GB2507532A

GB2507532A - Seabed receiver with arms having a tensioning element

Info

Publication number: GB2507532A
Application number: GB201219721A
Authority: GB
Inventors: Anthony Allan Greer; Jonathan Charles May
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-11-02
Filing date: 2012-11-02
Publication date: 2014-05-07
Also published as: GB201219721D0

Abstract

A seabed receiver for marine controlled source EM (CSEM) and magneto-telluric (MT) surveying has a central unit 10 and a plurality of sensor arms 16. Each arm is mounted to the central unit by an arm mount 42,44 via which the arm can rotate. Each arm mount cooperates with a tensioning element 50 which exerts an increasing corrective force on the arm when the arm is rotated away from horizontal towards the vertical, so the arm will always be urged to regain its horizontal position when it is aligned partly or fully upwards. This folding arm arrangement allows the receivers to be deployed from a moving vessel through a chute. When a receiver drops out of the end of the chute into the water, the arms only partly unfold, since drag during the descent through the water keeps them oriented upwards. Once the receiver lands on the seabed, the arms fold out to the horizontal position under torque exerted by the tensioners. During recovery, the arms fold down, speeding ascent of the receiver to the sea surface. Also disclosed is a chute for deploying seabed receivers from a marine survey vessel and a recovery buoy for seabed receivers.

Description

I

TtLE OF THE lNVENflON Seabed Recevers

BACKGROUND OF THE lNVENTON

The invention relates to the design, deployment and recovery of seabed receivers as used for geophysical surveying.

Seabed or seafloor receivers are used in the field of marine geophysical surveying. In particular, seafloor electromagnetic (EM) receivers are used in the field f mar-inc electromagnetic surveying, specificaUy for controlled source EM (CSFM) and magneto-telluric (MT) surveying. An MT survey involves the deployment of an array of receivers on the sea floor, typically 50 to 200 receivers. Whilst deployed on the seafloor the EM receivers can measure the earth's electric and/or magnetic fields depending on their design. The period of recording depends upon the aims of the survey. t may range from a number of hours up Lo weeks and months, or even longer for long-term monitoring applications. However for structural mapping in oil exploration, it typically ranges between three and seven days.

US5770945 (Constable) and US7116108 (Constable) describe conventional EM receivers. Conventional EM receivers include electric field sensors which are held at the ends of arms which are several metres long and extend from a central unit. An arm length of 3-10 m is usual with 5 m being typical. The central unit is most often of a generally box shape with width, depth and height dimensions in the range 1-3 m The arms are arranged in pairs extending horizontally in opposite directions from the central unit, electrodes at the end of each pair forming an electric dipole. Typically there are two pairs of arms to form two dipoles which are orthogonal to each other in the horizontal plane to detect electric field in the x and y directions as considered in a Cartesian coordinate system. In some designs, for example as shown in US71 16108, a vertical arm is also included to measure the vertical electric dipole component of the electric field, i.e. the electric field in the z direction. The vertical arm may be rigid, similar to the horizontal arms, or may be flexible and rely on a buoyancy element at its distal, upper end to keep it vertical when the receiver is on the seafloor.

Conventionally, the arms achieve the necessary rigidity being fabricated from a hollow rod plastics material, such as polyvinyl chloride (PVC) or polypropylene (PP), or from fibreglass, and extend out horizontafly, with some gravity-induced droop. A tube diameter for the rod of perhaps 5O1 00 mm is usual. The tubular nature of the rod allows the cable that connects the electrode at the end of the arm to the central unit to be routed through the tube which thereby acts as a protective sleeve.

W02007/141548A2 (Ellingsrud) discloses another design approach for the arms in which the cables are run alongside the rod. One advantage here is that the rod can be made thinner (and solid), which gives it less hydrodynamic drag during deployment and recovery. This not only improves the speed of deployment and recovery, but also allows more accurate deployment by reducing the amount of lateral drift as the receiver free falls from the sea surface to the seabed. Another advantage is that, for storage, the rods can be detached and separately stored from the rest of the receiver, which reduces the space needed to store the receivers.

Figure 1 shows a conventional receiver 1 as it is being held by a crane hook 2, for example as it would be hanging over the water immediately prior to being lowered into the sea for deployment. Most of the basic parts of the receiver are visible. A central unit 10 is made from a metal frame and includes near the top a set of floats 12. The central unit 10 accommodates one or more pressure vessels 8 for packages, i.e. the electronics needed to support the instruments, control the data acquisition, communicate with the vessel etc. A loop 14 is also arranged at the top of the central unit 10 via which the receiver can be handled by the crane hook 2. A first pair of arms 16 extends from the central unit 10 in the x-direction and a second pair of arms 18 extends from the central unit lOin the y-direction, each arm, 16, 18 having an electrode 20, e.g. silver-silver chloride, mounted at its distal end. Magnetic induction coil sensors 24 are also provided. A ballast weight 22 is releasably attached to the base of the receiver central unit, which assists in sinking the receiver for deployment, and when released allows the receiver to float back to the surface for recovery, the receiver without the ballast weight 22 being positively buoyant by virtue of the floats 12.

To deploy a seabed receiver, the current surveying process with a rigid arm receiver of the Constable type is as follows: o The survey vessel travels to the location where the receiver is to be dropped, as defined by the survey plan.

The vessel is slowed down from its previous transit speed.

With the vessel stationary or moving slowly a recelve is attached to the boom of a crane by engaging a hook into a lOop at the top of the receAver central unit.

The crane lifts the receiver over the side of the vessel to a suitable distance away from the vessel so that the receiver arms are well clear of the vessel.

The crane either reaches down or pays out wire to lower the receiver to the water leveL * When the receiver is in contact with the water it is released.

The receiver then sinks to the seafloor under its own weight which includes the weight of the ballast unit which is typically made of concrete.

The vessel then speeds up again to transit to the next receiver drop site.

After the receivers have collected all the data for the survey, they need to be recovered onto the vessel. To recover a seabed receiver, the current surveying process is as follows: * The vessel approaches the receiver site and sends an acoustic signal to the receiver which commands t to release its ballast weight. Upon release of the ballast weight the receiver is positively buoyant and floats to the sea surface.

Release of the ballast weight also frees up a stray line and buoy1. The stray line is a floating rope approximately 10 m long and has a float on the end to act as the buoy.

* When the receiver reaches the sea surface, it floats partially above the water and the stray line floats a small distance away from the receiver.

Upon approaching the receiver site the vessel slows down to a near standstill or completely stops.

