUVs 10 align in a predetermined further number of rows, for example rows R5-R8 as shown in Figure 4, on the seabed 30 as the deployment vessel 20 moves along the second pass 72. Upon completion of the second pass 72, the deployment vessel 20 reverses direction and moves in the first, inline, direction in the same direction as the first pass 71 to form a third pass 73 (refer to Figure 7). Such a process is repeated as necessary. The number of passes and rows deployed per pass may vary.

[0089] By deploying multiple AUVs 10 acting as receiver nodes concurrently it is possible to minimise the time for performing the seismic survey. It is possible to minimise the distance that the deployment vessel 20 has to travel to locate the array 80 of nodes 10 on the seabed 30. That is, the number of passes that the deployment vessel 20 undertakes is minimised. Use of AUVs 10 aids the deployment of multiple rows of receiver nodes concurrently. In one known method an ROV may be used which moves relative to the seabed in directions independent of a surface vessel; however the ROV must be operated to individually deploy and locates single seismic sensor devices on the seabed 30 prior to deploying the next seismic sensor device.

[0090] The AUVs 10 are deployed in a predetermined pattern on the seabed 30 to form the receiver array or grid 80. The AUVs 10 are disposed on the seabed 30 in rows, for example rows R1-R8, extending in the first, inline direction, which are spaced from the or each adjacent row in the second, cross-line, direction. Each AUV 10 in each row is spaced from the or each adjacent AUV 10 in the row.

[0091] As shown in Figure 3, the distance between each row of AUVs 10 is shown as distance DR. The distance between adjacent AUVs 10 in each row is shown as distance DA. In the receiver array configuration shown in the Figures, distance DA conforms to the spacing between adjacent columns of AUVs 10.

[0092] The spacing between the AUVs 10 in the receiver array 80 in the second, crossline, direction, that is distance DR, is denser than a spacing in the first, inline, direction, that is distance DA. In other words, the spacing DR between adjacent rows in the second, crossline, direction, is less than the spacing DA between adjacent AUVs 10 in each row in the first, inline, direction.

[0093] In the present arrangement, the distance DR between rows in the second, crossline, direction is half of the distance DA between adjacent AUVs 10 in each row. In one embodiment, the distance DR in the second, crossline, direction between rows of AUVs 10 is 50 metres, and the distance DA in the first, inline, direction between adjacent AUVs 10 in each row is 100 metres. In some embodiments, the distance DA between adjacent AUVs 10 in each row is a greater multiple of the distance DR between adjacent rows. For example, the spacing between rows DR may be 50 metres and the distance DA between adjacent AUVs 10 in each row may be 400 metres.

[0094] The method provides for limiting the number of passes whilst maximising the density of rows. The efficient deployment and recovery of the AUVs 10 as described herein provides a significant saving in both the required survey time and cost. By minimising the required number of AUVs 10 in each row it is possible to further maximise the efficient deployment of the seismic sensors to the seabed 30.

[0095] Following the operation of the AUVs 10 to locate on the seabed 30 the process moves to step SI 10. The operation of step S108 may be ongoing, as shown in Figures 6 and 7. At step SI 10 a plurality of shots 60 are undertaken. In Figures 6 to 8, the position at which each shot is generated is indicated by a cross-shaped icon. The shots 60 are performed in a predetermined pattern or grid relative to the seabed 30. As such, the shots 60 are generated along a predetermined pattern relative to the array of AUVs 10 located on the seabed. The shooting operation of the moveable transmitter 40 is operable upon completion of at least one row of AUVs 10 being located on the seabed 30.

[0096] The shots 60 are generated by the moveable transmitter 40. The seismic survey control module 50 is configured to operate the source vessel 41 to move the moveable transmitter 40 along a predetermined path. The source vessel 41 may be operated to account for drift resulting from one or more of water current, wind and wave actions.

[0097] The moveable transmitter 40 is moved along a predetermined shot path 75 which defines a plurality of adjacent shot lines each extending in the first, inline direction. The shot lines are indicated in Figures 6 to 8 by designations L1-L6. The predetermined shot path 75 includes a number of passes or courses through the survey area which each extend in the first, inline, direction. The courses are spaced from each other in the second, crossline, direction. The courses of the shot path 75 define the shot lines Ll- L6.

[0098] As the moveable transmitter 40 is moved along its shot path 75, the moveable transmitter 40 generates the series of shots 60. The series of shots 60 are generated at predetermined locations relative to the seabed 30, and therefore the AUVs 10 on the seabed. The shots 60 are spaced along each shot line L1-L6. The shots 60 are formed in a predetermined pattern. The shot lines are parallel to the rows of AUVs 10.

