Abstract:
A system for filtering elevation-sensitive data. The system generates coincident contour and attribute maps of a physical structure. A filter band that is limited to a certain range of contour values is used to filter the attribute values that are output to a CBA attribute map.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to the field of electronic data filtering. In another aspect the invention concerns a system for filtering contour-sensitive seismic data.  
           [0003]    2. Discussion of Prior Art  
           [0004]    The detection of hydrocarbon contacts, zones of diagenetic cementation, and anomalous pressure are common problems in oil and gas exploration and production. Seismic data can often yield insights as to where these geologic features occur in time or depth. However, one problem with using seismic data to detect hydrocarbon contact, zones of diagenetic cementation, and anomalous pressure is that often seismic attributes, such as amplitude, interval velocity, and AVO class, are localized and highly variable due to lateral lithology changes and seismic noise.  
           [0005]    Typically seismic attribute maps are presented as 2-D flat images or may be draped on the structural surface along which they were extracted. Typically, the filtering of data used to generate seismic attribute maps is done for a user specified area, without regard for time or depth. Such filtering without regard for time or depth can cause smearing of elevation-sensitive (i.e., contour-sensitive) data across structural (contour) boundaries.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0006]    It is, therefore, and object of the present invention to provide a data filtering system for contour-sensitive data that minimizes data smearing along structural (contour) boundaries.  
           [0007]    Other objects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments, claims, and drawing figures.  
           [0008]    In accordance with one embodiment of the present invention, there is provided a method of filtering contour-sensitive attribute data comprising the steps of: (a) generating a contour map of a physical structure; (b) generating an attribute map of the physical structure, with the attribute map being substantially coincident with the contour map; (c) defining a contour filter band that is limited to a certain range of contour values; and (d) filtering the attribute map using only attribute values that are located within the contour filter band.  
           [0009]    In accordance with a further embodiment of the present invention, there is provided a method of filtering contour-sensitive seismic data comprising the steps of: (a) generating a contour map of a subterranean structure; (b) generating a seismic attribute map of the subterranean structure, with the attribute map being defined by a plurality of attribute data points that are substantially coincident with the contour map; (c) selecting a reference point from the attribute data points; (d) defining a filter zone around the reference point; (e) defining a contour filter band within the filter zone, with the contour filter band being limited to a certain range of contour values; and (f) calculating a filtered attribute value for the reference point based on the values of all attribute data points located within the contour filter band.  
           [0010]    In accordance with another embodiment of the present invention, there is provided a computer-implemented method of filtering contour-sensitive seismic data comprising the steps of: (a) generating a topographical map of a subterranean structure, with the map being defined by a plurality of seismic data points each having a unique position relative to orthogonal X, Y, and Z coordinate axes, the X and Y axes having units of distance, the Z axis having units of contour value; (b) assigning a seismic attribute value to each seismic data point; (c) selecting a reference point from the seismic data points; (d) defining a filter zone around the reference point, with the filter zone being defined along the X and Y axes; (e) defining a contour filter band for the seismic data points located within the filter zone, with the contour filter band being defined along the Z axis; and (f) calculating a filtered attribute value for the reference point based on all seismic data points located within the contour filter band.  
           [0011]    In still another embodiment of the present invention, there is provided a program storage device readable by a computer. The device tangibly embodies a program of instructions executable by the computer for filtering contour-sensitive seismic data. The program of instructions comprises the steps of: (a) generating a contour map of a subterranean structure; (b) generating a seismic attribute map of the subterranean structure, with the attribute map being defined by a plurality of attribute data points that are substantially coincident with the contour map; (c) selecting a reference point from the attribute data points; (d) defining a filter zone around the reference point; (e) defining a contour filter band within the filter zone, with the contour filter band being limited to a certain range of contour values; and (f) calculating a filtered attribute value for the reference point based on the values of all attribute data points located within the contour filter band.  
