WO2013120963A2

WO2013120963A2 - Improvements in underwater surveying

Info

Publication number: WO2013120963A2
Application number: PCT/EP2013/053010
Authority: WO
Inventors: Øyvind ANDREASSEN; Øyvind GRANDUM; Jan Charles KIELLAND; Lars Ole LØSETH; Kjetill LØVBRØTTE; Bjørn Anders Pettersson REIF; Per Magnus Sparrevik; Carl Erik WASBERG; Harald Westerdahl
Original assignee: Statoil Petroleum As
Priority date: 2012-02-14
Filing date: 2013-02-14
Publication date: 2013-08-22
Also published as: GB201202505D0; WO2013120963A3; GB2499397A

Abstract

There is described an underwater positioning body, and a method and apparatus using an underwater positioning body. In an embodiment the positioning body is adapted to be coupled to a flexible elongate member. The elongate member is coupled to an underwater tow body. The positioning body may comprise means for changing or maintaining its horizontal position relative to the tow body for imparting a positioning force on the elongate member during towing.

Description

Improvements in underwater surveying

TECHNICAL FIELD The present invention relates to marine surveying, in particular underwater surveying. In particular, it relates to a device for positioning underwater equipment. In specific embodiments, it relates to marine seismic surveying. In yet other embodiments, it is concerned with marine controlled source electromagnetic (CSEM) surveying. BACKGROUND

In marine surveying, equipment may be towed through the water from a surface vessel. Such equipment may include instruments for detecting properties of the seabed or subsurface. For these purposes, it may be useful or necessary to tow such instruments at depth, close to the seabed. Deep towing may for example be useful to help collect relatively noise free data.

Example surveys include deep towed seismic surveys, where an acoustic source instrument may be used for generating an acoustic pulse, which propagates and interacts with the seafloor. A streamer is towed close to the seabed carrying measurement instruments (interferometers, hydrophones or the like) for detecting seismic P-waves and/or S-waves from the seafloor, in response to the acoustic pulse.

Marine controlled source electromagnetic (CSEM) surveys may be performed for investigating electrical properties, in particular resistivity, of the sea floor. In such surveys, a long electric dipole source antenna may usefully be towed under water, close to the sea floor. Whilst towing, the source antenna transmits an electromagnetic (EM) field. Receivers are typically located on the seafloor a distance away from the source and are used for measuring EM field components.

Deep water towing environments provide significant challenges. Long tow cables need to be used extending from the surface vessel down to the underwater equipment. Drag forces produced during towing can be significant. It also means that conventional surface towing arrangements cannot simply be applied in deep water. It can take a long time to deploy and recover equipment. There are greater weight and pressure requirements on equipment due to the depth. As a result, performing such surveys can be costly. Controlling the position of deep towed equipment and instruments is a challenge. SUMMARY

According to a first aspect of the invention there is provided apparatus for performing a towed marine electromagnetic (EM) survey, the apparatus comprising:

a tow body adapted to be coupled to a tow cable by which the tow body is towable from a surface vessel; and

at least one flexible elongate member coupled to the tow body to trail behind the tow body upon towing;

at least one positioning body coupled to said elongate member;

said positioning body being operable to change or maintain its horizontal position relative to the tow body during towing, for imparting a positioning force to the elongate member.

The first aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect.

According to a second aspect of the invention, there is provided apparatus for performing a towed underwater survey, the apparatus comprising:

at least one positioning body coupled to said elongate member;

said positioning body being operable to change or maintain its horizontal position relative to the tow body during towing, for imparting a positioning force on the elongate member.

The second aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect. According to a third aspect of the invention there is provided a method of towing equipment for performing a marine electromagnetic (EM) survey, the method comprising the steps of:

providing underwater a tow body adapted to be coupled via a tow cable to a surface vessel, at least one flexible elongate member coupled to the tow body, and a positioning body coupled to said flexible elongate member;

towing the tow body from the surface vessel, whereby the flexible elongate member trails behind the tow body; and

operating the positioning body to change or maintain its horizontal position relative to the tow body to impart a positioning force on the elongate member.

The third aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect. According to a fourth aspect of the invention there is provided a method of towing underwater equipment for performing an underwater survey, the method comprising the steps of:

The fourth aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect.

According to a fifth aspect of the invention there is provided an underwater positioning body, the positioning body adapted to be coupled to a flexible elongate member, the elongate member coupled to an underwater tow body, the positioning body comprising means for changing or maintaining its horizontal position relative to the tow body for imparting a positioning force on the elongate member. The fifth aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect.

According to a sixth aspect of the invention there is provided an underwater wing, the wing being adapted to be coupled to a flexible elongate member for positioning a towed flexible elongate member in an underwater survey, the wing comprising:

at least one wing surface configured to produce a component of force during towing for positioning the flexible elongate member. The sixth aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect.

According to a seventh aspect of the invention there is provided a method of manufacturing an underwater wing, the underwater wing being a wing according to the sixth aspects of the invention, the method comprising steps of:

forming a wing profile from a foam block; and

applying a coating or a composite shell over the formed wing profile.

The seventh aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect.

According to an eighth aspect of the invention there is provided a control system for controlling positioning of a towed flexible elongate member in an underwater survey, the system comprising:

an in/out device for receiving a position signal from a positioning body, the positioning body coupled to the flexible elongate member so as to be able to impart a positioning force thereto;

wherein the in/out device is further configured to send a control signal to the positioning body to change or maintain its horizontal position.

The eighth aspect may include further features or steps as defined in the claims appended hereto, in any appropriate combination and from any aspect.

Further aspects of the invention and further features and advantages are apparent from the description below, and specification as a whole. It can be noted that the term "horizontal" is used in the sense of parallel with reference to the Earth's surface, including for example the seabed or sea surface. The term "vertical" is used in the sense of perpendicular with reference to the Earth's surface, including for example the seabed or sea surface. Thus, horizontal and vertical directions are perpendicular to each other.

It can further be noted that the terms "horizontal position" and "vertical position" cover positions defined by horizontal and vertical coordinates respectively. These can be relative, for example specified as offsets from a moving reference point. It will be appreciated that different horizontal positions do not necessarily have to have the same vertical coordinate value. Conversely, it will be appreciated that different vertical positions do not necessarily have to have the same horizontal coordinate. Similarly, a "horizontal angle" of a line with respect to another line or a surface may be defined with reference to the horizontal coordinates even if the line itself is not oriented horizontally in space.

DESCRIPTION AND DRAWINGS There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of electric dipole source apparatus according to an embodiment of the invention;

Figure 2A is a schematic side perspective view of a positioning device for use with the electric dipole source apparatus of Figure 1 ;

Figure 2B is a schematic front-side perspective view of the positioning device of Figure 2A;

Figure 3A is a schematic representation of towed underwater apparatus, comprising a streamer according to an embodiment of the invention; Figure 3B is a schematic representation of towed underwater apparatus comprising a plurality of streamers in a parallel configuration according to an embodiment of the invention; Figure 3C is a schematic representation of towed underwater apparatus comprising a plurality of streamers in a fan formation according to an embodiment of the invention;

Figure 4 is a model of a symmetric Eppler-837 profile delta wing as used in the positioning device of Figures 2A and 2B;

Figures 5A and 5B comprise graphs of estimated lift (Figure 5A) and drag forces (Figure 5B) for the model of Figure 4 for different angles of attack;

Figure 6 is a graph of Lift/Drag ratio for different angles of attack for the model wing of Figure 4;

Figure 7 is a graph of the source-fish-tail-fish line angle with respect to travel direction against lift for different drag coefficients for two different towing speeds; Figures 8A and 8B comprise graphs of source position with depth for different source weights with a drag coefficient of 1.2 (Figure 8A), and for different drag coefficients with a source weight of 2 T (Figure 8B), for 3 km and 5 km umbilical lengths;

Figure 9 is a graph of response amplitude operator against surface wave period for different components of the source apparatus;

Figures 10A and 10B comprise graphs of vertical displacement for different lift forces (Figure 10A) by control surfaces providing vertical lift, and for response time (Figure 10B) when applying 1 kN vertical lift force;

Figures 1 1A and 1 1 B comprise respectively side and bottom view representations of the tail fish of Figure 1 , showing operational equipment mounted thereto, according to an embodiment of the invention; and Figure 12 is a schematic representation of a position control system incorporating a computer device according to an embodiment of the invention.

