1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic data processing, and more particularly, to estimating seismic data using interferometric methods.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath areas of land and bodies of water, various types of land and marine seismic surveys have been developed to determine the locations of these hydrocarbon deposits.
In a typical land seismic survey, seismic sensors are installed in specific locations around the land in which hydrocarbon deposits may exist. Seismic sources, such as vibrators, may move across the land and produce acoustic signals, commonly referred to as “shots,” directed down to the land, where they are scattered from the various subterranean geological formations. Scattered signals are received by the sensors, digitized, and then transmitted to a survey database. The digitized signals are referred to as seismograms and are recorded on the survey database.
In a typical marine seismic survey, seismic streamers are towed behind a survey vessel. The seismic streamers may be several thousand meters long and contain a large number of sensors, such as hydrophones, geophones, and associated electronic equipment, which are distributed along the length of the seismic streamer cable. The survey vessel may also include one or more seismic sources, such as air guns and the like. The seismic streamers may be in an over/under configuration, i.e., one set of streamers being suspended above another set of streamers. Two streamers in an over/under configuration, referred to as twin streamers, may be towed much deeper than streamers in a conventional single configuration.
As the seismic streamers are towed behind the survey vessel, acoustic signals, commonly referred to as “shots,” produced by the one or more seismic sources are directed down through the water into strata beneath the water bottom, where they are scattered from the various subterranean geological formations. Scattered signals are received by the sensors, digitized, and then transmitted to the survey vessel. The digitized signals are referred to as seismograms and are recorded and at least partially processed by a signal processing unit deployed on the survey vessel.
The ultimate aim of these processes is to build a representation of the subterranean geological formations beneath the land or beneath the streamers. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations.
Described herein are implementations of various technologies for removing non-physical wavefields from interferometrically obtained Green's functions. In one implementation, a method for estimating seismic data from sources of noise in the earth surrounding a first seismic receiver and a second seismic receiver may include calculating a Green's function G′(X1, X2) between the first seismic receiver X1 and the second seismic receiver X2 using interferometry. The first seismic receiver X1 is located at a distance away from the second seismic receiver X2. After calculating the Green's function G′(X1, X2), the method may include determining an estimate of one or more non-physical wavefields present in the Green's function G′(X1, X2) and determining a filter to remove the non-physical wavefields from the Green's function G′(X1, X2) based on the estimate of the non-physical wavefields. The filter may then be applied to the Green's function G′(X1, X2) to obtain a Green's function G″(X1, X2) free of non-physical wavefields.
In another implementation, a computer-readable storage medium may have computer-executable instructions which cause the computer to calculate a Green's function G′(X1, X2) between a first seismic receiver X1 and a second seismic receiver X2 using interferometry. The first seismic receiver X1 may be located at a distance away from the second seismic receiver X2, and after calculating the Green's function G′(X1, X2), the computer-readable storage medium may split one or more wavefields received between the first seismic receiver X1 and the second seismic receiver X2 into one or more direct wavefields G0(X1, X2) and one or more scattered wavefields Gsc(X1, X2). The direct wavefields G0(X1, X2) may be generated from one or more sources of noise surrounding the first seismic receiver X1 and the second seismic receiver X2, while the scattered wavefields Gsc(X1, X2) may be generated from one or more anomalies in a subterranean medium. The computer-readable storage medium may then determine a contribution of the direct wavefields and the scattered wavefields to the Green's function G′(X1, X2) such that the contribution may represent an estimate of one or more non-physical wavefields present in the Green's function G′(X1, X2). The contribution may then be used by the computer-readable medium to determine a filter that may be used to remove the non-physical wavefields from the Green's function G′(X1, X2). After determining the filter that may be used to remove the non-physical wavefields from the Green's function G′(X1, X2), the computer-readable medium may then apply the filter to the Green's function G′(X1, X2).
