WO2003005062A1 - Underwater survey apparatus and method - Google Patents

Abstract

A survey apparatus and method that includes the use of a remotely operated vehicle (ROV) (10) to deploy a survey apparatus (20) at an underwater location. The survey apparatus (20) includes a seismic source (40), and the ROV (10) is typically used to locate the survey apparatus (20) and/or the seismic source (40) in contact with or adjacent to an underwater bed (28). The seismic source (40) is actuated once in contact with or adjacent to the underwater bed (20) to generate seismic waves for the survey of underwater reservoirs, strata or structures. The ROV (10) can then be used to move the survey apparatus (20) to a new location and locate it with precision.

Description

UNDERWATER SURVEY APPARATUS AND METHOD

The present invention relates to a survey apparatus and method and particularly, but not exclusively, to a survey apparatus that includes a seismic source that is preferably, but not necessarily, in contact with the seabed.

A number of different types of apparatus and methods exist for taking seismic surveys to, for example, assess the volume of an underwater hydrocarbon reservoir. Some conventional methods use a vessel to illuminate the reserλroir using water gun seismic techniques, where others use airgun technologies.

These conventional methods and apparatus have a number of associated disadvantages in terms of efficiency and potential damage to the environment of the sea and the seabed, including wildlife.

According to a first aspect of the present invention there is provided an underwater survey apparatus comprising a seismic source and a deployment means for deploying the source and/or locating the source adjacent an underwater bed, wherein at least a portion of the apparatus is adapted to be in contact with or adjacent to the underwater bed in use.

The present invention also provides an underwater survey apparatus comprising a seismic source wherein at least a portion of the source is adapted to be in contact with the underwater bed in use.

The apparatus typically includes an anchor to engage an underwater bed. The anchor is preferably a suction anchor.

According to a third aspect of the present invention, there is provided a method of surveying an underwater bed, the method comprising the steps of locating a seismic source in contact with or adjacent to the underwater bed, and actuating the source to generate seismic waves.

The deployment means is preferably a remotely operated vehicle (ROV) or an autonomous vehicle (AUV) . Alternatively, the deployment means may comprise a crane, winch or the like provided on a surface vessel. An ROV is preferred as this allows the survey apparatus to be positioned or located more accurately.

Where an ROV is used as the deployment means, the apparatus typically includes a frame (e.g. an ROV sub frame) and the seismic source is typically coupled to the frame. The frame is typically detachable from the ROV. This helps to prevent the ROV from being damaged during actuation of the seismic source. The frame is typically coupled to the ROV by one or more latches. The latches may be, for example, API17H latches.

As an alternative to this, the frame may be suspended from the ROV using vibration-absorbing mounts.

The remotely operated vehicle (ROV) may comprise any work class ROV. The ROV typically has a power rating of between 100 and 200 horsepower.

The underwater bed is typically a seabed, but may comprise a lake or riverbed.

The seismic source can be of any conventional type such as hydraulic, piezeo, electrical or rheolitic. The seismic source preferably includes a base plate that is typically vibrated on one or more axes to generate the seismic waves. The base plate typically comprises the portion of the source that is in contact with or adjacent to the underwater bed. The base plate is typically vibrated using rams where the source is hydraulic or electrical. The rams typically act on an anvil that is coupled to the base plate. Six rams are typically provided; four rams typically vibrate the plate in a first plane, and two rams typically vibrate the plate in a second plane. The first plane is typically perpendicular to the second plane. Thus, P and S waves can be generated using the same source. The rams are typically hydraulic or electromagnetic rams. Alternatively, electromagnetic actuation pads may be used to vibrate the base plate. The base plate may be provided with^" a ribbed surface. This enhances the transmission and/or generation of S waves.

The seismic source is typically actuated for between 4 and 10 seconds, although this can be varied to suit the particular source and application. The apparatus may include a frequency sweeper to actuate the seismic source at frequencies of between 1Hz and 1kHz, although frequencies outwith this range may also be used. Higher frequencies give greater signal penetration and a reduction in extraneous interference (e.g. from ship propellers, wave action etc.) . Also, higher frequencies allow for a higher seismic frequency range to be attained, which in turn allows greater discrimination through gains in resolution of reservoir characteristics to be observed and recorded.

The seismic source is typically located within a housing, the housing typically being coupled to the frame. The apparatus typically includes a suction anchor, and in certain embodiment, the housing typically forms part of the suction anchor. The housing typically includes an open end and a closed end. The suction anchor typically includes a pump, and one or more fluid inlets/outlets in fluid communication with the pump. Four fluid inlets/outlets are typically provided, each inlet/outlet being symmetrically arranged at the closed end of the housing.

The pump is typically reversible so that it can remove fluids from and inject fluids into the housing. The pump can be of any conventional type and is generally capable of producing a pressure differential between an interior and exterior of the housing in the range of 0 to 5 bar, although pressure differentials outwith this range may be used. The housing is typically provided with one or more transducers or sensors to monitor the pressure differential. The transducers or sensors are typically coupled to a pump controller that controls the pump. This provides a feedback loop to the pump so that the pressure differential can be maintained at a substantially constant level.

The open end of the housing is typically adapted to be engaged in the underwater bed. This allows the base plate to be located adjacent the underwater bed, and preferably in cqntact with the underwater bed. The housing is typically circular in cross- section, although other shapes may be used..

The housing is optionally movable relative to the seismic source. Thus, the depth of. penetration of the housing into the underwater bed can be varied. The housing can be moved manually or automatically (e.g. by the use of hydraulic rams) .

The apparatus typically includes a responder so that the range and bearing to the apparatus can be measured. This allows the precise position of the apparatus adjacent the underwater bed to be measured relative to a reference (e.g. a transponder on a surface vessel) . The surface vessel is preferably provided with a positioning system (e.g. a global positioning system (GPS/DGPS) . This has the advantage that the position of the vessel is known so that the survey apparatus can be positioned at a specific target location adjacent the underwater bed. Other positioning systems may be used, such as ultra-short base line (USBL) , long base line (LBL) and long range real time kinematic (LRRTK) .

