Patent Application
20160202378
The invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to a marine seismic survey conducted using a first source transmitting a continuously varying seismic signal and a second source transmitting an impulsive seismic signal.
Seismic surveying is used to map the geology of formations in a survey area by way of detecting the elastic properties of different geological features. Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. Marine seismic surveying typically uses a marine survey vessel to tow a seismic source through a body of water over the survey area whilst actuating the seismic source at selected intervals of time. Making a marine seismic survey of the structure and character of a subsea geological formation relies on intermittently activating the seismic source to generate seismic waves which travel through the body of water and the seabed before being refracted through the earth or reflected at interfaces associated with geological features within the subsea geological formation. The various seismic signals are received by a plurality of seismic signal receivers or sensors and recorded for further analysis. In a marine environment, one or more streamer cables may be towed by the survey vessel or by another vessel over the survey area, each streamer cable comprising a plurality of seismic signal receivers arranged at spaced apart locations along the length of the streamer cable. Alternatively or additionally, the seismic signal receivers may be arranged on the sea bed and the seismic source may be towed behind a separate vessel
There are two types of seismic sources in use at this time. The first type is an ‘impulsive’ seismic source which is ‘shot’ at intermittent time intervals to produce ‘pressure waves’ or ‘P-waves’. Examples of an impulsive seismic source are explosive devices such as ‘air guns’, ‘gas guns’ or ‘sleeve exploders’ or implosive devices. When an airgun is shot, it discharges air under very high pressure into the water. The discharged air forms a bubble which oscillates at a given frequency which is a function of the size of the airgun and how far below the surface of the water the airgun is located at the time that it is fired. This air bubble generates a pressure wave that expands outwards into a large volume until it interacts with a geological feature and generates a reflected seismic signal. One problem with the use of air guns is the interaction of the pulse of air with the surface of the water which creates a reflected ‘ghost signal’ that interferes with the reflected seismic signals from the geological features of interest that are being surveyed. The seismic signal generated by impulsive seismic sources has a waveform that cannot be precisely controlled and is either on or off.
Due to the physical limitations associated with generating and collapsing a bubble of air in seawater, another problem with the use of conventional impulsive seismic sources is that airguns only generate significant energy above around 5 Hz. One of the limiting factors for air guns (and water guns) with respect to a lowest practical output frequency is the size of the chamber used to store the pressurized gas, air or water. The low frequency energy is further reduced by the destructive interference of the ghost signal reflected from the surface of the water. Consequently, conventional airguns are only capable of generating a seismic signal having bandwidth in the range of 10 to 250 Hz. Attempts have been made to overcome this inherent problem of not being able to generate low frequency seismic signals using airguns by making them bigger or towing them at lower depths below the surface of the water. However, the pressure on the air bubbles generated when conventional airguns are towed more deeply under the water is increased making it more difficult to generate a large air bubble of lower frequency, with the result that the capacity of the air guns must also be increased to generate a signal of equivalent size. Attempts have also been made to operate multiple air guns of different sizes in an attempt to cancel out unwanted ghost signals. Despite these attempts, a particular limitation to impulsive seismic sources known in the art is that they are not capable of generating low frequency seismic energy, typically less than about 5 Hz.
The second type of seismic source is a marine vibrator that generates a waveform that can be controlled (see, for example, U.S. Pat. No. 3,863,202, U.S. Pat. No. 3,349,367, U.S. Pat. No. 4,635,747, U.S. Pat. No. 4,918,668, and U.S. Pat. No. 8,102,731). Marine vibrators typically include a pneumatic or hydraulic actuator which causes an acoustic piston to vibrate at a selected frequency. The vibrations of the acoustic vibrator produce pressure differentials in the water which generate seismic pulses free from spurious bubbles. Vibrator-type seismic energy sources are programmed through control signals to generate energy in the seismic frequency bandwidth of between 10 and 190 Hz. At these frequencies, the reactive mass or diaphragm must be relatively large, and the amount of motion that must be imparted to the radiating surface is also relatively large. Controlling such motion so that it faithfully corresponds to the electrical control signal has proven to be difficult. Thus, one of the limiting factors of a seismic energy source such as a marine vibrator is the power required to move the driving element at low frequencies and to maintain a reasonably pure tone, because hydraulically driven devices in the vibrator may have difficulty shifting the required fluid volume for such low frequencies. Consequently, a particular limitation to vibrator seismic sources known in the art is that they are not capable of generating low frequency seismic energy, typically less than about 10 Hz.