In yet another implementation, a computer system has a processor and a memory having program instructions executable by the processor to calculate a Green's function G′(X1, X2) between a first seismic receiver X1 and a second seismic receiver X2 using interferometry. In order to calculate the Green's function G′(X1, X2), the first seismic receiver X1 may be located at a distance away from the second seismic receiver X2. After calculating the Green's function G′(X1, X2), the computer system may split one or more wavefields received between the first seismic receiver X1 and the second seismic receiver X2 into one or more direct wavefields G0(X1, X2) and one or more scattered wavefields Gsc(X1, X2). Here, the direct wavefields G0(X1, X2) may be generated from one or more sources of noise surrounding the first seismic receiver X1 and the second seismic receiver X2 and the scattered wavefields Gsc(X1, X2) may be generated from one or more anomalies in a subterranean medium. The computer system may then determine a contribution of the direct wavefields and the scattered wavefields to the Green's function G′(X1, X2) such that the contribution may represent an estimate of one or more non-physical wavefields present in the Green's function G′(X1, X2). Upon determining the contribution of the direct and scattered wavefields in the Green's function G′(X1, X2), the computer system may determine a filter to remove the non-physical wavefields from the Green's function G′(X1, X2) based on the estimate of the non-physical wavefields. The computer system may then apply the filter to the Green's function G′(X1, X2) and determine the filtered Green's function G′(X1, X2). The computer system may consider the filtered Green's function G′(X1, X2) to be the seismic data received between the first seismic receiver X1 and the second seismic receiver X2 without interference from non-physical wavefields.
The claimed subject matter is not limited to implementations that solve any or all of the noted disadvantages. Further, the summary section is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description section. The summary section is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
Implementations of various technologies will hereafter be described with reference to the accompanying drawings. It should be understood, however, that the accompanying drawings illustrate only the various implementations described herein and are not meant to limit the scope of various technologies described herein.
The discussion below is directed to certain specific implementations. It is to be understood that the discussion below is only for the purpose of enabling a person with ordinary skill in the art to make and use any subject matter defined now or later by the patent “claims” found in any issued patent herein.
A homogeneous Green's function (the Green's function plus its time-reverse) between two points has been proven to be constructed from records of the Green's functions between each of those points and a surrounding boundary of energy sources without the need of a “shot” at either point. Here, the homogenous Green's function may show that both monopolar and dipolar boundary sources are useful, but only monopolar sources may be necessary if the boundary is sufficiently far away from the two points such that energy paths to each leave the boundary noise source approximately perpendicularly to the boundary. If the boundary sources are fired individually and sequentially, recordings made at the two locations may be cross-correlated and summed over boundary sources to obtain the inter-receiver homogeneous Green's function, and hence the inter-receiver Green's function.
Equivalent results may hold for diffusive (e.g., highly scattered) wavefields. Under a unified formulation of the theory it has been shown that other types of Green's functions, such as electrokinetic Green's functions in poroelastic or piezoelectric media, can be retrieved. Similar results may also hold for dissipative media and for monopolar random noise sources provided either that the boundary connecting the noise sources is sufficiently irregular (i.e., the noise source locations are sufficiently random), or is sufficiently far away from the receivers as described above. If neither of these conditions holds, then dipolar noise sources may need to be obtained.
Impulsive or noise source versions of the above theory created a new schema with which synthetic wavefields between receivers could be modeled flexibly. In an exploration setting and in the case of borehole receivers and surface sources, seismic interferometry may be used to re-datum both sources and receivers into the borehole, which may remove many undesirable near-surface related effects from the data. Major body wave components of Green's functions could be estimated using background (passive) noise records in a particularly quiet area (i.e., where surface generated noise is at a minimum).
In order to build the representation of the subterranean geological formations, seismic data may be processed. It is desirable to be able to develop a method to process the data more efficiently and to have a method and an apparatus to make this possible.
The following paragraphs provide a brief description of one or more implementations of various technologies and techniques directed at removing non-physical wavefields from interferometrically obtained Green's functions, where interferometry cannot be applied exactly. In one implementation, two seismic receivers may be defined at two different locations in a seismic survey area such that a Green's function may be estimated between the pair of seismic receivers. The Green's function may be calculated using interferometry and the surrounding noise sources. The calculated Green's function, however, may be biased due to the non-identical strengths of the surrounding noise sources. When removing the bias in the calculated Green's function, one or more representations of physical and non-physical energies received by the receivers may be embedded within the calculated Green's function.
