Patent Application
20120213032
1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for 3-dimensional (3D) processing seismic data to separate up-going and down-going wave fields recorded by multi-component receivers underwater.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which are especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of the features causing such reflections. These features may be associated with subterranean hydrocarbon reservoirs.
A traditional system for generating the seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in
The seismic wave emitted by the source 16 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 16. Disturbances produced by the passing reflected seismic waves 24 (primary) are recorded by the various detectors 11 (the recorded signals are called traces), while disturbances produced by reflected seismic waves 26 (reflected at the water surface 14) are detected by the detectors 11 at a later time. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic or seismic waves), the reflected waves 26 travel back toward the detector 11 as shown in
Thus, every arrival of a marine seismic wave at detector 11 is accompanied by a ghost reflection. In other words, ghost arrivals trail their primary arrival and are generated when an upward traveling wave is recorded a first time on submerged equipment before being reflected at the surface-air contact. The now downward propagating reflected wave 26 is recorded a second time at detector 11 and constitutes the ghost. Primary and ghost (receiver side ghost and not the source side ghost) signals are also commonly referred to as up-going and down-going wave fields, respectively.
The time delay between an event and its ghost depends entirely upon the depth of the receiver 11, the wave velocity in water (this can be measured and considered to be approximately 1500 m/s), and the wave incidence angle. It can be only a few milliseconds for towed streamer data (depths of less than 15 meters) or up to hundreds of milliseconds for deep Ocean Bottom Cable (OBC) and Ocean Bottom Node (OBN) acquisitions. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches or gaps in the frequency content, and these notches cannot be removed without the combined use of advanced acquisition and processing techniques.
One popular technique for separating the up-going and down-going wave fields is called PZ-summation and applies to both OBC/OBN and streamer data. Here, the seismic wave field is recorded using co-located hydrophones (P) and vertical geophones (Z). In other words, the detector 11 shown in
However, there are some limitations for the existing techniques that are now discussed. While wave field separation techniques for two components (2C) OBS data were described in the literature, Soubaras I (“Ocean bottom hydrophone and geophone processing,” SEG, Expanded Abstracts, 15, 24, 1996) proposed a 1D 3-step procedure (referred herein as PZ summation) for separating the up- and down-going components. The PZ summation first calibrated the geophone, then separated the up-going and down-going wave fields by summing the hydrophone and the calibrated geophone, and last attenuated the water-bottom peg-legs by adaptively subtracting the down-going from the up-going (considering wave fields just above the sea bottom). Later, Soubaras II (“Multiple attenuation of multicomponent ocean-bottom data according to an elastic model,” EAGE, Extended Abstracts, 1-16, 1999) extended the 1D technique to 2D.
With an arbitrary geology, only the receiver-side peg-legs are attenuated with the existing techniques. If the geology is 1D, both the source-side and the receiver-side peg-legs are attenuated because of the adaptive subtraction, but with a first-order amplitude approximation.
The extension of the 2D method to 3D gathers is possible by adapting the existing mono-channel implementations to a tau-px-py domain (using a radon transform) (see, for example, Soudani et al., “3D Methodology for OBC Pre-Processing,” EAGE, Extended Abstracts, B0144, 2006). However, the tau-px-py transforms can be demanding on the computing device that performs the calculations and prone to artifacts (e.g., due to spatial aliasing).
The remaining source-side peg-legs can, for example, be modeled using the 3D wave equation and adaptively subtracted, regardless of the geology, if a reflectivity model of the superficial layers is available (see, for example, Pica et al., “3D SRME on OBS data using waveform multiple modeling,” SEG, Expanded Abstracts, 25, no. 1, 2659-2663, 2006). The other surface multiples can finally be addressed by SRME if streamer data are available (see, for example, Ikelle L., “Combining two seismic experiments to attenuate free-surface multiples in OBC data,” Geophys. Prosp., 47, 179-193, 1999).
However, as discussed above, the classic 3D PZ summation method needs to perform calculations in the tau-px-py domain, which places a high toll on the existing computing devices. Thus, it would be desirable to provide a method that can process 3D gathers free of the tau-px-py transformations, and that can improve the source-side peg-leg attenuation.
According to an exemplary embodiment, there is a method for de- pegging seismic data related to a subsurface of a body of water. The method includes receiving as input recorded seismic data (H, G), wherein the recorded seismic data (H, G) is recorded with a receiver having at least three components; extracting a three-dimensional (3D) gather from the recorded seismic data (H, G); separating up-going and down-going components (U, D) from the 3D gather using a 3D calibration operator (Gcal); and calculating de-pegged seismic data (P) based on the up-going and down-going components (U, D). The de-pegged seismic data (P) is calculated with no radon transform.
