System for depth control of a marine deflector

A system for adjusting a deflector in a seismic survey comprises a generally upright deflector body and at least one bridle connected to a seismic cable. The bridle includes an upper segment secured to an upper connection point on the deflector body, and a lower segment coupled to a lower connection point on the deflector body. The upper bridle segment, lower bridle segment and deflector body define a geometry between themselves. This geometry is adjustable by at least one actuator so as to control the tilt angle of the deflector body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for controlling the depth of a seismic deflector under tow through the water.

2. Background of the Related Art

The ability to conduct accurate seismic surveys may help improve the discovery rates and even the production of subsurface accumulations, such as hydrocarbons. Seismic surveying is a method of stimulating a geological subsurface formation with, e.g., electrical, magnetic, and/or acoustic signals to acquire seismic data about the formation. From this data, one can predict whether the formation contains hydrocarbon deposits and, if so, where those hydrocarbon deposits are located.

One type of seismic survey is generally referred to as a “marine” survey, because it is typically conducted at sea, although this is not necessarily always the case. During marine seismic surveys, seismic cable systems are deployed in the water behind a towing vessel.

Deflector devices, also known as deflector systems (collectively “deflector(s)” herein), are used between a towing vessel and a streamer located in water, in order to pull the streamer out to one side of the vessel. Control of the deflector allows the streamer to be positioned at a desired lateral offset from the course followed by the vessel. Seismic surveys are generally carried out with a number of streamers towed in substantially parallel paths behind a vessel.

For example,FIG. 1is an aerial view of a typical towed streamer array using a door type deflector. The system10includes a vessel12for pulling lead-ins14and streamers15through the water. A door deflector16is coupled by cables or “ropes”18to the streamers15or lead-ins14. As the deflector16is towed through the water in the direction of tow indicated by the arrow20, the force of the water against the surfaces of the deflector16allows the deflector to pull the streamers15out laterally to the side of the vessel12. This allows the streamers15to be appropriately spaced over a larger survey area. Typically, the streamer array is symmetrical about the central axis22that extends directly behind the vessel.

FIG. 2is a perspective view of a door deflector as shown inFIG. 1. The door deflector16is a traditional seismic deflector, also referred to as a vane, bi-vane, Baro-door, Baro vane, or paravane. The door deflector16comprises of a number of parallel vertical wings24mounted along side each other in a frame26that typically forms a rectangle. The door deflector16is normally towed with up to six bridle chains28, including one chain28from each corner of the rectangular door and often two extra chains28in the middle. The deflector16is completely submerged and positioned generally vertically in the water by suspending the deflector by a chain30coupled a surface float (not shown).

FIG. 3is an aerial view of towed streamers using a wing type deflector. The system40includes a vessel12for pulling lead-ins14and streamers15through the water. A wing deflector42is coupled between the lead-ins14and the streamers14and towed through the water in the direction of tow indicated by the arrow20. The force of the water against the deflector42allows the deflector to pull the streamers14out laterally to the side of the vessel12into an appropriate spacing for a survey. Typically, the streamer array is symmetrical about the central axis22that extends directly behind the vessel.

FIG. 4is a perspective view of a wing deflector as shown inFIG. 3. In use, the wing deflector42has a wing-shaped body44suspended by a chain or a rope30beneath a float (not shown) so as to be completely submerged and positioned generally vertically in the water. As the deflector device is pulled through the water, the wing-shaped body produces a sideways force, or “lift”, which moves the deflector laterally relative to the direction of tow. It is useful to define an “angle of attack” when discussing such lift, this angle being defined by the arc between the plane in which the trailing surface of the deflector body lies and the direction of tow through the water. The angle of attack will lie generally in a horizontal plane, although not necessarily so. Thus, inFIG. 4, the angle of attack is indicated as angle f between trailing deflector body surface44aand direction of tow20.

