Patent Application
20130201792
This disclosure relates generally to geophysical exploration systems, and more particularly to methods of processing seismic data obtained in geophysical surveys.
Petrochemical products such as oil and gas are ubiquitous in society and can be found in everything from gasoline to children's toys. Because of this, the demand for oil and gas remains high. In order to meet this high demand, it is important to locate oil and gas reserves in the Earth. Scientists and engineers conduct “surveys” utilizing, among other things, seismic and other wave exploration techniques to find oil and gas reservoirs within the Earth. These seismic exploration techniques often include controlling the emission of seismic energy into the Earth with a seismic source of energy (e.g., dynamite, air guns, vibrators, etc.), and monitoring the Earth's response to the seismic source with one or more receivers in order to create an image of the subsurface of the Earth. Each receiver may include, for example, a pressure sensor and a particle motion sensor in proximity to one another. The pressure sensor may be, for example, a hydrophone that records scalar pressure measurements of a seismic wavefield. The particle motion sensor may be, for example, a three-component geophone that records vectorial velocity measurements of the seismic wavefield. By observing the reflected seismic wavefield detected by the receiver(s) during the survey, the geophysical data pertaining to reflected signals may be acquired and these signals may be used to form an image indicating the composition of the Earth near the survey location.
In marine-based acquisitions, the receiver(s) may measure the seismic wavefield after it is reflected from the sub-surface of the earth. The reflection from the sub-surface may, however continue upwards to the surface of the water, where it may again be reflected by the boundary between the water and the air above the water. Because the water-air boundary is a near perfect reflector, the seismic wavefield reflected from the water-air boundary may have a reflection coefficient of minus one and its propagation direction may change so that it propagates back towards the sub-surface. The downwardly reflecting seismic wavefield is commonly known as a “ghost.” In some cases, the ghost may again reflect off of the sub-surface, and again reflect off of the water-air boundary, thus creating multiple reflections which may be referred to as surface multiples. Similarly, in land-based seismic acquisition, the recorded seismic wavefield may include surface related multiples, that are similar to the ghost and surface multiples encountered in marine-based applications.
Because the ghosts and/or surface multiples may distort or otherwise detract from the upgoing primary seismic wave of interest, it is common to “deghost” the wavefield in order to obtain a more accurate subsurface image from the measured seismic wavefield. This can be done, for example, by combining a pressure measurement from a hydrophone with one component (e.g., the vertical component) of a particle motion measurement from a geophone in order to separate the upgoing and downgoing components of the measured seismic wavefield. This is possible because both instruments can detect the changed reflection coefficient in the downgoing reflection from the water-air boundary and the particle motion sensor (e.g., geophone) can detect the change in propagation direction for the reflected seismic wavefield. By combining the pressure and particle motion measurements from both instruments, the wavefield can be deghosted by canceling the downgoing wavefield.
Because a geophone, for example, is a vectorial instrument, however, it is sensitive to the angle of incidence of the incoming seismic wave. Therefore, in order to accurately deghost the wavefield, the vectorial measurement of particle motion from a geophone may need to be corrected in order to account for the angle of incidence of the seismic wave, prior to combining the vectorial particle motion measurement with the scalar pressure measurement.
Methods for deghosting by combining the pressure and the corrected particle motion measurements are typically accomplished by transforming the measured data to the spatial and temporal Fourier domain, correcting the particle motion signal, and combining the two signals in the spatial Fourier domain. Transformation into the spatial Fourier domain, however, generally requires regularly and densely sampled measurements, typically from many different receivers. Obtaining regularly and densely sampled data may, however, be difficult in the cross-line direction for towed-streamer acquisitions or other acquisition systems, such as a nodal application. This lack of regularly and densely sampled data may prohibit effective deghosting at many angles of incidence in the Fourier domain. In order to overcome the lack of spatially regularly and densely sampled data, the existing data is sometimes interpolated in order to give the regular, dense samples needed for transformation into the Fourier domain. Such interpolation may, however, require additional data processing and, by its very nature, may introduce errors into the measured data set.
Therefore, methods and apparatuses are needed for deghosting of seismic wavefields where regularly and densely sampled data sets may not be available.