The step of locating the seismic source underwater typically comprises deploying the source from a surface vessel. The source may be deployed using any conventional means such as a crane, winch or the like, but is preferably located using an ROV.

The step of actuating the seismic source typically includes the step of vibrating a base plate on one or more axes. The base plate is typically vibrated on at least one and preferably three axes. Each axis is typically perpendicular to the others. The method typically includes the additional step of providing the base plate adjacent to, or preferably in contact with, the underwater bed. The step of actuating the seismic source typically includes the step of generating P waves. The step of actuating the seismic source typically includes the step of generating S waves. The step of actuating the seismic source may include the step of generating P waves, followed by the step of generating S waves, or vice versa. Optionally, the method may include the additional step of mounting the seismic source to the ROV using vibration-absorbing mounts.

The method optionally includes the additional step of actuating the seismic source two or more times. This provides for multiple seismic shots.

The method optionally includes the additional step of releasably attaching the seismic source to a remotely operated vehicle (ROV) . The method optionally includes the additional step of decoupling the ROV from the seismic source during actuation of the source. This can help prevent the ROV and its systems from being damaged by the vibrations caused by the seismic source.

The method preferably includes the additional step of locating the seismic source adjacent to or in contact with the underwater bed using the ROV. The method preferably includes the additional step of locating the seismic source at a target location on the underwater bed. The source can be positioned precisely using any positioning system, such as GPS/DGPS, USBL, LRRTK or LBL.

The seismic source is typically located in a housing, and the method typically includes the additional step of locating the seismic source in the housing. In this case, the method typically includes the additional step of engaging at least a portion of the housing in the underwater bed. The step of engaging the housing in the underwater bed typically includes the step of creating a pressure differential between an exterior and an interior of the housing. The step of creating the pressure differential typically includes the step of removing fluids from within the housing. The pressure differential causes the housing to be sucked into and thus become embedded in the underwater bed.

The method optionally includes the additional step of monitoring the pressure differential.

The method typically includes the additional step of removing the housing from the underwater bed. The step of removing the housing typically includes the step of reducing the pressure differential. The step of reducing the pressure differential typically includes the step of injecting fluids into the housing. This causes a reduction in the pressure differential that facilitates removal of the housing from the underwater bed. The method optionally includes the additional step of actuating the seismic source during removal of the housing from the underwater bed. This has the advantage that the vibrations caused by the seismic source tend to liquefy the underwater bed material, thereby reducing friction and easing the release from the underwater bed.

The method typically includes the additional step of re-coupling the ROV to the seismic source.

The method optionally includes one, some or all of the additional steps of a) moving the seismic source to another target location using the ROV, b) engaging the housing in the underwater bed, c) decoupling the ROV from the source, d) actuating the source at least once, and e) re-coupling the ROV to the source.

The method optionally includes the additional step of repeating steps a) to e) .

Embodiments of the present invention shall now be described, by way of example only, and with reference to the accompanying drawings, in which: - Fig. 1 is a side elevation of a remotely operated vehicle (ROV) provided with an embodiment of survey apparatus; Fig. 2a is a schematic representation of the survey apparatus of Fig. 1; Fig. 2b is a view of the survey apparatus of Fig. 2a from below; Fig. 3 is a schematic representation of part of the survey apparatus of Figs 1 and 2; Fig. 4 is a schematic representation of the survey apparatus of Figs 1 to 3; Fig. 5 is a side elevation of the ROV of Fig. 1 at a first location on the seabed ; Fig. 6 is a side elevation showing the ROV of Figs 1 and 5 engaged in the seabed; Fig. 7 is a side elevation showing the position of the ROV of Figs 1, 5 and 6 during actuation of the survey apparatus of Figs 2 to 4; Fig. 8 is a side elevation of the ROV of Figs 1 and 5 to 7 after it has docked with the survey apparatus of Figs 2 to 4; Fig. 9 is side elevation of the ROV of Figs 1 and 5 to 8 after it has moved to a second location on the seabed; Fig. 10 is a side elevation of the ROV that is similar to Fig. 7 when located at the second location; Fig. 11 is a side elevation of the ROV that is similar to Fig. 8 when located at the second location; and Fig. 12 is side elevation of the ROV of Figs 1 and 5 to 11 at the surface.

Referring now to the drawings, Fig. 1 shows an ROV 10 that in this example is a 200 horsepower (HP) multi-role vehicle (MRV) , but could be any working class ROV or equivalent. The ROV 10 is preferably rated between 100 and 200 HP (approximately 75kW to 150kW) , but any suitable rating can be used, depending upon the application and power requirements. A survey apparatus, generally designated 20 and best shown in Figs 2 to 4, is releasably attached to the ROV 10. The ROV 10 is preferably used to deploy the survey apparatus 20, as this allows for more precise location of the apparatus 20 adjacent an underwater bed (e.g. a seabed 28). However, the apparatus can be deployed from a surface vessel, platform etc. using any conventional means, such as a crane, winch or the like provided on the vessel.

Referring particularly to Figs 2a, 2b and 4, the survey apparatus 20 includes a. cylindrical housing 22 that is open at a first end 24, and closed at a second end 26. Housing 22 can be of any convenient size and shape and need not be cylindrical. For example, the housing 22 may be square, rectangular, triangular etc in cross-section. The open end 24 of the housing 22 is adapted to be engaged in the seabed 28, as schematically shown in Fig. 4, and thus a circular cross-section for the housing 22 is preferred. The closed end 26 of the housing 22 is adapted to be releasably attached to the ROV 10, as will be described. It should be noted that references to seabed 28 also include references to any underwater bed such as a river or lake bed, as the case may be.

The closed end 26 of the housing 22 is preferably provided with concentric and cross bracing 27, as best shown in Fig. 2b. The bracing 27 provides additional strength for the housing 22 so that it can be used in deep water where the pressure on the housing 22 can be significant, and also to help absorb normal operational stresses and strains. The bracing 27 can extend towards the open end 24 of the housing 22.

