Abstract:
Diagonal gather trace interpolation systems and methods are disclosed. In some embodiments, the method includes obtaining seismic traces from a marine seismic survey performed using typically two seismic sources fired in a flip-flop pattern. The seismic traces are processed in common field file trace groups from each cable by performing trace interpolation in opposing diagonal directions. Among other things, diagonal trace interpolation may improve azimuthal regularization. In any event, the disclosed interpolation methods maintain spatial bandwidth increased spatial resolution with increased inline and crossline sampling components. Diagonal gather trace interpolation exploits reduced input trace separation to provide improved stability and detection of a greater range of formation dip angles. After interpolation and regularization, the seismic traces may be imaged and interpreted for improved seismic exploration and monitoring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    Seismology is used for exploration, archaeological studies, and engineering projects that require geological information. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data, can provide information about the structure and distribution of rock types and their contents. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Most oil companies rely on exploration seismology to select sites in which to drill exploratory oil wells. 
         [0004]    Seismology employs artificially generated seismic waves to map subsurface structures. The seismic waves propagate from a source down into the earth and reflect from boundaries between subsurface structures. Seismic energy receivers detect and record reflected seismic waves for later analysis. Though some large-scale structures can often be perceived from a direct examination of the recorded signals, the recorded signals must be processed to remove distortion and reveal finer detail in the subsurface image. Among the available processing techniques for marine seismic surveys is a regularization technique to remove azimuthal effects on the seismic traces. As described by Michel Schonewille in “Regularization with azimuth time-shift correction”, SEG 2003, regularization eliminates survey artifacts due to the effect of azimuthal differences that arise when small navigational changes in position and direction occur. However, it has been discovered that the performance of some regularization algorithms is sensitive to the interpolation technique employed beforehand, and that existing interpolation techniques can be inadequate or expensive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    A better understanding of the various disclosed embodiments can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
           [0006]      FIG. 1  shows a side view of an illustrative marine seismic survey environment; 
           [0007]      FIG. 2  shows a top view of an illustrative marine seismic survey environment; 
           [0008]      FIG. 3  shows an illustrative seismic survey recording system; 
           [0009]      FIG. 4  shows an illustrative set of traces; 
           [0010]      FIG. 5  shows an illustrative midpoint pattern that result from flip-flop shots received by a given channel; 
           [0011]      FIG. 6  illustrates in-line and cross-line trace interpolation; 
           [0012]      FIG. 7  illustrates diagonal gather trace interpolation; 
           [0013]      FIG. 8  shows a flowchart of an illustrative seismic imaging method; and 
           [0014]      FIG. 9  shows an illustrative imaging system. 
       
    
    
       [0015]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0016]    Described herein are diagonal gather trace interpolation systems and methods that support enhanced performance of regularization algorithms. The disclosed interpolation methods provide better spatial bandwidth by increasing the inline and crossline sampling. In addition, the disclosed interpolation methods exploit reduced input trace separation leading to greater stability and the unwrapping of aliased frequencies in structurally complex zones. Smooth variation of midpoint increments are maintained within the diagonal gathers so that the effect of the acquisition footprint is minimized. The sorting of shot-ordered input data to diagonal gathers and back to shots again after trace interpolation only requires the storage in memory of a cable length along a sail line, allowing for efficient processing. 
         [0017]    Exploration seismology is routinely performed both on land and at sea. At sea, seismic survey ships deploy streamers behind the ship as shown in  FIG. 1 . Each streamer  110  trails behind the ship  100  as the ship moves forward (in the direction of arrow  102 ), and each streamer includes multiple evenly-spaced receivers  114 . Each streamer  110  may further include a programmable diverter  118  and programmable depth controllers that pull the streamer out to an operating offset distance from the ship&#39;s path (see  FIG. 2 ) and down to an operating depth ( FIG. 1 ). 
         [0018]    Streamers  110  may be up to several kilometers long, and are usually constructed in sections 25 to 100 meters in length that include groups of up to 35 or more uniformly spaced receivers. Each streamer  110  includes electrical or fiber-optic cabling for interconnecting receivers  114  and the seismic equipment on ship  100 . Data is digitized near the receivers  114  and transmitted to the ship  100  through the cabling at rates of 7 (or more) million bits of data per second. 