Whilst it is relatively easy to detect a geological interface where there are very sudden changes in the geological properties of the earth using conventional mid to high frequency seismic sources, it is only possible to detect gradual changes between the various layers in a formation at low frequencies. This limited range of bandwidths available with conventional seismic sources has the result that information regarding gradual changes in the geological features cannot be captured. When seismic survey data is collected, it is subjected to processing to attempt to recreate all the physical properties of the earth that relate to the propagation of seismic waves. One of the modelling techniques used during processing is called ‘Full Waveform Inversion’. Because it is not possible to measure low frequency information below 5 Hz using conventional seismic sources, the seismic data that is acquired using conventional seismic sources is subjected to months of processing to estimate such parameters as the velocity and density to allow Full Waveform Inversion to commence. In the absence of low-frequency observed data below 5 Hz, full wave form inversion algorithms will not converge on the correct earth model due to the problem of local minima. Current approaches use conventional velocity tomography to create a ‘starting model’ for Full Waveform Inversion containing low-frequency information not measured in the survey. This approach has several major shortcomings. It requires a full seismic processing and imaging workflow to be applied before the tomography step can be conducted. This is time consuming and expensive. The result is a model with features in the range of 0 to 2 Hz. However, tomography still cannot resolve features in the range of 2 to 5 Hz which means that the result is a poor starting model for Full Waveform Inversion and the local minima problem is not satisfactorily addressed.
There remains a need in the art for an alternative marine seismic surveying method and system source capable of supplying the lower frequencies required to more accurately characterise subsea geological features.
According to a first aspect of the present invention there is provided a method of acquiring marine seismic survey data comprising:
According to a second aspect of the present invention there is provided a method of processing marine seismic survey data comprising:
In one form of the first or second or third aspect, the second source transmits an impulsive seismic signal has a bandwidth frequency greater than 5 Hz to 300 Hz. In one form of the first or second or third aspect, the second source is an air gun, gas gun or sleeve exploder.
In one form of the first or second or third aspect, the method further comprises a marine seismic receiver for detecting the first received signal generated by the first source signal, and, the second received signal generated by the second source signal, to produce a set of acquired seismic survey data.
In one form of the first or second or third aspect, the step of processing comprises the step of applying Full Waveform Inversion to the first received signal and the second received signal to generate a final model. In one form of the first or second or third aspect, the step of processing comprises the step of applying Full Waveform Inversion to the first received signal to generate an initial model and using the initial model as a starting point for applying Full Waveform Inversion to the second received signal to generate a final model. In one form of the first or second or third aspect, the step of processing comprises applying Full Waveform Inversion processing to the first received signal data and then the second received signal data.
In one form of the first or second or third aspect, the marine seismic receiver is a single streamer towed from a seismic survey vessel and the method comprises recording the set of acquired seismic survey data on a data recorder, wherein the data recorder is positioned on a marine survey vessel or positioned at a remote location. In one form of the first or second or third aspect, the streamer is one of a plurality of streamers in a towed array. In one form of the first or second or third aspect, the marine seismic receiver is an ocean bottom cable arranged on the ocean floor. In one form of the first or second or third aspect, the ocean bottom cable is one of a plurality of ocean bottom cables in a seabed array.
In one form of the first or second or third aspect, the marine seismic receiver is a seabed array comprising a plurality of ocean bottom nodes. In one form of the first or second or third aspect, the first source is towed using a first source tow cable behind a marine survey vessel and the second source is towed using a second source tow cable behind the marine survey vessel. In one form of the first or second or third aspect, the second source and a towed array of streamers is towed behind a marine survey vessel and the first source is towed behind a second vessel. In one form of the first or second or third aspect, the first source is arranged at a fixed location on the ocean floor.