The housing 22 typically has an inner diameter (ID) of around 1500mm, an outer diameter (OD) of around 1503mm (equivalent to a wall thickness of 3mm) and a height of around 600mm. However, it will be appreciated that the particular dimensions of the housing 22 can be varied for the particular application, and especially to suit the type of seabed material and depth of water.

The housing 22 is typically attached to an ROV sub frame 12 (shown schematically in Fig. 4), or can be provided on a suitable underwater skid (e.g. for use with a crane, winch etc) . The sub frame 12 generally allows the housing 22 and the remainder of the survey apparatus 20 to be coupled to substantially any suitable work class ROV with sufficient power and data communication capability to the surface (typically via an umbilical 14) . The sub frame 12 and the other components of the survey apparatus 20 are preferably rated to at least 2500 metres water depth. The ROV sub frame 12 is generally provided with suitable clamps, systems and interfaces so that it can be coupled to any standard work class ROV 10. The sub frame 12 is constructed to withstand normal operational stresses, loads and strains, and is insulated from the survey apparatus 20 using a plurality of steel/rubber shock mounts 16 and/or tendons 56.

A number of two-way fluid inlets/outlets 30 are coupled to the housing 22 to inject fluids into and remove fluids out of the housing 22. Four inlets/outlets 30 are typically provided, each inlet/outlet 30 being circumferentially and equally spaced from the others. This provides the advantage that the removal and injection of fluids is substantially balanced within the housing 22.

Each inlet/outlet 30 is in fluid communication with a pump (not shown) via a number of proportionally controlled conduits 32, a conduit 32 being provided for each inlet/outlet 30. The pump is typically a low pressure reversible pump capable of generating a pressure differential of between 0 and 5 bar, although higher pressure differentials may be provided with an appropriate pump as required. Two separate pumps may be provided, one to pump fluids into the housing 22 and one to remove fluids therefrom. The pump can be provided as part of the apparatus 20 or on the ROV 10. The pump is typically reversible by actuation of a slide (not shown) that changes the direction of flow between removing and injecting fluids, and is typically controlled by a pump controller (not shown) that is in turn controlled by an ROV hydraulic control system (not shown) . The closed end 26 of the housing 22 is provided with a mud filter 34 that is used to filter out the larger particles of seabed material so that the particles do not adversely affect operation of the pump. The mud filter 34 generally extends across the entire surface area of an inner surface of the housing 22 at the closed end 26, but may be located only in the vicinity of the inlets/outlets 30.

The housing 22 and the pump together with the inlets/outlets 30 form a suction anchor, the function of which shall be described below.

A marine sound or noise generator, generally designated 40, is located within the housing 22. The noise generator 40 is typically a seismic sound source and can be of any conventional type, and will be referenced as a seismic source 40 hereinafter. Any suitable seismic, noise or other acoustic actuator or generator may be used. For example, the seismic source 40 can be hydraulic, electric or piezeo. In addition to this, a rheolitic hammer base plate system may also be used. In the example^' shown, the seismic source 40 is hydraulic and will be described in detail hereinafter.

For reference, an electrical seismic noise source typically comprises one or more electromagnetic actuation stators that are orthogonally mounted, and two vertical rams. Each stator is generally controlled by a proportional power management system and an independent control pod. The system can be configured to operate sequentially to provide P, S and shear and cross wave signatures without having to recover the source to the surface for reconfiguration.

A piezeo seismic noise source is typically an electro/mechanical seismic signal generator that comprises one or more piezoelectric actuation pads stacked vertically and controlled by a series of proportional power management systems via an independent control pod. This system can be configured to operate to give P wave signatures, and may also be capable of providing S wave function signatures.

Referring again to Figs 2a, 2b and 3, the seismic source 40 in this example is a hydraulic seismic source. This system typically comprises four actuation rams 42 (see Fig. 2b in particular) that are orthogonally mounted around an anvil 44 (i.e. each ram 42 is mounted on axis that is substantially perpendicular to the others), and two vertical rams 46 (only one shown) that are mounted above the anvil 44. The rams 42, 46 are generally controlled by a number of proportional valves (not shown) and an independent valve control manifold (not shown) .

The anvil 44 is attached to a base plate 48. The base plate 48 is typically of metal but can be of any suitable material, and preferably provides a good, solid contact with the seabed 28 in use so that the majority of the energy in the seismic vibrations generated by the source 40 is directed into the seabed 28.- The base plate 48 can be provided with ribs (not shown) so that S wave generation into the seabed 28 can be more effective, and also to minimise liquefaction effects due to vibrations of the base plate 48 at the interface between the plate 48 and the seabed 28. The ribs typically have a depth of around 20mm and are spaced-apart by around 100mm.

Although it is preferred that the base plate 48 is in direct contact with the seabed 28 in use, this is not essential, and the plate 48 need only be adjacent to or in close proximity with the seabed 28.

The two vertical rams 46 provide for S or shear wave generation by applying vertical vibrations to the anvil 44 that are transferred to the base plate 48. The anvil 44 and base plate 48 are typically vibrated in the z direction by the rams 46 (i.e. vibrated vertically with respect to the orientation of the source 40 in Figs 2a and 3) . The vertical rams 46 are actuated by a respective servo system 51 and typically operate at frequencies in the order of 1 to 1000 Hz. A frequency sweep function is typically provided so that the vertical rams 46 are driven at successive frequencies from 1Hz up to around 1kHz, in, for example, 1Hz intervals. The frequency sweep can be generated by any standard frequency controller that is controlled by any standard computer with appropriate software. Higher frequencies give greater signal penetration and a reduction in extraneous interference (e.g. from ship propellers, wave action etc.). Also, higher frequencies allow for a higher seismic frequency range to be attained, which in turn allows greater discrimination through gains in resolution of reservoir characteristics to be observed and recorded.

