RU2485554C1 - Method of conducting 3d submarine-subglacial seismo-acoustic survey using submarine vessel - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: geophysical 3D submarine-subglacial survey is carried out by moving an acoustic signal emitter mounted on a submarine vessel near the bottom of the investigated offshore zone. Linear bottom antennae consisting of digital seismo-acoustic receivers, which are used as self-contained receivers, are mounted at the bottom of the water area using the same submarine vessel. Further, the bottom of the water area is irradiated directly from the cruising submarine vessel with powerful wideband sources of coherent acoustic radiation, and the reflected radiation is picked up by said digital seismo-acoustic receivers.

EFFECT: high accuracy of obtained survey data.

8 cl, 3 dwg

Description

The present invention relates to geophysics, and in particular to the field of seismic or acoustic exploration of areas covered by water, and can be used for geophysical exploration of marine areas, in particular, lying under solid pack ice in the search for minerals in the seabed, for example, on the shelf of the northern seas .

The vast majority of known methods and devices for underwater-under-ice geophysical exploration of minerals in the seabed include towing by a surface vessel (icebreaker) of a seismic survey (or seismic survey), released into the water using special seismic winches (see, for example, US Pat. RU 2317572, IPC G01V 1/38 (2006.01), publ. 02.20.2008). Since the length of standard seismic surveys reaches 6000 m, their descent into the water from the side or from the hold of a surface vessel and their towing behind the vessel under ice reconnaissance in the northern latitudes is a difficult technical task, and in pack ice conditions it is simply an impossible task.

The growing interest in the development of methods of marine seismic exploration, including on the shelf of the Arctic regions, is also evidenced by patent RU 2407043 (publication date 12/20/2010), which proposes the use of an adapted design of a georadar towed by a surface ship when registering reflected electromagnetic waves stationary electromagnetic receivers installed at the bottom. The obtained information is proposed to be used to clarify the structure of the environment in areas with previously identified geological structure of interest for practical use. The previously obtained geophysical information is used to eliminate the ambiguity in the interpretation of electromagnetic surveys. This method has a drawback inherent in the technology of using ground penetrating radars: the strong absorption of electromagnetic radiation in porous media saturated with water limits the penetration depth of electromagnetic waves.

There is also a method of conducting 3D underwater-under-ice seismic exploration of minerals in bottom sediments on the seabed, in which a submarine, for example, a submarine, is used (US Pat. RU 2388022, IPC G01V 1/38 (2006.01), publ. 04/27/2010 ) In this method, the submarine in the operating mode moves with a lag, that is, it moves forward with one side and tows a set of seismic acquisition-emitting antenna streamers discharged from the other side of the vessel. In fact, this method with minor additions copies the standard methods for conducting marine seismic survey mentioned above, transferring the same configurations for placing equipment under water. To implement this known method requires a surface icebreaking escort vessel. The geophysical exploration submarine itself according to this method for lagging must be designed in such a way as to have a symmetrical streamlined skin profile both in the bow-feed direction and in the perpendicular direction. This method is selected as a prototype.

The disadvantage of the prototype method is that its implementation requires a radical alteration of the basic solution of submarines, which significantly reduces the cost-effectiveness of the method, and the long working movement of the submarine with a lag (if at all possible) makes increased demands on the stability of the vessel. In addition, during towing, long, flexible seismic acquisition-emitting antenna streamers are subject to vibrations caused by hydrodynamic forces, which leads to phase errors and additional flow noise. All this reduces the quality of the received signals.

The problem to which the present invention is directed, is to develop a more cost-effective and more technologically advanced method for conducting 3D geophysical exploration of the bottom of marine areas lying under the ice, providing a higher quality of the material obtained.

The technical result in the developed method is achieved by the fact that the developed method for conducting 3D underwater-under-ice geophysical exploration of bottom sediments of marine areas, as well as the prototype method, involves moving at least one acoustic signal emitter near the bottom of the studied marine area with using a submarine.

New in the developed method is that first, at the bottom of the studied water area using a submarine, linear bottom antennas from digital seismic receivers are installed. After that, the bottom of the water area is irradiated directly from a submarine moving relative to the above-mentioned linear bottom antennas using an emitter made in the form of at least one source of coherent broadband low-frequency acoustic radiation with a power of the order of more than 1 kW and installed permanently on the submarine. In this case, the reception and registration of reflected radiation is carried out by the mentioned digital seismic-acoustic receivers installed at the bottom, which at the end of underwater under-ice geophysical exploration are lifted aboard the vessel, and further processing of the received signals is carried out using the correlation method.