A grappling line is then thrown overboard to catch the stray line. The stray line is then fed onto the crane so that the receiver can be lifted out of the water and onto the deck.

The vessel then speeds up again and moves to the next receiver site.

in rough sea conditions it can become too dangerous to use the crane and recover receivers from the water. Receiver recovery operations then have to be halted, and this is a common cause of weather delays for the whole survey process.

US5770945 (Constable) and US7116108 (Constable) envisage that the arms are rigidly attached to the central unit and this has become the standard design.

However, it is also known, for example from Sinha et al and US6842006 (Confi & Nichols), to mount the receiver arms with a hinge or pivot at their proximal end where they are connected to the central unit.

In the receiver described by Sinha et al, which is a very early design, the proximal end of each arm is attached by a hinge to the ballast weight (which would be replaced by the central unit in more modern designs) allowing the arm to hinge through 800 consisting of 70° of freedom above horizontal and 100 of freedom below horizontal. The above4iorizontal movement allows the arms to fold upwards during its free fall to the seabed.

In the receiver described in US6842006 (Conti & Nichols), hinged arm mounts are also used, as shown in Figure 2, which illustrates an arm 16/18 being hinged by a pin 26 passing through through-holes in two stubs 28 extending from a face 30 of the central unit 10. The receiver design is otherwise the same as the Constable receiver.

The hinges have approximately ±900 of freedom from vertically up to vertically down.

During deployment, the arms initially hinge down near vertically under gravity as the receiver is craned out from the vessel and then immersed in the sea. Once the crane releases the receiver and it starts its descent through the water column to the seafloor, the arms hinge up near vertically because of the drag induced by the descent. The ability of the arms to pivot reduces the drag of the receiver, the drag reduction reducing the time it takes for the receiver to sink to the seafloor. During recovery, as the receiver (less ballast weight) ascends to the surface, the arms hinge down, once again reducing drag compared with a receiver with arms that are rigidly attached, thereby reducing the ascent time. When not in use, the arms can also be hinged up to reduce the storage footprint.

SUMMARY OF INVEN11ON

RECEIVER DESIGN

There is provided a seabed receiver comprising: a central unit; and a plurality of sensor arms extending outwardly from the central unit, wherein the seabed receiver comprises for each arm: an arm mqunt on which the associated arm is pivotally mounted on the central unit; and a tensioning element arranged to exert increasing corrective force on the arm when the arm is angularly displaced in at least one angular direction away from a defined angular position, forcing the ami to pivot about the arm mount to return to the defined angular position.

For each arm the tensioning element an be an elongate resilient member pivotaily mounted at one end to the central unit and at its other end to the associated artn Ma distance away from the arm mount. In this case, the tensioning element can be mounted to the arm at a proximal end of the arm. Although this is the preferred implementation, in principle the tensioning element-to-arm mount could be in an arbitrary position between the arm mount and the proximal end of the arm. The resilient members can be arranged to extend and tension when the arms are angularly displaced in said at least one angular direction away from said defined angular position.

Alternatively, for each arm, the tensioning element can be a torsion spring arranged integrally with the arm mount. The torsion springs can be arranged so they wind up when the arms are angularly displaced in said at least one angular direction away from said defined angular position.

Of course a mixture of resilient members and torsion springs could be used for different arms, or each arm could be provided with both acting cooperatively.

In one group of embodiments, the arms extend tangentially outwardly from the periphery of the central unit. Alternatively, the arms could extend radially outwardly from the central unit.

The receiver may also be provided with an arm locking mechanism for fixing the arms at a particular angular position. The arm locking mechanism may be some means for pinning the arms together in a vertical position (either vertically up or verticaHy down), such as a hoop, loop or band arranged around the arms, or may be some form of lock or clamp for the arm mounts to prevent them from pivoting from the locked position. An arm mount ock can in prindple lock the arms in any desired angular position of the arms, but in practice is intended to provide for locking the arms in a vertically up position in which the tensioning elements are in a state of tension so would otherwise exert a torque on the arms to move them back to horizontal.

In use, the arms are movable between: a horizontal position in which for each arm the tensioning element is not tensioned, or at least not tensioned enough to induce any angular displacement of the arm away from the horizontal position; and an upward position in which for each arm the tensioning element is tensioned to exert a force on the arm forcing the arm to pivot about the arm mount to return to the horizontal position.

Optionally, the tensioning elements may be such that the horizontal position is an equilibrium position in which perturbations of one of the arms away from the horizontal position in at east one angular direction tensions that arm's tensioning element, thereby exerting a corrective force to return that arm to the horizontal position.

in use, the arms may be additionally movable to a downward position in which for each arm the tensioning element is not tensioned, or at least not tensioned enough to induce any angular displacement of the arm away from the downward position Optionally, the tensioning elements may he such that the downward position is an equilibrium position in which perturbations of one of the arms away from the downward position in at least one angular direction tensions that arm's tensioning element, thereby exerting a corrective force to return that arm to the downward position.

By "upward position", we generally mean vertically up and by "downward position", we generally mean vertically down, although it will be appreciated that small angular variations from vertical are included so long as the distal ends of the arms remain relatively closely bunched together with a footprint comparable to that of the central unit, e.g. no more than 2 or 3 times the footprint of the central unit.

By horizontal, we do not necessarily mean precisely horizontal with some defined angular variation above and below that. This is because, in some implementations, the preferred horizontal position of the arms may be pointing slightly downwards to lightly press the arms onto the seabed when the receiver is deployed. The downward angle associated with the horizontal position will depend on factors such as the strength of the tensioning elements, the length of the arms and the height of the arm mounts on the central unit.

In use, the arms may also adopt a part upward position in which for each arm the tensioning element is in tension to balance drag force exerted upwardly on the arm as the receiver descends through a water column, such that when the receiver lands on the seabed and the drag force substantially ceases, the tension is released by the arm pivoting about the arm mount to the horizontal position.

As stated above, movements of the arms away from horizontal and downward positions may produce tension in the resilient member which will exert a corrective force to return the arm to its equilibrium position. However, It is worth noting this effect may only exist when the movement is in one direction. When the movement is in the opposite direction, there may or may not be a corrective effect depending on the design of the arm mount and also on whether the resilient member is capable of storing energy under compression. If the resilient member is like a rubber band, there can be no corrective effect, since it is not compressible and will just become slack, in which case the horizontal and downward positions are points of Inflection adjacent to a neutral equilibrium. If the resilient member is like an automotive coil-over-oil spring and cannot bend, then it will compress and a corrective effect analogous to the correction under tension can be provided, in which case the horizontal and downward positions can be made to be points of stable equilibrium. If the resilient member is a relatively stout helical spring or a relatively stiff rod, then it will bend and the force seeking to straighten it will provide some corrective more limited effect, in which case the horizontal and downward positions are points of stable equilibrium albeit asymmetric.