In order to remove the non-physical energies from the calculated Green's function, the non-physical energies may be estimated using a wavefield-separation based prediction method. The wavefield-separation based prediction method may analyze the contribution of the physical direct and scattered waves to interferometrically constructed wavefields. Interferometry between the various combinations of the direct waves and scattered waves may result in four separate contributing terms: T1, T2, T3, and T4. These terms may represent the contributions resulting, respectively, from the cross-correlation of the direct waves received by a first receiver with the direct waves received by a second receiver, the direct waves received by the first receiver with the scattered waves received by the second receiver, the scattered waves received by the first receiver with the direct waves received by the second receiver, and the scattered waves received by the first receiver with the scattered waves received by the second receiver. In order to remove the non-physical energies from the calculated Green's functions, the term T4 is adaptively subtracted from the sum of all of the terms T1-T4 (or where only scattered waves are desired, terms T2-T4) because the term T4 represents an estimate of the non-physical energies. Therefore, the non-physical energies embedded in the calculated Green's functions may then be removed using a filter that may have been created using the term T4.
One or more implementations of various techniques removing non-physical wavefields from interferometrically obtained Green's functions will now be described in more detail with reference to
The virtual source receiver X1 and each receiver X2 may represent a seismic sensor capable of measuring and recording seismic waves. In one implementation, the receivers X2 may be arranged in a line, but in other implementations the receivers X2 may be arranged in another manner. The virtual source receiver X1 may represent a location in which a source may be simulated in order to obtain a Green's function interferometrically. The virtual source receiver X1 and the receivers X2 may be placed on a land terrain, on a sea surface, on a body of water with the use of one or more streamers, on a seabed, or in the subsurface (e.g., within a well).
The scatterer location Sc1 may represent an anomaly in a particular geographical area of the Earth that may create a distortion in the seismic waves received by the receivers X2. In one implementation, the anomaly may represent a change in the nature of the subterranean makeup of the Earth. As a result, after seismic waves from the sources Si reach the scatterer location Sc1, they may no longer travel in an expected manner with respect to the uniform characteristics of the surrounding subterranean medium. Instead, scattered waves may be created from the scatterer location Sc1 and then received by the receivers X1 and X2. The scatterer location Sc1 may represent an isolated subterranean geological structure.
At step 210, seismic receivers may be defined at two or more locations such that a Green's function may be estimated between the receivers. In one implementation, a virtual source receiver X1 may be placed in a specified location to denote the location of a virtual source in estimating a Green's function. A second seismic receiver X2 may be placed at a distance away from the seismic receiver X1. In one implementation, the second seismic receiver X2 may consist of one or more seismic receivers arranged in a line such as the receiver X2 as described in
At step 220, a Green's functions G′(X1, X2) may be calculated between the virtual source receiver X1 and the receiver X2. In one implementation, the Green's function may be calculated using interferometry applied to the seismic recordings of the surrounding source signals Si from the receivers X1 and X2. Therefore, the Green's function G′(X1, X2) may be biased due to the non-identical strengths of the sources Si surrounding the receivers X1 and X2. In one implementation, the Green's functions G′(X1, X2) may be calculated in a space-time domain, but it may also be calculated in a variety of other domains such as the space-frequency domain and the like. After calculating the Green's function G′(X1, X2) in a particular domain, the Green's function G′(X1, X2) may be transformed into another domain (e.g. spatial wavenumber-frequency domain, time-radon domain) in order to perform additional data calculations or data processing.