The four actuation rams 42 provide for P wave generation by applying sideways vibrations on each perpendicular axis to the anvil 44, the vibrations being transferred to the base plate 48. Thus, the anvil 44 and the base plate 48 are shaken in the x and y directions by the four rams 42 (i.e. vibrated to the left and right, and also upwards and downwards with respect to the orientation shown in Fig. 2b) . The four actuation rams 42 are actuated by a respective servo system 52 and typically operate at frequencies in the order of 1 to 1000 Hz, similar to the vertical rams 46. The rams 42 are generally also provided with a similar frequency sweep function.

The actuation of the rams 42, 46 and thus actuation of the seismic source 40 to generate seismic waves to illuminate the seabed 28 can be anything from around a few seconds up to permanent actuation, or can be cycled (e.g. rams 42 are actuated followed by rams 46) . The ROV 10 and survey apparatus 20 are generally configured to the optimal configuration on board the surface vessel (not shown) before being launched. Suitable soil data of the seabed 28, water depth information and basic reservoir depth information and structure can be used (where available) to configure the ROV 10 and survey apparatus 20 before deployment.

Referring now to Fig. 1, the ROV 10 with the survey apparatus 20 coupled thereto is launched from the surface vessel, typically via a moonpool (not shown) . The ROV 10 is then piloted to the seabed 28 in any conventional manner as shown in Fig. 5. An advantage of embodiments of the present invention is that the position of the ROV 10 on the seabed 28 can be precisely known by the use of a positioning system such as a Global Positioning System (GPS/DGPS), ultra-short base line (USBL) acoustic positioning system, a long base line (LBL) system and/or a long range real time kinematic system (LRRTK) so that the survey apparatus 20 is located precisely at the target co-ordinates for each shot point. This has the advantage that the apparatus 20 can be positioned precisely at the particular target area of the seabed 28 to illuminate the part of the seabed 28 that is of interest. Thus, the energy generated by the survey apparatus 20 is precisely targeted at the specific location of the seabed 28, thus making the apparatus 20 more efficient. In addition to this, the use of the positioning system can provide for accurate placement of the ROV 10 and the survey apparatus 20 at precisely the same position on the seabed 28 any number of times to facilitate repeat surveys. This has the advantage that the data obtained from actuation of the survey apparatus 20 can be made more consistent giving a better indication of, for example, depletion of an underground reservoir. The survey apparatus 20 can also be used to monitor any migration of the oil, gas and condensate so that drilling and recovery operations can be more precisely directed, saving on time, costs and increasing productivity.

The GPS/DGPS (not shown) is generally provided on the surface vessel, which in itself is provided with a transponder or the like that can be used to record the range and bearing to the ROV 10 and/or the survey apparatus 20 on the seabed 28. This, together with the position of the surface vessel (e.g. from GPS/DGPS readings), facilitates the precise position of the ROV 10 and thus the survey apparatus 20 to be known. The positioning information can be used to give precise repeat surveys at the same location, offering the advantages set out above. The GPS/DGPS may be provided on the ROV 10 and/or the survey apparatus 20 where the apparatus 20 is being used in relatively shallow water (typically less than 100m in depth) . The position of the ROV 10 and the survey apparatus 20 can also be set precisely using a USBL responder link to the surface vessel or with respect to a previously established long baseline array. Inertial navigation may also be used, or a combination of these systems.

As the ROV 10 and survey apparatus 20 come to rest on the seabed 28, they sit thereon under their own weight. In this position, an annular seal 50 is created at the interface between an outer surface of the housing 22 at the open end 24 thereof and the seabed material. The seal 50 creates a barrier between the internal volume 52 of the housing 22 and the surrounding seabed material.

Once the ROV 10 is on the seabed 28, the pump is then actuated to remove fluids from within the housing 22. The mud filter 34 prevents large particles of the seabed material from being drawn in by the pump and thus reduces the likelihood that the pump and/or the conduits 32 will become blocked. A pressure differential is created between the interior volume 52 of the housing 22 and the surrounding seawater that initiates the suction anchor formed by the housing 22, the pump and the inlets/outlets 30. The suction anchor pump is proportionally controlled so that the pressure differential is increased gradually to around 3 to 5 bar. The pressure differential is increased gradually to prevent loss of the annular seal 50 during actuation of the pump.

The differential pressure _.not only facilitates actuation of the suction anchor to anchor the survey apparatus 20 to the seabed 28, but also provides a reaction force or mass for the seismic source 40. The reaction force or mass is provided by the differential pressure and is generally around 20 to 60 tonnes, being a function of the surface area of the suction anchor and the differential pressure. Other factors may also require to be taken into consideration, such as the water depth and the volume of the suction anchor (e.g. the internal volume 52 of the housing 22) .

The reaction force can be calculated using the relationship that the differential pressure is equal to the reaction force divided by the area of the top surface of the suction anchor, and thus the reaction force is equal to the differential pressure multiplied by the top area of the suction anchor. Thus, if the differential pressure is known and is limited to a certain value, the corresponding reaction force provided by the differential pressure can be calculated. If a particular reaction force is required by the seismic source 40, the required pressure differential and/or surface area can be calculated.

For the example of anchor described herein, the height is 600mm, the wall thickness is 3mm, the OD is 1503mm, the ID is 1500mm, and thus the inner radius (IR) is 750mm. The area of the top surface is 17671.46 cm², which is the equivalent of 1.767146 m². Where the reaction force required is 60 tonnes (60000 kg), then the required differential pressure is 3.395 kg/cm² or 3.46 bar or 48.28 psi.

However, this assumes that the full internal volume 52 of the suction anchor is available for the differential pressure, but in practice, the volume is reduced by the mass or volume of the seismic source 40. However, the differential pressure can be increased accordingly which in turn will increase the reaction force. Friction between the housing 22 and the seabed material has been neglected, but is typically in the order of 1% to 2% of the reaction force .

Reaction force is also a function of water depth, and the size of the surface area of the suction anchor can be varied to optimise operation. Table 1 below sets out the reaction force in varying water depths with three different surface areas, the pressure differential being in the order of 3 to 5 bar. The reaction forces are shown in tonnes.