           [0012]    In yet another embodiment of the present invention, there is provided an apparatus for filtering contour-sensitive seismic data. The apparatus comprises a computer programmed to carry out the following method steps: (a) generating a contour map of a subterranean structure; (b) generating a seismic attribute map of the subterranean structure, with the attribute map being defined by a plurality of attribute data points that are substantially coincident with the contour map; (c) selecting a reference point from the attribute data points; (d) defining a filter zone around the reference point; (e) defining a contour filter band within the filter zone, with the contour filter band being limited to a certain range of contour values; and (f) calculating a filtered attribute value for the reference point based on the values of all attribute data points located within the contour filter band. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0013]    The preferred embodiment of the invention is described in detail below with reference to the attached drawing figures, wherein:  
         [0014]    [0014]FIG. 1 is a computer flow chart outlining the inventive steps for filtering contour-sensitive seismic attribute data using coincident seismic contour and attribute maps;  
         [0015]    [0015]FIG. 2 is a computer generated contour map showing the time structure of a subterranean formation with the X and Y axes being in units of distance and the Z axis being in units of time;  
         [0016]    [0016]FIG. 3 is a computer generated attribute map that is coincident with (i.e., draped onto) the contour map shown in FIG. 2, particularly illustrating the BNA amplitude of the subterranean structure defined along the contour map;  
         [0017]    [0017]FIG. 4 is a computer generated contour-based attribute map generated using the system of the present invention, particularly illustrating contour-based BNA amplitude values that indicate upper and lower hydrocarbon boundaries;  
         [0018]    [0018]FIG. 5 is a simplified two-dimensional illustration showing a contour map of a physical structure;  
         [0019]    [0019]FIG. 6 is a simplified two-dimensional attribute map of the physical structure represented by the contour map in FIG. 5;  
         [0020]    [0020]FIG. 7 shows the contour and attribute maps of FIGS. 5 and 6 overlain on one another, with a reference point being selected and a filter zone being defined around the reference point;  
         [0021]    [0021]FIG. 8 is an enlarged view of the filter zone shown in FIG. 7, particularly illustrating that a filter band can be defined within the filter zone along a set range of contour values;  
         [0022]    [0022]FIG. 9 a  is a computer generated time structure contour map;  
         [0023]    [0023]FIG. 9 b  is a computer generated amplitude map corresponding to the contour map shown in FIG. 9 a;    
         [0024]    [0024]FIG. 9 c  is a computer generated contour-based amplitude map generated in accordance with the present invention;  
         [0025]    [0025]FIG. 10 a  is a computer generated time structure contour map;  
         [0026]    [0026]FIG. 10 b  is a computer generated amplitude map corresponding to the contour map shown in FIG. 10 a;    
         [0027]    [0027]FIG. 10 c  is a computer generated contour-based amplitude map generated in accordance with the present invention;  
         [0028]    [0028]FIG. 11 a  is a computer generated map showing the far amplitude of a subterranean structure; and  
         [0029]    [0029]FIG. 11 b  is a computer generated map of far amplitude (shown in FIG. 11 a ) minus contour-based far amplitude, particularly illustrating the manner in which the differenced amplitude map enhances fault detection. 
     
    
       [0030]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fees.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Referring initially to FIG. 1, in step  20 , a seismic contour map of a subterranean formation is generated in accordance with standard practice. Generation of the contour map can be performed manually, but it is preferred for step  20  to be a computer implemented process. Referring to FIG. 2, a computer generated time structure contour map is illustrated in relation to orthogonal X, Y, and Z coordinate axes. The X and Y axes of the contour map in FIG. 2 represent units of distance, while the Z axis represents units of time. Although FIG. 2 illustrates a time structure contour map, it should be understood that the contour map generated in step  20  (FIG. 1) can be selected from a variety of contour maps having X and Y axis units of distance and varying units for the Z axis. Examples of suitable contour maps that can be generated in step  20  of the present invention include time structure maps, depth structure maps, isochron maps, and isopach maps.  
         [0032]    Referring again to FIG. 1, in step  22 , a seismic attribute map coincident with the seismic contour map is generated in accordance with standard practice. FIG. 3 shows a Background Normalized Amplitude (BNA) seismic attribute map that is essentially BNA amplitude values draped onto the time structure contour map illustrated in FIG. 2. Thus, the attribute map illustrated in FIG. 3 and the contour map illustrated in FIG. 2. are substantially coincident with one another. Each data point defining the attribute map in FIG. 3 has a unique contour value based on its position relative to the Z axis.  
         [0033]    Referring to FIG. 5, a simplified two-dimensional contour map of a physical structure is provided to illustrate contour lines (labeled with contour values 100 through 600) of the physical structure. The contour map of FIG. 5 represents a view that is perpendicular to the X-Y plane, with the contour lines illustrating the position of the physical structure relative to the Z axis (defined by the range of contour values). Referring now to FIG. 6, a simplified two-dimensional attribute map of the same physical structure illustrated by the contour map in FIG. 5 is shown, with dashed lines (labeled with attribute values 10 through 60) showing the varying attribute values at locations coincident with the contour map illustrated in FIG. 5. When the attribute map (shown in FIG. 6) is a seismic map of a subsurface structure, the attribute values may be, for example, amplitude, internal velocity, AVO class, or instantaneous frequency. FIG. 7 illustrates the coincident attribute and contour maps from FIGS. 5 and 6 being overlain on one another.  