In one form of the first or second or third aspect, the first source is suspended to a pre-determined depth from a buoy arranged at a fixed location at the waterline. In one form of the first or second or third aspect, the first source is one of a plurality of first sources arranged in a first source array. In one form of the first or second or third aspect, the first source array includes one of the plurality of first sources towed behind a marine survey vessel and a second one of the plurality of first sources towed behind a second vessel. In one form of the first or second or third aspect, a first source is arranged at a long offset location and a second source is arranged at a short offset location. In one form of the first or second or third aspect, the second source is one of a plurality of second sources arranged in a second source array. In one form of the first or second or third aspect, the second source array is a phased array. In one form of the first or second or third aspect, the second source is arranged at a depth below the waterline of at least ten meters.
In one form of the first or second or third aspect, a first source array includes a first source suspended from a first buoy at a first predetermined depth below the waterline with another first source being suspended from a second buoy at a second predetermined depth (below the waterline. In one form of the first or second or third aspect, the first received signal is an encoded signal. In one form of the first or second or third aspect, the marine seismic survey is a narrow azimuth, wide azimuth, or multi-azimuth survey. In one form of the first or second or third aspect, the marine seismic survey is a coil, slanted cable, or ocean bottom survey. In one form of the first or second or third aspect, the marine seismic survey is 4D seismic survey.
In one form of the first or second or third aspect, the first source is a magneto-hydrodynamic seismic source. In one form of the first or second or third aspect, the magneto-hydrodynamic seismic source comprises:
In one form of the first or second or third aspect, the plurality of electromagnets includes a first electromagnet arranged along a first side of the channel and a second paired electromagnet arranged on a second opposing side of the channel, and, each of the plurality of electromagnets is a saddle-type electromagnet. In one form of the first or second or third aspect, each of the plurality of electromagnets is a superconducting electromagnet.
In one form of the first or second or third aspect, each superconducting electromagnets has a polarity and the polarities of the plurality of superconducting magnets arranged along the casing are paired and opposed such that a first inflow of seawater enters at the first end of the fluid flow channel, and a second inflow of water enters at the second end of the fluid flow channel, wherein the fluid flow channel includes a discharge port for producing an outflow of seawater in the form of a seismic signal and the discharge port is positioned a discharge location intermediate between the first end of the fluid flow channel and the second end of the fluid flow channel.
In one form of the first or second or third aspect, the discharge location is centrally located between the first end of the fluid flow channel and second end of the fluid flow channel. In one form of the first or second or third aspect, the discharge port takes the form of a plurality of spaced apart apertures such that the outflow of seawater is directed radially outwardly from the fluid flow channel.
In one form of the first or second or third aspect, the casing has a central longitudinal axis and the longitudinal axis of the fluid flow channel is parallel to or coaxial with the central longitudinal axis of the casing.
In one form of the first or second or third aspect, the first seismic signal has a broadband waveform. In one form of the first or second or third aspect, the first seismic signal has a coded waveform. In one form of the first or second or third aspect, the first seismic signal has a continuously varying waveform in the form of a spike, a narrow band signal, or a monochromatic waveform. In one form of the first or second or third aspect, the fluid flow channel is one of a plurality of fluid flow channels, each fluid flow channel having a first end and a second end and a set of first and second electrodes.
In one form of the first or second or third aspect, the casing has a central longitudinal axis and the longitudinal axis of each fluid flow channel is parallel to and radially offset from a central longitudinal axis of the casing so that the plurality of flow fluid channels is evenly spaced around the circumference of the casing.
In one form of the first or second or third aspect, the plurality of electromagnets is provided in the form of a plurality of electromagnet segments. In one form of the first or second or third aspect, there is a corresponding number of the plurality of fluid flow channels and the plurality of electromagnet segments. In one form of the first or second or third aspect, the plurality of electromagnet segments is a plurality of toroidal or solenoidal magnet segments, arranged around the circumference of the casing.