Underwater apparatus

With reference firstly to Figure 1 , there is shown underwater survey apparatus in the form of electric dipole source apparatus 1 under tow behind a tow vessel. The source apparatus is suitable for use in a marine CSEM survey. The source apparatus has a source fish 2 (constituting a "tow body") operatively connected to the vessel (not shown) an umbilical cable 3 providing power and data communication with the source fish 2. The source fish carries transformer and switching electronics for producing a suitable source signal. The source fish is deployed under water and has a suitable weight, optionally ballasted, to place it at the required depth. In this way, the source fish provides a first order control of the depth of deployment of the source apparatus. The source fish 2 is typically towed passively behind the vessel following the vessel along a tow path as a survey is performed. The apparatus 1 of Figure 1 is designed particularly for deep water use, such as depths below sea level of around 2000 m or more, although it may also be used at shallower depths.

The source apparatus 1 has first, front and second, rear electrodes 4, 5 for the dipole source. The electrodes 4, 5 define respective ends of the dipole. The electrodes are configured to transmit an electric field into the sea water for performing a CSEM survey. The first, front electrode 4 is coupled to the source fish 2 via a first antenna cable 6, and the second, rear electrode 5 is coupled to the source fish 2 via a second antenna cable 7.

In order to achieve specific source configurations, the rear electrode is additionally coupled to a tail fish 8 (constituting a "positioning device") at its far end. The tail fish is used for positioning the second electrode in an operative position 9, as shown in Figure 1 . In this position, the source is configured such that a source line 10 extending between the rear electrode 5 and the front electrode 4 extends across a travel direction of the first electrode, indicated by arrow 1 1 . In this example, the dipole source line 10 is oriented at an angle of approximately 45 degrees to the travel direction 1 1. As these are located close to the electrodes however, the electrodes will take a near identical orientation. This orientation of the source line may be beneficial for efficient acquisition of electric field components in a direction along and perpendicular to the tow path. It can be noted that source configurations with other angles of the source line to the travel direction can be obtained similarly, by using the tail fish 8 to position the second electrode in the desired operative position. For example, any angle up to around 50° may be achieved, on either side of a line of travel 12 of the first electrode 4 in the travel direction. It will be appreciated that in certain variants, the tail fish and source fish may be positioned to define a line extending with an angle of 45 degrees with respect a direction of travel of the source fish or other parts of the source. Indeed, as discussed below the navigation system is preferably implemented to position the source fish and tail fish with respect to each other.

It will be appreciated that the front electrode 4 may therefore follow the source fish passively through the water, whilst the second electrode is manoeuvred actively, relative to the first electrode, using the tail fish 8 to form the desired source configuration for a given survey. The electrodes can also define a travel direction aligned configuration, in which the front and rear electrodes 4, 5 are aligned with each other along the travel direction 1 1. In this configuration, the electrodes and tail fish 8 may follow passively behind the source fish. Alternatively, the tail fish may be used for maintaining, for example by dynamically positioning, the position of the second electrode 5 relative to the first electrode 4 in the travel direction aligned configuration, i.e. at 0 degrees to the travel direction. With further reference now to Figures 2A and 2B, the tail fish 8 for manoeuvring the rear electrode 5 is described in more detail. Movements in the reference frame of the tail fish can be defined by pitch, roll, and yaw axes as indicated. As can be seen, the tail fish 8 comprises a main hydrodynamic wing 13 coupled to the electrode 5. In order to position the second electrode in an operative position, the wing produces a lateral or horizontal component of force (lateral lift) which acts across the travel direction, to locate the electrode in the operative position. To do so, the wing is designed to be used in an upright, vertical configuration, as shown in the figures with lift surfaces 14a, 14b of the wing extending vertically. The lift force produced by the wing then acts against one of opposing lift surfaces 14a, 14b of the wing pushing the fish in a lateral or horizontal direction. In practice of course, the fish may experience a certain amount of rolling imparted by the water or by other components of the apparatus, such that perfect vertically may not be achieved. However, as explained further below, there are provided means for correcting and controlling the orientation of the fish by various control members. It can be noted that there is a natural tendency upon towing for the second electrode to align with the towing direction. The lateral lift force produced by the wing counteracts this, to keep the electrode positioned as explained above. In order to produce a suitable lift force, the wing needs to be oriented with a suitable pitch angle or "angle of attack" with regard to a direction of flow of water across its surface. The wing takes the form of a delta wing, having generally a D-shape or delta shape or similar. The delta wing is a high-lift wing, in this example taking the form of the Eppler- 837 profile (Eppler, R., Airfoil design and data, Springer-Verlag, Berlin, Hamburg (1990)). As can be seen, the opposing lift surfaces 14a, 14b are joined at a front end to form a leading edge 40. They extend from the leading edge 40 following a slight curved profile toward the rear of the wing where they join at a trailing edge 42. A surface section 43 where the opposing lift surfaces join along leading edge 40 is rounded to gently deflect the flow there past and over the lift surfaces 14a, 14b. The leading edge 40 follows a line intersecting the points of maximum curvature of the rounded portion between wing tips 45a, 45b. The leading edge comprises edge portions 41 a, 41 b extending in different directions to define a delta shape, meeting at a front nose 42 of the wing. The delta wing provides smooth stalling characteristics, i.e. sudden stall with lack of control is therefore not likely to occur, and a large range of attack angles producing large lateral lift forces. Accordingly, when using the apparatus of Figure 1 , the delta wing provides for a high lift/drag ratio. The delta wing construction is simple and robust. In particular, the tail fish is near neutrally buoyant with a separated centre of buoyancy (CoB) and centre of gravity (CoG) for stability, the CoG lying lower than the CoB.

The fish is coupled to the electrode 5 by a hinged fork coupling 16. The coupling allows the pitch or angle of attack of the wing to be changed. To this effect, the wing has upper and lower attachment means 15a, 15b which receives and connects to respective arms of the hinged fork coupling 16. The use of upper and lower attachment means facilitates roll stability. The fork coupling 16 is attached to the wing close to the hydrodynamic force centre. A change of the pitch angle out of equilibrium will then result in a restoring force that will pull the wing back into equilibrium. The fork coupling 16 is hinged to the wing along a hinge axis 24 that is perpendicular to the pitch axis. The hinge axis 24 also extends through a point ahead of the hydrodynamic force centre. The hinge axis 24 extends through a centre plane of the wing to minimize torque. The wing can rotate about the hinge axis to change its orientation and pitch angle whilst remaining connected.

The tail fish 8 also has various control members for controlling the position of the wing and the electrode. The control members perform different functions. As seen best in Figure 2A, the control members include pitch rudders 17a, 17b (constituting "constituting a controllable steering member") used for changing the pitch angle or angle of attack of the wing. To produce a sufficient torque, pitch rudders are mounted to a trailing end of the wing. They are movable to deflect the flow of water across the lift surfaces for changing the pitch angle. More specifically, the pitch rudders are rotatable with respect to the wing surfaces 14a, 14b. The pitch rudders can be controlled to move a specified amount for changing the orientation of the wing. Respective pitch rudders 17a, 17b are spaced apart from each other near upper and lower ends of the wing respectively. The pitch rudders can be used differentially, for example an individual pitch rudder can be operated without the other, for correcting the roll angle of the wing or to increase the roll stability.

