Patent Publication Number: US-10310111-B2

Title: Wave-fields separation for seismic recorders distributed at non-flat recording surfaces

Description:
BACKGROUND 
     Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for separating up-going and down-going wave fields from seismic data recorded underwater or under the surface of the earth by a seismic receiver. 
     Discussion of the Background 
     Offshore and onshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys to more accurately decide where to drill in order to avoid a well with no or non-commercial quantities of hydrocarbons. 
     Marine and land seismic data acquisition and processing generate an image of the geophysical structure (subsurface). While this image/profile does not provide a precise location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas. 
     For example, marine systems for the recording of seismic waves are based on towed streamers or on seafloor-deployed cables or nodes. An example of traditional marine system for recording seismic waves at the seafloor is illustrated in  FIG. 1  and this system is described in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, plural seismic receivers  10  are removably attached to a pedestal  12  together with a memory device  14 . Multiple such receivers are deployed on the bottom  16  of the ocean. A source vessel  18  tows a seismic source  20  that is configured to emit seismic waves  22  and  24 . Seismic waves  22  propagate downward, toward the ocean bottom  16 . After being reflected from a structure  26 , the seismic wave (primary) is recorded (as a trace) by the seismic receiver  10 , while the seismic waves  24  reflected at the water surface  28  are detected by the receivers  10  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 wave  24  travels back toward the receiver  10 . This reflected wave is traditionally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the receivers  10 , but with a different polarization and a time lag relative to the primary wave  22 . As the primary wave  22  moves in an upward direction toward the receiver  10 , this wave is sometimes called an up-going wave-field, and as the ghost  24  moves in a downward direction toward the receiver  10 , this wave is sometimes called a down-going wave-field. 
       FIG. 1  also shows the receiver  10  being configured to detach from the pedestal  12  and to rise to the water surface  28  to be retrieved by a collecting boat  30 . Based on the data collected by the receiver  10 , an image of the subsurface is generated by further analyses. 
     As discussed above, every arrival of a marine seismic wave at receiver  10  is accompanied by a ghost reflection. The same applies for every arrival of a land seismic wave recorded by a buried receiver. 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. 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. 
     The time delay between an event and its ghost depends entirely upon the depth of the receiver  10  and the wave velocity in water (this can be measured and is considered to be approximately 1500 m/s). 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. 
     Such advanced processing techniques include wave-field separation or wave-field decomposition or deghosting. These techniques require advanced data acquisition, i.e., multi-component marine acquisition. Multi-component marine acquisition uses receivers that are capable of measuring at least two different parameters, for example, water pressure (recorded with a hydrophone) and water particle acceleration or velocity (recorded with a geophone or accelerometer). Thus, multi-component marine acquisitions deliver, besides a pressure recording P, at least a vertical particle velocity (or acceleration) component Z. 
     A sensitive data-processing step for marine multi-component recordings is pre-stack wave-field separation. Wave-field separation allows the separation of the recorded wave-field into its individual parts: up-going and down-going waves. Various techniques are known in the field for wave-field separation, e.g., Amundsen, 1993 , Wavenumber - based filtering of marine point source data , Geophysics; or Ball and Corrigan, 1996 , Dual - sensor summation of noisy ocean - bottom data , SEG Ann. Mtg.; or Schalkwijk et al., 2003 , Adaptive decomposition of multi - component ocean - bottom seismic data into downgoing and upgoing P and S waves , Geophysics, the entire contents of which are incorporated herein by reference. 
     Regardless of the type of separation and of the details of the algorithm used, current separation algorithms assume that the recording surface is a planar surface. However, the ocean bottom is a non-planar acquisition surface. Alternatively, the towed-streamer depth may vary along its length, or buried receivers may be deployed at variable depth. Thus, for these situations, the planar surface assumption fails, and the collected data may generate spurious effects in the final image unless it is corrected. 
     Accordingly, it would be desirable to provide systems and methods that avoid the aforedescribed problems and drawbacks, e.g., take into account the non-flat acquisition surface. 
     SUMMARY 
     According to an exemplary embodiment, there is a method for separating up-going and down-going wave fields (U, D) in seismic data related to a subsurface of a body of water, or to a subsurface of a body of rock. The method includes a step of receiving seismic data (P o , Z o ) recorded in the time-space domain with seismic recorders distributed on a first datum, wherein the first datum is non-flat; a step of establishing a mathematical relation between transformed seismic data (P, Z) and the up-going and down-going wave fields (U, D) on a second planar datum; and a step of solving with an inversion procedure, run on a processor, the mathematical relation to obtain the up-going and down-going wave fields (U, D) for the second datum. The second datum is different from the first datum. 