In one implementation, the calculated Green's function G′(X1, X2) may contain representations of physical and non-physical energies of the Earth's subterranean medium as recorded by the receivers X1 and X2. Non-physical energy may represent spurious waves that were generated due to the non-uniform strengths of the source signals Si. The physicals waves are waves that would have been observed if a source had been placed at one of the receivers X1 and X2, and the non-physical waves are waves that appear in the interferometric estimates that would not have been observed if a source had been placed at one of the receivers X1 and X2. For instance, if the receivers X1 and X2 are surrounded by a set of evenly distributed source signals Si, the Green's function G′(X1, X2) may be created without the interference of the non-physical waves because non-physical waves created from one side of the source signals Si may be cancelled by equal non-physical waves created from the opposite side of the source signals Si. However, when one or more source signals Si are missing, or their amplitudes are weaker than the rest (as seen in
In one implementation, non-physical arrivals may be identified when a waveform arrives at the receivers X1 and X2 prior to the expected time in which the first physical waveform should arrive (e.g., prior to 0.1 seconds). These waveforms may be described as non-physical because they occur at times that are before the direct waves can physically arrive by physical wave propagation from the source signals Si.
At step 230, the non-physical energy or wavefields within the Green's function G′(X1, X2) may be estimated in order to create a filter to effectively remove the non-physical wavefields from the Green's function G′(X1, X2).
In one implementation, the non-physical wavefield may be estimated using a wavefield-separation based prediction method. The wavefield-separation based prediction method may split the surface wavefields, received by the receivers X1 and X2, into direct waves and scattered waves. The direct waves may be created from the source signals Si, but the scattered waves may be created due to the scatterer location Sc1. Then, the wavefield-separation based prediction method may analyze the contribution of the direct waves and scattered waves to interferometrically constructed wavefields. Interferometry between the various combinations of the direct waves and scattered waves may result in four separate contributing terms: T1, T2, T3, and T4, each of which will be described in more detail below. These terms may represent the contributions resulting, respectively, from the cross-correlation of the direct waves received by receiver X1 with the direct waves received by receiver X2, the direct waves received by receiver X1 with the scattered waves received by receiver X2, the scattered waves received by receiver X1 with the direct waves received by receiver X2, and the scattered waves received by receiver X1 with the scattered waves received by receiver X2.
In one implementation, after splitting the wavefields received by the receivers X1 and X2, the direct wavefields between the receivers X1 and X2 may be defined as G0(X1,X2) and the scattered wavefields between the receivers X1 and X2 may be defined as Gsc(X1,X2). The direct wavefields G0(X1,X2) and the scattered wavefields Gsc(X1,X2) may be determined using a windowing or some other wavefield separation scheme on the calculated Green's function G′(X1, X2). The Green's function G0(X1,X2) represents the direct wavefield or the seismic waves received by the receivers directly from the source signals Si, and the Green's function Gsc(X1,X2) represents the scattered wavefield or the seismic waves received by the receivers after they have been scattered from the scatterer location Sc1. In one implementation, the Green's function Gsc(X1, X2) may include data pertaining to the scattered waves from one or more scatterer locations Sc1.
The real energy recorded by the receivers should only include the Green's function G0(X1,X2) and the Green's function Gsc(X1,X2) because no other energy exists in the single scatterer model 100. However, after cross-correlation is completed between various combinations of the direct waves and scattered waves in the interferometric calculation of the Green's function G′(X1, X2) at each receiver (X1 and X2), scattered wavefields and non-physical wavefields may be visible in the obtained Green's function G′(X1, X2). The scattered wavefields and non-physical wavefields may also be denoted in the contributing terms.
Term T1 may then be estimated after the direct waves recorded at both receivers X1 and X2 have been cross-correlated in the interferometric process in creating the Green's function G′(X1, X2). Term T1 may be defined as:
T1=G0*(X1,X2)−G0(X1,X2)
where G0(X1,X2) represents the direct wavefield between receivers X1 and X2 and G0*(X1,X2) represents the time-reversed direct wavefield between receivers X1 and X2 created from the cross-correlation step in the interferometric process.