At upper and lower ends of the main wing, the tail fish is provided with winglets 18a, 18b. The winglets have flow surfaces 19a, 19b which face away from the wing ends. Depth rudders 20a, 20b (constituting controllable steering members) are mounted to the winglets at a trailing end of the winglets. The rudders can be used to control slip and depth motion. In this way, the wing may be movable to a different depth or its depth position can be adjusted. During operations, the tail fish will track the heavy and stable source fish such that depth corrections provided with the depth rudders are relatively minor. In embodiments where the wing has a buoyantly neutral trim, the depth rudders provide only small forces in order to generate the required vertical accelerations to make such corrections.

To increase the slip motion control, vortex generators may be attached closely after the wing leading edge. The number and size of these vortex generators should be experimentally optimized. The specific vertical control capacity is governed by the desired time for performing dive and recovery of the apparatus. In this regard, it can be noted that diving is mainly governed by the umbilical drag.

In order to produce vertical lift, retractable vertical lift members 21 a, 21 b (constituting controllable steering members) may also be provided which extend out of the lateral lift surfaces 14a, 14b of the wing. The retractable vertical lift members are attached inside the wing-hull, close to the wing centre. These members are retractable, in the sense that they can be pulled into the hull of the wing when not in use. When a vertical force is needed, the vertical lift member is moved out. A vertical lift member on one side of the wing may be configured for upward force and on the other side of the wing for downward force. They may also have a non-symmetrical profile. The magnitude of lift depends on the length of the vertical lift member protruding out from the wing surfaces 14a, 14b. The amount of lift is adjustable by adjusting the extension of the arms 21 a, 21 b. The vertical forces generated by the retractable vertical lift members 21 a, 21 b are for correction of depth and are small compared with the lateral lift forces of the wing. The vertical lift member is not extended to a length than the thickness of the wing, to keep torque effects small compared with other parts of the system and such that the overall stability of the wing is not affected. In addition, the pitch rudders are able to counteract this torque and maintain stability. Withdrawn vertical lift members are well protected, which makes this a mechanically robust solution. These surfaces facilitate positioning of the tail fish and the second electrode vertically. The vertical lift members may also comprise vertical lift rudders.

In particular, depth rudders 20a, 20b and vertical lift members 21 a, 21 b may facilitate positioning the second electrode at the same depth as the first electrode. Although they only provide relatively small forces vertically, these forces are sufficient for vertical control since the tail fish is near neutrally buoyant. The fork coupling may force the wing to rotate around the axis of the antenna cable if torque is present in the cable. If depth adjusting members such as these are used at the same time as there is a substantial angle of attack, a torque directed along the second electrode 5 and antenna cable 7 may be created, tending to cause the wing to rotate around a torque axis along the antenna cable. However, an asymmetric buoyancy composition of the wing (with CoG separated from and lower than the CoB) acts against that torque, for stabilizing the wing. The control members may provide a vertical force along an axis through CoG and/or CoB to change the elevation of the tail fish for minimising tilting. Yet further, the tail fish may includes control members in the form of speed breaks 22 mounted to a trailing end of the wing. These are operable from a collapsed position to an extended position where the surfaces protrude outwards with respect to the wing lift surfaces 14a, 14b. The function of the speed break is to adjust the drag; increased drag will reduce the angle of the source line to the travel direction for a given angle of attack, whereas reduced drag increases the source line angle to the travel direction with the same angle of attack. Speed breaks are small compared with the area of the pitch rudders where considerable forces are needed to set required angles of attack.

Examples of further embodiments of the invention can be noted with reference to Figures 3A to 3C. In these embodiments, instead of the source electrode cable, streamers are used, which may carry instruments for seismic surveying. Like features to those of the above described embodiments of Figures 1 , 2A and 2B are given the same reference numerals in Figures 3A to 3C but incremented by one hundred or a multiple thereof. In Figure 3A, seismic survey apparatus 101 is shown. A front tow fish 102 is deployed at the required depth, preferably close to the seabed, and is towed from the surface by a surface vessel. A streamer 107 is attached at one end to the front tow fish 102 (constituting a "tow body"), and at its other end, towards the rear of the apparatus, to a tail fish 108 (constituting a "positioning device"). The front tow fish 102 functions as a stable reference point and clump weight in the water. The tail fish 108 is steerable horizontally relative to the front tow fish during towing, to maintain or change its horizontal position, and in turn, position the streamer by way of its attachment to the streamer. The tail fish is able to move to a greater extent laterally than the front tow fish 8 during towing The tail fish is otherwise configured in the same way as the tail fish 8 described above.

In Figure 3B, a plurality of streamers 207 are attached to and trail a single front tow fish 202 during towing. In this case, each streamer has two positioning fishes, an intermediate fish 208a and a tail fish 208b. The two positioning fishes are configured as the tail fish 8. The fishes, 208a, 208b are controlled to maintain specific horizontal positions relative to the front tow fish 202, in order to achieve a desired configuration. More specifically, as can be seen in Figure 3B, the fish are controlled to place the streamers 207 so that the extend parallel to each other, in the section of the streamers between the intermediate and tail fishes 208a, 208b, permitting a parallel survey configuration to be achieved.

In Figure 3C, there are a plurality of streamers 307 connected to a single front tow fish 302, the streamers each having attached thereto two positioning fishes in a similar way to that of Figure 3B. In this case however, the positioning fishes are controlled so as to acquire positions for placing the streamers in a fan formation behind the front tow fish. The streamers extend behind the tow fish with different angles to each other. In other embodiments, further tail fishes may be coupled to the streamers 107, 207, 307. This may be desirable to increase positionability of the streamers. Similarly, further streamers may be coupled to the front tow fish. Such further fishes may be spaced apart along the streamer and may otherwise be configured in a similar manner to those of Figures 3A to 3C. The fishes may be positioned to produce a desired streamer formation, for example a fan formation, or other non-parallel formations or a parallel formation. The parallel streamer sections may have a uniform or non-uniform spacing from pair to pair.

It will be appreciated that the positioning fishes may equally well be applied with different kinds of underwater survey apparatus, for example cables, lines or other kinds of streamers etc.

Experimental results Modelling experiments have been performed to verify the suitability of the apparatus to produce the necessary lift and positioning, in particular the performance of the tail fish. Figure 4 shows the design of the wing 13 for the tail fish 8. The wing is symmetrical about the plane 30 and the plane 31. The plane 30 extends between the respective lift surfaces, being defined on either side of the plane 30. The leading and trailing edges at or along which the lift surfaces join may be considered to define the plane 30. 3D numerical simulations for investigating the hydrodynamic characteristics of the wing were performed, based on the governing Navier-Stokes equations for incompressible flow. Except for some specific cases with large attack angles, a steady solution is found. The lift, drag, and moment coefficients and resulting forces and moments on the wing were calculated based on the steady solution. For instance, the lift force L can be defined based on a relationship including the lift coefficient, density of water, the wing area, and the water speed.

Figures 5 and 6 provide graphs showing the lift, drag and the lift/drag ratio in water at 5 knots for the symmetric 3D wing. The required lateral lift forces for 45° offset with Cd = 1 .2 (highest antenna drag analysed) have been calculated to be 23 kN (5 knots towing speed) and 8 kN (3 knots) towing speed, as shown in the graph in Figure 6. In Figure 5, we can see that a lift force of 23 kN is obtained for an angles of attack equal to 10° with a drag of 2.2 kN and a lift to drag ratio of about 1 1 . If the antenna drag is less, the required lift force and angle of attack is reduced. Both the lift capacity of the wing and the required lift due to drag is proportional to the square of the towing speed thus in other embodiments, operation with a 45 degree source angle can be achieved for a 3 knot towing speed, with an angle of attack less than 10°. With the low drag resistance of the delta wing, the drag of the tail fish has only a limited effect on the required lift to obtain 45° offset.

In Figure 5, it can be seen that the lift capacity of the wings linearly increases also for angles of attack > 10° and the wing can have a smaller size if larger angle of attack is used. Thus, smaller wings with greater angles of attack could be used to place the electrode at the 45° angle to the travel direction provided wing stability is maintained. Angles of attack up to 10° are considered to provide good wing stability.