An exemplary wing deflector is described in detail in U.S. Pat. No. 5,357,892, which patent is incorporated by reference herein, and comprises a wing-shaped deflector body having a remotely-operable pivotal lever or “boom” which extends rearwardly from a point near the middle of the trailing edge of the wing-shaped body. The lift produced by the deflector can be varied by adjusting the angle of the boom from the vessel, thus permitting the lateral offset of the tow from the course of the vessel to be varied in use. The deflector device of U.S. Pat. No. 5,357,892 has been successfully commercialized by the Applicant as its MONOWING™ deflector device. In use, rolling stability of the device is provided by the connection to the float, while stability of the device about a vertical axis is provided by the drag produced by the tow.

A different version of the MONOWING exists where the angle of attack is controlled by other means than regulating the angle of the boom as described above and which relates to U.S. Pat. No. 5,357,892. In this system, a relatively long boom is rigidly fixed to the suction side of the wing and pointing rearwardly from the wing. In the rear end of this boom are mounted so called boom-wings that are adjustable in angle of attack and hence lift. By means of adjusting the lift of the boom-wing, new equilibrium positions in the so called yaw angle (rotation about the vertical axis) are found and the lift of the main wing is modified.

The MONOWING deflector devices in current use are very large, typically 7.5 m high by 2.5 m wide, and weigh several tons. They are usually suspended around 2 m to 8 m below the float by means such as a fiber rope, and are also provided with a safety chain intended to prevent separation of the float and wing-shaped body in the event that the rope breaks. In rough weather, the upper part of the wing-shaped body may rise up out of the water, allowing the rope connecting the wing-shaped body and the float to go slack. If the wing-shaped body then drops abruptly, the rope, and possibly even the safety chain, may break, and/or their attachment points on the wing-shaped body may be badly damaged.

The depth at which the current deflector devices operate is effectively determined by the length of the rope connecting the deflector to the float. As a result of this, the operating depth of the deflector device cannot readily be varied while the device is deployed in the water. And since the normal operating depth of the current deflector device is typically a few meters, in the event of the onset of bad weather during a survey, the device and all the streamers and other equipment directly or indirectly attached to it have to be recovered onto the towing vessel, and then re-deployed when the bad weather has passed, both of which operations are very time consuming.

Therefore, there is a need for a deflector that can be controlled to a given depth. It would be desirable if the depth were controllable on a continuous basis. It would be further desirable if the deflector was not directly affected by wave actions. It would be even more desirable if depth control could be used with existing deflector designs, including both door and wing deflectors.

SUMMARY OF THE INVENTION

The present invention provides a seismic survey system comprising a generally upright deflector body and an adjustable bridle coupled to the deflector body, wherein the adjustable bridle includes a connector for coupling the bridle to a cable, such as a lead-in, and wherein the adjustable bridle is capable of varying the tilt angle of the deflector body. The depth of the deflector body is controlled by varying the tilt angle of the deflector body being towed in water by a lead-in cable drawn behind a vessel. Preferably, the tilt angle is varied by pivoting the deflector about an axis that is generally transverse to the cable. The deflector may be pivoted through a number of mechanisms that alter the bridle geometry, such as by changing the length of one or more bridle segments or changing one or more angles between the bridle segments and/or the deflector body.

The deflector body may be of any known or later developed design, such as a wing deflector or a deflector door. Unlike existing deflectors, a preferred embodiment of the deflector is not suspended from a separate flotation device. Optionally, the deflector body may include a buoyancy element, preferably such that an upper end of the deflector has more buoyancy that the lower end of the deflector. In one embodiment, the deflector comprises a weight element in a lower end of the main body and a buoyancy element in an upper end of the main body. It is most preferable to make the deflector only slightly negatively buoyant.

The adjustable bridle is responsible for varying the tilt angle of the deflector body. In a preferred embodiment, the bridle has an upper segment coupled to an upper connection point on the deflector body and a lower segment coupled to a lower connection point on the deflector body. The segments of the bridle may be made from rigid members, flexible members, or a combination thereof. Nonlimiting examples of rigid members includes rods, beams and actuated cylinders, and nonlimiting examples of flexible members includes ropes, chains and cables. In one embodiment, the tilt angle of the deflector is adjusted by changing the length of the upper segment(s) of the bridle relative to the length of the lower segment(s) of the bridle. In another embodiment, the tilt angle of the deflector is adjusted by changing an angle between upper and lower bridle segments or between a bridle segment and the deflector body. The latter change in angle between a bridle segment and the deflector body may be accomplished by moving the connection-point of the bridle segment to the deflector body upward or downward.