     According to another exemplary embodiment, there is a computing device for separating up-going and down-going wave fields (U, D) in seismic data related to a subsurface of a body of water or to a subsurface of a body of rock. The computing device includes an interface configured to receive seismic data (P o , Z o ) recorded in the time-space domain with seismic recorders distributed on a first datum, wherein the first datum is non-flat; and a processor connected to the interface. The processor is configured to receive a mathematical relation between transformed seismic data (P, Z) and the up-going and down-going wave fields (U, D) on a second planar datum, and solve with an inversion procedure the mathematical relation to obtain the up-going and down-going wave fields (U, D) for the second datum. The second datum is different from the first datum. 
     According to still another exemplary embodiment, there is a computer readable medium including computer executable instructions, wherein the instructions, when executed by a processor, implement instructions for separating up-going and down-going wave fields (U, D) in seismic data related to a subsurface of a body of water or to a subsurface of a body of rock. The instructions include receiving seismic data (P o , Z o ) recorded in the time-space domain with seismic recorders distributed on a first datum, wherein the first datum is non-flat; establishing a mathematical relation between transformed seismic data (P, Z) and the up-going and down-going wave fields (U, D) on a second planar datum; and solving with an inversion procedure, run on a processor, the mathematical relation to obtain the up-going and down-going wave fields (U, D) for the second datum. The second datum is different from the first datum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram of a conventional seismic data acquisition system having plural seismic receivers provided at the ocean bottom; 
         FIG. 2  is a schematic diagram illustrating plural seismic receivers provided on a non-flat datum and a flat datum at which up- and down-going wave-fields are calculated according to an exemplary embodiment; 
         FIG. 3  is a flowchart illustrating a method for separating up- and down-going wave-fields according to an exemplary embodiment; 
         FIGS. 4 and 5  are graphs illustrating synthetic P and Z components according to an exemplary embodiment; 
         FIGS. 6 and 7  are graphs illustrating up- and down-going wave-fields separated according to an exemplary embodiment; 
         FIG. 8  is a flowchart of another method for separating up- and down-going wave-fields according to an exemplary embodiment; and 
         FIG. 9  is a schematic diagram of an apparatus configured to run a separation method according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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 deghosting algorithms for separating up-going and down-going wave-fields that are recorded by plural seismic receivers provided on the ocean bottom at different depths relative to the surface of the water. However, the embodiments to be discussed next are not limited to receivers placed on the ocean bottom but may also be applied to streamers that have the receiver placed at different depths or to receivers that are buried in land below the earth&#39;s surface at different depths. 
     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, a novel method for separating up- and down-going components includes a first step (i) of determining equations relating the desired separation results on a planar acquisition datum to the available multi-component recordings. Such equations can be formulated in the f-k (frequency wave-number) domain, in the tau-p domain, or other equivalent domains, or a combination of these domains. These equations involve wave-field extrapolation terms (to be discussed later) and are a function of the corresponding media properties (e.g., sound velocity in water or in rock layers). 
     The novel method further includes a step (ii) of inverting the equations from step (i) to find the desired separation results as a function of the available recordings. This inversion step can be carried out using a variety of algorithms, for example, analytically or by means of a least-squares inversion. It is noted that the amount of seismic data that is used with the equations and the inversion process require specialized computer software to be implemented on a computing device. As the volume of seismic data that needs to be processed for separating the up-going and down-going components is large, it is impractical, if not impossible, for a human being to do all these calculations in his or her mind. 
     One example of a non-flat and non-planar acquisition surface is an ocean-bottom acquisition system in which the nodes are deployed on a seabed with a rough topography.  FIG. 2  illustrates a non-flat ocean bottom  40  on which plural receivers  42 - 50  have been distributed. A body of water  52  is located above the receivers and has a water/air interface  54 . A structure  56  is buried below the ocean bottom  40  and it is desired to be imaged with the novel method. Another example of a non-flat acquisition surface is the situation when towed-streamer depths vary along their lengths, or when receivers are buried beneath the earth&#39;s surface at variable depths. 
     Considering that each receiver  42 - 50  is configured to record a water pressure P and a particle velocity Z along a z-axis, a possible example of equations (mathematical relation) relating (i) the up-going U and down-going D waves at a planar (and also flat) datum  60  and (ii) the recorded and transformed P and Z seismic data on a non-flat datum  62  is given by: 
     