         [0019]    As shown in  FIG. 1 , seismic survey ship  100  also tows a source  112 . Source  112  may be an impulse source or a vibratory source. The receivers  114  used in marine seismology are commonly referred to as hydrophones, and are usually constructed using a piezoelectric transducer. Various suitable types of hydrophones are available such as disk hydrophones and cylindrical hydrophones. Source  112  and receivers  114  typically deploy below the ocean&#39;s surface  104 . Processing equipment aboard the ship controls the operation of the source and receivers and records the acquired data. 
         [0020]    Seismic surveys provide data for imaging below the ocean surface  104  and include subsurface structures such as structure  106 , which lies below the ocean floor  108 . Certain seismic characteristics of recorded seismic data are indicative of oil and/or gas reservoirs. 
         [0021]    To image the subsurface structure  106 , source  112  emits seismic waves  116  that are reflected where there are changes in acoustic impedance contrast due to subsurface structure  106  (and other subsurface structures). The reflected waves are detected by a pattern of receivers  114 . By recording the elapsed time for the seismic waves  116  to travel from source  112  to subsurface structure  106  to receivers  114 , an image of subsurface structure  106  can be obtained after appropriate data processing. 
         [0022]      FIG. 2  shows an overhead view (not to scale) of the seismic survey ship  100  towing a set of streamers  110  and two sources  112 . As the ship  100  moves forward, the sources are triggered alternately in a so-called flip-flop pattern, as discussed below. Programmable diverters are used to provide roughly even spacing between the streamers. The receivers at a given position on the streamers are associated with a common field file trace number or common channel  202 . This grouping of receivers is useful for the explanations that follow. 
         [0023]      FIG. 3  shows an illustrative seismic survey recording system having the receivers  114  coupled to a bus  302  to communicate digital signals to data recording circuitry  306  on survey ship  100 . Position information and other parameter sensors  304  are also coupled to the data recording circuitry  306  to enable the data recording circuitry to store additional information useful for interpreting the recorded data. Illustratively, such additional information may include array orientation information and velocity information. 
         [0024]    A general purpose digital data processing system  308  is shown coupled to the data recording circuitry  306 , and is further shown coupled via bus  302  to positioning devices  310  and seismic sources  112 . Processing system  308  configures the operation of recording circuitry  306 , positioning devices  310 , and seismic sources  112 . Recording circuitry  306  acquires the high speed data stream(s) from receivers  114  onto a nonvolatile storage medium such as a storage array of optical or magnetic disks. Positioning devices  310  (including programmable diverters and depth controllers) control the position of receivers  114  and sources  112 . 
         [0025]    The seismic recording system of  FIG. 3  may include additional components not specifically shown here. For example, processing system  308  may include a user interface having a graphical display and a keyboard or other method of accepting user input. Each streamer  110  may have an independent bus  302  for coupling to the data recording circuitry. Processing system  308  may further include a network interface for communicating stored seismic survey data to a central computing facility having powerful computing resources for processing the seismic survey data. 
         [0026]      FIG. 4  shows illustrative seismic signals (also called “traces”) detected and sampled by receivers  114 . The signals indicate some measure of seismic wave energy as a function of time (e.g., displacement, velocity, acceleration, pressure), and they are digitized at high resolution (e.g., 24 bits) at a programmable sampling rate. Such signals can be grouped in different ways, and when so grouped, they are called a “gather”. For example, a “shot gather” is the group of signals recorded for a single firing of the seismic source. 
         [0027]    For quality control purposes, the averaging, or “stacking”, of traces that share a common midpoint is normally carried out to improve the signal-to-noise ratio. Such traces may have different offsets (i.e., distances between source and receiver), which implies that the seismic waves have propagated at different angles to and from the formations of interest. To compensate for the resulting differences in travel time, traces are commonly subjected to a normal move-out (“NMO”) correction that applies a changing time shift to each trace based on the offset distance. Stacking may then be performed with the NMO-corrected traces. 
         [0028]    Whether or not common midpoint stacking is employed, seismic surveys are often analyzed in terms of the resulting midpoint pattern to ensure that adequate coverage is being obtained.  FIG. 5  shows an illustrative midpoint pattern  502  resulting from four receivers on a common field file trace number or common channel  202 . The receivers are in motion, and are shown at four time instants corresponding to the source firings  504 . Sources  112  are also in motion and are being fired alternately as shown by the pattern of source firings  504 . In  FIG. 5 , each source firing produces four traces (one for each of the four receivers). The location of one midpoint is shown where ray  506  intersects the midpoint pattern  502 , and the location of a subsequent midpoint is shown by ray  508 . Note that the alternating firing pattern of sources causes the midpoints for traces measured by a given receiver to be staggered, so that the midpoints have a diamond pattern. It may be noted that similar patterns will be generated for other receiver channels, and in the overall survey pattern, those patterns will be intermixed with the pattern shown. 