In order to achieve a 45° source line angle with a source line distance of 200 m, the tail fish needs to overcome the drag exerted by the source apparatus 1 as a whole during towing. Figure 7 shows the lift forces required for achieving different source angles for different drag coefficients Cd and for towing performed at 3 and at 5 knots. The drag of the wing is very small compared with that of the umbilical, and the umbilical 3 is a dominating factor. The drag coefficients as used in the simulations of Figure 7 represent a realistic range where the highest coefficients are conservative in the sense that they partly include also the increased drag due to possible vortex shedding (vibrations in lines causing a higher effective diameter and larger drag). Figure 7 shows that: 1 ) the required lift force of the tail fish is 12-23 kN for typical drag coefficients of the towing umbilical for a velocity of 5 knots; and 2) the required lift force of the tail fish is 4.5-8.1 kN for typical drag coefficients of the towing umbilical for a velocity of 3 knots. Figures 5 and 6 show that these forces can be produced by the wing 13. As outlined above, the desired lift for the symmetric wing may be obtained simply by increasing the wing area.

Figure 8 shows a graph of example plots showing the sensitivity to towing depth with respect to potential source fish weights and umbilical drag factor (Cd). Sensitivity tests have shown that the static draught of the system is mainly dependent upon the following four system properties:

• towing velocity (3-5 knots);

• drag coefficient of towing umbilical (0.6-1 .2);

· weight of source (2-5 tonnes) including the tail fish weight; and

• length of towing umbilical (2000-4000 m).

As can be seen in Figure 8, in order to tow the array at a minimum of 2000 metres using a 4T source fish, the length of the towing umbilical should be minimum 3000 meters long. At a towing speed of 5 knots, a longer umbilical is however required. Further, the figure shows that the draught of the system is very sensitive to the drag coefficient of the umbilical.

It can be seen that the draught is particularly sensitive to drag of the umbilical cable. An option for reducing the drag may be to use a cable with fairing. In this regard, it can be noted that the drag coefficient, Cd, is normally 1 .2 for unfaired (bare) cable, 0.6-0.7 for cable with flexible (ribbon or hair) fairing and 0.2 for cable with hard fairing (spoilers). Preferably, a flexible fairing is used, in which case a Cd of 0.8 (or less) can be expected to apply. Thus, for a towing speed of around 3 to 4 knots it can be feasible to go deeper than 2500 m using a faired umbilical and source weight of 4-5 T.

The effects of side currents have also been considered in the simulations. In particular, the effect of a 90° side current of 0.1 -1 knots was considered. No instabilities were observed. However, a greater lift force is needed in order to position the second electrode. It is found that the system is displaced in a manner that reduces the angle of the source line defined by the electrodes, as indicated below:

• a 1 knot side current reduces the offset angle from 45° to 28°, for a towing velocity of 3 knots.

• counteracting a 1 knot side current at 5 knots towing velocity, requires approximately 27 kN total lateral lift to achieve a 45° offset. • counteracting a 1 knot side current at 3 knots towing velocity, requires approximately 15 kN total lift to achieve a 45° offset.

The response to surface wave motions transferred to the tail fish and source fish were also investigated. Figure 9 shows results for a towing velocity of 3 knots and incident plane harmonic waves against the travel direction of 4 m height and periods of 4-20 s. The figure shows Response Amplitude Operators (RAOs), that is the object response relative to wave excitation. The largest responses are for wave periods of 8-12 seconds. It could be concluded as follows:

• The towing vessel, (which is a typical 100m offshore vessel) has a natural pitch period of about 10 seconds. The largest responses for the individual components in the system are found for wave periods of 8-12 seconds.

• The response of the transponder/front electrode cable is small, max 9% of the vertical response of the towing vessel in the given frequency range. The rear electrode cable has a negligible vertical response.

• The source vertical response is 60-100% of the vertical response of the towing vessel (in the Vessel-Umbilical junction point).

• The response of the tail fish is dominated by horizontal oscillation, but is limited at maximum 15% of the vertical response of the towing vessel (for wave periods in the range; T= 8-10 sec). The dynamic oscillations of the tail fish do not have significant effects on the lift of the tail fish.

• No instabilities in the system were encountered. Wave motions are significantly damped for the electrode parts of the towing system including the tail fish, thus the dipole antenna appears not to be sensitive to sea state and especially stable for long period swells (> 12 sec period).

The displacement response to applying a vertical lift force to the tail fish using the control members has also been analysed. This is particularly relevant to the ability to provide dynamic control of the tail fish. The results are summarized by Figures 10A and 10B. A 1 kN (100 kg) lift would be able to displace the tail fish at a rate of at least 1 m/sec. The rate of displacement as shown in this figure decreases as a greater elevation force is applied and the difference in elevation increases. This is because the antenna cable is exposed to a cross flow as the tail fish rises. However, when towing, the cross flow will work the opposite way adding to the lift to equalise the elevation (or working against any elevation difference between source and tail fish). When the tail fish follows the source fish upwards, this cross flow effect may help to bring the tail fish to the same elevation as the source fish after a period of time. The control members may then be used primarily to boost the speed of elevation adjustment.

Control system

Tail fish control is to provide means of by active positioning of the source, in particular the second source electrode, by controlling the tail fish. Active positioning is provided by the control members provided on the hydrofoil shaped delta wing of the tail fish. A position control system is used to control the control members of the tail fish for positioning the tail electrode. The control system includes topside control of both the source fish and tail fish as well as closed loop control of tail fish (autopilot).

The control system may operate in accordance with a control model implemented via a control program. With reference to Figure 12, the control system may include a computer device 70 having a processor 73 and an In/Out device 72 for sending and receiving instructions to the source and tail fish 8, and/or controllable components thereof. Memory 74 may be used to store the control program which upon execution by the processor 73 may produce the instructions necessary to control the positioning of the tail fish. The In/Out is used for sending these instructions to the tail fish control members. Position information from the tail fish may be sent in the opposite direction and received by the In/Out device. Received signals and data may be processed using the processor and output to a display 75 for viewing by an operator.

The position control system is designed to produce and ensure that the source apparatus maintains a particular source line angle of up to and including 45° in the horizontal plane relative to a travel direction of the first electrode or source fish, preferably within a 1 ° precision. Elevation of the tail fish (and antenna) is controlled preferably to within ±3.5 m relative to a reference transponder at the source electrode. The distance between electrodes may typically be approximately 200 m.

Control and manoeuvring of the tail fish is implemented by controlling the tail fish position relative to the source fish, which typically follows the tow vessel passively as a clump weight in the water. The antenna cable 7 takes a curved shape between the source fish and the tail fish. The antenna cable 7 interconnecting the source fish and the tail fish limits the degree of freedom for movement of the tail fish 8. However, the tail fish can be considered to have two degrees of freedom in it terms of positioning, assuming an idealized model and steady state condition. This can be considered as vertical position and transverse angle of source line relative to a travel or tow direction.

Since lift relates to the speed by the power of two, the control system may also take into account the tow speed as part of the control model. The control model may also be based on the angle of attack (AoA) of the wing being kept below an angle at which stall or other unstable conditions of the wing may occur.

The control system utilises the lateral lift capacity of the delta wing to provide the controlling horizontal forces and by controlling the pitch rudders provided on the wing can move the source into different configurations in the horizontal plane. By controlling the depth rudders attached to small control areas on winglets at the top and bottom of the wing at the trailing edge of the wing, vertical or elevational control can be implemented. Retracting vertical lift surfaces may also be used for vertical control. Vertical control involves less force (0.01 -0.1 kN) as mentioned above, particularly in the examples described where the tail fish 8 and antenna cable 7 are near neutrally buoyant.