At any point in time, the bridle is connected to the deflector in a manner defining a geometry therebetween that established the tilt angle of the deflector relative to the cable. The bridle geometry is made adjustable by including an actuator means or motor means that can controllably adjust a connection-point or effective length of one or more segments of the bridle. The term “connection point”, as used herein, means the point along the length of the deflector where the bridle segment connects with the deflector body. The term “effective length”, as used herein, means the distance between the real or imaginary point where the bridle segments converge and the real or imaginary point where the bridle segment is secured to the deflector body. Preferably, upper and lower segments of the bridle are connected to the body of a wing deflector on a line extending parallel to the longitudinal axis of the deflector body. Preferably, upper and lower segments of the bridle are connected to the body of a door type deflector along multiple lines extending parallel to the longitudinal axis of the deflector body.

The deflector or deflector system may further include a controller for adjusting the tilt angle. The controller may be of any type, such as digital, analog or a combination thereof, and may be located with the deflector, with the vessel, or constitute distributed control with different steps or actions taking place in different locations yet collectively serving as a controller. Where a controller is located within the deflector for controlling the tilt angle or depth of the deflector, the system will preferably further comprise a remote controller, such as on the vessel, for providing a tilt angle or depth setpoint to the local controller. In one embodiment, the deflector or deflector system comprises a sensor for measuring the actual depth of the deflector, an actuator for adjusting the bridle, and a controller for providing a command to the actuator upon input from the sensor to achieve or maintain a desired depth of the deflector.

In a preferred embodiment, the controller causes the actuator to vary the angle between the deflector and the cable so that the vertical component of lift from the deflector is substantially equal to the vertical component of gravity (adjusted for buoyancy) minus the vertical component of tension in the cable.

In a particularly preferred embodiment, the deflector may additionally include means for controlling the cross-line position of the deflector. For example, the deflector may include an adjustable lever pivotally connected by a first pivotal connection to the deflector body adjacent the rear edge of the deflector body and extending rearwardly thereof, and an attachment point on the rear end of the adjustable lever for connecting a rearwardly extending cable. Typically, the adjustable lever includes an actuator for adjusting the position of the attachment point with respect to the deflector body. Specifically, the adjustable lever may comprise a second pivotal connection on the adjustable lever, and an adjustable mechanism mounted on the deflector body and operatively connected to the second pivotal connection for adjusting the rear end of the adjustable lever by pivoting the second pivotal connection about the first pivotal connection between the adjustable lever and the deflector body. The adjustable mechanism adjusts the angle of the adjustable lever with respect to the deflector body. In a practical seismic survey system, the system will include a seismic cable link between forwardly and rearwardly extending cables for bypassing the deflector body. Those skilled in the art will understand that “forward” or “forwardly” in this specification means the direction from the center of the deflector generally towards the towing vessel during operation and that “rear” or “rearwardly” in this specification means the direction from the center of the deflector generally away from the towing vessel during operation. The system will also include power means operatively connected between the deflector body and the adjustable mechanism for operating the adjustable mechanism. A controller is also typically included for controlling the adjustable mechanism.

In one embodiment, the deflector with an adjustable bridle is used in combination with a pivot float attached at a position on the cable, such as the lead-in, that is forward of the deflector body. Accordingly, the pivot float serves as a pivot point from which the deflector pivots when the deflector depth is adjusted. Generally speaking, by adjusting the tilt angle of the deflector, the deflector can be made to move in an arc about the pivot point defined by the pivot float in order to achieve a given depth.