       
         
           
             
               
                 
                   
                     
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     It is noted that the original seismic data P o  and Z o  is a function of the position x i  and z i  of each receiver  42 - 50  and also a time t at which the data is recorded. For simplicity, the y i  component is considered to be zero in the example shown in  FIG. 2 . However, the method is applicable for all three spatial components of the position of the receivers. Further, it is noted that the method is also applicable to a situation in which the seismic data is recorded with only one-component receivers, i.e., P o  or Z o  or other component. In this situation, an equation relating the recorded component to the up-going wavefield on a flat datum can be derived and inverted. The inversion result might, in this case, contain noise due to the presence of receiver ghost notches. However, this noise can be effectively reduced or removed by the process of stacking or using traditional noise attenuation and signal enhancement techniques. 
     The original seismic data P o  and Z o  is transformed (from the time-space domain), in this example, with a temporal Fast Fourier transformation (FFT) so that the time component t is now an angular frequency component ω. Thus, the P and Z components in equations (1) and (2) are the temporal FFT of P o  and Z o . Further, it is noted that the U and D components in equations (1) and (2) are written in the f-k domain (with f being the frequency corresponding to the angular frequency ω, k being the horizontal wave-number and k z  being the vertical wave-number) and these are the up-going and down-going wave-fields desired to be calculated. As noted above, the f-k domain is one possible transformation. Other transformation or transformations may be used. 
     The vertical distance Δz i  in equations (1) and (2) is the depth difference between the planar datum  60  and the non-planar datum  62  at receiver i. The density of the water is represented by ρ, and N k  is a normalization factor related to the number of receivers at the ocean bottom. The terms e j2πkx     i    present in both equations (1) and (2) are related to a spatial inverse FFT that transforms the wave-numbers k to the spatial coordinates of the sensors. 
     Wave-field extrapolators for the up-going and down-going wave-fields are also present in equations (1) and (2). In fact, the wave-field extrapolators can be found in the equations relating the P and Z components to the U and D components irrespective of the transformation domain employed. For the present exemplary embodiment, the wave-field extrapolators are given by e ±jk     z     Δz     i   . The wave-field extrapolators have opposite signs for the U and D components, and they depend from the vertical wave-number and the depth difference between the planar datum  60  and the non-planar datum  62  at receiver i. The wavefield extrapolators in this example apply to acoustic propagation with a constant velocity. The extension to the situation of variable velocity is well known in the field of seismic data processing. It is noted that the planar datum  60  at which the U and D fields are calculated can be above or below one or more of the receivers  42 - 62 . The embodiment shown in  FIG. 2  illustrates the planar datum  60  above the receivers. Further, it is possible to have the planar datum  60  to have a flat shape. Furthermore, it is possible that the planar datum  60  is above but close to the receivers  42 - 62 . 
     It is noted that equations (1) and (2) are linear in U and D and, thus, the equations can be inverted using a variety of known algorithms. The details of these algorithms are omitted herein. The novel process discussed above may be implemented in a computing device that is provided with dedicated software for separating the up- and down-going components. The computing device is discussed later with regard to  FIG. 9 . 