In one form of the first or second or third aspect, the casing is cylindrical and has a circumference, and, the plurality of fluid flow channels is arranged at radially evenly spaced intervals around the outside of the circumference of the casing. In one form of the first or second or third aspect, the plurality of fluid flow channels comprises a first subset of fluid flow channels and second subset of flow fluid channels and the power source is configured so that the direction of the inflow into the first subset of fluid flow channels is reversed relative to the direction of the inflow of seawater into the second subset of fluid flow channels.
In one form of the first or second or third aspect, the electrical field being generated across each set of first and second electrodes is tuned to adjust the inflow and outflow of seawater through each of the plurality of channels to counteract an overall drag force experienced by the magneto-hydrodynamic source when towed behind a marine vessel in use.
In one form of the first or second or third aspect, the electrical field being generated across each set of first and second electrodes is tuned to adjust the inflow and outflow of seawater through each of the plurality of channels so that the magneto-hydrodynamic source can be self-propelling.
In one form of the first or second or third aspect, the power source is one of a plurality of power sources.
According to a fifth aspect of the present invention there is provided a system for acquiring marine seismic survey data, comprising:
In one form, the first source transmits a continuously varying seismic signal having a bandwidth frequency in the range of 0-5 Hz. In one form, the second source transmits an impulsive seismic signal has a bandwidth frequency greater than 5 Hz. In one form, the second source transmits an impulsive seismic signal has a bandwidth frequency greater than 5 Hz to 300 Hz. In one form, the second source is an air gun, gas gun or sleeve exploder.
In one form, the system further comprises a marine seismic receiver for detecting the first received signal generated by the first source signal, and, the second received signal generated by the second source signal, to produce a set of acquired seismic survey data.
In one form, the marine seismic receiver is a streamer towed from a seismic survey vessel and the method comprises recording the set of acquired seismic survey data on a data recorder, wherein the data recorder is positioned on a marine survey vessel or positioned at a remote location. In one form, the streamer is one of a plurality of streamers in a towed array. In one form, the marine seismic receiver is an ocean bottom cable arranged on the ocean floor. In one form, the ocean bottom cable is one of a plurality of ocean bottom cables in a seabed array. In one form, the marine seismic receiver is a seabed array comprising a plurality of ocean bottom nodes.
In one form, the first source is towed using a first source tow cable behind a marine survey vessel and the second source is towed using a second source tow cable behind the marine survey vessel. In one form, the second source and a towed array of streamers is towed behind a marine survey vessel and the first source is towed behind a second vessel. In one form, the first source is arranged at a fixed location on the ocean floor. In one form, the first source is suspended to a pre-determined depth from a buoy arranged at a fixed location at the waterline.
In one form, the first source is one of a plurality of first sources arranged in a first source array to generate directional acoustic energy form a plurality of continuously varying seismic signals. In one form, the first source array includes one of the plurality of first sources towed behind a marine survey vessel and a second one of the plurality of first sources towed behind a second vessel. In one form, a first source is arranged at a long offset location and a second source is arranged at a short offset location. In one form, the second source is one of a plurality of second sources arranged in a second source array. In one form, the second source array is a phased array.
In one form, the second source is arranged at a depth below the waterline of at least ten meters. In one form, the first source array includes a first source suspended from a first buoy at a first predetermined depth below the waterline with another first source being suspended from a second buoy at a second predetermined depth (below the waterline. In one form, the first received signal is an encoded signal.
In one form, the first source is a magneto-hydrodynamic seismic source. In one form, the magneto-hydrodynamic seismic source comprises:
In one form, the plurality of electromagnets includes a first electromagnet arranged along a first side of the channel and a second paired electromagnet arranged on a second opposing side of the channel, and, each of the plurality of electromagnets is a saddle-type electromagnet.
In one form, each of the plurality of electromagnets is a superconducting electromagnet.