In another embodiment, a deflector comprises a deflector body, perhaps selected from a wing deflector and a deflector door, a connector coupling the deflector body to a cable being towed in water behind a vessel, a first actuator for varying the angle of attack of the deflector body, and a second actuator for varying the tilt angle of the deflector body, wherein the first and second actuators are operated independently. This deflector is characterized in that the depth of the deflector body is controlled by varying the tilt angle of the deflector body. The tilt angle is varied by pivoting the deflector body about an axis that is generally transverse to the cable. Optionally, the second actuator is coupled to a first controllably movable flap to one side of the deflector center of lift.

In yet another embodiment of the invention, a deflector comprises a generally upright deflector body including at least one connection point for coupling to at least one cable, and at least one controllably movable flap coupled to the deflector body to vary the tilt angle of the deflector body.

The invention also includes a method for controlling the depth of a deflector under tow. The method comprises varying the tilt angle between the deflector and cable, wherein a change in the tilt angle causes the deflector to change depth, and wherein the tilt angle is varied about an axis that is generally transverse to the cable. Preferably, the method includes controlling the tilt angle to change the depth, such as by changing the length of one or more bridle segments or changing one or more angles between the bridle segments and/or the deflector body. Alternatively, the method may include controllably adjusting the angle of flaps on upper and lower segments of the deflector to varying the tilt angle of the deflector. In this instance, the tilt angle may be adjusted by using flexible bridle members and permitting one or more of the members to go slack while other members remain in tension A preferred method includes measuring the depth of the deflector, and providing a command for the deflector to achieve a different depth or maintain the same depth. It should be recognized that changing ocean current or wave conditions, as well as changing tow speed, may require continuous adjustments in the tilt angle in order to even maintain a depth already achieved. In one embodiment, the method includes coupling a float to the cable upstream of the deflector, wherein a change in the tilt angle causes the deflector to pivot about the float.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention provides a marine deflector for a seismic survey system. The deflector has a generally upright deflector body that controllably tilts about an axis that is generally transverse to a cable that pulls the deflector through the water. In one embodiment, an adjustable bridle is coupled to the deflector body, wherein the adjustable bridle includes a connector for coupling the bridle to the cable, such as a lead-in, and wherein the adjustable bridle is capable of varying the tilt angle of the deflector body. The depth of the deflector body is controlled by varying the tilt angle of the deflector body. Preferably, the tilt angle is varied by pivoting the deflector relative to an axis that is generally transverse to the cable.

The deflector may comprise a so called wing deflector, e.g. the WesternGeco Monowing, or it may comprise a so called deflector door, frequently called a door or a Barovane comprising a series of hydrofoils mounted within a rectangular frame. Regardless of the type of deflector, the present invention allows for the tilt angle of the deflector to be adjusted by one or more of a variety of methods or means. One embodiment includes adjusting wing flaps as known from airplanes. The flaps re-distribute the lift of the wing along the span so as to create a moment of force that results in a tilt angle. A second embodiment includes manipulating the geometry defined by the deflector body and the towing bridle segments. Finally one may also imagine a combination of bridle adjustment and flap adjustment.

The preferred method of using the deflector involves controlling the tilt angle, also called heel angle or roll angle, of the deflector in such a way that the vertical component of the lift force generated by the wing changes with the tilt. The wing will find its equilibrium in depth when the vertical component of the wing lift plus the vertical component of the gravity force equals the vertical component of the tension in the tow wire or lead-in. By changing the tilt angle, the wing will find a new equilibrium depth.

Unlike deflectors currently on the market, the present depth controllable deflectors should be close to zero buoyant. Preferably, the deflectors should be slightly negative buoyant in order for changes in water speed to have minimal influence on the depth of the deflector. Furthermore, since a near-zero buoyant deflector does not require a direct connection to a surface float, i.e., a hanging support connection, the typical strains on the direct cable connection due to wave loads on the surface float are eliminated. Therefore, maintenance intervals may be increased and the risk for catastrophic failures of this cable connection is eliminated.