In one group of embodiments, a seabed receiver of the kind comprising a central unit and a plurality of sensor arms extending outwardly from the central unit is provided in which each arm has an arm mount on which the associated arm is pivotally mounted part way along its length and in which a variable length strut, preferably formed as a tensioning element, is pivotally mounted at one end to the central unit and at its other end to the associated arm at a distance away from the arm mount. The arms may extend tangentially outwardly from the periphery of the central unit, the struts being mounted on the central unit angularly displaced along the periphery from their respective arm mounts. Alternatively, the arms may extend radially outwardly from the central unit, the struts being mounted on the central unit radially inwardy of their respective arm mounts. If the variable length struts are tensioning elements, these can be any form of resilient members that vary in length as a result of being tensioned (or compressed) by loads exerted via the arms, for example rubber or rubber-like rods or helical springs.

In one group of embodiments, a seabed receiver of the kind comprising a central unit and a plurality of sensor arms extending outwardly from the central unit is provided in which an arm mount on which the associated arm is pivotaily mounted on the central unit; and a torsion spring is arranged integrally with the arm mount to exert increasing corrective force on the arm when the arm is angularly displaced in at least one angular direction away from a defined angular position, forcing the arm to pivot about the arm mount to return to the defined angular position.

In summary, the receiver is provided with articulated sensor arms, the arms being pivotable at a coupling with the receiver body, such as rotatable about an axle or hinge, or more freely pivotable about a ball joint. Through the deployment, recording and recovery stages of a survey, the arms are in different orientations relative to the central unit.

When folded upwards, the arms need to be retained by an external force countering the force exerted by the tensioner. This can be provided by a retention loop arranged around the receiver arms whilst on deck, or by the deployment chute during deployment. Alternatively, this can be provided by some locking mechanism for the arm mount to prevent it pivoting the arms away from the upwards position.

A receiver with the arms in either the vertically up or down positions has a footprint substantially the same as the central unit. For example, if the central unit has a footprint of 1 m x 1 m and the receiver arms are 5 m long, then the footprint is only 1 m x 1 m, rather than 11 m as it would be if the arms were fixed horizontally. An order of magnitude reduction in the storage footprint is therefore achieved.

RECEIVER DEPLOYMENT

There is provided a Kit for a survey vessel comprising: a set of seabed receives of a given type, each comprising a central unit and a plurality of sensor arms; and a chute which is attachable to or adjacent to a deck of a vessel, the chute having an entrance, a guiding porton and an exit dimensioned respectively to permit receivers of the given type to be inserted in, pass through and leave the chute with the sensor arms exterdina behind the receiver.

The kit may further comprise a deployment buoy which is attachable to the chute so as to permit a receiver leaving the chute exit to commence tree descent through the water column. The deployment buoy can have a hull with, an aperture and is attachable to the chute so that the chute exit is aligned with the aperture, so that a receiveris free to pass through the chute and the aperture. The hull of the deployment buoy can have a twin hull structure with first and second hull portions, the aperture being a gap formed between the first and second hull portions. A single hull design could also be used. The deployment buoy may include a steering device which steers the deployment buoy away from the vessel when it is being towed beside the vessel and at least one tether for limiting the distance between the vessel and the deployment buoy as the deployment buoy is being towed by the vessel.

Opposing forces of the steering device, e.g. a keel or rudder, and the towing cable can he used to control the deployment buoy's distance from the vessel by paying out more or less tether cable.

There is provided a method of deploying seabed receivers from a survey vessel, comprising: attaching a chute as specified in the kit of claim 1, to or adjacent to a deck of the survey vessel; and sailing the vessel at a desired deployment speed along a desired deployment route while placing the receivers into the entrance of the chute in sequence at desired intervals, so the receivers descend through the water and fall to the seabed at a plurality of survey locations along the route in readiness to acquire survey data.

The method may further comprise attaching a deployment buoy as specified in the above kit to the chute exit, and placing the deployment buoy in the water to one side of the survey vessel.

There is provided a survey vessel comprising: a set of seabed receivers of a given type, each comprising a central unit and a plurality of sensor arms; arid a chute attached to or adjacent to a deck of the vessel, the chute having an entrance, a guiding portion and an exit dimensioned respectively to permit receivers of the given type to be inserted in, pass through and Heave the chute with the sensor arms extending behind the receiver.

The survey vessel may further comprise a deployment buoy attached to the chute so as to permit a receiver leaving the chute exit to commence free descent through the water column.

Receivers with a variety of foldable mounts for the arms can be used with the deployment chute and buoy system, including simple hinges, axles or ball joints to provide pivoting or rotation as desired.

RECEIVER RECOVERY

There is provided a recovery buoy for seabed receivers of the type comprising a central unit and a plurality of elongate sensor arms, comprising: a hull shaped to travel through the water in an intended direction of travel and defining a waterline; and a plurality of capture veins fixed to the hull and aligned with the direction of travel, the capture veins extending at an angle forwards and downwards below the waterline, and being spaced apart transverse to the direction of travel.

The hull of the recovery buoy can have a twin hull structure with first and second hull portions, the capture veins being arranged between the first and second hull portions.

A single hull design could also be used. The recovery buoy can include a steering device which steers the recovery buoy away from a vessel when it is being towed beside the vessel and at least one tether for limiting the distance between the vessel and the recovery buoy as the recovery buoy is being towed by the vessel.

There is provided a kit for a survey vessel comprising: a set of seabed receivers of a given type, each comprising a central unit and a plurality of sensor arms foldably or detachably mounted to the central unit: a recovery buoy as specified in the above kit, wherein the veins are separated by a distance sufficiently small to capture receivers Hying in the water. The vein separation is preferably sufficiently large to permit the sensor arms to pass freely between them when the sensor arms are oriented downwards from the central unit.