In one form, wherein each superconducting electromagnets has a polarity and the polarities of the plurality of superconducting magnets arranged along the casing are paired and opposed such that a first inflow of seawater enters at the first end of the fluid flow channel, and a second inflow of water enters at the second end of the fluid flow channel, wherein the fluid flow channel includes a discharge port for producing an outflow of seawater in the form of a seismic signal and the discharge port is positioned a discharge location intermediate between the first end of the fluid flow channel and the second end of the fluid flow channel. In one form, the discharge location is centrally located between the first end of the fluid flow channel and second end of the fluid flow channel. In one form, the discharge port takes the form of a plurality of spaced apart apertures such that the outflow of seawater is directed radially outwardly from the fluid flow channel.
In one form, the casing has a central longitudinal axis and the longitudinal axis of the fluid flow channel is parallel to or coaxial with the central longitudinal axis of the casing.
In one form, the first seismic signal has a broadband waveform. In one form, the first seismic signal has a coded waveform. In one form, the first seismic signal has a continuously varying waveform in the form of a spike, a narrow band signal, or a monochromatic waveform.
In one form, the fluid flow channel is one of a plurality of fluid flow channels, each fluid flow channel having a first end and a second end and a set of first and second electrodes.
In one form, the casing has a central longitudinal axis and the longitudinal axis of each fluid flow channel is parallel to and radially offset from a central longitudinal axis of the casing so that the plurality of flow fluid channels is evenly spaced around the circumference of the casing.
In one form, the plurality of electromagnets is provided in the form of a plurality of electromagnet segments. In one form, there is a corresponding number of the plurality of fluid flow channels and the plurality of electromagnet segments. In one form, the plurality of electromagnet segments is a plurality of toroidal or solenoidal magnet segments, arranged around the circumference of the casing. In one form, wherein the casing is cylindrical and has a circumference, and, the plurality of fluid flow channels is arranged at radially evenly spaced intervals around the outside of the circumference of the casing.
In one form, the plurality of fluid flow channels comprises a first subset of fluid flow channels and second subset of flow fluid channels and the power source is configured so that the direction of the inflow into the first subset of fluid flow channels is reversed relative to the direction of the inflow of seawater into the second subset of fluid flow channels.
In one form, the electrical field being generated across each set of first and second electrodes is tuned to adjust the inflow and outflow of seawater through each of the plurality of channels to counteract an overall drag force experienced by the magneto-hydrodynamic source when towed behind a marine vessel in use.
In one form, the electrical field being generated across each set of first and second electrodes is tuned to adjust the inflow and outflow of seawater through each of the plurality of channels so that the magneto-hydrodynamic source can be self-propelling.
In one form, the power source is one of a plurality of power sources.
The manner in which the objectives of the invention and other desirable characteristics can be obtained is explained in the following description and attached figures in which:
Particular embodiments of the present invention are now described. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs.
The term ‘marine seismic survey’ refers to a seismic survey conducted in a marine environment. Note that marine surveys may be conducted not only in saltwater environments, but also in fresh or brackish waters.
The term ‘towed streamer survey’ refers to one or more streamers, each streamer comprising multiple sensors or receivers, being towed behind a survey vessel. When more than one streamer is used, the term ‘towed-array streamer survey’ is used.
The term ‘seabed survey’ or ‘ocean-bed survey’ refers to one or more seismic cables, each cable comprising multiple sensors or receivers, being laid on the ocean floor, or sea bottom. When more than one cable is used, the term ‘seabed-array survey’ is used.
The term ‘source ghost signal’ is used to refer to the energy reflected at the interface between the surface of a body of water and the pressure wave emitted by a seismic source.
