Seabed monitoring seismic cable

The present invention relates to a technique for seismic exploration and seismic imaging of subsurface layers. In particular, but not exclusively, it relates to apparatuses for underwater seismic surveys.

In conventional seismic methods of research of underground layers below the seafloor use of seismic waves and measuring the response from the underground layers. The seismic wave may be simple or complex and may be formed at sea level, beneath the surface or seafloor. Response fix several spaced receivers which can be located on a cable or "seykosah" towed below the sea surface for research vessel, they can also be located on the seabed. When placing a stationary receivers on the seabed after the step of detecting may need to move them to another location to repeat the process or removed, if the research is completed.

The response to a seismic wave in the solid rock at the bottom surface includes a longitudinal wave (P-wave) and shear wave (S-wave). It is believed that the P-waves is well suited to imaging structures while the combination of S-waves is well suited for determining the characteristics of the rock and fluids. P-waves travel through rock and sea water while S-waves travel through rock only. Thus, if the receivers are hydrophones located at or beneath the surface, they will detect only the P-wave. In order to detect S-waves, it is necessary to use a hydrophone located at the seabed.

It is found that the best seismic image can be obtained by using P- and S-waves in so-called 4C seismic image, where 4C stands for "four component", one of P-wave and S-wave three. To effectively detect the S-wave requires three independent orthogonal stationary hydrophone in each location registration. However, recently, it became possible to use a detecting apparatus located at a short distance from the seabed, which monitors the movement of the particles on the surface and detects both P- and S-waves, for example, such a detecting apparatus described in WO 2004/003589.

In the devices described WO 2004/003589 entitled "Apparatus for recording oscillations seabed" (OBM) they detect P-waves and S-waves at the location a short distance from the seabed by measuring the response of particles on the seabed to seismic waves.

4C seismic image of the subsurface and may provide additional information to better study due to high recording S-waves at the seafloor. Unfortunately, 4C-image has drawbacks such as high cost of research variable results and uncertainty in predicting results. This is due to the fact that the placement and movement to another place on the seabed hydrophones was very expensive and inaccurate.

The present inventors have found that the effectiveness of the implementation of the seismic image can be improved, if the technique is available, providing a continuous controlled motion of the respective receivers P- and S-wave relative to the seabed. However, the detection distance above the seafloor has problems in that the detecting devices are exposed to ocean currents, which has a negative effect on the effective positioning apparatus and introduces noise into measurements, making correlation of the results difficult.

Currently known streamers are designed to tow a little below the sea level and operated in a lateral direction. In some systems, streamer depth of immersion is adjustable and can reach several meters below sea level. Such systems are not suitable for use near the seafloor in deep water, and they are not easily deployed in these areas.

In connection with this aspect of the invention is to provide a device for underwater research, which does not need to lift, move and change the position of the sensors is strictly on the seabed, which is used for towing the receivers on the adjustable depth a short distance above the seafloor.

The present invention provides a system that includes a research vessel; at least one streamer, the streamer adapted to towing vessel: streamer associated with a detection system controller depth and at least one setter height above the bottom and the control system, the controller depth and at least one setpoint height above bottom adapted to retain the streamer at a position near the seafloor in its towing vessel; a detection system configured to detect and record data while the streamer motion relative to the seabed during its towing vessel.

The invention is particularly applicable to seismic exploration, but also can be applied to any type of study that may benefit under such controlled towing devices for research. For example, such a device can be successfully used in studies that determine the location of potential deposits, foreign objects near the seabed pollutants.

Streamers towed device is adapted to operate under high pressure. The detecting apparatus may include instruments for detecting P- and S-waves, for example, hydrophones to detect waves P-waves in the water, and a detecting apparatus of the type described in WO 2004/003589, for the detection of P- and S-waves in the sea day (OBM). The number and position of the devices depends on the research carried out. The detecting apparatus may include means for compensation of its own motion, produced when the detection and recording of data.

Streamers may be short, for example 50 m long, non-rotating section located immediately after, for or between successive pieces of equipment attached to the streamer, such as appliances and setting devices of the height above the bottom. Non-rotating section can be equipped with longitudinal wings, to prevent rotation of the streamer. Winding a cable coating must be performed so that the change does not cause rotation of the load. Streamers may also include vibration isolation modules (VIM-IVI) to reduce the noise in the data.

The system according to the invention makes it possible to deploy streamer at a depth of several thousand meters below the sea level, with adjustable immersion depth streamers. A lot of streamers can be towed in the same group. The length of each streamer can be from 3000 to 8000 m, 5000 m for example. Driving towing streamers is dictated by the nature of the research undertaken and can be adapted to take account of bathymetric data in the study area.