According to another exemplary embodiment, there is a computing device for de-pegging seismic data related to a subsurface of a body of water. The device includes an interface configured to receive as input recorded seismic data (H, G), wherein the recorded seismic data (H, G) is recorded with a receiver having at least three components; and a processor connected to the interface. The processor is configured to extract a three-dimensional (3D) gather from the recorded seismic data (H, G), separate up-going and down-going components (U, D) from the 3D gather using a 3D calibration operator (Gcal), and calculate de-pegged seismic data (P) based on the up-going and down-going components (U, D). The de-pegged seismic data (P) is calculated with no radon transform.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:
The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of PZ summation algorithms for separating interfering up-going and down-going wave fields that are recorded by the same receivers. However, the embodiments to be discussed next are not limited to these dimensions, but may be extended to the X and Y directions, where the X, Y and Z directions determine a Cartesian system of reference.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
According to an exemplary embodiment, novel techniques are presented next that achieve up-down separation on OBS and/or marine streamer data. The techniques involve the recording of additional geophone (or other sensors, e.g., accelerometers) channels which measure particle velocity in a horizontal radial direction (X-component) and/or in a horizontal transverse direction (Y-component) besides the vertical direction (Z-component). The recorded 3D data is processed without the need for tau-px-py transforms. While existing methods may attenuate the source-side multiples (by relying on a first order approximation for the amplitudes and on the 1D geology assumption), by introducing a simultaneous 3D predictive deconvolution operator in the peg-leg attenuation formulation, the present embodiments remove the first order approximation, and the attenuation in the case of deviations from the 1D assumption is improved, as discussed later.
Before proceeding further with the novel embodiments, a brief discussion of a peg-leg is believed to be in order. A peg-leg may be defined as a multiply-reflected seismic energy, having, for example, an asymmetric path. The peg-leg amplitudes are added to primary reflections and they tend to come from shallow subsurface phenomena and highly cyclical deposition, and can be suppressed by seismic processing as discussed next. In some cases, the period of the peg-leg multiple is so brief that it interferes with primary reflections, and its interference causes a loss of high frequencies in the wavelet. Peg-leg multiple reflections may also be defined to be those multiples that undergo one reflection in the sedimentary sequence and other reflections in the near surface.
As the exemplary embodiments to be discussed next introduce novel deghosting techniques, synthetic data is used for illustrating the power of the new techniques. Synthetic data is defined as data generated, for example, on a computer, and it is considered to describe a possible subsurface. However, the synthetic data does not include measured data. The exemplary embodiments of deghosting techniques produce wave fields to be used for producing a final image of the subsurface. All these novel techniques are implemented in a computing device, for example, a processor, and the deghosted data is used to generate an image of the surveyed subsurface.
According to an exemplary embodiment, the up-going/down-going separation is obtained by summing/subtracting the calibrated geophone data to/from the hydrophone data. Cross-ghosting is a commonly used calibration method described in Soubaras I and II noted above. Such calibration is achieved by searching for a short matching filter g0. Considering H to be the hydrophone recorded data, G the geophone recorded data, Z a water layer propagation operator, and I the unity matrix, the cross-ghosted versions H′ and G′ of H and G, respectively, are as follows:
H′=H·(I+Z), (1) and
G′=G·(I−Z), (2)
where (I+Z) and (I−Z) are the deterministic vertical velocity and pressure ghost operators. The filter g0 is found by resolving equation:
g
0=argming∥H′−g·G′∥2. (3)
Having the filter g0, the calibrated ghost G is determined based on:
G
cal
=g
0
·G. (4)
The up- and down-going operators U and D are determined as follows:
U=H+G
cal and D=H−Gcal. (5)
The above method is now extended to 3D receiver gathers, using true 3D filters and operators, and 3D convolutions. As a consequence, the 3D calibration operator matches not only the normal incidence response, but also the directivity diagram of the geophone to the hydrophone.
The propagation operator Z in equations (1)-(5) is 3D, but it is built assuming a locally 1D geology. However, if cross-ghosting is not required (e.g., for deep-water data, and in general when using an estimation window without multiples) the method is valid for any geology.
The above equations are now extended for 3D peg-leg attenuation. In addition, the equations are extended by incorporating a 3D predictive deconvolution operator. The receiver-side peg-leg attenuation is obtained by adaptively subtracting the down-going from the up-going as illustrated in
f
0=arg minf∥U−f·D∥2, and (6)
P=U−f
0
·D, (7)
where P is the de-pegged data, and f0 is a filter that represents the reflectivity of the (possibly complex) water bottom. It is noted that the example used herein to determine the above equations assumes that the receiver 66 is part of the OBC. However, the equations may be easily adapted for the situation when the receiver is part of a streamer. The filter f0 may be 1D (Soubaras I), 2D (Soubaras II), and also 3D. In 3D, the filter f0 contains the angle-of-incidence dependent reflectivity information (in 2D or 1D it is noted that all the waves are in a single plane, while in 3D each wave may be in a different plane, thus, the need of having the angle-of-incidence information). It is noted that any receiver-side peg-leg is present on both the up-going and down-going wave fields, while primaries (or source-side peg-legs) are present only on the up-going fields.