Using standard correlation signal processing in the developed method leads to a gain in the signal-to-noise ratio due to correlation compression during convolution of the received signal with the reference signal (signal supplied to the emitter). Registration of the three components of the displacement (vector reception) and pressure by each of the seismic-acoustic receivers that make up the linear bottom antenna allows for additional selection of the types of received (reflected and refracted) wills, which ensures a higher quality of the material obtained. The effectiveness of the vector technique is shown, for example, in the books: Halperin E.I. "Vertical seismic profiling." M .: Nedra, 1982, second edition, and “Vertical Seismic Profiling: Experience and Results”. - M .: Nauka, 1996.

In addition, the developed method involves the use of repeatedly tested basic engineering solutions in the field of underwater shipbuilding, which is fundamentally important for ensuring the safe operation of equipment. The lack of receiving towed antenna braids in the first place leads to the exclusion of hydrodynamic noise around the flow and the appearance of additional phase distortions due to fluctuations of long flexible braids during towing. Thus, the measurement scheme itself in the developed method is aimed at improving the quality of received signals while also ensuring economic efficiency and manufacturability of geophysical exploration of the bottom of ice-covered sea areas.

In the first particular case of the implementation of the method, it is advisable first to install two linear bottom antennas at an angle to each other of approximately 90 ° using an underwater vessel at the bottom of the water area, operating autonomously.

In the second particular case of the implementation of the method, it is advisable to first install four linear bottom-line antennas operating autonomously at the bottom of the water area using an underwater vessel, which should be placed on the bottom in the form of a rectangle or square.

In the third particular case of the implementation of the method, it is advisable to provide each digital seismic-acoustic receiver for autonomous operation with its own information storage device.

In the fourth particular case of the implementation of the method, it is advisable to provide each linear bottom antenna with a common storage of information for all digital seismic acoustic receivers of this antenna.

In the fifth particular case of the implementation of the method, it is advisable to irradiate the bottom of the water area using an omnidirectional emitter made in the form of a source of coherent broadband low-frequency acoustic radiation with linear frequency modulation in the frequency band 60-120 Hz.

In the sixth particular case, using an emitter containing a set of several sources of coherent broadband low-frequency acoustic radiation, it is advisable to form a radiation pattern for irradiating the bottom of the water area in nadir.

In the seventh particular case, it is advisable to additionally receive reflected radiation with digital acoustic receivers located in two rows along both sides of the submarine

In all special cases of the method implementation, the signals are recorded by multichannel vector receivers deployed at the bottom in the form of a set of parallel and / or perpendicular linear bottom antennas from digital seismoacoustic receivers operating autonomously. The autonomous operation of the seismic-acoustic receivers eliminates the presence of a long signal cable, which would lead to limited mobility and the potential for emergency situations. Signals are recorded in digital form with the maximum possible bit depth (for example, 32 bits) to magnetic or other storage devices (for example, high-speed flash cards with large capacities) designed for long-term storage of information. Such information storages consuming low energy can be autonomous and used to store data of either each receiver, as stated in claim 4, or all of the receivers of a linear bottom antenna at once, as stated in claim 5. The recording format must allow conversion to the SEG-Y format, which is used in exploration geophysics and with which numerous application programs work. Each of the seismic-acoustic (vector) receivers should be equipped with an orientation system for interlinking signals of horizontal (in the bottom plane) and vertical projections of velocities (or accelerations in the case of using accelerometers as geophones).

Currently, there are technologies for laying cable with coordinate binding. As an example, we can point to the US-Canadian project Theseus, based on the use of an underwater autonomous vessel for laying cable routes in the Arctic basin. Detailed information in English can be found at: www.ise.bc.ca/Theseus.htm. Vector receivers based on accelerometers (digital seismoacoustic receivers) with a frequency band from units of hertz to kilohertz integrated into a cable with a working depth of up to a kilometer are manufactured, for example, by the Norwegian company RXT (www.rxt.com). Thus, the installation of a geophysical cable to the bottom in the Arctic basin is a solvable technological problem.