Table 1

The pressure differential that needs to be between the interior volume 52 and the surrounding seawater is generally dependent upon the type of seabed material, but is generally around 3 to 5 bar. Where the seabed material comprises large grains of sand for example, the depth of penetration of the survey apparatus 20 (in particular the housing 22) into the seabed 28 is generally less than that required -for soft mud and sand. Thus, the pressure differential will generally be less where the seabed material is large sand than that for soft mud and sand so that the depth of penetration of the housing 22 into the seabed material can be controlled. The housing 22 need only penetrate the seabed material until the base plate 48 is in contact with the seabed material. Thus, the housing 22 penetrates until the base plate 48 is at least adjacent to the seabed 28, and preferably just in contact with the seabed 28. Alternatively, the pressure differential can be increased so that the housing 22 penetrates the seabed 28 so that the surface of the seabed 28 is above the base plate 48. In the latter cases, the base plate 48 will still provide a good contact with the seabed for efficient transmission of the seismic waves generated by the noise generator 40 into the seabed 28. The pressure differential can be monitored using one or more differential pressure transducers or sensors 29 (Figs 2a and 2b) mounted between the cruciform and concentric bracing 27 or otherwise in the internal volume 52 of the housing 22. The transducer 29 provides feedback to the pump controller so that the pressure differential can be monitored and maintained throughout the actuation of the seismic source 40. It is advantageous to monitor the pressure differential because this generally reduces over time due to fluid ingress past the seal 50. Also, the transducer 29 allows for continual monitoring of the pressure differential to ensure that the base plate 48 remains adjacent to or in contact with the seabed 28. A differential pressure transducer or sensor may also be located outwith the housing 22 (e.g. on the ROV 10) for reference purposes.

The survey apparatus 20 can be provided with a hydraulic system (not shown) that allows the depth of penetration of the housing 22 to be adjusted. It will be appreciated that the depth of penetration can be adjusted manually. In the case of soft sand, silt or mud, the housing 22 need not penetrate so far into the seabed 28, and thus the depth of penetration of the housing 22 can be reduced. In the case of large sand, the depth of penetration should generally be increased.

The changes in penetration of the housing 22 can be achieved either hydraulically (e.g. by using hydraulic cylinders 35 that are coupled to the housing 22 so that it can be moved) or manually. In this case, the housing 22 would be mounted independently of the seismic source 40 so that it can be moved relative thereto. Indeed, a cone penetrometer tester (CPT) can be -mounted on the ROV 10 or apparatus 20 to confirm the soils type from which the depth of penetration can be calculated, and this information can be fed back to a user to provide for automatic and/or manual adjustment of the penetration by moving the housing 22 accordingly. Alternatively, or additionally, one or more strain gauges can be mounted to the outer surface of the housing 22 to measure the sleeve friction to give an indication of penetration. As a further alternative, a transponder, may be attached internally of the housing 22 to measure the distance between the base plate 48 and the seabed 28 to ensure that the base plate 48 is in close proximity to or makes contact with the seabed 28, or one or more load cells may be provided at the open end 24 of the housing 22 to measure the reaction force generated by the housing 22 as it penetrates the seabed 28.

If the circumference, depth, radius or other dimensions of the housing 22 are changed, this will change the volume of the housing 22. Changes in the volume of housing 22 will change the pressure differential that is required and also cause changes in the reaction force. Thus, movement of the housing 22 relative to the noise source 40 will have an effect on all of these factors, and the pressure differential is preferably monitored to ensure that it remains at the appropriate level to keep the base plate 48 adjacent to or in contact with the seabed 28.

Fig. 6 shows the housing 22 embedded in the seabed 28 after actuation of the pump. Once the pressure differential has been created, the pump is stopped and the pressure differential maintains the suction anchor in the seabed 28. The pump is actuated until the base plate 48 of the seismic source 40 is adjacent to or in contact with the seabed 28, or until the housing 22 has achieved the desired level of penetration. These can be monitored using any of the devices or systems described above (e.g. the transponder, strain gauges, load cells etc). Indeed, an underwater camera (provided on the ROV 10 for example) may be used to confirm the required penetration or that the base plate 48 is adjacent to or in contact with the seabed 28 (e.g. using appropriate markings on the outer surface of the housing 22) . The pressure differential transducer 29 can be used to continually monitor the pressure differential to keep it substantially constant by automatic or manual control of the pump.

Referring now to Fig. 7, the ROV 10 is preferably detached from the survey apparatus 20 during actuation thereof. This prevents damage being caused to the ROV 10 or any of its equipment and instrumentation by the actuation of the seismic source 40 and the vibrations caused by the source 40. The ROV 10 and survey apparatus 20 are provided with a plurality of docking latches 54, with a latch 54 typically being located at each corner of a lower portion of the ROV 10 so that four latches 54 are generally provided. It will be appreciated that the number of latches 54 can be varied. The latches 54 could be, for example, API17H latches. The latches 54 typically comprise a docking probe 54p provided on one of the ROV 10 and the apparatus 20 (in this example, the ROV 10) and a docking cone (not shown) provided on the other of the ROV 10 and the apparatus 20 (in this example, the apparatus 20) .

As an alternative to decoupling the frame 20 from the ROV 10, the frame 20 can be suspended beneath the ROV 10 using vibration absorbing mounts.

Although the ROV 10 is effectively decoupled from the survey apparatus 20, it typically remains coupled thereto via a plurality of tendons 56 or the vibration absorbing mounts. The number and placing of the tendons 56 can be varied, but four tendons 56 are generally provided (only two shown) , with a tendon 56 being placed at the front, rear and each side of the ROV 10 and survey apparatus 20. In addition to this, a number of cables 58 (e.g. electrical, hydraulic etc) remain between the ROV 10 and the survey apparatus 20 so that power, data and command signals etc can be transferred therebetween. The tendons 56 are typically of carbon fibre, but may be of any flexible material such as steel wire or Kevlar.