         [0029]    Midpoint patterns such as pattern  502  have traces for specific midpoints. For various reasons, it may be desirable to determine traces for positions other than those already available. Such reasons may include a desire to correct for survey problems to replace missing midpoints or to adjust midpoints that have been shifted during the survey process. Another reason may be a desire for a different midpoint pattern than that provided by the survey if, for example, an imaging algorithm has specific input requirements. Yet another reason may be to increase the spatial sampling frequency of the survey. The determination of such traces may be performed by interpolation, preferably after NMO correction. See, e.g., U.S. Pat. No. 5,648,938 (Jakubowicz, Jul. 15, 1997). Various interpolation techniques exist and may be employed. Suitable interpolation techniques apply a spatial (between traces) and time-based (along the traces) filter to the existing traces to generate the interpolated traces. Suitable filters include linear, quadratic, or spline interpolation filters as well as FK and FX based filters that unwrap spatially aliased frequencies. 
         [0030]      FIG. 6  shows a midpoint pattern  602  that can be derived from the midpoint pattern  502  ( FIG. 5 ) by interpolating between existing traces in an inline direction  604  and/or in a crossline direction  606 . The resulting interpolated traces may be located at points  608  halfway between existing midpoints to obtain a rectangular grid pattern of midpoints. (It is noted that interpolating in this fashion provides predicted traces that would have resulted from firing the sources at the additional firing positions  610  for the existing moving receivers  114  and/or corresponding additional receiver locations  611 ). 
         [0031]    Interpolation in the fashion of  FIG. 6  may be unsatisfactory if the spacing of the input midpoints becomes undesirably large for accurate trace interpolation. Moreover, the midpoint pattern created after the interpolation can be very dense making it expensive to process. 
         [0032]    Accordingly,  FIG. 7  shows a preferred midpoint pattern  702  prior to regularization that can be derived from the midpoint pattern of  FIG. 5  by interpolating between existing traces in diagonal directions  704  and  706 . The resulting interpolated traces may be located at points  708  halfway between existing midpoints to obtain an open-cell diamond pattern of midpoints. (It is noted that interpolating in this fashion provides predicted traces that would have resulted from firing the sources at additional firing positions  710  and receiving the signals at additional exclusive receiver positions  712  and  714 .) 
         [0033]    The diagonal interpolation approach exploits the fact that in the diagonal direction, the input midpoint trace spacing is smaller than in the inline direction for common channel interpolation. In addition, this approach does not depend on the shot/receiver ratio being a positive integer (unlike some interpolation techniques performed in another sort domain). The reduced spacing enables more robust interpolation. In addition, the interpolated traces have both inline and crossline components that are not present in the original midpoint pattern. As mentioned previously, these additional components may prove helpful for certain regularization methods. 
         [0034]      FIG. 8  shows a flowchart of an illustrative exploration method employing diagonal gather trace interpolation. In block  802 , a computer obtains the seismic survey data. The seismic survey data, which may have been gathered as outlined previously, is generally stored in a structured form in a disk array to enable high bandwidth accessibility by a parallel processing computer system. In block  804 , the computer obtains a velocity cube, i.e., an indication of seismic velocities at each point in the region of interest. The velocity cube can be derived from the seismic survey data using known techniques. See, e.g., Jon F. Claerbout,  Fundamentals of Geophysical Data Processing , p. 246-256, which is hereby incorporated herein by reference. Alternatively, the velocity cube may be independently available from other sources, e.g., well logs. 
         [0035]    In block  806 , the computer applies a normal move-out correction to the seismic traces. As previously mentioned, the normal move-out (NMO) correction is a time-shifting operation to account for time delays attributable to offset distance between the source and receiver. In block  808 , the computer performs trace interpolation along the diagonals of the midpoint array for a common field file trace number or common channel from each cable (e.g.  202  shown in  FIG. 2 ). Such interpolation may be performed using a spatial dealiasing operator that is designed and applied within overlapping windows both in space and time. 
         [0036]    In some embodiments, diagonal gather trace interpolation is first performed to replace any traces that are missing or corrupted in the seismic survey data. (After such replacement, diagonally adjacent points correspond to signals received from different sources  112 .) Diagonal gather interpolation can be applied to a set of common channel input traces that have a complete or “filled in” midpoint pattern. The resulting midpoint array preferably includes the midpoints for the original traces and at least one additional midpoint for an interpolated trace between each original pair of diagonally adjacent midpoints. In some method embodiments, there is exactly one additional trace generated having a midpoint halfway between each such pair, while in other embodiments, there are multiple additional traces generated having midpoints evenly spaced between each such pair. 