Along with the registration of signals by stationary vector (seismoacoustic) receivers installed at the bottom, it is advisable to install a set of hydroacoustic receivers (hydrophones) rigidly connected to the vessel on the submarine itself. Such receivers constitute a group of several (two to four) hydrophones and are mounted rigidly on the sides, above and below the submarine, as indicated, for example, in paragraph 8 of the formula and in FIG. 2. Hydroacoustic receivers can be used both as auxiliary recorders of reflected signals with subsequent joint processing with vector receivers installed stationary at the bottom, and for organizing vector receivers configured to register signals reflected from bottom sediments. To install the geophysical cable to the bottom and subsequent movement of the emitter in the developed method, it is proposed to use an underwater autonomous vessel of small tonnage. In this case, due to the limited length of the submarine, groups of hydroacoustic receivers do not form deployed antennas and can only play an auxiliary role of interferometer receivers.

The technical result is an increase in economic efficiency, manufacturability and quality of the material obtained is provided in the developed method due to the use of coherent accumulation of complex waveforms by continuous radiation proposed by the authors. In contrast to the prototype method, instead of pulsed devices in the form of air guns (see, for example, Telegin AN / Red. Marine seismic exploration. - M .: Geoinformmark LLC, 2004) it is proposed to use sources of coherent low-frequency broadband acoustic radiation with power of the order and more than 1 kW. Coherence of radiation also implies coherent accumulation and processing of received signals. The general principles of coherent processing are known and described, for example, in the Handbook of sonar. / Ed. Kolesnikova A.E. - L .: Shipbuilding, 1988. Second edition. Chapter 4 “Hydroacoustic signals”. Moreover, the ratio of the duration of the coherent radiation in the developed method to the radiation duration of the aforementioned pulsed devices (about 0.1 second) is of the order of 100 and, possibly, more, which means that the radiation energy gain in the developed method is the same amount. Important is the environmental aspect. In recent years (see, for example, materials from the last congresses of the American Acoustic Society, section “Marine Acoustics”), scientists have noted the negative impact of sound noise from intense sources (sonars and air guns) on the life of marine inhabitants. The use of coherent radiation sources makes it possible to reduce the instantaneous radiation power and localize it in the frequency range below the kilohertz range, in which the activity of marine life is mainly concentrated. In the context of the fragile ecological balance of the Arctic basin, the environmental aspect takes on additional significance.

In addition, the coherence of radiation allows you to organize directed radiation and reception, for example, predominant radiation in the direction of the nadir, as stated in claim 7, and the reception of signals reflected mainly by the bottom of the water area with attenuation of reflections from the surface covered with ice. When using the developed method of powerful sources of directional coherent acoustic radiation, there is no need, in contrast to the prototype, to summarize a large number of signals of paired receivers to isolate the signals of the reflected waves from the noise background. The autonomy of each of the information storage devices and the coherence of the radiation provide significant flexibility in processing the received signals and organizing the reception of signals from certain directions. This circumstance, among other things, provides high economic efficiency and manufacturability of the developed method. When using the coherence of the emitted and received signals, there is no need to use a large number of paired receivers to ensure a high signal-to-noise ratio, which eliminates the cost of creating multi-kilometer underwater seismic acoustic streamers. The second, no less significant, feature of the proposed method is the use of proven basic solutions of submarines with the necessary modifications, which, when implementing the developed method, move in the usual marching mode. The ability to use proven basic solutions of submarines is provided due to the rigid fastening of the radiating system on a submarine. As a result, the hydrodynamics of the submarine, its maneuverability and safety of operation, do not suffer. All this is important for ensuring the autonomous operation of the submarine and conducting 3D seismic by sequentially recording the received signals for a set of seismic traces. The quality of the material obtained is ensured through the use of coherent sources of broadband acoustic radiation and receivers with digital conversion directly in the elements of a linear bottom antenna.