Term T2 may be estimated after the direct waves recorded at the first receiver X1 and the scattered waves recorded at the second receiver X2 have been cross-correlated in the interferometric process in creating the Green's function G′(X1, X2). Term T2 may be defined as
T2=Gsc*(X1,X2)+Gnp1(X1,X2)
where Gsc*(X1,X2) represents the time-reversed scattered wavefield between receivers X1 and X2 and Gnp1(X1,X2) represents the non-physical wavefield between receivers X1 and X2 created from the cross-correlation step in the interferometric process.
Term T3 may be estimated after the scattered waves recorded at the first receiver X1 and the direct waves recorded at the second receiver X2 have been cross-correlated in the interferometric process in creating the Green's function G′(X1, X2). Term T3 may be defined as
T3=−Gsc(X1,X2)+Gnp2(X1,X2)
where Gsc(X1,X2) represents the scattered wavefield between receivers X1 and X2 and Gnp2(X1,X2) represents the non-physical wavefield between receivers X1 and X2 created from the cross-correlation step in the interferometric process. Gnp2(X1,X2) and Gnp1(X1,X2) can contain different non-physical arrivals, but the combination of both of these terms represents the total non-physical wavefield.
Term T4 may be estimated after the scattered waves recorded at the first receiver X1 and the direct waves recorded at the second receiver X2 have been cross-correlated in the interferometric process in creating the Green's function G′(X1, X2). Term T3 may be defined as
T4=−Gnp1(X1,X2)−Gnp2(X1,X2)
where Gnp1(X1,X2) represents the non-physical wavefield between receivers X1 and X2 and Gnp2(X1,X2) represents the non-physical wavefield between receivers X1 and X2 created from the cross-correlation step in the interferometric process. Hence the term T4 represents the total non-physical wavefield. When interferometry is applied, the term T4 mutually cancels the non-physical terms T2 and T3.
In one implementation, if the source signals Si have a uniform strength distribution at each location, the non-physical wavefield Gnp1(X1,X2) and the non-physical wavefield Gnp2(X1,X2) should cancel out when all of the terms T1, T2, T3, and T4 are added together. Therefore, the result of the summation of the terms T1-T4 (or T2-T3) should include the direct and scattered wavefields without the non-physical wavefields. In certain configurations, such as when a scatterer location Sc1 lies on the extension of the inter-receiver path which may extend out beyond the receiver in either direction, the term T4 may also contribute towards the physical wavefields (direct or scattered) even though it may otherwise include non-physical wavefields. In such cases, the physical wavefield contribution is expected to be very small.
However, if the source signals Si do not have a uniform strength distribution at each location, then the amplitudes of the four terms T1-T4 may vary, and the non-physical wavefields may not necessarily cancel out which may result in the introduction of the non-physical wavefields into the interferometric Green's function estimates.
In another implementation, the non-physical wavefields may also be estimated using a moveout-based prediction method as described in
At step 240, a filter may be created to remove the non-physical wavefields from the Green's function G′(X1, X2) based on the estimate of the non-physical wavefields obtained in step 230. In one implementation, the filter may be designed to adaptively subtract the term T4 from the sum of all of the terms T1-T4 because the term T4 represents an estimate of the non-physical wavefields. The filter may be created in the same domain in which the Green's function G′(X1, X2) may have been transformed to in step 220. The filter may be created using matching filters, least-square filters, helical filters, or any other appropriate type of filter.
At step 250, the filter may be applied to the Green's function G′(X1, X2) to obtain a filtered Green's function G″(X1, X2). The filtered Green's function G″(X1, X2) may include less non-physical wavefields as compared with the original Green's function G′(X1, X2) between receiver X1 and X2. In one implementation, the filtered Green's function G″(X1, X2) may then be transformed back into the domain in which the Green's function G′(X1, X2) may have been originally obtained in at step 220.
In another implementation,
The receivers Xi may represent one or more seismic sensors capable of measuring and recording seismic waves. In one implementation, the receivers Xi may be arranged in a line, but in other implementations the receivers Xi may be arranged in another manner.
At step 510, a central receiver (X1) of the receivers Xi may be designated to be a virtual source. Although, in this implementation a central receiver may have been selected to be the virtual source, in other implementations, the virtual source may be selected to be any one of the receivers Xi.