Research vessel, preferably equipped with at least one winch towing and outboard winch system for lowering, towing and recovery from the depths of the streamer (streamers). The conditions in the study area affect the optimum length, width, number of instruments and streamer depth research group, which in turn determine the nature and number of items of towing equipment required.

The vessel is preferably provided with GPS and sonar positioning device, which is preferably a multi-path.

Each streamer is equipped with certain items of equipment. The immersion depth is regulated by the front end of the streamer depth controller, which is preferably a multihull manoeuvrable depth controller and being operable to have an optimal position of 50 to 100 m above the seafloor, depending on the terrain. Depth controller may optionally be monohull regulator depth depending on the instrument requirements and conditions of the study.

Preferably, each streamer attached by a cable to a separate controller maneuverable depth, which provides independent control of depth of each streamer. Controller depth is actively controlled device and can perform vertical and horizontal maneuvers. Alternatively or additionally, can be applied additional separate items of equipment which affect the horizontal displacement of the streamer in its use, such as paravane-type equipment.

The controller can be equipped with a depth control surfaces, including the adjustable wings, balance wheel and rudder. Preferably it can be provided with a depth sensor, altimeter, fiber optics, gyro, altimeter, sonar transponders positioning and / or alternative devices control the position and / or velocity, such as a camcorder. There is also a depth controller control system, which can communicate with the master control unit and the power distribution controller for activating depth rudder surfaces depth controller when required.

The main goals of the controller consist of depth to omit streamers as close as possible to the seabed with security research, taking into account the surrounding landscape and providing a horizontal management streamers. Controller depth can be controlled actively, providing the soft descent or climb, which can be combined with the rise or dive with the help of the towing winch, allowing safely to perform research in the areas of the mainland and from it. The altimeter in the controller provides the depth of the input information to determine a safe altitude for the setpoint height of the streamer above the bottom. In this role, altimeter depth controller acts as a device for early detection and prevention, and helps prevent unnecessary dives and ascents.

Streamer may also be provided with ballast, in order to facilitate a submerged section of the system, and a ballast release system which is activated acoustically, allowing to perform lifting subsea system in an emergency.

Streamers preferably equipped with one or more of the height setters attached at intervals along the streamer. The interval between each pair of height setting devices arranged in series, may be from 200 to 500 m, for example 250 m.

Zadatchik height preferably in the form of winged fish that can be monohull or multihull. Setpoint height more precisely controlled than controllers depth and can be used to reduce any rotation of the streamer as well as to hold it at a distance of 2 to 50 m above the seafloor during the study, preferably from 5 to 30 m, or 5 to 20 m above the seafloor. To use specific tools to find the P- and S-wave is required to streamer as close as possible to the bottom of the sea, but without the risk of damage to equipment.

The minimum number and the optimum number of altitude setting devices required for a streamer depend on the length, sea currents and other operational conditions. Each dial is preferably equipped with sonar positioning device and / or alternative devices control the position and / or velocity, such as cameras, as well as the management system set point height, which can communicate with the control system controller depth and the main control system directly or via a control system controller depths. Each is also equipped with a height adjuster control system to activate the control surfaces when required. Each dial contains the height of a number of instruments that provide the data management system and which can also store data in height adjuster, devices that measure data, including bathymetry and log data for Doppler velocity.

In addition, any of the items of equipment relating to the streamer, including for example, setting devices of the height and the sensing equipment can also carry auxiliary measuring apparatus for recording data regarding any of the following: water pressure, temperature, depth, salt concentration, speed submarine currents streamer vibrations, oscillations research devices, reflectance seafloor (using, e.g., white light or laser light), geometry of towing profile.

The control system preferably consists of a main control system and a number of auxiliary systems. The multiplexer can be used for communication between the underwater sections of the systems and control systems, on the surface. Main control system receives input from a GPS and sonar vessel controller (pit) depth sonar equipment positioning, bathymetric and speed data from underwater devices. Home Control System provides instructions for control of winch and control systems for underwater device to adjust the height and depth of the dive streamer (streamers), before, during and after the investigations and to prevent a collision with the seabed streamers.

Winch control system controls the main control system, but further possible manual control in an emergency.

The depth controller control system preferably inputs data from the position sensors on the depth controller to the main control system. Based on these data and other information, the main control system can then communicate with the depth controller control system, which provides instructions to the depth controller control surfaces to keep the depth controller in a horizontal plane and optimize depth adjustment.

Manage wings controller depth, preferably related to the management of the winch so that if you want a quick lift, winch command is issued on the rise with a maximum speed control surfaces at the same time the depth of the controller tilt in a situation in which the power of lowering the minimum, for a quick recovery. The controller may also include the depth of ballast, which can be reset if needed a rapid rise in an emergency.