The invention provides the ability to effectively control the depth of the deflector while the deflector is deployed. Controlling the depth of the deflector means that you can control the depth of the streamer near the connection to the deflector. For example, the front end of the streamer may be controlled at a fixed or changing depth. One advantageous application for controlling the depth includes lowering the front end of a streamer during periods of strong waves in order to reach a depth where the wave action is insignificant. It should be noted that wave action decays exponentially with depth. Consequently, the seismic vessel can leave the equipment out in the water through much worse wave conditions with little or no damage. This also reduces the number of retrievals and deployments and opportunities for equipment damage and personnel injuries that can occur during retrievals and deployments. Productivity is increased because the system is quickly ready for production when the waves calm down simply by returning the streamer to the desired operating depth. Furthermore, depth control of the deflector and the front end of the streamer can have a positive effect on the seismic data quality, since the front end may be operated at the same depth as the desired streamer depth resulting in less noise on the front sections of the streamer.

FIG. 5is an aerial view of an exemplary configuration of a seismic survey system50having a deflector52in accordance with the present invention. The deflector52is coupled by a bridle53to a lead-in cable54that is pulled through the water55behind a vessel (not shown). The lead-in cable54is on the upstream end of the seismic cable from the deflector and is shown including an optional pivot float58. A streamer56is attached to the lead-in cable54, but could also be coupled directly to the deflector52. The streamer56is preferably coupled as close to the deflector52as possible for effective depth control of the front end of the streamer56. The deflector52should be generally upright during operation, since a large tilt angle will reduce the horizontal lift force on the deflector that is needed to achieve the desired separation of multiple streamers in an array. In addition, it is beneficial for the bridle53to be as perpendicular to the deflector as possible. The latter two characteristics of the system are achievable when the deflector is balanced so that it is slightly heavier than water when submerged in water. In this case, a change in lift force by e.g. change in water speed and/or change in angle of attack will have as little impact on the depth of the wing as possible.

FIGS. 6A-6Care schematic diagrams of the system50as viewed from behind the deflector52looking in the direction of tow. The deflector is viewed from below the surface of the water55and from a downstream location. The diagrams illustrate the force equilibrium on the deflector52that is used to control the deflector depth, d. The lead-in (tow cable)54and bridle53are shown coming in from the left and being attached to the deflector52. The lead-in54is coupled to the optional surface float or pivot float58that establishes a local pivot point from which the outer part of the lead-in54, the bridle53and the deflector52are pivoted when depth is adjusted. The deflector52remains in an equilibrium position as long as the resultant force, R, between the lift force, L, and the gravity force (i.e., weight) resulting from the application of gravity on the deflector mass, Mg, is in-line with the lead-in tension force, T. It is useful to define a “tilt angle” for this discussion this angle being defined by the arc between the plane in which the trailing surface of the deflector52lies and the gravity vector (i.e., vertical). The tilt angle will lie generally in a vertical plane, and is indicated as angle θ inFIGS. 6B-6C. Changing the tilt angle, θ, of the deflector will result in a change in direction of the resultant force, R, such that the whole system from the pivot buoy or float58to the deflector will rotate about the pivot float58until a new equilibrium position or depth is established.

FIG. 7Ais a perspective view of a deflector wing60having a deflector body62and adjustable flaps64in accordance with one embodiment of the invention. Rotating the upper and lower flaps64in opposite directions or in the same direction to different degrees, i.e., independently, will create a hydrodynamic tilt-moment that will make the wing60tilt. Alternatively, a tilt-moment can be created by rotating a single flap64without movement of the other flap(s)64. The deflector60may have any number of flaps, even a single flap, so long as one or more of the flaps can produce a tilt-moment. The deflector60is shown inFIG. 7Acoupled to a bridle having an upper segment66and a lower segment68converging and coupling to the lead-in cable54at a point69. In this instance, the tilt angle is adjusted by using flexible bridle segments66,68and permitting one of the segments to go slack while the other segment remains in tension.