[0099] As the moveable transmitter 40 moves along a first course 76, the moveable transmitter 40 undertakes a series of spaced shots 60 to form shot line LI . The shots 60 in shot line LI are spaced apart in the first, inline, direction. Upon completion of the first course 76 to form the first shot line 75, the moveable transmitter 40 reverses direction and moves in the first, inline, direction in the opposing direction to form a second course 77, as shown in Figure 6. The second course 77 is parallel to, and spaced from, the first course 76 in the second, crossline, direction. The shots 60 generated as the moveable transmitter 40 moves along the second course 77 forms the shot line L2. The second shot line L2 is transposed over the second row R2 of AUVs 10. [00100] Upon completion of the second course 77, the moveable transmitter 40 reverses direction and moves in the first, inline, direction in the same direction as the first course 76 to form a third course 78 (refer to Figure 7). The shots 60 generated as the moveable transmitter 40 moves along the third course 78 forms the shot line L3. The shots 60 generated as the moveable transmitter 40 moves along a fourth course 79 forms the shot line L4. Such a process is repeated as necessary. The number of courses and performed shots per course may vary.

[00101] It will be understood that AUVs 10 continue to deploy to the seabed 30 to form further rows as the moveable transmitter 40 generates the shot lines. Specifically, Figure 7 shows the deployment vessel 20 deploying the AUVs 10 in the third pass 73, at the same time as the moveable transmitter 40 generates the series of shots 60 in the shot line L4.

[00102] Figure 7 indicates an Active Area: this is an area which covers all the AUVs 10 in the inline direction and at least three rows of AUVs 10 in the crossline direction. The Active Area indicates a set of rows of AUVs 10 (in this case rows R5-R7) which are sufficiently close to detect seismic data from at least some of the shots of the shot line L4.

[00103] The Active Area in Figure 7 is shown to be symmetrical about the shot line L4. However, it may be asymmetrical.

[00104] The Active Area for a given seismic survey may be predefined. A predefined Active Area ensures that the moveable transmitter 40 starts a shot line when all AUVs 10 within the Active Area are deployed and ready to detect seismic data.

[00105] In the example of Figure 7, all deployed AUVs 10 remain on the seabed until all shot positions 60 have been populated.

[00106] Figure 11 shows a process flow SlOOa which is similar to the process flow SI 00 of Figure 10, and the same reference numbers are used to indicate equivalent steps, which will not be described again.

[00107] At step SI 08a a minimum of two receiver rows are deployed on the seabed. After the rows are deployed in step SI 08a, the movable transmitter 40 starts moving along a path firing shots 60 in an inline direction. Concurrently, the deployment vessel continues to deploy AUV’s 10 in step SI 08b which descend to land on the seabed 30 to form more rows, and onwards until all the future AUV locations are populated.

[00108] The movable transmitter 40 continues to follow the path firing shots in the inline direction. Thus self-propelled receiver nodes continue to deploy to the seabed to form a further row or further rows in step SI 08b as the moveable transmitter generates the shot lines in step SI 10.

[00109] Once all receiver positions are occupied by AUVs 10, then deployment of AUVs 10 stops at step S109. Similarly, the movable transmitter 40 stops operating at step SI 12 once all the shot positions have been populated.

[00110] It is not necessary for all deployed AUVs 10 to remain on the seabed 30 until all receiver positions are populated. It can even be advantageous to retrieve and re deploy AUVs 10 from behind the Active Area to in front of the Active Area to perform a so-called rolling survey as shown in Figure 8.

[00111] Figure 8 shows current AUV (node) locations 10 with circles, shot locations with crosses 60, previous AUV (node) locations 10b with squares, and future AUV (node) locations 10c with triangles. Figure 8 is a snapshot showing the current status of the survey as the movable transmitter 40 generates the shot line L5. The Active Area is shown in Figure 8 as rows R6 to R8. Previous AUV locations 10b are shown as rows R1 to R4, and future AUV locations 10c shown as rows R10 to R13. Future AUV locations 10c are locations where AUVs behind the Active Area will re-deploy.

[00112] Optional 'buffer' rows R5 and R9 are provided behind and in front of the Active Area respectively.

[00113] Figure 12 shows a specific process flow SlOOb associated with Figure 8. The process flow SlOOb is similar to the process flows SI 00 and SI 00a of Figures 10 and 11, and the same reference numbers are used to indicate equivalent steps, which will not be described again. [00114] At step SI 08c, the moveable transmitter 40 has passed the receiver rows placed during step SI 08a. The deployment vessel 20 (or another vessel) acts as a re deployment vessel in steps SI 08c and S108d.

[00115] First it retrieves the AUVs from rows placed during step S108a (for instance rows R1-R4) which have detected seismic data (for instance seismic data from shot lines L1-L4). The AUVs may be retrieved by operating the AUVs to operate their thrusters to cause the AUVs to ascend from the seabed to the vessel 20, and then retrieving them with the towed apparatus 21 of the vessel 20. The AUVs may be retrieved by the towed apparatus 21 as it is being towed in the first, inline, direction.

[00116] The towed apparatus 21 may then be lifted onto the deck of the vessel. The AUV batteries may be re-charged on-board the vessel; and/or seismic data may be downloaded from the AUVs on-board the vessel.

[00117] At step S108d the re-charged AUVs are re-deployed by the same vessel 10 in front of the movable transmitter 40 to form further rows (for instance rows R10-R13).