At step 520, a Green's function G′(X1, Xi) may be interferometrically calculated between the receiver X1 at the virtual source location and each receiver Xi.
At step 530, the wavefield-separation terms T2 and T3 may be calculated using the direct wavefields and the scattered wavefields received between the receiver X1 and the receiver Xi as described in step 230 of
At step 540, the sum of the wavefield-separation terms T2 and T3 calculated at step 530 may be plotted on a graph. The plotted graph with the sum of the wavefield-separation terms T2 and T3 for the Greens' function G′(X1, Xi) of the two-scatterer model 600 is illustrated in
At step 550, the virtual source and receiver locations may be switched and a Green's function G′(Xi, X1) between each receiver Xi (new virtual source location) and the receiver X1 may be calculated using interferometry.
At step 560, the wavefield-separation terms T2 and T3 may be calculated using the direct wavefields and the scattered wavefields received between the receiver Xi and the receiver X1 as described in step 230 of
At step 570, the wavefield-separation terms T2 and T3 may be plotted on a graph. The plotted graph with the sum of the wavefield-separation terms T2 and T3 for the Greens' function G′(Xi, X1) of the two-scatterer model 400 is illustrated in
At step 580, the difference between the plotted graph with the sum of the wavefield-separation terms T2 and T3 for the Greens' function G′(X1, Xi) determined at step 540 (
In another implementation,
The system computer 930 may be in communication with disk storage devices 929, 931, and 933, which may be external hard disk storage devices. It is contemplated that disk storage devices 929, 931, and 933 are conventional hard disk drives, and as such, will be implemented by way of a local area network or by remote access. Of course, while disk storage devices 929, 931, and 933 are illustrated as separate devices, a single disk storage device may be used to store any and all of the program instructions, measurement data, and results as desired.
In one implementation, seismic data from the receivers may be stored in disk storage device 931. The system computer 930 may retrieve the appropriate data from the disk storage device 931 to process seismic data according to program instructions that correspond to implementations of various technologies described herein. Seismic data may include pressure and particle velocity data. The program instructions may be written in a computer programming language, such as C++, Java and the like. The program instructions may be stored in a computer-readable memory, such as program disk storage device 933. Such computer-readable media may include computer storage media and communication media.
Computer storage media may include volatile and non-volatile, and removable and non-removable media implemented in any method or technology for storage of information, such as computer-readable instructions, data structures, program modules or other data. Computer storage media may further include RAM, ROM, erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other solid state memory technology, CD-ROM, digital versatile disks (DVD), or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computing system 900.
Communication media may embody computer readable instructions, data structures, program modules or other data in a modulated data signal, such as a carrier wave or other transport mechanism and may include any information delivery media. The term “modulated data signal” may mean a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media may include wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of the any of the above may also be included within the scope of computer readable media.
In one implementation, the system computer 930 may present output primarily onto graphics display 927. The system computer 930 may store the results of the methods described above on disk storage 929, for later use and further analysis. The keyboard 926 and the pointing device (e.g., a mouse, trackball, or the like) 925 may be provided with the system computer 930 to enable interactive operation.
The system computer 930 may be located at a data center remote from the survey region. The system computer 930 may be in communication with the receivers (either directly or via a recording unit, not shown), to receive signals indicative of the reflected seismic energy. After conventional formatting and other initial processing, these signals may be stored by the system computer 930 as digital data in the disk storage 931 for subsequent retrieval and processing in the manner described above. While
While the foregoing is directed to implementations of various technologies described herein, other and further implementations may be devised without departing from the basic scope thereof, which may be determined by the claims that follow. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.
This application claims priority to U.S. provisional patent application Ser. No. 61/099,845, filed Sep. 24, 2008, titled INTERFEROMETRIC DIRECTIONAL BALANCING, NON-PHYSICAL WAVE PREDICTION AND REMOVAL which is incorporated herein by reference.
Number | Date | Country | |
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61099845 | Sep 2008 | US |