Each control system is preset setpoint height of the height that it should support as appropriate the surrounding landscape, and any obstacles when towing. Using the topography below the depth of the controller, the main control system may provide instructions to the system control setting devices via the height control system controller depth to prevent a collision with the seabed streamers.

The present invention also provides a method of underwater exploration, the method comprising the steps of towing at least one streamer behind a survey vessel, the streamer is connected to the detecting apparatus, a depth controller, a set point altitude and the steering system; maintaining a streamer in a position near the bottom of the sea, using the controller setpoint depth and height for trailering streamer vessel; control the direction of motion of streamers from the steering system; and detecting and recording data using the detecting apparatus while the streamer is in motion relative to the seabed with its towing vessel. Preferably, the method further comprises the excitation of the seismic waves with marine seismic source. Source type and the method used will depend on the geography of the area and the type of research performed by research.

Thus, according to the present invention can measure small movements and seismic waves, including P and S-waves, on the seafloor when the unit is moved in the water above the seafloor.

Allowing marine streamers rather than stationary seafloor detectors means that relatively large sea areas can be investigated in a relatively short time and more cost-effective.

The invention may be practiced in various ways, one embodiment will be described below, using the relevant drawings.

1 - schematic diagram of the research system according to the invention;

2 - an example of the depth of immersion of the controller;

3 - an example of the setpoint height.

1 shows a survey system in accordance with the invention. The research system includes a vessel 10 at 19 Sea, which tows 11 streamers attached cable 12 to 13. The tow rope tow rope 13 can be equipped with one or more devices (not shown), "softening" flows, devices can be placed at regular intervals along the tow line and serve to suppress vibrations of a cable due to turbulence generated for trailering. Adaptations can be made in the form of wedge shaped plates attached to the cable. Such devices can reduce noise in research by reducing fluctuations in the system, attached to the tow rope. Seismic streamer 11 contains a detection system (not shown). Immersion depth streamer 11 is regulated by the regulator 14 multihull depth and height of the setters 16, which are located along the length of the streamer 11. Distance control multiple effect 14 depth to the farthest from the controller 14 setpoint height of about 5,000 m. Height setting dial 16 are placed every 500 meters along the 11. The length of the streamer cable 12 has a length of approximately 250 m and tow rope 13 of about 150 m length. Streamer 11 is provided with a drogue (not shown) at the end remote from the multiple effect depth regulator, to stabilize the streamer.

Immersion depth streamer 11 is regulated by the regulator 14 multiple effect depth, which can perform independent maneuvering. Adjustable steering control surface depth include wings 15, as well as a balanced rudder and the rudder (not shown). The controller is equipped with a depth sensor, altimeter, a position sensor and sonar positioning transponders.

After the controller 14 streamer depth 11 brought into position close to the seabed 18, the height setting devices 16 maintain and operate small correction of position adjustment streamer 11 above the seafloor 18. There are ten independent altitude setting devices 16 (eight not shown) spaced at equal intervals along the streamer 11 to control the position of the streamer along its length. Each uses a dial height control surfaces 17 for holding the height of the streamer and make minor adjustments.

The steering system is coordinated streamers main control system 20 research vessel 10 which receives data from GPS-receivers 21 and 22 of the sonar vessel. The main control system 20 also receives data from the winch control system 23, the system controller 24 controls the depth of immersion and control systems 25 height setting devices, which in this embodiment communicate with the main control system via the control system control the depth of immersion. Control system 25 setters height also interact directly with each other to keep the depth of immersion streamer 11 at the desired level.

23 System Management winch control system 24 and control depth of 25 setters height control controls the main control system 20 for safe diving, towing and lifting streamer 11 and connected with her devices intended for research.

2 shows a depth controller suitable for use in a system according to the present invention, a depth control. The controller 30 is equipped with a depth control surfaces 31 capable of changing the position of the controller in water depth. The regulator has a depth of 32 towing point from which it can be attached to the towing vessel, such as towing cable (Figure 1). The depth gauge is also equipped with immersion devices previously described, and a control system (not shown).

3 shows a height adjuster, suitable for use in the system according to the present invention, which has the shape of a winged fish towed. Towed fish 40 comprises two approximately symmetrical shape, which are provided with control surfaces 41. The two parts of the fish towed 40 separated by a gap 42 and are connected to one another only in the fixing unit 44, the approximate location of where noted. When towed fish is used, the cable 43 is put into the slot 42 and is attached to the fish in the towed unit 44 attachment. Towed fish is also provided with the appropriate equipment, mentioned above, and a control system (not shown).

It will be appreciated that there are many alternative methods for control systems to adjust the depth of immersion underwater section of the research system. The equipment required to maintain the depth streamer may also vary and depend on the type of research and studies carried out. The detection equipment associated with each streamer will also depend on the type of research. Many types of sensors can be used to characterize the field, including magnetic sensors and devices for registration P- and S-waves.