There is provided a method of recovering a seabed receiver of the type comprising a central unit and a plurality of sensor arms pivotally mounted to the central unit, comprising: deploying a recovery buoy as specified in the above kit from the vessel; saling the vessel to a vicinity where a seabed receiver is floating on the sea sur ace ready for recovery; sailing the recovery buoy to the receiver so that the receivers centre! unit engages with the angled portion of the capture veins owing to the foard motion of the recovery buoy, thereby to capture the receiver; and lifting the receiver from the recovery buoy onto the vessel.

it is noted the recovery system can be used for any design of receiver in which the arms can be in a downward position at the time of recovery. For examp!e, it can be used with receivers with hinged arms as described in US6842006 (Conti & Nicho!s) which have no tensioning elements, t could also be imagined that it cOuld he used for receivers with arms that are detachable after the end of data acquisition, for example which release from rigid attachment to the central unit at the same time as detaching the ba!last weight and then dangle freely being retained by a tether between the proximal arm ends and the centra! unit.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, the invention is described with reference to the following drawings aR of which are schematic.

Figure 1 is a perspective view of a receiver as described in US5770945 (Constable) with arms rigidly mounted to a central unit.

Figure 2 is a perspective view showing an arm hinged at the central unit as described in US6842006 (Conti & Nichols) which is a modification of the design of Figure 1.

Figure 3 is a plan view of a seabed receiver according to an embodiment of the invention with the arms in a horizontal position.

Figure 4A is a more detailed plan view of the receiver of Figure 3.

Figure 4B is a side view of the receiver shown in Figure 4A.

Figures SA to SD show the receiver arms in four different positions which are adopted at different times during use of the receiver.

Figures 6A to 6D correspond to Figures 5A to SD respectively, but are zoomed out to show the four positions more clearly in the context of use during deployment, data acquisition and recovery.

Figure 7 shows an alternative embodiment with a box shaped, i.e. square section, central unit.

Figures 8A and 83 are plan and side views of an alternative tensioner design using a torsion spring.

Figures 9A and 9B are schematic side and plan views of a survey vessel with a receiver deployment buoy and chute.

Figures bA, lOB and IOC are schematic side views of a survey vessel with recovery buoy at three different stages during recovery of a receiver from the sea.

DETAILED DESCRIPTION

Firstly, it will be understood that references herein to seafloor, seawater etc. should not be regarded as limiting and should be interpreted as covering lakebed, riverbed etc., since the apparatus and methods described herein are equally applicable to surveying of freshwater, for example in lakes and estuaries.

RECEIVER DESIGN

Figure 3 is a plan view from above of a seabed receiver 1. The receiver I comprises a cylindrically shaped central unit 10 and a plurality of elongate sensor amis 16, 18 extending outwardly from the central unit. More specifically, the arms 16, 18 extend tangentially outwardly from the periphery of the central unit 10. The arms are arranged in first and second pairs 16 and 18 respectively extending horizontally in opposite directions from the central unit 10. An electrode (not shown in Figure 3) is situated at the distal end of each arm 16, 18, with the two electrodes of each pair of arms forming an electric dipole. The electrodes may be silver-silver chloride for example. There are two pairs of arms to form two dipoles which are orthogonal to each other in the horizontal plane to detect electric field in the x and y directions as considered in a Cartesian coordinate system.

Figure 4A is a more detailed plan view from above of the receiver. For simplicity, only one of the arms 16 is shown, although it will be appreciated that this is the same four-arm receiver as shown in Figure 3. Moreover, only the portion of the arm 16 nearest the central unit 10 is illustrated. The arm is in a horizontal orientation, i.e. the same orientation as in Figure 3.

Figure 48 is a side view of the receiver with the arm in the same position as shown in Figure 4A.

The central unit 10 is approximately cylindrically shaped with the cylinder shape being defined by a cage defined by frame pieces 32, 33, 34, 35, 36, 37 made of metal, e.g. stainless steel, or other suitably strong and corrosion resistant material.

An upper hoop 36 and a lower hoop 37 are connected to each other by metal parts 32, 33, 34, 35 having vertical portions (see Figure 4B), horizontal top portions (see Figure 4A) and horizontal base portions (not visible), wherein the junction between the vertical and horizontal portions is formed by a 90 degree bend. The horizontal portions extend radially inwardly, in the manner of spokes of a cartwheel, from the circumference (i.e. periphery) of the central unit 10 towards the central axis of the cylinder shape and are commonly terminated at the top and bottom with fixings to a spine piece 38 which extends axially through the cylinder. The parts 3237 are iflustrated as sheets, but could be bars, or any other suitable construction. In its interior, the central unit 10 accommodates one or more floats (not visible) for buoyancy and one or more electronics packages (not visible) for the instruments and for control, e.g. to support the instruments, to control the data acquisition and to communicate with the vessel. A loop (not shown) is also arranged at the top of the central unit 10 via which the receiver can be handled by a crane hook.

A baflast weight 22, which may be made of a bio-degradable concrete, is releasably attached to the base of the central unit 10, which assists in sinking the receiver for deployment, and when released allows the receiver to float back to the surface for recovery, the receiver without the ballast weight 22 being positively buoyant by virtue of the floats provided in the interior of the central unit 10. The release mechanism (not shown) is controllable via a controller, which in turn has a communication link for establishing communication with the vessel.

The arm design is now described in more detail.

As is known in the art, the arm 16 is fabricated from a hollow or solid plastics material, such as polyvinyl chloride (PVC) or polypropylene (PP), or from fibreglass, for example. With a hollow construction, cables can be routed inside the arm, With a solid construction, cables will be routed outside the arm.

The arm 16 in the position illustrated extends tangentially to the circumference of the cylindrical central unit lOin a horizontal direction. However, the arm is not fixed in the illustrated position. Rather, it is rotatably mounted on an axle 44 which defines a pivot axis, which makes the arm rotatable in a plane that is tangentially aligned relative to the surface of the nominal cylinder. The arm is able to rotate from the illustrated horizontal position to a vertically up position or to a vertically down position (at least when the ballast weight is not present). The arm 16 is mounted to the axle 44 with a Tbar shaped sleeve arrangement 42. The axle 44 and sleeve arrangement 42 form an arm mount on which the arm 16 is rotatably mounted on the central unit 10 part way along its length. The main portion of the length of the arm, perhaps 3-10 m in length, extends towards the electrode, but there is a shorter arm portion the other side of the pivot axis that is connected at its end (or at least some distance away from the pivot axis) via a connector 40 to one end of a tensioner 50, the other end of which is secured to the central unit 10 ata connector 39. The connector 39 s shown as being secured to the upper frame hoop 36 and the axle 44 is shown as being secured to the lower frame hoop 37. The tensioner 50 is illustrated as a resient rod, e.g. a rubber or rubberlike rod, which stores energy when extended in tension, but which is relatively resistance-free in compression. Various implementations could be used such as elastic rod, as mentioned, twisted elastic band. a spring in partcular a helical spring, or a suspension strut. Springs could be made of a non4errous metal or ceramic to inhibit seawater corrosion. It will be appreciated that the state of tension of the tensioner varies with the rotational position of the arm, and this is described in more detail below Rotation of the arms between various positions is now described.

