Abstract:
A unique method has been developed which can show the presence of fractures in an Earth formation as a mappable attribute. This method uses the frequency spectra derived from P-wave seismic data over a pair of specific time windows above and below a seismic horizon or reflector of interest to infer the presence or absence of these geologic fractures based on an attenuation of high frequencies. The method produces a parameter value (t*) which is proportional to the shift in frequency spectra amplitudes (i.e., energy) from higher frequencies to lower frequencies, that is, from a time-window above a horizon or reflector of interest to a time-window below the horizon or reflector of interest.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a utility application of prior pending provisional application Ser. No. 60/353,698 filed Jan. 31, 2002, and entitled “Detecting Fractures with frequency data derived from P-wave seismic data”. 

   BACKGROUND 
   The subject matter of the present invention relates to a method and apparatus responsive to a plurality of seismic data for generating a map illustrating data representative of a frequency shift of a plurality of seismic signals when said seismic signals propagate through a layer of fractured rock in an Earth formation. 
   Geologic formations containing fractures are an important source of hydrocarbon accumulations and an interesting target for geophysical exploration. The presence of fractures in a geologic formation will act as a high-cut filter with respect to the seismic wave that is propagating through the layer of fractured rock in the Earth formation. This produces a measurable and mappable change in the frequency spectra of the seismic signal propagating above the fractured zone compared to the frequency spectra of the seismic signal propagating below the fractured zone. 
   SUMMARY 
   Accordingly, in accordance with the present invention, a unique method has been developed which can show the presence of fractures in an Earth formation as a mappable attribute. This method, described in the Detailed Description hereinbelow, uses the frequency spectra derived from P-wave seismic data, comprised of a plurality of seismic traces, over a pair of specific time windows, which are located above and below a seismic horizon or reflector of interest, to infer the presence or absence of these geologic fractures, in a layer of fractured rock, based on the preferential attenuation of high frequencies. The method produces a parameter (t*), the parameter t* being proportional to the shift in frequency spectra amplitudes (i.e., energy), from higher frequencies to lower frequencies, when the plurality of seismic traces propagate from the time-window located above the seismic horizon or reflector of interest, through the layer of fractured rock, to the time-window below the seismic horizon or reflector of interest. A map is generated based on the computation of t* for all seismic traces in the seismic data. 
   Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinbelow, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein: 
       FIG. 1  illustrates a workstation with a CD-Rom adapted to be inserted therein for loading a workstation software package known as the ‘Fracture Detection Software’ in accordance with the present invention; 
       FIG. 2  illustrates a layer of fractured rock in an Earth formation; 
       FIGS. 3 and 9  illustrate the layer of fractured rock including a first window above the fractured rock zone and another, second window below the fractured rock zone; 
       FIG. 4  illustrates the frequency spectrum of the seismic signal in the first window of  FIG. 3 ; 
       FIG. 5  illustrates the frequency spectrum of the seismic signal in the second window of  FIG. 3 ; 
       FIGS. 6 and 10  illustrate the frequency spectra of  FIGS. 4 and 5  superimposed upon one another defining six measurement values; 
       FIG. 7  illustrates formula for defining ‘F high’, ‘F low’, and t*; and 
       FIGS. 8 and 11  illustrate a map of the fractured rock zone of  FIGS. 2 and 3 , which maps the attribute t*. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a workstation or other computer system  30  is illustrated. The computer system  30  may be, for example, a personal computer, a workstation, a mainframe, etc. Examples of possible workstations include a Silicon Graphics Indigo  2  workstation and a Sun SPARC workstation. The computer system  30  stores and executes a plurality of instructions that are used to detect fractures in an Earth formation in response to a plurality of seismic data  33 , the seismic data  33  being provided as ‘input data’ to the computer system  30 . The computer system  30  of  FIG. 1  includes a processor  30   a , a recorder to display device  30   b , a memory  30   c  which is adapted to store the ‘Fracture Detection Software’  30   c   1  in accordance with the present invention, and a system bus  30   d  to which the Processor  30   a  and the recorder or display device  30   b  and the memory  30 c are connected. A CD-Rom  32  stores the ‘Fracture Detection Software’  30   c   1  of the present invention, and, when the CD-Rom  32  is inserted into the computer system  30 , the Fracture Detection Software  30   c   1  is loaded from the CD-Rom  32  into the memory  30   c  of the computer system  30 . The processor  30   a  can now execute the ‘Fracture Detection Software’  30   c   1  instructions and perform its fracture detection for detecting fractures in an Earth formation. 
   The process steps being practiced by the ‘Fracture Detection software’  30   c   1  of the present invention, when the instructions of the fracture detection software  30   c   1  are being executed by the processor  30   a  of the computer system  30  of  FIG. 1 , will be set forth below followed by an explanation of each of those process steps. 
   Process Steps 
   The following steps represent the process steps practiced by the Fracture Detection software  30   c   1  of FIG.  1 : 
   1. Interpret the reflector (horizon) on the seismic data, recording the two-way seismic travel time. 
   2. The interpreter specifies the length of the time window (e.g., 100 milliseconds) to extract the frequency spectra. 
   3. The same time window length is recommended, but not required, on the seismic trace above and below the reflector. This window will be relative to the travel time of the interpreted horizon (see step 1); that is, the window will be parallel to the geologic structure. 
   4. For every seismic trace where the horizon is interpreted, the interpreter generates two spectra; that is, a first spectra located above the horizon, and a second spectra located below the horizon. This operation can be performed using any number of transforms which result in a frequency representation of the data, i.e., Fast Fourier Transform, Wavelet Transform, Cosine Correlation, etc. 
   5. The interpreter extracts amplitudes for two specific frequencies (i.e., 10 Hz and 30 Hz) from the spectra above and below the horizon. The objective is to select a high and low frequency from the spectra of each window (above and below the horizon) which are separated as far as possible in the usable bandwidth of the signal yet still contain valid amplitude (energy) above the background noise level. This can be generalized to any technique that measures the change in the energy (amplitude) distribution for the window above the horizon and the window below the horizon. 
   6. The amplitude values are used as input to the algorithm which compute the ‘t*’ parameter for that seismic trace. The computation of t* is as follows:
 