In the developed method, the primary correlation processing of signals is carried out in a standard way (see, for example, Skuchik E. Fundamentals of acoustics. - M .: Mir, 1976, T.1; Handbook of hydroacoustics. / Ed. Kolesnikov A.E. L .: Shipbuilding, 1988. Second edition, Chapter 4, “Hydroacoustic signals.”), Which, as shown by practical experience (Lebedev A.V., Malekhanov A.I. Coherent seismoacoustics. // Izv. Universities. Radiophysics. 2003. 46. No. 7. 579-597), allows to implement sounding modes uncharacteristic for traditional seismic exploration and increase the resolution my ability. As a result of correlation processing, the impulse response of the medium under study is determined, which, due to the small size of deformations and the obvious linearity of the propagation medium (bottom sediments) formally coincides with the impulse response obtained in a standard way (for example, in marine seismic exploration using pulsed sources). Therefore, the interpretation obtained after correlation processing of ice reconnaissance data using coherent radiation and reception is carried out in the same way as for surface marine seismic data. Consequently, it is possible to use all the accumulated experience, applications, computer centers, etc. services.

The invention is illustrated by drawings.

Figure 1 presents a diagram of a method for conducting 3D underwater-under-ice seismic-acoustic reconnaissance using an underwater vessel.

Figure 2 presents two types (side view and top view) of the basic layout of the submarine seismic-acoustic reconnaissance vessel for implementing the developed method.

Figure 3 presents a diagram of the movement of a submarine relative to linear bottom antennas from digital seismic receivers.

The seismic reconnaissance submarine (see FIGS. 1 and 2) includes the submarine 1 itself, which has a sealed compartment 8 for positioning there and transporting it in the “on-the-go” position of the emitter 2, made from at least one source 3 coherent broadband low-frequency acoustic radiation with a power of the order of or more than 1 kW. The emitter 2 is mounted on the rod 9, which is pivotally mounted in the compartment 8 with the possibility of fixing in two positions: in the "marching" position, in which the rod 9 is located in the compartment 8 parallel to the bottom of the vessel, and in the working position, in which the rod 9 is located outside the hull perpendicular to its bottom (see figure 2).

In accordance with the scheme developed in this method for 3D underwater under-ice seismic exploration of hydrocarbon minerals (see Fig. 1), linear bottom antennas 4 from digital seismic receivers 5 with information storage devices 7 are installed at the bottom of the water using an underwater vessel 1. Information 7 can be both common to all digital seismic-acoustic receivers 5 of the linear bottom antenna, and installed inside each of them.

In addition, the submarine 1 is additionally equipped with hydroacoustic receivers (hydrophones) 10, mounted on a cable cable 11, which is rigidly connected with the submarine vessel 1. The number of sonar receivers 10 is determined by the ratio of the length of the cable cable 11 and the maximum operating frequency of the radiation. For example, for a maximum frequency of 120 Hz with a sound wavelength of about 12 meters, hydrophones 10 can be placed no more than 6 meters later. With the length of an autonomous submarine of small tonnage of 15-20 meters, the number of auxiliary sonar receivers 10 in each group does not exceed two or three hydrophones.

As sources 3 of acoustic coherent broadband low-frequency radiation, deep-sea sources of coherent broadband acoustic radiation with a power of the order of more than 1 kW manufactured at IAP RAS (Nizhny Novgorod) can be used. A brief description of the sources is presented in the review (Lebedev A.V., Malekhanov A.I. Coherent seismoacoustics. // Izv. Universities. Radiophysics. 2003. 46. No. 7. 579-597).

As digital hydrophones 10, hydrophones produced, for example, by the Monitoring company (IAP RAS, Nizhny Novgorod), the description of which is presented in the works (Korotan P.I., Kiyashko B.V., Chashchin A.S., Kharitonov AV Digital hydroacoustic receiver. - Journal “Instruments and systems for exploration geophysics” 2007, No. 1, pp. 35-39 and Korotan PI, Chashchin AS, Eidelman ES. Digital data acquisition system marine seismic exploration. - Journal “Instruments and systems for exploration geophysics” 2007, No. 3, p.16-18). Registration and conversion of signals from spherical seismic receivers 5 can be organized in a similar way.

The developed method is implemented as follows.