Equations (6) and (7), while being capable of handling 3D gathers, attenuate only the receiver-side peg-legs in the case of an arbitrary underlying geology. When the geology is close to 1D, the adaptive subtraction described above attenuates both the source-side and receiver-side peg-legs by overestimating f0, but with a first order amplitude approximation (the full peg-leg attenuation requires a second order term).
Thus, according to an exemplary embodiment, a 3D predictive deconvolution operator F is introduced to specifically target the multiples. The predictive deconvolution operator F is simultaneously estimated with the reflectivity operator f as follows:
(f0, F0)=arg minf,F∥(I+F)·(U−f·D)∥2. (8)
Having determined F0 and f0, the de-pegged data P is given by:
P=(I+F0)·(U−f0·D), (9)
where F0 is the Green's function of the medium, as seen from the shot point (see, for example,
The algorithm characterized by equations (8) and (9) is now tested based on synthetic data. The synthetic data includes a 3D receiver gather including 121×121 traces, the offset on x and the offset on y ranging from −1500 m to +1500 m. Three interfaces (reflectors 44 in
In a first test illustrated by
In a second test illustrated by
A real data example is illustrated in
Thus, one or more of the exemplary embodiments discussed above has advantageously extended to 3D receiver gathers the existing methods and algorithms of Soubaras I and II, i.e., method for up-going/down-going wave field separation and peg-leg attenuation of 2C OBS data. The 3D gathers can be processed in one pass with the novel approach, using full 3D operators, and without the need of splitting the operators into individual 2D shot lines or using tau-px-py transformations. In addition, the novel approach introduces a 3D predictive deconvolution operator for the peg-leg attenuation part that results in a better removal of peg-legs under a 1D assumption.
The novel algorithm discussed above is now discussed with regard to a flow chart that is illustrated in
In step 904, the 3D peg-leg attenuation is calculated, i.e., de-pegged data P. The entire algorithm for determining the P data uses 3D operators and matrices, and there is no need for a radon transform or other transforms for dealing with 3D data. Further, a 3D predictive deconvolution operator F is introduced and used to de-peg the original data, both at a source-side and a receiver-side. This step may include plural sub-steps, as discussed with reference to equations (8) and (9).
The de-pegged data P obtained in step 904 may then be used in step 906, for example, to determine an image of the surveyed subsurface. Depending on the application and the need of the operator of the survey, the de-pegged data P may be used for other purposes.
An example of a representative computing device capable of carrying out operations in accordance with the exemplary embodiments discussed above is illustrated in
The exemplary computing device 1000 suitable for performing the activities described in the exemplary embodiments may include server 1001. Such a server 1001 may include a central processor unit (CPU) 1002 coupled to a random access memory (RAM) 1004 and to a read-only memory (ROM) 1006. The ROM 1006 may also be other types of storage media to store programs, such as programmable ROM (PROM), erasable PROM (EPROM), etc. The processor 1002 may communicate with other internal and external components through input/output (I/O) circuitry 1008 and bussing 1010, to provide control signals and the like. The processor 1002 carries out a variety of functions as are known in the art, as dictated by software and/or firmware instructions.
The server 1001 may also include one or more data storage devices, including hard disk drives 1012, CD-ROM drives 1014, and other hardware capable of reading and/or storing information such as a DVD, etc. In one embodiment, software for carrying out the above-discussed steps may be stored and distributed on a CD-ROM or DVD 1016, removable media 1018 or other form of media capable of portably storing information. These storage media may be inserted into, and read by, devices such as the CD-ROM drive 1014, the hard disk drive 1012, etc. The server 1001 may be coupled to a display 1020, which may be any type of known display or presentation screen, such as LCD or LED displays, plasma displays, cathode ray tubes (CRT), etc. A user input interface 1022 is provided, including one or more user interface mechanisms such as a mouse, keyboard, microphone, touch pad, touch screen, voice-recognition system, etc.
The server 1001 may be coupled to other computing devices via a network. The server may be part of a larger network configuration as in a global area network (GAN) such as the Internet 1028.
As also will be appreciated by one skilled in the art, the exemplary embodiments may be embodied in a wireless communication device, a telecommunication network, as a method or in a computer program product. Accordingly, the exemplary embodiments may take the form of an entirely hardware embodiment or an embodiment combining hardware and software aspects. Further, the exemplary embodiments may take the form of a computer program product stored on a computer-readable storage medium having computer-readable instructions embodied in the medium. Any suitable computer-readable medium may be utilized, including hard disks, CD-ROMs, digital versatile discs (DVD), optical storage devices, or magnetic storage devices such a floppy disk or magnetic tape. Other non-limiting examples of computer-readable media include flash-type memories or other known types of memories.
The disclosed exemplary embodiments provide an apparatus and a method for seismic data processing. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.
Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.
The present application is related to and claims the benefit of priority of U.S. Provisional Application No. 61/445,177, having the title “PZ Summation of 3D WAZ OBS Receiver Gathers,” and being authored by Hugonnet et al., the entire content of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61445177 | Feb 2011 | US |