[00118] This is a rolling process which repeats steps SI 08c and S108d. Once deployed rows fall outside the Active Area, the AUVs which make up these rows are retrieved, re-charged and re-deployed in front of the movable transmitter 40. This continues until no further rows of receivers are required to be deployed and the movable transmitter 40 has moved sufficiently far away (the source offset distance), shown at step S108e. AUVs populating these rows are then recovered to the vessel 20.

[00119] It will be understood that AUVs 10 continue to deploy to the seabed 30 to form further rows and/or be retrieved from the seabed 30 as the moveable transmitter 40 generates the shot lines. Specifically, surface vessel(s) can be deploying, re-charging and/or re-deploying the AUVs 10, at the same time as the moveable transmitter 40 produces the series of shots 60.

[00120] Returning to Figure 8, an example of the above process can be understood. As the shot line L5 is generated, a surface vessel (typically the deployment vessel 20) retrieves the AUVs from rows R1-R4 and re-deploys those AUVs to rows R10-R13. In this example, four rows are retrieved and re-deployed together, but the number of rows may be more or less (including one row). In this example there is only a single buffer row R5, R8 either side of the Active Area, but the number of buffer rows may be greater.

[00121] This process can then repeat as the movable transmitter 40 transmits further shot lines. It can be seen that the total number of AUVs needed to complete the survey is reduced.

[00122] The benefit of the above process flow is that the movable transmitter 40 can continue at a consistent pace and a small number of AUVs 10 can be re-deployed around the position of the movable transmitter 40 ensuring a fast survey over a relatively large area in comparison to the number of AUVs with a good accuracy.

[00123] It is beneficial to use surface vessels to recover AUVs so that seismic data can be downloaded and batteries can be recharged however this is not always necessary. In an alternative example, the AUVs can retrieve themselves with their own propulsion system, travel to a new location, and re-deploy themselves.

[00124] Figure 9 shows a schematic view of a completed survey indicating the respective positions at which the AUVs 10 and the shots 60 are located. It will be understood that the number of nodes and shots, relative distances, and respective positioning of nodes and shots will vary.

[00125] As shown in Figure 9, the distance between each shot line is shown as distance D _L . The distance between adjacent shots 60 in each shot line is shown as distance Ds.

[00126] The spacing between shots 60 in the second, cross-line, direction, that is distance D _L, is less dense than the spacing between the position of each adjacent pair of the series of shots transmitted in each shot line in the first, inline, direction, that is distance Ds _. In other words, the spacing Ds between shots 60 in the first, inline, direction is less than the spacing D _L between successive shot lines in the second, cross-line, direction.

[00127] In the present arrangement, the distance D _L between successive shot lines in the second, crossline, direction is at least twice the distance Ds of the spacing between the position of each adjacent pair of shots 60 taken in each shot line. In one embodiment, the distance D _L between adjacent lines is 400 metres, and the distance Ds between adjacent shots 60 in each shot line is 50 metres. The moveable transmitter is efficiently able to undertake shots along each line as it moves in a course along each shot line, however by minimising the number of shot lines along which shots 60 are required to be generated, it is possible to minimise the number of courses that the moveable transmitter 40 has to undertake, and therefore to minimise the distance that the moveable transmitter 40 has to be moved in order to complete the seismic survey.

[00128] It is known to use a dense shot grid and a sparse receiver grid due to the time it takes to deploy the receivers to the seabed. As such, the time taken to conduct the survey is dictated by the number of shots required through use of a dense shot grid. The applicant has recognised that densely deploying the receivers in the cross-line direction allows the density of shots in this direction to be reduced. As each line of shots requires the source vessel to perform an additional pass if you reduce the density of shots in the crossline direction, then it is possible to reduce the time required to shoot the survey.

[00129] In embodiments, the distance DA between each adjacent pair of AUVs 10 in each receiver row in the first, inline, direction corresponds to the distance DL between adjacent shot lines in the second, cross-line, direction; with the distance Ds between each adjacent pair of shots 60 in each shot line in the first, inline, direction corresponding to the distance DR between adjacent receiver rows in the second, cross- line, direction. With such a method of conducting a seismic survey in the marine environment it is possible to provide a dense spacing of seismic sensors in the second, cross-line, direction and a dense spacing of shots in the first, inline, direction, forming an orthogonal survey, with the density of the seismic sensors and the generated shots being perpendicular to each other. This enables a high resolution seismic survey to be conducted whilst minimising the required number of AUVs and efficient use of both the deployment vessel 20 and moveable transmitter 40.

[00130] Although in the above described embodiments, one moveable transmitter is used, it will be understood that at least two movable transmitters may be used to each transmit a series of shots in a predetermined pattern to be detected by the self-propelled receiver nodes in the receiver array. Each movable transmitter may move along independent paths in which each movable transmitter transmits the series of shots along their path to define a plurality of successive shot lines, with each shot line extending in the first, inline, direction. In such an arrangement, the shot lines may not overlap. Each movable transmitter may move along a path covering a distinct portion of the array.

[00131] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.