Figures 5A to SD show the receiver arms in four different positions which are adopted at different times during use of the receiver. Reference numerals are for the most part omitted, since it is the same receiver as shown in the previous figures.

Figure 5A shows the arms oriented vertically upwards. In this position the tensioner is in a high degree of tension. To maintain this position, for example as desired for storage, it is required to restrain the arms in some way. In the illustrated design, the arms are shown restrained by a loop 45 arranged some distance above the top of the central unit and enclosing the arms. The ioop 45 may be made of rigid or flexible material and can be unfastened at one or more points so that it can be removed and replaced as desired. Other ways to restrain the arms include provision of a clamping mechanism to clamp the arms to the frame of the central unit, or a locking mechanism to lock the arm mount to the axle in the case that the arm mount is journalled on a fixed axle 44. If the axle 44 is fixed to the arm mount and rotates in a bush or bearing in the central unit, then it is only required to lock the axle to prevent it from rotating.

Figure 5B shows the arm oriented partly upwards. This position is adopted when the receiver is falling through the water column under gravity and an equilibrium is reached between hydrodynamic drag on the receiver arms and tension in the tensioner. In this position, the amount of tension in the tensioners is less than in the vertically upward arm position of Figure 5A. During descent, the arms are not completely vertical, but have a relatively small horizontal component of extent which provides some additional stability.

Figure SC shows the arm in the horizontal position shown in Figure 3 and Figures 4A and 4B. In this position, there is little or no tension in the tensioner. The design may make the zero tension position when the arm is precisely horizontal. However, as illustrated, the design may make the zero tension position when the arm is pointing down slightly, e.g. 5-10 degrees below horizontal, so that when the receiver is on the seabed the distal end of the arm is lightly pressed down onto the seabed. By trigonometry, the most appropriate angle will depend on the height of the pivot axis above the base of the ballast weight and the length of the arm portion from the pivot axis to the distal end of the arm as well as the strength of the tensioner. The precise implementation may also depend on the rigidity of the arm, since if the arm is relatively floppy, gravity may be sufficient to ensure that the distal end of the arm rests on the seabed, whereas if the arm is relatively stiff, the distal end of the arm may not rest on the seabed if the equilibrium position of the tensioner is with the arm precisely horizontal.

Figure 5D shows the arm oriented vertically downwards. The ballast weight is not shown, since in use this arm position is expected when the receiver is floating back to the surface during recovery. In this position, there is little or no tension in the tensioner. The design preferably makes the zero tension position when the arm is precisely vertically downwards. Depending on the shape and size of the ballast weight, this arm position may or may not he adoptable with the ballast weight attached.

Figures 6A to 6D correspond to Figures SA to 5D respectively, but are zoomed out to show the four different arm positions more clearly in the context of use during deployment, data acquisition and recovery.

When on the vessel prior to deployment, and also during the initial phase of deployment, the arms are vertically upwards as shown in Figure 6A.

As the receiver descends through the water column towards the seabed during deployment, an equilibrium is reached between hydrodynamic drag on the receiver arms and tension in the tensioners and the part upward position is adopted as shown in Figure 6B. The direction of descent is shown by the arrow. Some curvature in the arms is also shown, indicating their semi-rigid nature.

Once the receiver central unii lands on the seabed. the tension in the tensioners is released rotating the arms to fold them out into their desired orientation for data acqusthon, i.e. horizontal or sVghtly below horizontal in opposed pairs as shown in Figure SC.

When the data collection has been completed and the receiver is ready to be recovered, an acoustic signal is sent to the receiver to cause the receiver to release its ballast weight. After release from the ballast weight, the receiver is positively buoyant and ascends to the sea surface. When the receiver moves away from the seabed, drag on the anrns folds them into the vertically downward position as illustrated. The direction of ascent is shown by the arrow. WhUst the receiver is moving upwards thorough the water column, hydrodynamic drag exerts a downwards force on the arm. The arm tensioners are effectively slack and do not act in this direction, therefore the arms remain in the verticafly downwards position. As the ballast weight has been left at the seafloor, its wider footprint does not inhibit the receiver arms from orientating vertically downwards.

A benefit of this configuration whilst the receiver is in the water column is that it has a much reduced hydrodynamic drag compare with a fixed arm receivers meaning that, for a given positive buoyancy. the articulated arm receiver will rise more quickly through the water column than a fixed arm receiver. This brings operational and buoyancy design benefits.

It will be understood that the central unit does not need to be cylindrical.

Figure 7 shows an alternative embodiment with a box shaped, i.e. square section, central unit 10 which is comparable to Figure 3, i.e. with the arms 16, 18 in the horizontal deployed position.

Figures BA and SB are plan and side views of an alternative tensioner design using a torsion springS The side view of Figure SB is a view from inside the receiver central unit looking out. The arm is shown in the horizontal position, but aligned slightly below horizontal (same position as in Figure SC). A torsion spring 52 has a central helical winding 59 arranged around the axle 44 and two straight end portions 55 and 57, one of which 55 is fixed to the receiver, for example by a spigot 54 protruding inwardly from the frame pieces 32/37, and the other of which 57 bears on a spigot 56 protruding inwardly from a rotation plate 58 which is rotatably arranged, for example with a bearing or bushing, in the frame pieces 32/37. The arm 16/18 is held in a 1-bar shaped sleeve arrangement 42 which has a similar shape and function to the corresponding part in the above described embodiment using a resilient rod. The T-bar sleeve can be retained on the axle by a circlip arrangement 51, for example, as schematically illustrated in Figure BA. The arm 16/18 extends to the right in Figure CA and Figure BB. (Compared with the above described embodiment using a resilient rod, in this embodiment there is no need for the arm to extend through the axis of rotation, and in Figure BA and 8B no such extension is shown.) The torsion spring arrangement is such that in the position illustrated in Figure 8B, the torsion spring is in equilibrium with no stored energy. If the arm is rotated clockwise from the illustrated position, the spigot 56 moves out of contact with the end portion 57 of the torsion spring and the arm can freely rotate to a downwards position, e.g. under gravity. If the arm is rotated anticlockwise from the illustrated position, the spigot 56 forces the end portion 57 of the torsion spring to follow it and the helical part 59 of the torsion spring is wound up to place the arm in increasing amounts of tension.