The seismic source(s) 102 may be anything that emits seismic energy. For example the sources 102 may include one or more air guns (e.g., for use in a marine towed-streamer acquisition), one or more vibrators (e.g., vibe trucks for use on land), and so forth. In some examples, the seismic sources 102 may be naturally occurring, such as a geological disturbance, background seismic noise, or seismic activity induced by hydraulic fracturing. As illustrated in
Seismic energy emitted by the seismic sources may be detected by one or more seismic receiver(s) 103. Each seismic receiver includes one or more sensors that detect a disturbance of a medium at one or more points in time. For example, a seismic receiver 103 may include a pressure sensor such as a hydrophone in some embodiments. A hydrophone detects amplitudes of a pressure wavefield over time. Another example of a seismic receiver 103 may include a pressure gradient sensor, which detects the rate of change of a pressure wavefield over time. The pressure gradient sensor may detect the rate of change of pressure in one, two, or three directional components.
A seismic receiver may alternatively or additionally include a motion sensor, such as a geophone or an accelerometer. A motion sensor detects the motion of particles or of an elastic medium over time. A motion sensor may detect velocity, acceleration, or displacement, or some combination of these, and may do so in one, two, or three directional components. In an acoustic medium, such as water, particle motion may be proportional to the gradient of the pressure wavefield, and thus data acquired using a pressure gradient sensor may be used interchangeably with data acquired using a particle motion sensor.
In some examples, a seismic receiver 103 may be multi-component in that the receiver detects more than one type of disturbance—for example, a multi-component receiver towed in a streamer in a marine acquisition may include a hydrophone to detect pressure variations and three particle motion sensors to detect three components of motion of the water particles.
The seismic receivers 103 may be positioned proximate the seismic sources 102 during a seismic survey. During the seismic survey, one or more seismic sources 102 may be fired, and the one or more seismic receivers 103 may measure one or more disturbances and may generate one or more traces, which are sequences of measurements over a period of time. In general, each component of each sensor may generate a trace—so a multi-component receiver with a pressure sensor and three particle motion sensors may generate four traces. Each trace may include or may be associated with corresponding positional information, which may be provided by a navigation system (not shown in
The seismic traces generated by the seismic receivers 103 may be provided to the data storage 106 in some embodiments. The data storage 106 may be a local data storage 106 near the seismic receiver 103 and may record seismic traces from a single receiver 103 in some examples, or may be a bulk data storage 106 located at a central station and may record seismic traces from a plurality of different receivers 103 in other examples. The data storage 106 may include one or more tangible mediums for storing the seismic traces, such as hard drives, magnetic tapes, solid state storage, volatile and non-volatile memory, and so forth. In some examples, the seismic traces from the seismic receivers 103 may bypass the data storage 106 and be provided directly to the data processing apparatus 108 in order to at least partially process the seismic traces in real-time or substantially real-time (e.g., to provide quality control information).
The data processing apparatus 108 may be any computing apparatus that is adapted to process and manipulate the seismic traces from the seismic receivers 103, and, in some embodiments, the seismic source data from the seismic sources 102. The data processing apparatus 108 may be a single computing device, or may be distributed among many computing nodes in some examples. In some examples, different computing apparatuses perform different data processing operations. For example, a first may deghost seismic traces, and another may migrate seismic traces to obtain an image of the earth's subsurface. An image of interest may be a spatial indication of discontinuities in acoustic impedance or the elastic reflectivity of the subsurface, and may be displayed on a tangible medium, such as a computer monitor or printed on a piece of paper. While some embodiments of the data processing apparatus 108 may process the seismic traces until a migrated image is obtained, in other examples, the data processing apparatus 108 may only partially process the seismic traces—for example, the data processing apparatus may merely deghost the seismic traces, and provide the processed and deghosted seismic traces to another process flow for further processing.
In operation 210, a set of vectorial measurements of a seismic wavefield and a set of scalar measurements of the seismic wavefield are obtained at a seismic receiver, such as receiver 403 (
Each vectorial measurement in the set of vectorial measurements may include a plurality of components. The first component may be a z-component, which may represent a vertical direction. The second and third components may be x- and y-components, which may represent two orthogonal horizontal directions.