An alternative design is shown inFIG. 7B. The bridle including segments66and68shown inFIG. 7Bare coupled to each other to form a single cable that is in engaged with a pulley65secured to the end of cable54. In this manner, the desired tilt angle can be achieved with a lower magnitude of the tilt-moment. The deflector embodiment ofFIG. 7Balso includes a sensor63for measuring the actual depth of the deflector. This sensor will cooperate with an actuator (not shown) for adjusting the bridle, and a controller (not shown) that provides a command to the actuator upon input from the sensor to achieve or maintain a desired depth of the deflector. The actuator and controller are described further below with reference toFIGS. 9C and 9D.

FIG. 8Ais a perspective view of a wing deflector70with a bridle formed by chain segments72and74extending from the deflector body76to a connection point78. A controller and actuator (not shown) can manipulate the geometry bound by the deflector and the bridles. This geometry comprises the triangle that is bound by bridle segment72, bridle segment74, and the deflector segment79extending therebetween. As with the embodiment ofFIG. 7A, the tilt angle may be adjusted in the embodiment ofFIG. 8Aby using flexible bridle segments and permitting one of the segments to go slack while the other segment remains in tension.

In this particular embodiment, the deflector70is shown equipped at its upper end with an elongated, streamlined float77that is rigidly secured to the deflector body76so that the body depends downwardly from the float like the keel of a boat. The float may be constructed of a similar material to the body, e.g., titanium, but may otherwise be made from a fiber-reinforced composite material. A weight element79is also secured to deflector body76, preferably to compensate the buoyancy force provided by float77to produce a slight negative buoyancy overall on deflector70.

FIG. 8Bis a perspective view of a deflector door80with a bridle formed by chain segments81,82,83,84(and optionally, segments85,86) extending from the deflector body87to a connection point88. For the door deflector80, a controller and actuator (not shown) can manipulate the similar geometry as inFIG. 8A, except that the upper segments81,82must act as a pair and the lower segments83,84must act as a pair for depth control. Specifically for the purpose of changing the tilt angle of the door87, the tetrahedral bound by the door87and the outer bridle members81,82,83,84are manipulated. The tilt angle of the deflector87is changed by altering the ratio of the length of the upper segments81,82to the length of the lower segments83,84. However, it should be recognized that it is also possible, either sequentially or simultaneously with tilt angle adjustment, to adjust the angle of attack of the deflector87by altering the ratio of the length of the front segments81,83,85to the length of the trailing segments82,84,86. When the tilt angle and angle of attack are both being controlled, the lengths of all bridle segments may be different at any point in time.

FIGS. 9A through 9Gare schematic diagrams of a number of means for changing or manipulating the geometry bound by the bridles and the deflectors of the present invention. It should be recognized that the triangles shown in these Figures are side views that apply equally to either a wing deflector having two bridle chains72,74as inFIG. 8Aor a deflector door having four bridle chains81,82,83,84as inFIG. 8B. In regard to a deflector door, the upper segment of the triangle inFIGS. 9A through 9Grepresents all upper segment chains, such as segments81and82ofFIG. 8B, and the lower segment of the triangle represents all lower segment chains, such as segments83and84ofFIG. 8B. The invention may be equally applied to bridles containing any number of segments.

FIG. 9Aillustrates a simple system90where the length of the lower bridle segment94is adjusted by means of a hydraulic cylinder or actuator96overcoming the tension in the bridle.

FIG. 9Bshows a system100comprising a first hydraulic cylinder102coupled between an upper portion of the deflector108and the upper bridle segment104, and a second hydraulic cylinder102coupled between a lower portion of the deflector108and the lower bridle segment106. Both bridle segments104,106are coupled at a connection point to the lead-in cable54. This system is more energy efficient as the hydraulic pump only has to overcome the force equal to the tension difference in the bridle members and not the total tension as in the system ofFIG. 9A. This is referred to as the principle of load-balancing.