The hinged fork coupling 16 may influence the vertical control, for example an isolated tilt in the towing direction will not be possible due to the coupling 16, such that the coupling axis may need to be incorporated and taken into account in the control model. In addition, the control model may take into account the effect of the fork coupling and the split CoG/ CoB configuration in tending to stabilise the pitch of the wing in the presence of torque. Control of the tail fish may be implemented by closed loop control, as indicated below, in three control levels:

• Control member position: The position of each rudder may be measured and the rudder positions compared to a reference. A controller, for example a computer device, may then transmit a signal to reduce a discrepancy between the measured position and reference position.

• Angle of attack of the wing: The orientation of the wing relative to a direction of flow over the wing will be measured. The orientation is compared to a reference. A computer device executing a program containing a control algorithm may transmit signals to operate the control member or rudder positions to reduce any discrepancy between the measured position and reference position. The control algorithm may be executed to operate the control member or rudder positions on a continuous basis.

• Tail fish position: The position of the wing relative to the source fish may be measured, and the measured position compared to a reference position. A computer device executing a program containing a control algorithm may transmit signals to operate orient the wing, for example to operate the control members and rudders, to reduce the discrepancy between the measured and reference positions.

It will be appreciated that for each control level, several closed-loops may be required to move the various surfaces about different axes simultaneously. An antecedent level may also be determined for each following level.

Accordingly, the position of each rudder may be monitored. Actuators for the rudders are provided on the tail fish and are connected to the rudders for moving them. Direct and continuous monitoring of rudder positions can be derived from the actuator position which is a known parameter, in view of for example historical movements. The rudder positions may need to take account for non-linear mechanisms such as rudder arms etc. In this way, continuous measurement and monitoring of rudder positions can be achieved.

The angle of attack of the wing may be measured or estimated. A direct way for doing this is by use of an acoustic Doppler current profiler onboard the tail fish. This will provide a direct reading of a water flow vector (speed and direction) relative to the tail fish. Hence, the angle of attack can be determined and the lateral lift generated by the fish 8 can be controlled. The position of the tail may also be measured, and monitored. The position may be controlled according to a specified precision and bandwidth. Typically, the positioning error of the tail fish 8 should be less than ±3.5 m vertically and ±1 ° of the source line angle horizontally with respect to the travel direction of for example the source fish or first electrode. The 1 ° error limit corresponds to 3.5 m at a source line distance of 200 m. To be able to control the tail fish in this envelope, the precision sought from the position measurement is one tenth of the specified positioning errors or better. In this example, the precision from the position measurement needs to be 0.35 m to achieve the specified relative position reference. The bandwidth of such a xyz-position may be 1 Hz or better. Positions obtained from inertial positioning sensors in combination with acoustic positioning devices may achieve this.

Control components, sensors, and navigation The electrodes are 12m 028m standard Cu electrodes assemblies. The tail electrode which is located between the tail fish and the antenna towing cable 7 is reinforced by Dyneema/Spectron ropes in order to maintain similar tension capacity as for the antenna cable itself (BS 15 T). The electrode cable between the source fish and the steerable tail fish is neutrally buoyant with aluminium core for 1500 A rating the OD is 63mm. The electrode cable includes a control cable for control of the steerable tail-fish which may be strapped to or integrated in the EM electrode cable. The control cable is continued across the tail electrode and reinforced/ strain relieved adequately for towing by the tail fish. Key requirements for the control cable include:

• Power requirements: Typically 600 VAC single phase

• Signal carrier: Optical fibre, single mode (9/125 μηη). All signals can be multiplexed onto one optical fibre, although multiple fibres are preferred for redundancy and the ability to add future equipment.

With reference now to Figure 1 1 , the tail fish 8 is shown with example physical components for operation of the tail fish. In particular, the tail fish 8 has: a termination box 34 for electrical termination of the antenna cable 7 and/or control cable; a control canister 35 housing control electronics; actuators for moving the pitch rudders 17a, 17b, depth rudders 20a, 20b and speed breaks 36; an acoustic navigation transceiver 37 for positioning of the tail fish relative to the source fish; and a inertial navigation unit

38 for determining the heading and orientation of the source fish.

The tow / control cable is mechanically terminated in the hinged fork coupling 16. From the mechanical termination, the cable is routed to the termination box 34, where the electrical and fibre optical leads are terminated. The termination box 34 may also contain a transformer for transformation of power from the tow cable to local system power for tail fish components, typically 1 10 VAC. The termination box comprises an oil-filled enclosure containing a power transformer and terminals for power connections. The control cable from the source fish is routed into the canister via a cable gland. Low voltage instrument power from the transformer secondary winding is routed to the control system via a dedicated subsea connector. Optical fibre runs from the termination canister to the control canister in a dedicated fibre harness with pressure resistant penetrators. A spring-loaded rolling diaphragm compensator can maintain a 0.5 bar overpressure with dielectric fluid in the transformer canister.

The control canister 35 is a one-atmospheric canister containing electronic devices, converters, power and signal distribution. The canister comprises a pressure tolerant cylinder with sealed end caps. Required electrical connections and penetrators will be mounted in the end cover. The control canister may be configured with the following specfication:

• Power input: 1 10 VAC.

• Power output: 1 10 VDC, 24 VDC, 12 VDC and 5 VDC as required by sensors and actuators.

· Fibre optic communication link: Serial/ Ethernet as required for sensors, and for control.

• Fuses, terminals end power management systems as required: ground fault detection, water alarm and power measurement, power relays etc. The control canister 35 also houses a telemetry system. The purpose of the telemetry system is to ensure data flow between sensors and control system, between the control system and the actuators and between source/ tail fish and the surface vessel. The telemetry system provides transparent data links (serial and Ethernet) over the fibre lines of the control cable between tail fish, source fish and surface. A large number of data channels can be provided by optical multiplexing, as well as time- domain multiplexing of serial lines. Ethernet provides the control system communication "backbone". A high speed Ethernet link is provided to the surface vessel for the control system only. Sensor data can be sent either to the control system directly for executing local control loops or to a surface component of the control system for any sensors that provide data to surface only, or that require topside units to operate. The telemetry system may comprise a limited number of boards accomodated into the control canister. At least two fibre lines may be provided between tail fish and source fish, and between source fish and surface vessel, for redundancy purposes.

The actuators 36 are compact units configured for multi-axis closed loop rudder control and are based on permanent magnet brushless DC armature. The units are capable of providing a 100 Nm torque using a slack free harmonic gear solution. A dedicated electronic drive unit reads the motor sensors and controls the electrical power driving the unit. The drive unit features required electronic and software to form a closed loop position (servo) control of the rudder position. The drive unit electronics (axis controller) is designed to operate in an oil filled environment that is compensated to the ambient sea pressure. In many applications, it may be practical to install the controller stack as an extension to the electronics control canister that will provide the power and communication. This combined unit will reduce the number of required cables and save space.

The sensors required will depend heavily on the control strategy for the tail fish. The set of sensors described below is used as "base case" for the control system for the tail fish.

In order to determine the position of the tail fish a transceiver based navigation system is used including the transceiver 37 on the tail fish. For example, a navigation system such as the Ranger or Fusion USBL tracking and positioning system as marketed by Sonardyne may be applied. In this case, the transceiver 37 is an inverted ultrasonic baseline (iUSBL) transceiver, for example Sonardyne's Inverted USBL (iUSBL) Type 8091 tranceiver. This system may further include:

topside software GUI by Sonardyne; topside hardware, such as for example Sonardyne's Navigation Controller Unit and Navigation Computer, or Data Fusion Engine (DFE);

source fish and electrode mounted responders, for example Wide Band Sub- Mini Responder (WSM Type 8070); and

a vessel mounted directional responder (WSM Type 8070)

Note that the functions of a responder are similar to a transponder. However by direct cable based power supply, the acoustic signal is stronger and thereby both range and accuracy is increased (a higher signal to noise ratio can be obtained). With a positioning accuracy of 0.1 % slant range for the iUSBL system, the tail fish with 0.2 m accuracy relative to the source fish.