The presently claimed invention is based in part on a realisation that acquiring low-frequency seismic data at the same time as conducting a conventional seismic survey will allow purely data-driven Full Waveform Inversion techniques to be successfully applied for the first time. As illustrated in
A key feature of the system and methods of the present invention is that the first source transmits a continuously varying seismic signal. Preferably, the continuously varying seismic signal has a bandwidth frequency in the range of 0-5 Hz. The continuously varying seismic signal may be transmitted using a mechanical or electrical marine vibrator configured to generate a continuously varying seismic signal having a bandwidth frequency in the range of 0 to 5 Hz. In a preferred embodiment, the first source is a magneto-hydrodynamic seismic source configured to generate a continuously varying flow of water. The continuously varying flow of water is oscillated preferably in the bandwidth range of 0 to 5 Hz. Such magneto-hydrodynamic seismic sources are not known to be used in the art of marine seismic surveying. Accordingly, embodiments of a magneto-hydrodynamic seismic source which is capable of being used as the first source for the present invention are described in detail below.
The second source is a conventional impulsive seismic source, such as an air gun, gas gun or sleeve exploder. A conventional air gun relies on transmitting a pulse or ‘shot’ of compressed air into the marine environment which forms an air bubble that expands under water. Impulsive seismic sources such as an airgun operate while underwater by opening a set of valves to a chamber containing air under pressure. When the valves are open, the pressure supplied to the water peaks initially and decays as the air supply is exhausted. When an airgun is discharged, there is a period required to recharge the chamber with compressed air, which limits the operating time available to discharge the next shot. Consequently, such impulsive seismic sources produce a high amount of acoustic energy over a short time on an intermittent basis. The impulsive seismic signal transmitted from the second source propagates in all directions with a bandwidth frequency greater than 5 Hz, typically in the range of 5 to 300 Hz.
One embodiment of a system (20) for acquiring marine seismic survey data is now described with reference to
The type of marine seismic receiver (32) may be any known device used in the art of marine seismic surveying to detect seismic energy, including pressure or pressure time gradient responsive sensors, particle motion responsive sensors, or combinations thereof. Conventional seismic signal receivers known in the art, such as; geophones, accelerometers, multi-component sondes, or hydrophones, may be used for detecting the first received signal (34) and the second received signal (36). During a marine seismic survey, each signal detected by the marine seismic signal receiver is logged along with the orientation and position of each seismic source relative to each signal receiver.
In the embodiment illustrated in
In an alternative embodiment illustrated in
In the embodiment illustrated in
In the embodiment illustrated in
In the embodiment illustrated in
In the embodiment illustrated in
Any of these source configurations can be used in combination with any of the marine seismic receiver configurations described above.
Now that the system has been described, a flow chart of one embodiment of the method of acquiring marine seismic survey data (110) is illustrated in
The set of acquired seismic survey data may be acquired by a first party and made available for processing by a second party. Referring to the flow charts illustrated in
The storage (136) is also encoded with an operating system (142), user interface software (144), and an application (146). The user interface software (144), in conjunction with a display (148), implements a user interface (150). The user interface (150) may include peripheral I/O devices such as a keypad or keyboard (152), a mouse (154), or a joystick (156). The processor (134) runs under the control of the operating system (142), which may be any operating system known to the art. The application (146) is invoked by the operating system (142) upon power up, reset, or both, depending on the implementation of the operating system (146). The application (146), when invoked, performs the method of the present invention. The user may invoke the application in conventional fashion through the user interface (150).
Note that there is no need for the set of acquired seismic data (138) to reside on the same computing apparatus (130) as the application (146) by which it is processed. Some embodiments of the present invention may therefore be implemented on a computing system, e.g., the computing system (160) in
The application (146) operates on the set of acquired seismic data (138) to extract the first received signal (34) and the second received signal (36), and, preferably, to perform Full Waveform Inversion processing on one or both of the extracted first received signal and extracted second received signal. As described above, the set of acquired seismic data (138) is acquired using one of the embodiments of the method of acquiring seismic data of the present invention already describe in detail above.
As is apparent from the discussion above, some portions of the detailed descriptions herein are presented in terms of a software implemented process involving symbolic representations of operations on data bits within a memory in a computing system or a computing device. These descriptions and representations are the means used by those in the art to most effectively convey the substance of their work to others skilled in the art. The process and operation require physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical, magnetic, or optical signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.