FIGS. 9C and 9Dillustrate other systems that use the load balanced principle. InFIG. 9C, the system110includes rotatable connection points or towpoints112coupled to the deflector114. The bridle segments111,113are attached to the outer lever arms, and between the inner lever arms are attached a rod, chain, or other connecting means116that transfer the loads between the two rotating towpoints112. A linear actuator118is connected to this middle member116to control the rotation of the towpoints112. In this embodiment of the invention, the deflector further includes a controller119for providing commands to the linear actuator. The controller may be of any type, such as digital, analog or a combination thereof, and may also be located on the vessel, or constitute part of a distributed control system with different steps or actions taking place in different locations yet collectively serving as a controller. Where the controller is locally positioned within the deflector, as depicted inFIG. 9C, for controlling the tilt angle or depth of the deflector, the system will preferably further comprise a remote controller (not shown), such as on the vessel, for providing a tilt angle or depth setpoint to the local controller.

FIG. 9Dillustrates another system120that utilizes the same principle of load balancing, but the bridle segments121,122,123comprise a closed loop that forms a triangle extending around a block or wheel124at each towpoint125. In the same manner as above, a linear actuator118adjusts the position of the bridle by being connected to the bridle segment122between the towpoints125.

FIG. 9Eillustrates a system130having sliding towpoints132that secure the ends of the upper bridle segment134and lower bridle segment136to the deflector body138. Sliders as applied in modern sailing yachts may be used, as well as any actuator or motor. While it is optional to use only one sliding towpoint132, the system would not be load balanced. Rather, it is preferred to use two sliding towpoints132that slide in the same operation so that the system will be load balanced.

FIG. 9Fshows a system140with the bridle segments142,144connected to an inverted toothed wheel146that is engaged with a toothed wheel148that is rotatably driven by an actuator149. By adjusting this actuator149the attachment point where the lead-in53is attached to the inverted toothed wheel146is effectively adjusted, resulting in the tilting of the deflector.

FIG. 9Gillustrates a system150in which the angle, α, at which the lead-in54is connected to the bridle segments152,154is altered by an actuator or cylinder156. A frame is formed at the outer end of the bridle and includes rigid members157,158,159that are pivotally connected. Energizing the actuator156applied a force on rigid member159, which member then causes rotation of member157at point155.

FIG. 10illustrates the construction of a deflector200that provides adjustable depth according to the invention, as well as adjustable cross-line positioning. The deflector body202that acts as a kind of otter board attached through the bridle members to the lead-in54that extends from the towing vessel (not shown) to the streamer56, i.e. the equipment that is towed behind the vessel through the water. The three attachment points to the deflector are indicated at207,208and214. A streamer cable that performs the necessary communication with the tow is led along the lead-in54and the streamer56and extends therebetween as indicated by206. It is separated from the lead-in54in the area of the deflector and reconnected with the streamer56some distance after the connecting point214. This cable section206is slack, in order to prevent it from influencing or restricting the movements of the deflector200.

The deflector202is also coupled to an adjustable bridle at two points207,208along a line generally parallel to the deflector's vertical axis, x. Here, an upper bridle segment215is coupled via an actuator217to the deflector body at point207and a lower bridle segment216is coupled via an actuator218to the deflector body at point208. As describe previously, this arrangement allows the tilt angle of the deflector to be adjusted, resulting in control of the deflector depth.

At the rearward end of the deflector202, the fitting209is equipped with an angle lever210that is rotatable or pivotable about a point219on the fitting. The attachment point214is located at the end of lever210. The angle lever210is connected by its second leg220via a pivotable connection213with the side of the fitting209. For adjusting purposes, there can be provided here an adjustable piston cylinder mechanism211that can cause a forward and backward movement of the pivotable connection213of leg220of lever210. This adjusting mechanism can also be of a different shape to that of a piston cylinder, and the device can be capable of being operated by a motor drive, e.g. a hydraulic motor in the deflector, a battery-driven motor or it could be adjusted before being deployed. Additional description of the operation of such a deflector is found in U.S. Pat. No. 5,357,892, which patent is incorporated by reference herein.

The invention also includes a method of performing a marine seismic survey, the method including towing a plurality of laterally spaced seismic steamers over an area to be surveyed, wherein the depth of at least one of the streamers is controlled by a deflector device in accordance with any one of the preceding statements of the invention.

It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.