For tail fish positioning and "auto pilot" function the stable source fish is used as the tracking target and equipped with a Wide Band Sub-Mini (WSM) responder. To check possible offsets and obtain exact position of the front electrode an additional responder is attached and towed directly behind the front electrode. These data are not used directly for tow fish control but can be used for exact electrode positions in later EM survey processing. The global accuracy is obtained by the vessel responder and strapped down to the UTM coordinates by input from the vessel GPS and AHR systems. Due to the long range this accuracy is limited to 4 m at 4 km distance (0.1 % slant range). At this distance the acoustic signal should be as strong as possible. Responders are therefore used with power and communication provided through the towing umbilical and following the antenna cables. The responder only sends out the acoustic tracking signal using fibre optic communication to minimize any response lag in the system. For long range a directional beam shape there could be used for example a WSM Type 8070 responder with an acoustic cover of ±20°.

The tail fish is also provided with an inertial navigation system, including the inertial navigation unit 38 mounted on the tail fish 8. The navigation unit 38 is a Motion Reference Unit for providing dynamic data of heading, pitch and roll (attitude). For example, the Lodestar Inertial Navigation System (INS) and heading reference system marketed by Sonardyne may be used. The Sonardyne system Type 8084-000-16 may be used which has a depth rating of 3000 meters. Alternatively, the PHINS inertial navigation system by Ixsea may be suitable. It can be noted that stated accuracy of the positioning can be verified and the system calibrated in-situ by installing one or more reference transponders at the seabed in the survey area. The stationary seabed transponder(s) are boxed in to high precision and used as references when the source apparatus passes over them. The iUSBL transceiver 37 on the tail fish may then need to be towed close to the seabed such that the transponder signal is within the iUSBL transceiver range (For example, Model 8091 has an operating range up to 700m and acoustic cover of ±80°). The iUSBL concept is based on removing the transceiver from noisy environment at the surface (thrusters noise etc.) in order to increase the signal to noise ratio (SNR). Increasing the SNR not only increases the maximum trackable range, but also the angular resolution (accuracy). In the described set up, the relative position between source fish and tail fish can be determined with very high accuracy due to a limited separation (the vessel is not included in this relative positioning). The tail fish is also provided with an altimeter for sensing the height of the tail fish above the seabed. The purpose of this is primarily to ensure collision with seabed does not occur. A typical sensor for this purpose may be the Tritech PA-500 altimeter.

In addition, the tail fish is provided with a depth sensor for measuring the depth of the tail fish below sea level. The depth sensor may be used as input to a control loop for keeping the tail fish within a vertical window of ±3.5 m of the source fish. If so, the sensor should have an accuracy of an order of magnitude higher than the "window size" (i.e. approximately 0.7 m). For a 2000 m depth of deployment, this gives an accuracy requirement of 0.035 % of full deployment depth. A Digiquartz or similar type of sensor may be suitable for this purpose. The depth may be controlled independently by a Digiquartz or similar sensor on both tail and source fish.

Furthermore, the tail fish is provided with an acoustic Doppler current profiler. This may operate as an acoustic Doppler velocity logger (DVL). Such a profiler detects the velocity vector of the tail fish moving across the sea floor, and is used for determining the angle of attack and orientation of the tail fish.

Materials and construction The tail fish 8 is preferably constructed from light structural materials and is designed with a sufficient volume to provide buoyancy. For example, the wing can have an internal framework made from aluminium and may be filled with syntactic foam. The internal framework may be covered with a composite material providing a smooth skin to the wing. The wing may be constructed by means of a laminated sandwich construction with a syntactic foam core and synthetic outer layer. This construction may save weight compared with traditional wing design.

The construction may be tailored to provide buoyancy to balance the weight of the materials and other components mounted on the tail fish. At large depths, air/gas cannot be used in order to create the buoyancy. However, syntactic foam that is incompressible can be used. For operating the tail fish in deep water at depths of for example 2000 m, the density of such foam may be 500 kg/m³. The syntactic foam material for the wing construction consists of a base polymer mixed with microspheres, small hollow glass spheres between 20 and 150 microns in diameter, and provides a specific gravity of 0.4-0.5 for deep water compressive strength (3000m). This syntactic foam also facilitates machining into complex shapes for example to house tail fish components inside the wing.

The skin may be provided in the form a polyurethane elastomeric layer which protects the syntactic foam against impacts and abrasions. The elastomeric layer may be manually sprayed on and subsequently polished to produce a high gloss finish. The skin thickness may be typically 6 to 30 mm. Multi-component polyurethane paint will provide colour stability and ultraviolet protection. Alternatives to spray applied polyurethane include composite shells, for example glass/ epoxy, Kevlar®/ epoxy, or carbon/ epoxy composites or the like. A composite shell of this type would increase the structural strength and when used in combination with syntactic foam may reduce metal reinforcement or render it unnecessary. Thus, the wing could be formed from a composite shell filled with syntactic foam. The shell is laminated by hand and the syntactic foam core is used as form work and must have the precise shape of the desired profile (Eppler-837 wing). Typically, A-class 3D surfaces are the basis for the numerical milling. A multi-axial and high-quality milling machine will reproduce the 3D surfaces in syntactic foam very precisely according to the steps: S1. Glue syntactic foam plates together to obtain a solid block of required size to accommodate the complete wing profile;

52. Mill out the shape of one side (half-shell); and

53. Turn the block and mill out the other side to produce a complete foam profile. Appropriate compartments for housing components of the fish inside the wing may be milled out as part of the process. The coating or composite shell can then be applied over the milled foam profile for the wing. Some manual sanding and grinding of the milled surfaces might also be necessary before applying a spray or laminated coating to the foam profile. Further sanding and polishing is applied to the coating in order to achieve a high gloss finish. The last stage includes applying two-component paint over the coating.

For operation, it is desirable that the fish has neutral or small positive buoyancy at the relevant operational depth. This may help with vertical positioning and recovery of the fish. Buoyancy can be provided by the volume and constructional materials of the wing, and by operational components mounted thereto. The appropriate buoyancy may also be provided by adding weight to the tail fish as ballast. The added weight can help to increase stability. The weight distribution of the tail fish structure generally is such that the CoG of the tail fish is well separated from the CoB along the span 39 of the wing, parallel to the pitch axis, to provide good stability and self-righting ability when deployed under water. Furthermore, the physical components mounted on the wing and/or any such additional ballast may be distributed to facilitate stability and self- righting of the wing when deployed under water. Providing the appropriate stability and buoyancy requires balancing of the wing with all its components, distribution of buoyancy materials and ballast whilst also taking into account structural strength.

Performance of the wing is not very sensitive to the maximum wing thickness (within reasonable limits) as long as the selected profile is maintained. The thickness may therefore be adjusted to accommodate internal instrumentation or buoyancy volume. For increased stability of the wing, the top winglet may be designed for maximum buoyancy and the bottom winglet may be ballasted. The total buoyancy of the winglets may be slightly negative. Simplified calculations have been made for buoyancy of the high lift wing profile with approximately 1 .7 m3 of available volume which indicate that when submerged, this volume will displace 1 734 kg of salt water. In order to achieve a net buoyancy of 50 kg the dry weight of the steerable tail fish cannot exceed 1 684 kg. The symmetrical profile used in practice has a larger volume, but this example is illustrative. A typical average density for the tail fish and its components may be around 1 600 kg/ m3.

The wing construction above provides a number of advantages. Syntactic foam is in itself a structural material, and can be appropriate for the wing construction without additional metallic or composite stiffeners. Machining is possible to very fine tolerances so that unwanted water-filled cavities can be avoided. In addition, moulding fasteners, bearings etc. can be provided directly to the syntactic foam. In this way, the tail fish can provide net buoyancy with respect to the described mechatronic layout and design, and provides a material distribution for strength and stability. Tow body fish

The source fish comprises a carrier frame, carrying one or more of the following components:

• Umbilical termination including a termination junction box.