In operation 212, an incidence vector of the seismic wavefield at the seismic receiver at a particular instance of time is calculated, with the incidence vector derived from a measure of correlation of at least one vectorial measurement of the set of vectorial measurements. The measure of correlation may in some embodiments be a measure of covariance. For example, the incidence vector may be an eigenvector of a covariance matrix derived from the components of at least one vectorial measurement. The covariance matrix may be generated using only the at least one vectorial measurement, or alternatively, the covariance matrix may be generated using two or more vectorial measurements from the set of vectorial measurements. In still other embodiments, the measure of correlation such as the covariance matrix may be generated from a range of vectorial measurements in a time-window, with the angle of incidence being found by sliding the time-window along the time axis. In general, the measure of correlation may be a measure of the covariance between the measurements of the different components of a single vectorial measurement, or between a plurality of vectorial measurements (such as an average among two, three, four, or even more vectorial measurements). Deriving the measure of correlation from a plurality of vectorial measurements may reduce the effects of noise that may otherwise dominate a single vectorial measurement. The time-window, when used, may be chosen based on the dominant frequency of the seismic wave of interest, and may be in some examples between 10 milliseconds and 200 milliseconds.
In still other examples, the measure of correlation may be derived from a range of vectorial measurements in a space-window. For example, vectorial data or a measure of correlation based on the vectorial data may be averaged over several different seismic receiver stations in order to reduce noise. In still other examples, the measure of correlation may be derived from a range of vectorial measurements in both a space-window and time-window.
The eigenvector of the covariance matrix used as the incidence vector may be the largest eigenvector of the covariance matrix in some embodiments, whereas in other embodiments, the incidence vector may be determined by using the two smallest eigenvectors of the covariance matrix, with the incidence vector being normal to a plane defined by the two smallest eigenvectors of the covariance matrix. In still other embodiments, the incidence vector may be related to, but not identical to, one of the eigenvectors of the covariance matrix.
The direction of the largest eigenvector of the covariance matrix may approximately correspond to the propagation direction of a seismic wavefield at a particular instance of time at the seismic sensor from which the data is being used, particularly if the seismic wave is linearly polarized (as the case may be for many seismic events in water). In some embodiments, only the angles that are related to seismic events are used to generate the covariance matrix and calculate the eigenvectors, as opposed to the noise in between or during the seismic events. This may be accomplished, for example, by using thresholding, or using local slope estimates.
In some examples, the magnitudes of one or more eigenvectors (or other measures of correlation) may be used to help separate seismic events of interest from noise. In some embodiments, the relative magnitude of one eigenvector compared to other eigenvectors of a covariance matrix may help determine whether a particular vectorial and corresponding scalar measurement corresponds to a seismic event of interest or to noise. For example, if the magnitude of the largest eigenvector is a certain multiple (e.g., 2×, 3×, 4×, etc.) of the magnitudes of the other eigenvectors, that measurement may be deemed a seismic event of interest, whereas if the magnitude of the largest eigenvector is only slightly larger (e.g., less than 0.5×) than the magnitudes of one or both other eigenvectors, that measurement may be deemed mere noise. In still other embodiments, the absolute magnitude of the largest eigenvector may be compared with other eigenvectors as a function of time to help separate seismic events of interest from noise. For example, if the absolute magnitude of the largest eigenvector of a covariance matrix (for a given window of samples) is plotted as a function of time, large amplitudes (with respect to the other, largest eigenvector magnitudes over time) may indicate a seismic event of interest.
In operation 214, an angle of incidence of the seismic wavefield at the seismic receiver at the particular instance of time is determined from the calculated incidence vector. In some embodiments, the angle of incidence may be the angle between the calculated incidence vector and the vertical axis.
In operation 216, one component of the vectorial measurement at the particular instance of time is corrected with the angle of incidence determined in operation 214. For example, the vertical component of the vectorial measurement may be scaled as a function of the angle of incidence. The vertical component of the vectorial measurement may, for example, be scaled by dividing it by the cosine of the angle of incidence. In other embodiments, one or more of the horizontal components of the vectorial measurement may be corrected using the angle of incidence.