As can be seen from Fig. 7, the survey apparatus 20 remains substantially embedded in the seabed 28, whilst the ROV 10 remains above it. The combined mass of the ROV 10 and the survey apparatus 20 give the ROV 10 a negative buoyancy to facilitate landing of the ROV 10 and the apparatus 20 on the seabed 28 under their own weight. Indeed, the ROV 10 and in particular the survey apparatus 20 remain in contact with the seabed 28 due to this negative buoyancy. However, when the ROV 10 detaches from the survey apparatus 20 as shown in Fig. 7, most of the combined mass of the ROV 10 and survey apparatus 20 remains in and/or on the seabed 28 (i.e. the apparatus 20 remains on the seabed 28) . Thus, the ROV 10 is provided with a positive buoyancy and floats towards the surface, but it is prevented from doing so by the tendons 56 and/or the cables 58,. The ROV 10 thus remains in position above the survey apparatus 10 but isolated therefrom.

At this point, and once ROV decoupling is confirmed (using any conventional method) , the seismic source 40 is actuated in the conventional manner as described above (depending upon the particular type of source 40) to illuminate the seabed 28 with seismic noise, focussing on the particular target area of interest (e.g. oil, gas, condensate or water strata) . The source 40 can be triggered remotely from the surface vessel or via the ROV 10 with a reference time pulse being provided to the recording system on the basis of read after write. Associated signature data can be forwarded concurrently or after completion of the seismic source generation sequence. All data is preferably time tagged on a common base or reference (e.g. provided by the GPS/DGPS system) . The source signature acquisition can be obtained through a control pod and confirmed with a remote broad band transducer recording data and providing the time offset for subsequent processing. The data can be recorded onto any medium (e.g. magnetic, CDROM, computer memory)- and can be simultaneously transmitted to a seismic wave recording system. The timing is undertaken by conventional electronic methods or otherwise through the umbilical 14, and is transmitted to the receiving vessel or facility. This can be achieved through use of standard electronic packages.

At this point, and depending upon seabed conditions, the pump that drives the suction anchor may be continually actuated to maintain the .pressure differential and. thereby ensure that the base plate 48 remains in contact with the seabed 28. The transducer 29 can be used to provide feedback to the pump controller to maintain the desired pressure differential within acceptable limits.

The seismic vibrations are conveyed directly into and thus penetrate the seabed 28 where the base plate 48 is in direct contact with the seabed. Even in the case where the base plate 48 is adjacent the seabed 28, the majority of the energy generated by the seismic source is conveyed into the seabed 28. Having the base plate 48 in direct contact with the seabed 28 or in close proximity thereto provides numerous advantages over conventional systems . In particular, the efficiency of the apparatus 20 is much greater as around 80% to 90% or more of the energy generated by the seismic source 40 is conveyed directly into the seabed 28. Also, the impact on the environment in the vicinity of the source 40 is also reduced. In particular, the impact on biological life such as cetacean and other marine mammals is minimised.

In addition to this, the ROV deployed apparatus 20 is low in terms of energy loss due to spread in the seawater and along the seabed 28, and thus the source 40 can generate less energy when compared to conventional systems whilst still providing a higher level of energy in the seabed 28. Some conventional systems also suffer from interference with one another as the seismic vibrations generated thereby can travel large distances. The energy generated by the seismic source 40 is generally less that that when compared to conventional systems, and there is no or very little affect on the performance of the apparatus 20, and in some cases, the data that is obtained can be of a higher quality, but is at least comparable.

The seismic source 40 typically provides a broad pulse of energy that is between 5Hz and 300kHz, and between 5kJ and 20 kJ (5kW to 20kW) against the reaction force provided by the pressure differential and/or the column of water above the apparatus 20. Relative efficiency of the system generally improves with water depth. The energy levels required to illuminate the area of seabed and the reservoir vary and are generally dependent upon the structure characteristics, depth and signal required to ensure coherent data can be received and recorded at, for example, the surface or using geophones, cables etc on the seabed 28. Such receiving systems may be provided on the ROV 10 for example to receive the generated signature waves and transmit the data to a surface vessel (e.g. via the umbilical 14).

The potential power output of the seismic source 40 is generally dependent upon the drive frequency, mass and drive stroke of the base plate 48. Theoretical drive can be at approximately 90% of the potential reaction force, but the power generation and mass of the rams 42, 46 and base plate 48 are factors, as is seabed compressibility, which is not readily quantifiable without geotechnical information.

Typical depths below the seabed 28 for deepwater hydrocarbon reservoirs vary from around 1000 to 1500 metres in Africa, to a median depth of 2000 metres, with some reservoirs at depths up to 8000 metres for high pressure temperature reservoirs in the Gulf of Mexico. The deepwater reservoirs are often complex in structure, and typically comprise sand bodies of 20 to 30 metres in depth and running in channels that may be 50 to 100 metres across. The reservoirs can be stacked or otherwise arranged in various and often complex patterns, and can cover areas that are many kilometres in surface spatial dimensions. The reservoirs can also be hundreds of metres deep. Thus, a controlled and relatively wide spectrum of energy is generally used so that the desired resolution can be defined and measured to determine the strata.

The survey apparatus 20 is generally capable of providing both P and S wave signatures by sequential actuation of the rams 42, 46. Thus, there is no requirement to recover the apparatus 20 to the surface for reconfiguration. The apparatus 20 can be cycled so that the P waves are generated for a specified time (e.g. 4 to 8 seconds) and thereafter S waves are generated for a specified period of time (e.g. 4 to 8 seconds) . The ability to generate different signatures (i.e. P and S waves) provides the advantage that different signatures can be obtained that has the potential to produce better discrimination between the resultant data.

The seismic source 40 is typically actuated for between 4 and 8 seconds, although the time period of actuation can be chosen to suit the particular application. The survey apparatus 20 can remain permanently positioned to give continuous or periodic illumination of the seabed 28. The seismic waves generated by the seismic source 40 that are reflected by the various interfaces and strata can be received by any seismic wave recording system. The waves are generally reflected at each interface within the seabed (e.g. between different materials of the seabed, and also between different oil, gas, condensate and water strata) . Geophones, cables on the seabed, microphones, hydrophones and the like can be used to receive the signature waves.