         [0034]    Referring again to FIG. 1, in step  24 , a reference point is selected from the attribute data points that define the attribute map. In step  26 , a spatial filter zone around the reference point is defined. As shown in FIG. 7, the filter zone is generally an area defined relative to the X and Y axes. For example, if the referenced point is located at x=200 and y=500, the boundaries of the filter zone may be defined by x=200+/−20 and y=500+/−15. Although the filter zone illustrated in FIG. 7 has a generally rectangular shape, it should be understood that the filter zone can have any of a variety of shapes including a square or circular shape.  
         [0035]    Referring again to FIG. 1, in step  28 , a contour-based filter band is defined within the filter zone by setting a range of contour values around the contour value at the reference point location. FIG. 8 shows a filter band defined by the contour value at the reference point (i.e., contour value =200) plus and minus a contour value of 20. FIG. 8 also shows that a plurality of attribute data points land within the filter band. Preferably, at least 5 attribute data points land within the filter band. More preferably, at least 10 attribute data points land within the filter band. In step  30  (FIG. 1), a filtered attribute value is computed based on all the attribute values located within the filter band. Thus, only attribute data points having contour values falling within a certain range of the contour value of the reference points are used to compute the filtered attribute value to be associate with the reference point. The manner in which the filtered attribute value is computed can vary greatly. For example, the filtered attribute value can simply be an average of all the attribute values for attribute data points located within the filter band. However, the filtered attribute value can also be computed as a mode, median, or some other type of a statistical representation of the attribute values within the filter band.  
         [0036]    In step  32  (FIG. 1), the filtered attribute value computed in step  30  can be written to a contour-based attribute map at a location corresponding to the location of the reference point. In step  34 , the computer asks whether or not more attribute data points in the attribute map need to be filtered. If not all attribute data points in the attribute map have been filtered, the reference point is set to the next attribute data point in step  36 . Steps  26  through  32  can then be performed using the next attribute data point as the reference point. Once all the attribute data points from the attribute map have been filtered and written to the corresponding contour-based attribute map, the contour-based attribute map can be viewed in step  38  using conventional seismic viewing tools. FIG. 4 shows an exemplary contour-based attribute map computed from the contour and attribute maps illustrated in FIGS. 2 and 3. The color bands of FIG. 4 represent ten millisecond contours and the bright red zones are coincident with areas of wavelet tuning associated with hydrocarbon fill. The upper and lower white bands represent the upper and lower hydrocarbon limits.  
         [0037]    Referring to FIGS. 9 a - c , a contour-based attribute map (shown in FIG. 9 c ) can be generated from the contour map (shown in FIG. 9 a ) and the attribute maps (shown in FIG. 9 b ) using the inventive method outlined in FIG. 1. The resulting contour-based attribute map (shown in FIG. 9 c ) is a more accurate indicator of hydrocarbon rich locations than conventional filtered amplitude maps.  
         [0038]    Referring now to FIGS. 10 a - c , a contour-based attribute map (shown in FIG. 10 c ) can be generated using the contour map (shown in FIG. 10 a ) and the attribute map (shown in FIG. 10 b ). FIG. 10 b  shows that the down dip amplitude is variable near the flat clipping plane (black) as the red-yellow boundary appears to move up and down along the field&#39;s edge. The contour-based attribute filtered amplitude in FIG. 10 c  shows a more consistent red color near the flat clipping plane. However, the amplitude cut-off is still not exactly coincident with the structure, which in this case, is consistent with a tilted water contact seen in area wells.  
         [0039]    Referring now to FIGS. 11 a  and  11   b , the contour-based attribute map can be subtracted from the unfiltered attribute map to aid in fault or edge detection. FIG. 11 a  shows the unfiltered far amplitude map. FIG. 11 b  shows the map generated by differencing the contour-based attribute far offset amplitude map and the unfiltered far amplitude map. It can be seen that fault trends (shown in red on FIG. 11 b ) are accentuated by differencing the contour-based map and the original attribute map.  
         [0040]    The present invention finds application in a variety of areas of seismic data interpretation. In particular, the present invention can be especially helpful in identifying hydrocarbon contact locations, zones of diagenetic cementation, zones of anomalous pressure, and fault locations. Although the present invention has been described herein primarily with reference to the filtering of contour-sensitive seismic data, it should be understood that the invention may find application in a variety of areas where contour-sensitive data requires filtering.  
         [0041]    The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.  
         [0042]    The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.