In some embodiments, the one component of the vectorial measurement may also be multiplied by a scalar such as the acoustic impedance of the medium. For water, the acoustic impedance is equal to the product of the density of the water (which may be denoted p) and the velocity of seismic waves in the water (which may be denoted c). Multiplying the one component of the vectorial measurement by a scalar such as the acoustic impedance of water may allow the component of the vectorial measurement to be combined with a scalar (omnidirectional) measurement in operation 218, such as a pressure measurement.
In operation 218, the corrected component of the vectorial measurement at the particular instance of time is combined with the corresponding scalar measurement for the particular instance of time. The combined scalar measurement and corrected component of the vectorial measurement may represent, for example, the deghosted, upgoing portion of the seismic wavefield, or it may represent the downgoing portion of the seismic wavefield. Whether the combined measurements represent the upgoing or the downgoing portion of the seismic wavefield, or something else altogether, may depend, for example, on how the corrected component of the vectorial measurement and scalar measurement were combined. For example, if the corrected vertical component of the vectorial measurement is subtracted from the corresponding scalar measurement, the result may be the downgoing portion of the seismic wavefield, as shown in the following equation:
where D(t) is the downgoing portion of the seismic wavefield at instance of time t, P(t) is the pressure measurement at instance of time t, and Vz(t) is the vertical component of the particle motion (e.g., velocity) measurement at instance of time t, c is the velocity of acoustic waves in water, ρ is the density of the water, and θ(t) is the angle with the vertical at instance of time t. D, P and V may all be related to the same measurement location in space.
As another example, if the corrected vertical component of the vectorial measurement is added to the corresponding scalar measurement, the result may be the deghosted upgoing portion of the seismic wavefield, as shown in the following equation:
where D(t) is the downgoing portion of the seismic wavefield at instance of time t, P(t) is the pressure measurement at instance of time t, and Vz(t) is the vertical component of the particle motion (e.g., velocity) measurement at instance of time t, c is the velocity of acoustic waves in water, ρ is the density of the water, and θ(t) is the angle with the vertical at instance of time t. D, P and V may all be related to the same measurement location in space.
Although the operations 200 illustrated in the flowchart of
Also, although the operations 200 illustrated in the flowchart of
Although the above description refers to using the covariance matrix and one or more eigenvectors of the covariance matrix, it is to be understood that the incidence vector and/or the angle of incidence of the seismic wavefield may be determined in alternate ways, such as by using a correlation matrix of the set of vectorial measurements, or by using any type of polarization filter analysis on the set of vectorial measurements, or by doing any type of statistical analysis on the set of vectorial measurements, and so forth. Furthermore, specific eigenvectors need not be used to determine the incidence vector, but rather, any method may be used to find trends in the set of vectorial measurements (and/or the pressure measurements in some embodiments) in order to estimate the propagation direction of the seismic wavefield. Furthermore, as described above, different types of sensors other than a geophone and hydrophone may be used. For example, a pressure gradient sensor may be used, in which case the covariance matrix may be generated based on the vertical gradient of the pressure (because the vertical gradient of the pressure measured by a pressure gradient sensor may be proportional to the vertical component of the particle motion measured by a particle motion sensor). In general, many different types of seismic sensors may be used, and many methods may be used to estimate the propagation direction of the seismic wavefield to scale a vectorial measurement with the angle of incidence of the seismic wavefield in order to be able to effectively deghost a seismic wavefield.
As mentioned above, the operations 200 may be carried out entirely in the space and time domain in some embodiments, and the operations 200 may be considered a single station system and method. In other embodiments, however, one or more of the operations 200 may be carried out in either the spatial or the temporal Fourier domain, or one or more of the operation 200 may be carried out in the time domain and combined with a separate Fourier domain analysis of the measured vectorial and/or scalar measurements from the one or more seismic receivers.
In operation 310, a covariance matrix of a set of measured vectorial data from a seismic receiver is generated as a function of time. Similar to operation 212, the covariance matrix may be generated from a single vectorial measurement, or from a plurality of vectorial measurements over a period of time.