· High voltage transformer system.

• Telemetry system.

• Positioning and navigation sensors.

The source fish acts as a depressor, providing a significant weight for lowering the source apparatus underwater to large depths. The total volume and weight of the source fish, includes that of the source equipment and transformers. Typically, the source fish does not have depressor foils to increase operational depth, as these have limited effect, although this may be an option. Instead, it is typically better and more practical to simply add ballast weight, if required, for operation in deep water with a depth of, say, more than 2500 m. As an example, the source fish may have the following characteristics:

• Size: LxHxW = 4.5 x 2 x 1 m.

• Weight: 4000 kg, approximately.

• Material: Framework of painted steel. Glass reinforced plastic panels.

· Towing by OD 29mm armoured tow umbilical with adjustable

attachment point (providing increased angle of incidence with increased depth).

In contrast with the tail fish, the source fish does not have a flight control system implemented. It lacks therefore steering and/or positioning capability when deployed. However, it provides a stable reference point. Typically, it does not have any steerable control members and is not to be dynamically positioned. If required however, the source fish can have a simple actuator and rudder to provide control in the horizontal plane to compensate for example for a static offset caused by side currents. Depth is simply controlled by the amount of towing umbilical paid out and the angle of incidence for the attachment point.

The source is provided by high voltage current through the umbilical (typically 3000 VAC) in order to keep the power conductors in the umbilical as small as possible. This reduces umbilical diameter and drag forces.

The source fish carries the source equipment including high voltage transformers/thyristors to produce a 1500 Amp pulsed output through the electrodes (relatively heavy components). The source acts as a clump weight with respect to towing. The position of the source fish therefore stays relatively constant, and does not change quickly. As such, it provides a useful reference point about which the tail fish can be manouevred, and relative to which the tail fish can be positioned.

The source electronics are not described but typical source electronics as available on the market today can be used.

Deployment

The present source apparatus for providing a "two-component" source configuration in the tow direction can be operated from a construction vessel or a survey vessel. The source fish itself does not differ much in dimension and weight compared to existing CSEM sources and can be deployed by crane, A-frame or dedicated cantilever launch and recovery system.

The tail fish is lifted and deployed in an upright position and is self-stabilizing once placed in the water with the upper winglet at the sea surface.

Once deployed in the water, the tail fish is initially towed in with electrodes in a travel path aligned mode (breaks engaged and rudders in zero position) at moderate speed whilst paying out the antenna towing cable 7. The deployment is performed cruising at low speed preferable against the waves and wind. As the CoB is above the CoG and the cross section in the water (added mass) is small, the tail fish itself is robust with respect to installation and sea state. The payout is conducted by a linear spooling device or winch (for the latter the antenna cable must be connected to the source fish after being spooled out). The source fish is deployed hooked up in-line between the antenna cable and the main towing umbilical. The main steps during sea launch and dive are as follows:

T1 . Lifting the tail fish in the water by crane or dedicated LARS.

T2. Paying out rear antenna cable 7, cruising forward at DP low speed with the tail fish in inline mode and brakes engaged.

T3. Launching the source fish by crane or dedicated LARS. The vessel should be stationary during the launch to minimize drag from the antenna cable.

T4. Releasing the crane (or LARS), transferring load to the main umbilical winch and sheave. Start diving by paying out umbilical.

T5. Deploying the full length of antenna cable with the tail fish in neutral position near the surface and the source fish is deployed as described in steps S2-S4.

T6. Paying out the umbilical, lowering the source fish. Meanwhile, the tail fish is controlled to track the dive of the source fish. The vessel is cruising at low speed DP in order to minimize drag and lift on the source fish.

T7. Performing step S6 until the source apparatus is at operational depth (or slightly above at a "safe" distance from the seabed).

T8. Operating the rudders and brakes on the tail fish to steer the antenna to desired offset position (45° for 2 component signals). T9. Making final adjustments at the towing speed for survey, putting the tail fish into auto-pilot tracking mode, actively controlling the tail fish to maintain the source configuration. For recovery, the installation procedure is reversed. During dive and recovery the tail fish operated in a manual operation mode.

The auto pilot control is engaged when running a survey line. However, at the end of the survey line manual control may be used whilst turning for adjusting and possibly reducing the turning radius including the tail fish. During turns in the survey, the tail fish offset with respect to the source fish will change, depending upon which way the turn is made (with or against the offset) offset to either side. In practice, the most favourable side with respect to side currents may be selected. In order to compensate for any side currents the tail fish rudders are used to maintain 45° offset to the towing direction. In order to compensate for possible position offset (parallel to the towing line), source fish rudder can be used and/or simply apply an offset for the vessel towing line at the surface.

In addition to the passive robustness of the wing (high CoB and Low CoG), the tail fish control will include a fail safe functionality. The most important function is the speed brakes on the tail fish which can be engaged if something unexpected happens (for example severe stalling). The increased drag at the tail fish will immediately stabilize the towed source apparatus and the tail fish (acting as a drag anchor), and the offset and depth will be reduced which is generally desirable in a contingency situation.

Further advantages

This provides for more efficient and flexible survey possibilities, in particular efficient survey geometries may be formed to maximise data. The use of a hydrodynamic designed tail fish with control surfaces for dynamic elevation and horizontal manoeuvrability with respect to the source fish provides good accuracy and simple and robust design. Operation at in deep water at depths of 2000 m or more can be achieved. Various modifications and improvements may be made without departing from the scope of the invention herein described.

Claims

1 . Apparatus for performing a towed marine electromagnetic (EM) survey, the apparatus comprising:

at least one positioning body coupled to said elongate member;

2. Apparatus as claimed in claim 1 , wherein the flexible elongate member comprises an electrode for an EM source.

3. Apparatus as claimed in claim 1 or claim 2, wherein the tow body comprises a source fish.

4. Apparatus as claimed in any of claims 1 to 3, wherein the flexible elongate member comprises a cable to which is coupled at least one receiver for detecting an EM field component.

5. Apparatus as claimed in claim 1 , having a first source electrode coupled to the tow body and a second source electrode coupled to the flexible elongate member for forming a horizontal electric dipole source, wherein said positioning force is imparted to the elongate member to move the second source electrode into a position in which a dipole source line extending between the first and second electrodes forms a horizontal angle with respect to a direction of travel of the tow body.

6. Apparatus as claimed in claim 5, wherein the horizontal angle is an angle of between around -50 and +50 degrees.

7. Apparatus as claimed in claim 2 or in claims 4 or 5, wherein the positioning body is steerable by means of at least one controllable steering member to position the source electrode in a desired position.

8. Apparatus as claimed in any of claims 1 to 7 configured as underwater apparatus.

9. Apparatus for performing a towed underwater survey, the apparatus comprising:

at least one positioning body coupled to said elongate member;

10. Apparatus as claimed in any preceding claim, wherein said positioning body is arranged behind the tow body during towing, and is spaced apart from the tow body along the flexible elongate member.

1 1 . Apparatus as claimed in any preceding claim, wherein said at least one positioning body comprises a plurality of positioning bodies coupled to said flexible elongate member behind the tow body, and spaced apart from the tow body and from each other along the flexible elongate member.

12. Apparatus as claimed in any preceding claim, wherein the or each positioning body is non-integral with the elongate member.

13. Apparatus as claimed in any preceding claim, wherein the or each positioning body is coupled to the flexible elongate member by a coupling member.

14. Apparatus as claimed in claim 13, wherein the or each coupling member permits rotation about a vertical axis of the positioning body relative to the flexible elongate member.

15. Apparatus as claimed in any preceding claim, wherein in said changed or maintained horizontal position, a line extending between the positioning body and the tow body forms a horizontal angle with respect to a direction of travel of the tow body upon towing of between -50 and +50 degrees.