Other variations of the receiver design are also possible.

For example, vertical sensor arms can be included as desired for detecting the z-component of electric field, as described in US71 16108 (Constable). The vertical arm could stay vertical in the phases shown by Figures 6A, 68 and 6C. However, during the recovery phase shown in Figure 60, it may be beneficial to incorporate some design features to allow the vertical arm to aUgn downwards with the other arms.

Moreover, although the main use of the arm design is for electric field detectors, the arm construction is generic to the type of detector, so could be used for magnetic

field detectors or any other detector type.

The embodiments have been described with axle mounting of the aims to allow arm rotation in a single plane. In other designs, a ball joint could be used, leaving the arms to pivot more freely.

For storage of the receivers on the vessel when not in use there are two main options, namely the receivers can be stored with the arms vertically upwards as in Figure 5N6A or vertically downwards as in Figure 5D/60, Storage in Inc vertically up position means that the tensioners are under tension and would require restraint to keep them in position. Depending on the amount of stored energy, there could also be degradation and potentiaiiy also safety considerations.

However, with this storage position, the receivers are ready for deployment, thereby avoiding any risk that the receivers would not be ready to deploy when required.

Storage in the vertically down position means that the tensioners are not under tension, or at least not much tension. However, the arms would need to be cranked over through 180 degrees to the vertically up position prior to deployment.

With both storage options it is envisaged that the receivers will be stored lying on their sides with the arms streaming out horizontally together. In addition, the receivers can be horizontally stacked by arranging the central unit of one receiver in between the arms of the next receiver to achieve a high storage density.

RECEIVER DEPLOYMENT

Receiver deployment is now described with reference to Figures 9A and 9B which are schematic side and plan views of a vessel 5 with a purpose-built receiver deployment buoy 80 and chute 70 deployed from its side. One end 72 of the chute 70 is attached to the operating deck 6 of the vessel. The other end of the chute 76 is attached to the deployment buoy 80. The chute 70 is dimensioned so that receivers, with their arms folded behind them (Figure 5A), can be dropped into the chute 70 and then guided away from the side of the vessel to the deployment buoy 80 where they drop out of the end of the chute 76 into the water. The chute 70 thus has an entrance 72, a guiding portion 74 and an exit 76. The deployment buoy 80 serves to keep the exit 76 of the deployment chute away from the side of the vessel to ensure that receivers dropping out of the chute into the water do riot interact with the side of the vessel or the vesseVs propulsion systems.

The cross-section of the main guiding portion 74 of the chute is matched to the shape and size of the receiver central unit (plus vertically folded arms if they protrude beyond the periphery of the central unit), i.e. has a circular cross-section for the cylindrical receivers described above with the section sized to be a little larger than the section of the receiver, e.g. approximately 5-20% larger than the receiver central unit plus vertically folded arms. The entrance portion 72 of the chute may be larger and, for example, taper down into the guiding portion.

The chute exit 76 is arranged in or adjacent the deployment buoy 80 so as to permit receivers leaving the chute to pass freely into the water column and commence their descent to the sea floor. In this embodiment, to achieve this, the deployment buoy has a twin hull structure with first and second hulls 82 and 84 joined by lattice frame pieces 86 a gap being formed between the hulls. The end of the chute 76 is aligned with the gap between the hulls 82 and 84, so that a receiver is free to pass through the chute and the gap between the hulls into the water. in other designs, the end of the chute may be to one side of the buoy, e.g. to the side of a hull in a single hull structure on the side farther away from the vessel.

The deployment buoy also has a keel (or two keels in the present case of a twin hull catamaran design). The keel serves to pull the base of the chute away from the side of the vessel and is counteracted by a wire or cable tether 60 from the vessel 5 to the deployment buoy 80 which limits the maximum distance of the deployment buoy from the vessel. The deployment buoy can therefore be kept at a fixed distance from the side of the vessel.

Optionally, the deployment buoy can be provided with a rudder to provide a steering mechanism which is controllable from the vessel, e.g. wirelessly with an RF transmitter. For example, a hand-held control unit can be provided to establish a communication link with the deployment buoy to control the steering mechanism by an operator on the operating deck.

The deployment chute and buoy enables transfer of receivers from the vessel operating deck to the sea surface whilst the vessel is in motion at considerable speeds, e.g. up to 5 or 10 knots. This allows a significant time saving to be gained compared with the situation where the vessel needs to be stopped or slowed to a very slow speed for each receiver deployment. For MT surveys in particular, the number of receivers that need to be deployed can be quite large 100-200 so the time savings are then considerable.

As a variation, it is noted that receiver deployment with a chute and no deployment buoy could be envisaged. This would function so long as the chute was long enough and/or inflexible enough to ensure that the chute exit could be kept far away from the vessel.

During a survey, the receiver deployment process is as follows. flA

The survey vessel is steamed to near the start of a desired receiver deployment route along which the receivers will be dropped at certain locations or at certain intervals.

The seabed receivers, for example 5O2O0, are readied for deployment by placing them somewhere convenient on the operating deck, each of the receivers having its arms in the vertically upwards position retained by a retention loop (Figure 5A) and a ballast weight fitted.

The survey vessel is stopped or slowed to a ow speed to deploy the deployment buoy and associated chute. The deployment chute is attachid at one end to the operating deck and at its other end to the deployment buoy. The deployment buoy with the end of the chute attached to it is lowered over the side of the vessel by a crane (not shown), for example a knuckle crane.

The survey vessel is then sped up to a receiver deployment speed, e.g. 5-10 knots, and the deployment buoy is maintained a desired lateral distance from the vessel, e.g. 5 m. The position on the deployment route for dropping the first receiver is then approached.

The first receiver is lifted by the crane 104 from its resting position on the deck and held in the entrance portion of the chute such that the retention loop, which is 1 or 2 metres above the top of the central unit, is still accessible to an operator standing on the operating deck. The retention loop is then manually removed, releasing the arms which then become pinned by the walls of the chute. The receiver is then released by the crane and drops through the guiding portion of the chute, leaving the exit of the chute with the sensor arms pivoted vertically behind the receiver. (An automatic release mechanism could be envisaged, whereby the retention loop is caught and broken open in the entrance portion of the chute when the receiver is dropped.) The arms then partially unfold under action of the tensioner, but remain predominantly vertical as a result of the hydrodynamic drag induced by the gravity induced fall through the water column (Figure 6B).