It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated or otherwise as may be apparent, throughout the present disclosure, these descriptions refer to the action and processes of an electronic device, that manipulates and transforms data represented as physical (electronic, magnetic, or optical) quantities within some electronic device's storage into other data similarly represented as physical quantities within the storage, or in transmission or display devices. Exemplary of the terms denoting such a description are, without limitation, the terms “processing,” “computing,” “calculating,” “determining,” “displaying,” and the like.
Note also that the software implemented aspects of the invention are typically encoded on some form of program storage medium or implemented over some type of transmission medium. The program storage medium may be magnetic (e.g., a floppy disk or a hard drive) or optical (e.g., a compact disk read only memory, or “CD ROM”), and may be read only or random access. Similarly, the transmission medium may be twisted wire pairs, coaxial cable, optical fiber, or some other suitable transmission medium known to the art. The invention is not limited by these aspects of any given implementation.
Embodiments of a magneto-hydrodynamic device that is suitable for use as the first source for the system and methods of the present invention are now described, by way of example only.
In broad terms, the magneto-hydrodynamic source of the present invention relies on an electrical field being applied transversely to a volume of seawater within a uniform, intense magnetic field. The resulting Lorenz force drives a flow of seawater at right angles to the electric and magnetic fields. The Lorenz force over a region of uniform electric and magnetic field can be calculated by the formula:
F=J×B×V,
As the resulting force on the seawater is directly proportional to the applied electric field, the movement of the flow of seawater is able to be precisely controlled by varying the applied current supplied to the electrodes. The Lorentz force acting on the seawater is produced by the electric field applied at right angles to the magnetic field. The conductive seawater will move rapidly in the direction of the Lorentz force when the electric field is applied. With the magnetic field being held at a constant magnetic flux, the flow of water from the magneto-hydrodynamic source is a linear function of the applied electric field making it possible to generate a low frequency flow of seawater that overcomes the limitations of prior art impulsive and vibration sources. The movement of the flow of seawater generated using the ‘magneto-hydrodynamic source’ of the present invention creates a pressure wave suitable for marine seismic surveying. To generate a seismic signal, the applied electric field is continuously varied.
A first embodiment of a magneto-hydrodynamic seismic source for use in the method and system of the present invention is now described with reference to
It is to be understood that the present invention is not limited to the use of a pair of electromagnets. Any number of electromagnets may be included in the plurality of electromagnets provided only that a uniform magnetic field is induced at right angles to the bore of the casing. Superconducting electromagnets are the preferred kind of electromagnets for the magneto-hydrodynamic seismic source of the present invention because once a superconductor magnet is powered up, they are capable of retaining a uniform intense magnetic field for an extended period of time. This time can be maximised by circulating a coolant such as liquid nitrogen through the superconducting electromagnets to improve performance. Other kinds of electromagnets other than superconducting electromagnets, for example, toroidal or solenoidal electromagnets, may be used. It is to be further understood that the fluid flow channel need not be cylindrical in cross-section, provided only that a flow of seawater can pass through the fluid flow channel from the first end to the second end.
In the embodiment illustrated in
By way of example only, assuming that the casing is one metre long and the internal diameter of the bore of the casing is 0.25 metres, with a uniform magnetic field inside the bore of the casing of 5 Telsa, and an applied current of 2000 A, the Lorentz force acting on the seawater inside the chamber will be 2500 N. Assuming no frictional losses and assuming a seawater density of 1000 kg/m3 and no interference with the ocean as the magneto-hydrodynamic source is towed through it, a flow of 2.5 m3/s of seawater will be produced from the fluid flow channel under steady conditions. To create a sufficient signal for marine seismic surveys, it is estimated that a flow rate of 3 m3/s is required to for frequencies below 2 Hz. This is achieved using the simplified example given above by increasing the current applied to the electrodes.