In operation 312, an incidence vector of a seismic wavefield at the seismic receiver is calculated from the covariance matrix as a function of time. In some embodiments, the incidence vector may be the largest eigenvector of the covariance matrix. As in operation 212, the incidence vector may correspond to the propagation direction of the seismic wavefield as a function of time.
In operation 314, a corrected set of vectorial data is generated as a function of the calculated incidence vector and the set of measured vectorial data. For example, the corrected set of vectorial data may include only the vertical component of the vectorial data scaled by a function of the incidence vector for each instance in time.
In operation 316, the corrected set of vectorial data is combined with a set of measured scalar data from the seismic receiver.
As with the operations 200, the operations 300 illustrated in the flowchart of
Furthermore, the angle of incidence of the seismic wavefield calculated according to the operations 212, 214, 312 described herein may be used in other ways, including in other ways to deghost a seismic wavefield. As just one example of another way that the angle of incidence of the seismic wavefield may be used to deghost a seismic wavefield, the angle of incidence may be used to calculate an operator that predicts the arrival of the seismic wave (or a part of the seismic wave, such as a downgoing ghost) at one or more seismic receivers. In general, the angle of incidence of the seismic wavefield at one or more seismic receivers at one or more instances of time may be provided as input to the operator, and the operator may predict the arrival of the seismic wave or a part of the seismic wave at one or more seismic receivers (which may or may not be the same as the one or more seismic receivers whose measurements are provided to the operator to determine the angle of incidence of the seismic wavefield). In those instances where the operator uses an angle of incidence at a first seismic receiver to predict the arrival of the seismic wave or part of the seismic wave at a second seismic receiver distinct from the first seismic receiver, the operator may be a space-domain operator. In those instances where the operator uses an angle of incidence at a first instance of time to predict the arrival of the seismic wave or part of the seismic wave at a second instance of time after the first instance of time, the operator may be a time-domain operator. Furthermore, the operator may be both a time- and a space-domain operator. The operator may be used, for example, in a space and/or time deconvolution-type approach to deghosting the seismic wavefield instead of a weighted sum of pressure and motion measurements for a single instance of time.
During operation, the source 402 may emit or “fire” seismic energy (e.g., through an air gun), which may reflect off various portions of the Earth 404 and may be received back at the receivers 403. The signal received and processed at the receivers 403 may indicate the composition of various portions of the Earth 404 proximate the location where the signal was reflected and indicate an oil and/or gas reservoir 405. In some embodiments, the signal received at the receivers is transmitted to a storage medium on the vessel towing the receivers for storage. The storage medium can be part of a comprehensive data processing system or can take the form of a stand alone data logging storage device. The received and stored signal may, in some embodiments be processed by computers or servers on-board the vessel in real-time, near real-time, or in some cases may not be processed at all on-board but simply recorded for processing at a later time. Alternatively, the signals can be transmitted from the vessel to a remote location for processing.
The operations 200, 300 described above in connection with
In some embodiments, the computer system 635 may be a personal computer and/or a handheld electronic device. In other embodiments, the computer system 635 may be an implementation of enterprise level computers, such as one or more blade-type servers within an enterprise. In still other embodiments, the computer system 635 may be any type of server. The computer system 635 may be onboard a vessel (such as vessel 301 shown in
A keyboard 640 and mouse 641 may be coupled to the computer system 635 via a system bus 648. The keyboard 640 and the mouse 641, in one example, may introduce user input to the computer system 635 and communicate that user input to a processor 643. Other suitable input devices may be used in addition to, or in place of, the mouse 641 and the keyboard 640. An input/output unit 649 (I/O) coupled to the system bus 648 represents such I/O elements as a printer, audio/video (A/V) I/O, etc.
Computer 635 also may include a video memory 644, a main memory 645 and a mass storage 642, all coupled to the system bus 648 along with the keyboard 640, the mouse 641 and the processor 643. The mass storage 642 may include both fixed and removable media, such as magnetic, optical or magnetic optical storage systems and any other available mass storage technology. The bus 648 may contain, for example, address lines for addressing the video memory 644 or the main memory 645.