     The novel process is now illustrated based on the flowchart of  FIG. 3 . In step  300 , seismic data (at least two components are recorded, e.g., P and Z) is recorded with corresponding seismic sensors that are provided on the bottom of the ocean. The seismic data is transformed in a desired first domain in step  302 . For example, the first domain may be the space-frequency domain. In step  304 , equations relating (1) the seismic data transformed in the first domain to (2) up- and down-going wave-fields in a second domain are established. The second domain is different from the first domain and may be, for example, the f-k domain. The up- and down-going wave-fields correspond to a desired planar acquisition datum, while the transformed seismic data corresponds to a non-flat datum. Other domains for the first and second domains are possible. 
     The equations are inverted in step  306  to find the desired separation results as a function of the available recordings. Then, after various processing steps which are known in the art and not repeated herein, an image of the surveyed subsurface is generated in step  308  based on the separated U and/or D. 
     The method noted above is now applied to a set of synthetic P and Z data. The synthetic P data is illustrated in  FIG. 4 , and the synthetic Z data is illustrated in  FIG. 5 . The data is generated as being recorded with a certain offset (distance along X axis) from the source and at a time t (on Y axis) from a non-flat acquisition datum. The data shown in  FIGS. 4 and 5  is calculated, for example, via acoustic modeling over a half-space and, therefore, it includes only direct arrivals. In this respect, it should be noted that the direct arrival is the most difficult type of event to separate, because its propagation angles are normally wider than for other events. After establishing the equations noted in step  304  in  FIG. 3 , and solving the up- and down-going components U and D as noted in step  306 , the U and D components are illustrated in  FIGS. 6 and 7 , respectively.  FIG. 7  shows that the down-going wave-field is symmetric, while  FIG. 6  shows that the up-going wave-field is complex due to the reflection at the non-flat datum (i.e., ocean bottom). 
     A method for separating up-going and down-going wave-fields (U, D) from seismic data related to a subsurface of a body of water or rock is now discussed with reference to  FIG. 8 . The method includes a step  800  of receiving seismic data (P o , Z o ) recorded in the time-space domain with seismic recorders distributed on a first datum, wherein the first datum is non-flat; a step  802  of establishing a mathematical relation between transformed seismic data (P, Z) and the up-going and down-going wave-fields (U, D) on a second plane datum; and a step  804  of solving with an inversion procedure, run on a processor, the mathematical relation to obtain the up-going and down-going wave-fields (U, D) for the second datum. The second datum is different from the first datum. 
     An example of a representative computer system capable of carrying out operations in accordance with the exemplary embodiments discussed above is illustrated in  FIG. 9 . Hardware, firmware, software or a combination thereof may be used to perform the various steps and operations described herein. 
     The exemplary computer system  900  suitable for performing the activities described in the exemplary embodiments may include a server  901 . Such a server  901  may include a central processor unit (CPU)  902  coupled to a random access memory (RAM)  904  and to a read-only memory (ROM)  906 . The ROM  906  may also be other types of storage media to store programs, such as programmable ROM (PROM), erasable PROM (EPROM), etc. The processor  902  may communicate with other internal and external components through input/output (I/O) circuitry  908  and bussing  910 , to provide control signals and the like. The processor  902  carries out a variety of functions as are known in the art, as dictated by software and/or firmware instructions. 
     The server  901  may also include one or more data storage devices, including hard disk drives  912 , CD-ROM drives  914 , 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  916 , removable media  918  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  914 , the drive  912 , etc. The server  901  may be coupled to a display  920 , 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  922  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  901  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  928 . 
     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 as 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.