16. Apparatus as claimed in any preceding claim, wherein the positioning body is controllable to maintain a specified horizontal position relative to the tow body.

17. Apparatus as claimed any preceding claim, wherein the positioning body comprises a wing for producing a horizontal component of force when suitably oriented during towing.

18. Apparatus as claimed in any preceding claim, wherein the positioning body is steerable by means of at least one controllable steering member operable to cause a change in orientation of the positioning body.

19. Apparatus as claimed in claim 18, wherein said at least one steering member is operable according to a control program to change or maintain the horizontal position relative to the tow body.

20. Apparatus as claimed in any preceding claim, wherein the positioning body is further controllable to maintain or change its vertical position relative to the tow body during towing.

21 . Apparatus as claimed in any preceding claim, wherein the positioning body is a wing.

22. A method of towing equipment for performing a marine electromagnetic (EM) survey, the method comprising the steps of: providing underwater a tow body adapted to be coupled via a tow cable to a surface vessel, at least one flexible elongate member coupled to the tow body, and a positioning body coupled to said flexible elongate member;

23. A method of towing underwater equipment for performing an underwater survey, the method comprising the steps of:

24. A method as claimed in claim 22, including the step of providing a plurality of elongate members coupled to the tow body, wherein said plurality of elongate members comprises a first elongate member having coupled thereto first and second steerable positioning devices, and a second elongate member having coupled thereto third and fourth positioning devices, and the method includes steering each of the positioning devices so that the devices acquire respective horizontal positions, in which a line extending between the first and second positioning devices is parallel with a line extending between the third and fourth devices.

25. A method as claimed in claim 22, including the step of providing a plurality of elongate members coupled to the tow body, wherein said plurality of elongate members comprises a first elongate member having coupled thereto first and second steerable positioning devices, and a second elongate member having coupled thereto third and fourth positioning devices, and the method includes steering each of the positioning devices so that the devices acquire respective horizontal positions, in which a line extending between the first and second positioning devices is non- parallel with a line extending between the third and fourth devices.

26. An underwater positioning body, the positioning body adapted to be coupled to a flexible elongate member, the elongate member coupled to an underwater tow body, the positioning body comprising means for changing or maintaining its horizontal position relative to the tow body for imparting a positioning force on the elongate member.

27. A positioning body as claimed in claim 26 being a wing.

28. An underwater wing, the wing being adapted to be coupled to a flexible elongate member for positioning a towed flexible elongate member in an underwater survey, the wing comprising:

at least one wing surface configured to produce a component of force during towing for positioning the flexible elongate member.

29. An underwater wing as claimed in claim 27 or claim 28, comprising opposing wing surfaces joined to each other to define a leading edge at a front end of the wing for deflecting an oncoming fluid flow over the wing surfaces for producing said component of force

30. An underwater wing as claimed in claim 29, wherein the leading edge is defined by a line through points of maximum curvature of a surface portion joining the opposing wing surfaces.

31 . An underwater wing as claimed in claim 30, wherein the joining surface portion provides a smooth and continuous join of the opposing wing surfaces.

32. An underwater wing as claimed in claim 27 to 31 , wherein the leading edge defines a plane on either side of which said wing surfaces are defined, the wing being configured to be oriented with said plane extending substantially vertically when deployed under water.

33. An underwater wing as claimed in claim 32, wherein the wing surfaces are substantially symmetrical and said plane is a plane of symmetry.

34. An underwater wing as claimed in any of claims 27 to 33, wherein the opposing wing surfaces are joined to define trailing edge at a rear end of the wing, opposite to the front end.

35. An underwater wing as claimed in claim 34, wherein the join of wing surfaces at the trailing edge is less smooth than a joining surface section at the leading edge.

36. An underwater wing as claimed in claim 34 or 35, wherein the leading edge and the trailing edges extend between a first wing tip region and a second wing tip region opposite to the first wing tip region.

37. An underwater wing as claimed in claim 36, wherein the leading and trailing edges meet each other at the first wing tip region and at the second wing tip region.

38. An underwater wing as claimed in any of claims 27 to 37, wherein the length of the wing in a length direction is greater than a thickness of the wing in a thickness direction substantially normal to the length direction.

39. An underwater wing as claimed in claim 38, having a width direction normal to said length and thickness directions, the width in the width direction being greater than said thickness of the wing.

40. An underwater wing as claimed in any of claims 34 to 39, wherein the trailing edge defines a plane on either side of which said opposing wing surfaces are defined, the wing configured to be oriented with said plane extending substantially vertically when deployed under water.

41 . An underwater wing as claimed in any of claims 27 to 40, wherein the leading edge has first and second leading edge portions extending in different directions along the leading edge to define a front nose of the wing.

42. An underwater wing as claimed in any of claims 27 to 41 , wherein the coupling means is configured to permit movement of the positioning body with respect to the flexible elongate member about a vertical axis, during towing, for facilitating changing an orientation of the leading edge for controlling the horizontal lift force produced by the wing surfaces.

43. An underwater wing as claimed in any one of claims 27 to 42 wherein the wing surfaces define a delta or generally D-shape.

44. An underwater wing as claimed in any of claims 27 to 43 wherein the wing takes the form of the Eppler E-837 delta wing profile.

45. An underwater wing as claimed in any of claims 27 to 44 comprising a single wing for producing the horizontal lift force component.

46. An underwater wing as claimed in any of claims 27 to 45 having its centre of mass spaced apart from its centre of volume to generate an orienting force upon deployment underwater for orienting the wing vertically.

47. An underwater wing as claimed in any of claims 27 to 46, wherein the wing has a hollow body portion for housing operational equipment.

48. An underwater wing as claimed any of claims 27 to 47, wherein the wing has its weight distributed for separating the centre of mass and the centre of volume.

49. An underwater wing as claimed in claim 48 including at least one controllable steering member for orienting the wing.

50. An underwater wing as claimed in any of claim 49, wherein the at least one controllable steering member comprises at least one pitch rudder rotatable about a pitch axis for changing an orientation of the wing with respect to a flow during towing for changing or maintaining a horizontal position. .

51 . An underwater wing as claimed in any of claims 49 or 50, wherein the at least one controllable steering member comprises at least one elevation member for changing an elevation of the wing.

52. An underwater wing as claimed in claim 51 , wherein said elevation member comprises an elevation rudder rotatable about an elevation rudder axis, and wherein said pitch rudder axis and said elevation rudder axis are perpendicular to each other.

53. An underwater wing as claimed in claim 52, wherein the pitch axis is arranged substantially vertically and the elevation rudder axis is arranged substantially horizontally when deployed under water.

54. An underwater wing as claimed in any of claims 27 to 53, wherein the wing comprises a syntactic foam core.

55. An underwater wing as claimed in any of claims 27 to 54, wherein the wing has a synthetic outer layer.

56. An underwater wing as claimed in claim 55, wherein the outer layer comprises a composite shell.

57. An underwater wing as claimed in any of claims 27 to 56, wherein the syntactic foam has a density of 500 kg per cubic metre.

58. An underwater wing as claimed in any of claims 27 to 57, wherein the wing is configured to produce a lift force of up produce a lift force of approximately 10 to 30 kN with a tow speed of 5 knots.

59. An underwater wing as claimed in any of claims 27 to 58, wherein the wing is configured to produce a lift force of up produce a lift force of approximately 4 to 18 kN with a tow speed of 3 knots.

60. An underwater wing as claimed in any of claims 27 to 59 configured to have neutral or small positive buoyancy at an operating depth of at least 2000 m below sea level.

61 . A method of manufacturing an underwater wing, the underwater wing being a wing as claimed in any of claims 27 to 60, the method comprising steps of:

forming a wing profile from a foam block; and applying a coating or a composite shell over the formed wing profile.

62. A control system for controlling positioning of a towed flexible elongate member in an underwater survey, the system comprising:

63. A control system as claimed in claim 62, comprising a processor for processing the received position signal for determining a position of the positioning body.