         [0037]    In block  810 , the computer performs offset regularization so that an accurate azimuthal regularization can be subsequently carried out. Offset regularization is typically performed for each shot/cable gather although other sort domains may also be used. It is noted that after offset regularization, the coordinates of the traces will have been modified including their midpoint positions. 
         [0038]    After the offsets have been regularized, azimuthal regularization is carried out in block  812 . Such regularization may be performed as described by Michel Schonewille in “Regularization with azimuth time-shift correction”, SEG 2003. The spatial bandwidth of this type of regularization is fully maintained when preceded by diagonal trace interpolation as described above with reference to block  808 . 
         [0039]    The regularized seismic traces can be used to great benefit for subsequent processing and interpolation (blocks  813  to  816 ) as the acquisition footprint has been removed. The final imaged volume can be examined in block  816  by a user or in some cases a computer to locate hydrocarbon reservoirs or mineral deposits within the mapped region of interest. 
         [0040]    It is contemplated that the operations shown in  FIG. 8  may be implemented in the form of software, which can be stored in computer memory, in long-term storage media, and/or in portable information storage media. It should be noted that illustrative method of  FIG. 8  is provided as an explanatory aid. In practice, the various operations shown in  FIG. 8  may be performed in different orders and are not necessarily sequential. For example, seismic data processing can benefit substantially from parallelism. In some processing method embodiments, data from different survey regions may be processed independently. In other embodiments, the operations may be “pipelined” so that individual trace data experiences the operations in the sequence shown, even though the operations are all occurring concurrently. Even for individual traces, the operations are not necessarily performed in the order shown. Additional operations may be added to the illustrative method and/or several of the operations shown may be omitted. 
         [0041]      FIG. 9  shows an illustrative computer system  900  for performing seismic data processing including diagonal trace interpolation. A personal workstation  902  is coupled via a local area network (LAN)  904  to one or more multi-processor computers  906 , which are in turn coupled via a storage area network (SAN)  908  to one or more shared storage units  910 . Personal workstation  902  serves as a user interface to the processing system, enabling a user to load survey data into the system, to retrieve image data from the system, and to configure and monitor the operation of the processing system. Personal workstation  902  may take the form of a desktop computer with a graphical display that graphically shows survey data and 3D images of the survey region, and with a keyboard that enables the user to move files and execute processing software. 
         [0042]    LAN  904  provides high-speed communication between multi-processor computers  906  and with personal workstation  902 . The LAN  904  may take the form of an Ethernet network. 
         [0043]    Multi-processor computer(s)  906  provide parallel processing capability to enable suitably prompt conversion of seismic trace signals into a survey region image. Each computer  906  includes multiple processors  912 , distributed memory  914 , an internal bus  916 , a SAN interface  918 , and a LAN interface  920 . Each processor  912  operates on an allocated portion of the input data to produce a partial image of the seismic survey region. Associated with each processor  912  is a distributed memory module  914  that stores conversion software and a working data set for the processor&#39;s use. Internal bus  916  provides inter-processor communication and communication to the SAN or LAN networks via the corresponding interfaces  918 ,  920 . Communication between processors in different computers  906  can be provided by LAN  904 . 
         [0044]    SAN  908  provides high-speed access to shared storage devices  910 . The SAN  908  may take the form of a Fibrechannel or Infiniband network. Shared storage units  910  may be large, stand-alone information storage units that employ magnetic disk media for nonvolatile data storage. To improve data access speed and reliability, the shared storage units  910  may be configured as a redundant disk array. 
         [0045]    Shared storage units  910  initially store a velocity cube and seismic traces. Each processor  912  retrieves a group of seismic traces and the velocity cube from shared storage units  910 . For NMO correction, diagonal trace interpolation, and regularization operations (blocks  806 - 812 ), each processor may retrieve seismic traces in common field file trace or channel groups. The common channel groups may be divided into (slightly overlapping) spatial regions, with the processing for blocks  806 - 812  being performed within each region and stored back to shared storage  910  before the next region is processed. Thereafter, the traces can be sorted back to shot and/or CMP gathers for QC purposes and further processing. In response to a request from the workstation  902 , the map data can be retrieved by computers  906  and supplied to workstation for conversion to a graphical image to be displayed to a user. 
         [0046]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.