The receiver lands on the seabed and the arms unfold under action of the force from the tensioner.

Acoustic signals are used to communicate the location of the receiver on the seabed to the vesseL The survey vessel then continues to the next receiver deployment location and the process is repeated, dropping the next receiver through the chute and so forth. The receivers may be dropped at precise locations, or simply at roughly equal time intervals along the deployment path, depending on the survey specification.

Once all the receivers are deployed, the survey vessel is stopped or slowed to a ow speed to recover the deployment buoy and associated chute using the crane.

RECEIVER RECOVERY

Figures bA, lOB and IOC are schematic side views of a survey vessel with recovery buoy a three different stages during recovery of a receiver from the sea.

Recovery of a receiver 1 floating on the sea surface is carried out with a receiver recovery buoy 90.

The receiver recovery buoy 90 is tethered to the side of the survey vessel 5 by wires or cables 92 and 94 attached to front and rear (bow and stern) parts of the hull structure of the receiver recovery buoy 90 from the operating deck 6 of the vessel 5.

The tethers 92, 94 permit the recovery buoy to be towed beside the vessel 5. The receiver recovery buoy 90 has a hull shaped to travel through the water in an intended direction of travel. In the illustrated design a twin hull catamaran hull structure is used comprising first and second hulls 96 and 98 interconnected by lattice frame pieces 100.

Between the two hulls 96 and 98 a plurality of parallel receiver capture elements 102, such as veins, fins, rods or blades, are arranged. From the capture function of the capture elements 102, it will be understood that their precise shape can be varied widely. The illustrated elements 102 are vertically aligned fins, so we use the term fin in the following, but it will be understood that the fin could be exchanged for a rod or any other suitable form of capture element.

In the illustrated twin hull structure, the fins 102 are arranged between the first and second hulls 96 and 98. The fins 102 are collectively fixed to the hull structure and aUgned with the direcdon of traveL The fins 102, specifically their upper or leading edges, slope at an angle forwards and downwards below the waterhne, and are spaced apart transverse to the direction of travel. The fins 102 are separated train each other lateraUy by a distance sufficiently small to capture receivers 1 lying in the water and sufficiently large to permit the arms to pass freely between them. In other words, the separation is somewhat smaller than the size of the central unit, for example between perhaps 10% and 60% of the width, depth or diameter of the central unit, but much larger than the diameter or other relevant width dimension of the arms. The total number of fins may be 201 00 for example with the total width of the array of fins being perhaps 10-20 times the width of the receiver's central unit.

The forward (bow) end of the fins' leading edges extend beneath the sea surface, i.e. the waterline of the hull of the recovery buoy, to a depth that is at least as deep as, preferably somewhat deeper than, the bottom of a receiver's central unit floating at the surface, so that, when the buoy is being towed forwards to capture a receiver floating in the water, the fins will engage with the receiver's central unit part way up the sloping part of the fins.

The back (stern) end of the sloping part of the fins' leading edges extend to above the sea surface, i.e. the waterline of the hull of the recovery buoy, by some distance, so that after a receiver has been captured by the moving recovery buoy, the central unit rides up the fins and comes to rest above the sea surface, either on an upper part of the sloping portion of the fins or further up on a level portion of the fins. The resting position may he stable, i.e. maintained even if the vessel stopped, or unstable and maintained by the forward motion of the recovery ouoy continuously pushing the receiver's central unit up the sloping fins.

The recovery buoy also has a keel (or two keels in the present case of a twin hull catamaran design). The keel serves to pull the recovery buoy away from the side of the vessel and is counteracted by the tethers 92 and 94, so that the recovery buoy is kept a fixed distance from the side of the vessel. Optionally, the recovery buoy can be provided with a rudder to provide a steering mechanism which is controllable from the vessel, e.g. wirelessly with an RF transmitter. For example, a hand-held control unit can be provided to establish a communication link with the recovery buoy to control the steering mechanism by an operator on the operating deck.

The receiver recovery process takes place as follows.

The survey vessel is steamed to near the start of a desired receiver recovery route, based either on the receiver deployment route defined in the survey specification, or the actual receiver locations obtained from acoustic communication between the vessel and the receivers.

The vessel is brought to a stop or near stop and the recovery buoy is deployed, eg.

by being placed into the water from the side of the vessel by a crane 104 on the operating desk 6, such as a knuckle crane. The vessel is then sped up to a receiver recovery speed of for example 5 knots with the recovery buoy being towed to the side of the vessel with the distance being maintained by the tethers 92 and 94.

Once the vessel is approaching the first seabed receiver, a command is sent to the receiver to separate from its ballast weight and float to the surface. The receiver's arms will be pointing down (Figure 6D).

Once the receiver position is detected, e.g. visuaUy on the surface, the vessel is carefully steered to intercept the receiver. Figure 1OA shows this stage of the capture process shortly before the receiver contacts the recovery buoy. At interception, the receiver's central unit engages with and rides part way up the fin array of the recovery buoy owing to the forward motion of the recovery buoy. Figure 106 shows this stage of the capture process. The arms remain dangling down through the gaps between the fins, but the forward motion of the vessel may result in the arms trailing back somewhat under drag as schematically illustrated in Figure lOB.

The captured receiver is then lifted onto the operating deck 6 of the vessel by the crane 104. Before lifting off the captured receiver, the recovery buoy can be pulled into contact with the side of the vessel by reeling in the tethers 92 and 24 or by some other means. Vertical struts (not shown) can also be provided on the recovery buoy rising from the vessel-side of the buoy to rest directly against the vessel's side when the recovery buoy has been reeled in, which inhibit the recovery buoy from bobbing up and down relative to the vessel. These measures will reduce relative motion between the recovery buoy and vessel which makes it easier to couple the crane hook to the top of the receiver central unit, in particular whilst the vessel is in forwards motion. Figure 1OC shows this stage in the capture process, with the recovery buoy 90 alongside the vessel and the receiver 1 being lifted onto the deck by the crane 104.

After lifting the receiver off the recovery buoy, the recovery buoy is now free to capture another receiver, if the recovery buoy has been puHed closer to or abngsde the vesseL it is released back to its capUre distance from the side of the vessel.

The vessel is then sailed to the next seabed receiver 2nd the process repeats until all the receivers are recovered.

The lack of relative motion between the vessel and receiver means that a craning operation is feasible whilst the vessel continues its transit to the next pickup site.