Submarine 1, while in autonomous navigation, in marching mode enters the area of geophysical exploration. First, at the bottom of the water area using an underwater vessel 1, linear bottom antennas 4 are installed from digital seismic-acoustic receivers 5 with information storage 7 (see Fig. 1). Then from the sealed compartment 8 through the rod 9 bring into operation the emitter 2, consisting of one or more sources 3 of coherent broadband low-frequency acoustic radiation (see figure 2). Since the acoustic radiation sources 3 and hydroacoustic receivers 10 are small and fixed on the submarine 1 stationary and rigidly, the hydrodynamics of the submarine and its maneuverability are not violated, and the submarine 1 can move over the surveyed area at a speed of up to 40 knots, i.e. . almost in march mode. In this mode, three-dimensional 3D geophysical exploration in the surveyed area is carried out by sequentially recording the received signals for a set of relative positions of the emitter 2 and the linear bottom antennas 4 from digital seismic receivers 5 (see figure 3). Moreover, through sources 3, the bottom of the water area is irradiated into nadir with coherent low-frequency broadband acoustic radiation 6 with a power of the order of or more than 1 kW. And the radiation reflected from the seabed and from inhomogeneities (bottom sediments) under the seabed is received by vector digital seismic receivers 5, equipped with information storage devices 7. Using, for example, two perpendicular linear bottom antennas 4 or a square composed of four linear bottom antennas 4, allows to evaluate the possible slopes of the boundaries of geological formations by identifying the corresponding in-phase lines (for example, Telegin AN, ed. Marine seismic exploration. - M .: Geoinformmark LLC, 2004; Hatton ., Werdington M., Meikin J., Processing of seismic data, theory and practice. - M.: Mir, 1989; Gamburtsev G.A. et al. Correlation method of refracted waves - a guide for seismic engineers. - M.: Iz - at the USSR Academy of Sciences, 1952). After lifting the linear bottom antennas 4 and the information storage devices 7 or only the information storage devices 7 to the surface, data is read. Processing the received signals is carried out using the well-known correlation method (see, for example, Skuchik E. Fundamentals of Acoustics. - M .: Mir, 1976, T.1) to calculate impulse responses. Then the primary data thus obtained are converted into the standard geophysical format SGY, the processing of which is carried out by qualified personnel of geophysical computer centers using specialized programs.

In the developed method, due to the coherence of the radiation and accumulation of the received signals, the energy of the probe pulses increases, which eliminates the use of long (about 6000 m) standard seismic surveys and uses proven basic solutions of submarines with the necessary modifications to create an underwater geophysical exploration complex, which ensures solution of the task.

Claims (8)

1. A method of conducting 3D underwater-under-ice geophysical exploration using an underwater vessel moving at least one acoustic signal emitter near the bottom of the investigated marine area, characterized in that first, linear bottom antennas from digital seismic acoustic instruments are installed at the bottom of the water area using an underwater vessel receivers, after which the bottom of the water area is irradiated directly from a submarine moving relative to the said linear bottom antennas using an emitter made in the form of at least one source of coherent broadband low-frequency acoustic radiation with a power of the order of more than 1 kW and installed permanently on an underwater vessel, while receiving and recording the reflected radiation is carried out by the mentioned digital seismic acoustic receivers installed at the bottom, and further processing of the received signals is carried out using correlation method. 2. The method according to claim 1, characterized in that first, at the bottom of the water area using a submarine, two linear bottom antennas are installed at an angle to each other of about 90 °, working independently. 3. The method according to claim 1, characterized in that first, at the bottom of the water area using an underwater vessel, four linear bottom-line antennas are installed, working autonomously, which are placed on the bottom in the form of a rectangle or square. 4. The method according to claim 1, or 2, or 3, characterized in that each digital seismic-acoustic receiver of linear bottom antennas for autonomous operation is equipped with its own information storage device. 5. The method according to claim 1, claim 2, claim 3, characterized in that each linear bottom antenna is equipped with a common storage of information for all digital seismic acoustic receivers of this antenna. 6. The method according to claim 1, characterized in that the bottom of the water area is irradiated with a radiator made in the form of a source of coherent broadband low-frequency, acoustic radiation with linear frequency modulation in the frequency band 60-120 Hz. 7. The method according to claim 1, characterized in that the bottom of the water area is irradiated with an emitter containing several sources of coherent broadband low-frequency acoustic radiation, which allows you to create a radiation pattern for irradiating the bottom of the water area in nadir. 8. The method according to claim 1, characterized in that the additional reception of the reflected radiation is carried out by digital acoustic receivers located in two rows along both sides of the submarine.

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