A second embodiment of the present invention is illustrated in
In a third embodiment illustrated in
The precision with which the power supply applies a continuously varying electric current across the first and second electrodes of the magneto-hydrodynamic seismic source is able to be controlled is far higher for the magneto-hydrodynamic source of the present invention, than the mechanical switching and movement involved in opening an airgun. Impulsive seismic sources such as an airgun operate while underwater by opening a set of valves to a chamber containing air under pressure. When an airgun is discharged, there is a period required to recharge the chamber with compressed air, which limits the operating time available to discharge the next shot. When the valves are open, the pressure supplied to the water peaks initially and decays as the air supply is exhausted. In contrast, the magneto-hydrodynamic seismic source of the present invention can operate continuously as there is no need for recharge, and the flow rate can be varied during each transmission event by changing the current supplied to the electrodes. As there is no requirement to provide a compressed air supply to the magneto-hydrodynamic seismic source of the present invention, the magneto-hydrodynamic seismic source is more portable and adapted for use in other situations where an air gun is used, such as vertical seismic profiling on drilling rigs, or using a vessel of opportunity to operate a source array independently of a hydrophone or geophone array.
As set out above in the ‘Background to the Invention’, conventional seismic sources used for marine seismic surveys have a limited bandwidth due to the finite volume of air that can be supplied for an individual pulse and the effect of increasing pressure if the source of the air is towed deeper below the surface of the water. As no bubble of air is used to move the water using the magneto-hydrodynamic seismic source of the present invention, the magneto-hydrodynamic seismic source overcomes the low frequency limitation of prior art devices. Advantageously, the magneto-hydrodynamic seismic source is able to continuously generate a flow of water as long as power is supplied to the electrodes. As the magneto-hydrodynamic seismic source can vary continuously, the potential to use multiple sources in phased arrays to simultaneously illuminate subsea geological features exists. The ability to transmit a complex waveform enables an array of magneto-hydrodynamic seismic sources to reconstruct a variety of wavefields below the array. These can be optimised to illuminate selected regions of the subsurface by adjusting the waveforms of the outlet flows of seawater so that they arrive at a selected target location associated with a subsea geological feature in phase at a selected time. One of the limitations of an airgun is caused by the interaction of the pressure wave generated by the airgun with the surface of the ocean. The energy travelling upward after the airgun discharges is reflected downward again from the surface of the water in the form of a ghost signal. This reflected, downward travelling ghost signal interferes with the rest of the pressure wave, and causes a loss of low frequency energy for prior art impulsive sources. Because the seismic signal generated by the magneto-hydrodynamic seismic source of the present invention is generated by moving an outflow pulse of seawater through seawater, the compressibility problems associated with the air bubbles generated by conventional air guns are avoided. The magneto-hydrodynamic seismic source of the present invention is able to transmit an arbitrary waveform which can be tuned to deliver a seismic signal with a waveform that interferes constructively with the ghost signal to eliminate its effects. By controlling the power supplied to the electrodes, the seismic signal generated by the magneto-hydrodynamic seismic source can have a magneto-hydrodynamic waveform in the form of a series of spikes an oscillating wave form, a sinusoidal wave form, a broadband signal, a narrow band signal, an array-based signal, a phased-array signal, a coded wave form (which is a random wave form generated according to a rule), or a monochromatic waveform. One of the main advantages of being able to generate broadband seismic signals using the magneto-hydrodynamic seismic source of the present invention is that noise becomes more random, making it easier to deal with.
In the embodiment illustrated in
In use, one or more power sources (338) in electrical communication with each of the first and second electrodes is actuated to apply an electrical current across the first and second electrodes to generate a controlled electric field extending from each first electrode (330) towards each second electrode (334) in the direction generally designated by the arrow labelled with the reference label ‘E’ in
In the embodiment illustrated in
Although only a few embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this invention. By way of example, the marine survey vessel may be caused to traverse a non-linear sail line such as a curved sail line, a sinusoidal sail line or a circular sail line, during the marine seismic survey. Accordingly, all such modifications are intended to be included within the scope of this invention.
It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the statement of invention and description of the invention which follow, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013901181 | Apr 2013 | AU | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU2014/000368 | Apr 2014 | US |
| Child | 14873300 | US |