The system bus 648 also may include a data bus for transferring data between and among the components, such as the processor 643, the main memory 645, the video memory 644 and the mass storage 642. The video memory 644 may be a dual-ported video random access memory. One port of the video memory 644, in one example, is coupled to a video amplifier 646, which is used to drive one or more monitor(s) 647. The monitor(s) 647 may be any type of monitor suitable for displaying graphic images, such as a cathode ray tube monitor (CRT), flat panel, or liquid crystal display (LCD) monitor or any other suitable data presentation device.
The computer system includes a processor unit 643, which may be any suitable microprocessor or microcomputer. The computer system 635 also may include a communication interface 650 coupled to the bus 648. The communication interface 650 provides a two-way data communication coupling via a network link. For example, the communication interface 650 may be a satellite link, a local area network (LAN) card, a cable modem, and/or wireless interface. In any such implementation, the communication interface 650 sends and receives electrical, electromagnetic or optical signals that carry digital data representing various types of information.
Code received by the computer system 635 may be executed by the processor 643 as the code is received, and/or stored in the mass storage 642, or other non-volatile storage for later execution. In this manner, the computer system 635 may obtain program code in a variety of forms. Program code may be embodied in any form of computer program product such as a medium configured to store or transport computer readable code or data, or in which computer readable code or data may be embedded. Examples of computer program products include CD-ROM discs, ROM cards, floppy disks, magnetic tapes, computer hard drives, servers on a network, and solid state memory devices. Regardless of the actual implementation of the computer system 635, the data processing system may execute operations that allow for processing seismic data, including for example the operations illustrated in
The apparatuses and associated methods in accordance with the present disclosure have been described with reference to particular embodiments thereof in order to illustrate the principles of operation. The above description is thus by way of illustration and not by way of limitation. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. Those skilled in the art may, for example, be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles described and are thus within the spirit and scope of this disclosure.
For example, although the above description details one type of seismic signal processing that may be done in the space and time domain, this space and time domain signal processing may be combined with signal processing done in other domains, such as the spatial and/or temporal Fourier domains. The combination of processing in multiple domains may help, for example, to resolve any ambiguities when multiple events happen in proximity (in time) to one another on a single seismic sensor. In some examples, data processed in one domain may be weighted and combined with weighted data processed in another domain—for example, data processed in the space-time domain may be assigned a certain weight based on an estimate of the number of overlapping events, data processed in one or both of the temporal and spatial Fourier domains may be assigned a certain weight based on the geometry and spacing of the seismic receivers, and then the weighted data from the two different domains may be summed together. In another example, small time windows may be analyzed in both domains—for example, data obtained during a first time window may be analyzed in the space-time domain to determine an angle of incidence using the methods described herein, and the same data from the first time window may be analyzed in the spatial and/or temporal Fourier domains using the determined angle of incidence to remove a ghost signal.
Also, in some embodiments, the angle of incidence (and/or incidence vector) calculated in operations 212, 214, 312 may be smoothed prior to or contemporaneous with correcting or combining components. Also, although
Accordingly, it is intended that all such alterations, variations, and modifications of the disclosed embodiments are within the scope of this disclosure as defined by the appended claims.
In methodologies directly or indirectly set forth herein, various steps and operations are described in one possible order of operation, but those skilled in the art will recognize that the steps and operations may be rearranged, replaced, or eliminated without necessarily departing from the spirit and scope of the disclosed embodiments.
All relative and directional references (including: upper, lower, upward, downward, upgoing, downgoing, left, right, top, bottom, side, above, below, front, middle, back, vertical, horizontal, and so forth) are given by way of example to aid the reader's understanding of the particular embodiments described herein. They should not be read to be requirements or limitations, particularly as to the position, orientation, or use of the invention unless specifically set forth in the claims. Connection references (e.g., attached, coupled, connected, joined, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, connection references do not necessarily infer that two elements are directly connected and in fixed relation to each other, unless specifically set forth in the claims.
This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 61/594,535 filed Feb. 3, 2012, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61594535 | Feb 2012 | US |