Recovery of receivers is possible with this design whilst the vessel is in motion, e.g. 2-5 knots, which provides a significant time saving for the survey operations, Further time is saved, since, compared with the prior art, the use of the receiver capture buoy system negates the need for the receivers to have stray lines and floats for capture by grappling hook.

t wiD be appreciated that the same crane can be used for deployment and recovery.

REFERENCES

1. US5770945 (Constable) 2 US7116108 (Constable) 3. W02007/1 41 548A2 (Euingsrud) 4. Sinha, Patel, Unsworth, Owen and MacCormack, An Adilve Source E'ectromagnetic Sounding System for Marine use, Marine Geophysical Research, volume 12, pages 59-68, 1990 5. US6842006 (Conti & Nichols)

Claims

CLAS1. A seabed receiver cornpsing: a centra; unt; ana a plurality of sensor arms extending outwardly from the central unit, wherein the seabed receiver comprises for each arm: an arm mount on which the associated arm is pivotally mounted on the central unit; and a tensioning element arranged to exert increasing corrective force on the arm when the arm is angularly displaced in at east one angular direction away from a defined angular position, forcing the arm to pivot about the arm mount to return to the defined angular position.
2. The receiver of claim 1, wherein for each arm the tensioning element is an elongate resilient member pivotally mounted at one end to tho central unit and at its other end to the associated arm at a distance away from the arm mount.
3. The receiver of claim 2, wherein the resilient members are arranged to extend and tension when the arms are angularly displaced in said at least one angular direction away from said defined angular position.
4, The receiver of claim 1, wherein for each arm the tensioning element is a torsion spring arranged integrally with the arm mount.
5. The receiver of claim 4, wherein the torsion springs are arranged so they wind up when the arms are angularly displaced in said at least one angular direction away from said defined angular position.
6. The receiver of any of claims ito 5, wherein the arms extend tangentially outwardly from the periphery of the central unit.
7. The receiver of any of claims ito 5, wherein the arms extend radially outwardly from the central unit.
8. The receiver of any preceding claim, further comprising an arm locking mechanism for fixing the arms in a defined angular position.
9. The receiver of any preceding claim, wherein the arms are movable between: a horizonta position in which for each arm the tensioning element is not tensioned, or at least not tensioned enough to induce any angular displacement of the arm away from the horizontal position; and an upward position in which for each arm the tensioning element is tensioned to exert a force on the arm forcing the arm to pivot about the arm mount to return to the horizontal position.
10. The receiver of claim 9, wherein the arms are additionally movable to a downward position in which for each arm the tensioning element is not tensioned, or at least not tensioned enough to induce any angular displacement of the arm away from the downward position 11 A kit for a survey vessel comprising: a set of seabed receivers of a given type, each comprising a central unit and a plurality of sensor arms; and a chute which is attachable to or adjacent to a deck of a vessel, the chute having an entrance, a guiding portion and an exit dimensioned respectively to permit receivers of the given type to be inserted in, pass through and leave the chute with the sensor arms extending behind the receiver 12. The kit of claim 11, further comprising: a deployment buoy which is attachable to the chute so as to permit a receiver leaving the chute exit to commence free descent through the water column.13. The kit of claim 12, wherein the deployment buoy has a hull with an aperture and is attachable to the chute so that the chute exit is aligned with the aperture, so that a receiver is free to pass through the chute and the aperture.14. The kit of claim 13, wherein the hull of the deployment buoy has a twin hull structure with first and second hull portions, the aperture being a gap formed between the first and second hull portions.15. The kit of claim 12, 13 or 14, wherein the deployment buoy includes a steering device which steers the deployment buoy away from the vessel when it is being towed beside the vessel and at least one tether for limiting the distance between the vessel and the depoyrnent buoy as the deployment buoy is heng towed by the vessel.16. A method of deploying seabed receivers from a survey vessel, comprising: attaching a chute as specified in the kit of daim 11, to or adjacent to a deck of the survey vessel; and sailing the vessel at a desfted deployment speed along a desired deployment route while placing the receivers into the entrance of the chute in sequence at desired intervals, so the receivers descend through the water and fafl to the seabed at a plurality of survey locations along the route in readiness to acquire survey data.17. The method of claim 16, further comprising attaching a deployment buoy as specified in the kit of claim 12 to the chute exit, and placing the deployment buoy in the water to one side of the survey vessel.18. A survey vessel comprising: a set of seabed receivers of a given type, each comprising a central unit and a plurality of sensor arms; and a chute attached to or adjacent to a deck of the vessel, the chute having an entrance, a guiding portion and an exit dimensioned respectively to permit receivers of the given type to be inserted in, pass through and leave the chute with the sensor arms extending behind the receiver.19. The survey vessel of claim 18 further comprising: a deployment buoy attached to the chute so as to permit a receiver leaving the chute exit to commence free descent through the water column.20. A recovery buoy for seabed receivers of the type comprising a central unit and a plurality of elongate sensor arms, comprising: a hull shaped to travel through the water in an intended direction of travel and defining a waterline; and a plurality of capture veins fixed to the hull and aligned with the direction of travel, the capture veins extending at an angle forwards and downwards below the waterline, and being spaced apart transverse to the direction of travel.21. The recovery buoy of claim 20, wherein the hull of the recovery buoy has a twin huh structure with first and second hull portions, the capture veins being arranged between the first and second huh portions.22. The recovery buoy of claim 20 or 21, wherein the recovery buoy includes a steering device which steers the recovery buoy away from a vessel when it is being towed beside the vessel and at least one tether for ilmiting the distance between the vessel and the recovery buoy as the recovery buoy is being towed by the vessel.23. A kit for a survey vessel comprising: a set of seabed receivers of a given type, each comprising a central unit and a plurality of sensor arms foldably or detachably mounted to the central unit; a recovery buoy as specified in any of claims 20 to 22, wherein the veins are separated by a distance sufficiently small to capture receivers lying in the water.24. The kit of claim 23, wherein the vein separaticn is sufficiently large to permit the sensor arms to pass freely between them when the sensor arms are oriented downwards from the central unit.25. A method of recovering a seabed receiver of the type comprising a central unit and a plurality of sensor arms pivotally mounted to the central unit, comprising: deploying a recovery buoy as specified in any of claims 20 to 22 from the vessel; sailing the vessel to a vicinity where a seabed receiver is floating on the sea surface ready for recovery; sailing the recovery buoy to the receiver so that the receivers central unit engages with the angled portion of the capture veins owing to the forward motion of the recovery buoy, thereby to capture the receiver; and lifting the receiver from the recovery buoy onto the vessel.