The source 40 can be actuated any number of times to produce several seismic shots at the same location (e.g. for comparison purposes).

The seismic programming of the seismic source 40 required to obtain the optimum system performance requires that the acquisitio is designed correctly. The example or analogy is that of a land seismic programme where initial field parameters are known, and the design of the subsequent acquisition of seismic data can be optimised to the best resolution (e.g. <10m in amplitude). This gives a better understanding of reservoir performance, and in some circumstances, it may be possible to observe the oil/gas/water interfaces during depletion of the field.

For example, higher frequencies can give greater signal penetration and thus better resolution. Indeed, if the system is optimised to give narrower strata limits (e.g. <10m in width), then the strata can be defined more precisely. After actuation of the noise source 40 and collection of the data, the ROV 10 is then re- coupled with the survey apparatus 20, as shown in Fig. 8. The ROV 10 is directed towards the survey apparatus 20 using appropriate thrusters, and an alignment system is provided to facilitate alignment of the docking probe 54p with the docking cone. Any alignment system can be provided such as a long pin for rough, initial alignment and a shorter pin for fine alignment. Additionally, a funnel or the like can be provided at each docking cone to guide the docking probe 54p into alignment therewith, the funnel providing for rough initial alignment and fine alignment.

Once docking has been confirmed, using any conventional means such as a camera or the like, the slide on the pump is moved so that the water is injected or pumped into the internal volume 52 of the housing 22. As the water is pumped in, the pressure within the internal volume 52 increases and the pressure differential between the interior volume 52 and the surrounding seawater is reduced. The rate at which fluid is pumped into the housing 22 is not critical, and indeed it may be advantageous to pump the fluid in at high speed and/or pressure to aid in freeing the housing 22 from the seabed 28. The increase in pressure tends to pump the housing 22 out of the seabed 28 by increasing the pressure in the internal volume 52 with respect to the seabed 28. The thrusters on the ROV 10 can be used to assist in releasing the housing 22 from the seabed 28.

In addition to this, the seismic source 40 can be actuated during this period so that the vibrations from the source 40 help liquefy the seabed material thereby reducing friction and facilitating release of the housing 22 from the seabed 28.

The ROV 10 is then piloted to a new target location using the GPS/DGPS system or the like and the transponder on the surface vessel to facilitate precise and accurate positioning of the ROV 10 and the survey apparatus 20 on the seabed. Fig. 9 shows a similar view to Fig. 7 where the ROV 10 has been moved to the second location and the suction anchor actuated (by actuating the pump to remove fluids from within the housing 22) so that the base plate 48 is adjacent to or contacts the seabed 28, as described above. Thereafter, the ROV 10 is decoupled from the apparatus 20 (Fig. 10), and the seismic source 40 is actuated, as described above.

This process can be repeated any number of times at any number of different locations. The ROV 10 and thus the survey apparatus 20 can be accurately and precisely positioned at each location, making the overall system more accurate and efficient.

The survey apparatus 20 may be provided with one or more hydraulic accumulators (not shown) that can be used in the event of an emergency to release the survey apparatus 20 from the seabed 28.

An advantage of using the overall system is that the apparatus 20 can be accurately positioned at the same location on the seabed 28 any number of times, providing the advantage that repeat surveys can be more precisely undertaken, leading to more consistent acquisition of data.

The overall system including the ROV 10 and the survey apparatus 20 has many different applications. The system can be used to support on-bottom seismic data acquisition (OBC) to gather data or to act as a quality control source. The system can also be used for field depletion studies and monitoring of field oil, gas and condensate migration during the exploitation and hydrocarbon recovery from the field.

Further, the system can be used to enhance existing seismic studies in areas where conventional systems are difficult to use or the results are difficult to interpret because of multiples or blanks in the seismic records. For example, the system can be precisely placed giving P and S wave performance to enhance seismic records prior to subsequent analysis. Additionally, the system can be used for shallow refraction seismic under platforms and other structures using refraction seismic where access for conventional systems can be difficult or time consuming. The system can be used to provide accurate and precise placement of the seismic source 40 (using the apparatus 20) so that it can be permanently installed in permanently instrumented fields . The system can be coupled to the field facilities as required.

The system also has applications in borehole seismic where a geophone is located in the borehole, of for vertical seismic profiling (VSP) surveys.

Advantages of the system in certain embodiments include that the noise generator may be configured to provide P, S, cross and shear wave characteristics from the same source without having to retrieve it to the surface for reconfiguration. Thus, the system in certain embodiments is more efficient and less time consuming to use.

Certain embodiments of the present invention provide the advantage that the seismic source can be precisely positioned relative to a target location on the seabed using GPS/DGPS for example, so that the source is provided at the precise target location on the seabed, and so that repeat surveys can be precisely undertaken, leading to a greater consistency in the collected data.

As the seismic source is in close proximity to or in direct contact with the seabed, the majority of the energy generated by the source is conveyed directly into the seabed. This has the advantage that more energy from the source is used to illuminate the target area of interest, thereby .making embodiments more efficient. In particular-, there is reduced energy loss by spread along the seabed when compared to conventional systems, as the energy is concentrated at a particular location on the seabed. The increase in efficiency does not lead to a reduction in performance, where certain embodiments can provide performance that is at least comparable with land seismic surveying equipment. Indeed, the overall data quality that can be obtained with embodiments of the present invention can be superior to conventional seismic generation and data acquisition systems.

In certain embodiments, the coupling of the seismic source to the seabed offers the advantage that the coupling is predictable and controllable, and the recovered data is not degraded by Doppler shifts.

Embodiments of the present invention, also offer the advantage that the seismic source or generator can be deployed from a wider range of vessels- than some conventional systems that require specialised vessels. Thus, certain embodiments of the present invention provide greater flexibility for the end user.

Certain embodiments can also be used to provide a seismic source that is conveyed to the seabed using an ROV and left permanently installed on the seabed. Embodiments of the present invention also provide the advantage that the impact on the subsea environment can be significantly reduced. In particular, the impact on cetacean and marine mammals (e.g. whales, dolphins etc) is minimised.

The system in certain embodiments is less prone to interference from adjacent and remote sources as the energy is directed into the seabed.

Certain embodiments can also be used to provide a long term seismic source position on the seabed and located by means of a suction anchor arrangement, or specially designed adaptive piles, capable of installation, maintenance and replacement during the life of the field.

Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.

Claims

CLAIMS 1. Underwater survey apparatus comprising a seismic source (40) and a deployment means (10) for deploying the source (40) and/or locating the source (40) adjacent an underwater bed (28), wherein at least a portion of the apparatus is adapted to be in contact with or adjacent to the underwater bed (28) in use.

2. Apparatus according to claim 1, wherein the deployment means comprises a remotely operated vehicle (10).

3. Apparatus according to either preceding claim, wherein the seismic source (40) includes a base plate (48) that is capable of being vibrated on one or more axes to generate seismic waves.

4. Apparatus according to claim 3, wherein the base plate (48) comprises the portion of the source (40) that is in contact with or adjacent to the underwater bed (28) .

5. Apparatus according to any preceding claim, wherein the seismic source (40) is located within a housing (22) .

6. Apparatus according to claim 5, wherein the apparatus includes a suction anchor.

7. Apparatus according to claim 6, wherein the housing (22) forms part of the suction anchor.

8. Apparatus according to claim 6 or claim 7, wherein the housing (22) includes an open end (24) and a closed end (26) .

9. Apparatus according to any one of claims 6 to 8, wherein the suction anchor includes a pump, and one or more fluid inlets/outlets (30) in fluid communication with the pump.

10. Apparatus according to claim 9, wherein four fluid inlets/outlets (30) are provided, each inlet/outlet (30) being symmetrically arranged at the closed end (26) of the housing (22) .

11. Apparatus according to claim 9 or claim 10, wherein the pump is reversible so that it can remove fluids from and inject fluids into the housing (22) .

12. Apparatus according to any one of claims 9 to 11, wherein the housing (22) is provided with one or more transducers or sensors (29) to monitor a pressure differential between an interior (52) and an exterior of the housing (22) .

13. Apparatus according to claim 12, wherein the or each transducer or sensor (29) is coupled to a pump controller that controls the pump.

14. Apparatus according to any one of claims 8 to 13, wherein the open end (24) of the housing (22) is adapted to be engaged in the underwater bed (28).

15. An underwater survey apparatus comprising a seismic source (40) wherein at least a portion of the source (40) is adapted to be in contact with or adjacent to an underwater bed (28) in use.

16. Apparatus according to claim 15, wherein the apparatus includes an anchor to engage the underwater bed (28).

17. Apparatus according to claim 16, wherein the anchor is a suction anchor.

18. Apparatus according to claim 17, wherein the seismic source (40) is located within a housing (22) .

19. Apparatus according to claim 18, wherein the housing (22) includes an open end (24) and a closed end (26) .

20. Apparatus according to claim 19, wherein the suction anchor includes a pump, and one or more fluid inlets/outlets (30) in fluid communication with the pump.

21. Apparatus according to claim 20, wherein four fluid inlets/outlets (30) are provided, each inlet/outlet (30) being symmetrically arranged at the closed end (26) of the housing (22),.

22. Apparatus according to claim 20 or claim 21, wherein the pump is reversible so that it can remove fluids from and inject fluids into the housing (22) .

23. Apparatus according to any one of claims 20 to 22, wherein the housing (22) is provided with one or more transducers or sensors (29) to monitor a pressure differential between an interior (52) and exterior of the housing 22.

24. Apparatus according to claim 23, wherein the or each transducer or sensor (29) is coupled to a pump controller that controls the .pump.

25. Apparatus according to any one of claims 18 to 24, wherein the open end (24) of the housing (22) is adapted to be engaged in the underwater bed (28) .

26. A method of surveying an underwater bed, the method comprising the steps of locating a seismic source (40) in contact with or adjacent to the underwater bed (28), and actuating the source (40) to generate seismic waves.

27. A method according to claim 26, wherein the step of actuating the seismic source (40) includes the step of vibrating a base plate (48) on one or more axes.

28. A method according to claim 27, wherein the method includes the additional step of providing the base plate (48) in contact with or adjacent to the underwater bed (28) .

29. A method according to any one of claims 26 to 28, wherein the method includes the additional step of locating the seismic source (40) at a target location on the underwater bed (28).

30. A method according to any one of claims 26 to 29, wherein the seismic source (40) is located in a housing (22), and the method includes the additional step of engaging at least a portion of the housing (22) in the underwater bed (28).

31. A method according to claim 30, wherein the step of engaging the housing (22) in the underwater bed (28) includes the step of creating a pressure differential between an exterior and an interior (52) of the housing (22) .

32. A method according to claim 31, wherein the step of creating the pressure differential includes the step of removing fluids from within the housing (22) .

33. A method according to claim 31 or claim 32, wherein the method includes the additional step of monitoring the pressure differential.

34. A method according to any one of claims 31 to 33, wherein the method includes the additional step of removing the housing (22) from the underwater bed (28).

35. A method according to claim 3-4, wherein the step of removing the housing (22) includes the step of reducing the pressure differential.

36. A method according to any one of claims 26 to 35, wherein the step of locating the seismic source (40) comprises locating the source (40) using a remotely operated vehicle (10) .

37. A method according to claim 37, wherein the method includes the additional step of decoupling the seismic source (40) from the remotely operated vehicle (10) during actuation of the source (40).

38. A method according to claim 37, wherein the method includes the additional step of re-coupling the seismic source (40) to the remotely operated vehicle (10) .

39. A method according to claim 38, wherein the method includes one, some or all of the additional steps of a) moving the seismic source (40) to another target location using the remotely operated vehicle (10), b) engaging the housing (22) in the underwater bed (28), c) decoupling the remotely operated vehicle (10) from the source (40) , d) actuating the source (40) at least once, and e) re- coupling the remotely operated vehicle to the source (40) .

40. A method according to claim 39, wherein the method includes the additional step of repeating steps a) to e) .