 F (high)= Fa (high)/ Fb (high);
 
 F (low)= Fa (low)/ Fb (low)
 
 t *={ln [ F (high)]−ln [ F (low)]}/(high−low)
 
where;
         F(high) is the ratio of the amplitudes for the higher frequency selected by the user (30 hz for the example) taken from the window above, Fa(high), and the window below, Fb(high);   F(low) is the ratio of the amplitudes for the lower frequency selected by the user (10 hz for the example) taken from the window above, Fa(low), and the window below, Fb(low); and   t* is the computed attribute taken from the difference in the natural log (ln) of F(high) and F(low) and this difference then scaled (divided) by the difference in frequency between the measurement points on the spectra (for the example: 30 hz−10 hz=20 hz, 20 was used in the denominator of the t* formula).       

   7. These steps are applied to every interpreted seismic trace. 
   8. The results (i.e., the t* parameter) are plotted on a map of the seismic survey. Areas of large t* values are more likely to contain a fractured formation. This map is generated using existing software, provided by Schlumberger Technology Corporation of Houston, Tex., for visualizing a seismically derived attribute in a spatial context. 
   Explanation of the Process Steps 
   Referring to  FIG. 2 , a layer of fractured rock in an Earth formation is illustrated. In  FIG. 2 , a layer of fractured rock  34  is located beneath the Earth&#39;s surface  36 . Assume that an acoustic or explosive energy source  38  generates sonic vibrations or sound waves  40  and those sound waves  40  reflect off a horizon  42  in the Earth&#39;s formation. The reflected sound waves  40   a  are received by a geophone  44  and, as a result, a plurality of seismic traces are recorded in a recording truck  46 . Lets examine carefully only ‘one such seismic trace’ among the plurality of seismic traces recorded in the recording truck  46 . 
   Referring to  FIGS. 3 and 9 , the ‘one such seismic trace’  48  is illustrated in connection with the layer of fractured rock  34  in the formation of FIG.  2 . In accordance with the novel method of the present invention, begin the steps of that method by selecting a window  50  along the seismic trace  48  which is disposed above the fractured rock zone, and another window  52  along the seismic trace  48  which is disposed below the fractured rock zone. 
   Referring to  FIG. 4 , generate a frequency spectrum of that portion of the seismic trace  48  which is disposed in the window  50  above the fractured rock zone  50 . That frequency spectrum, which is associated with that portion of the seismic trace  48  which is disposed inside the window  50  above the fractured rock zone  50  (hereinafter referred to as “Above”) is illustrated in FIG.  4 . The frequency spectrum “Above” of  FIG. 4  can be generated by using the Fast Fourier Transform or a ‘Cosine Correlation Transform’. One example of the use of the Fast Fourier Transform is illustrated in U.S. Pat. No. 5,870,691 to Partyka et al, the disclosure of which is incorporated by reference into this specification. In addition, one example of the use of the ‘Cosine Correlation Transform’, is disclosed in U.S. patent application Ser. No. 10/017,565, filed Dec. 14, 2001, entitled “Seismic signal processing method and apparatus for generating correlation spectral volumes to determine geologic features”, the disclosure of which is also incorporated by reference into this specification. 
   Referring to  FIG. 5 , generate a frequency spectrum of that portion of the seismic trace  48  which is disposed in the window  52  below the fractured rock zone  52 . That frequency spectrum, which is associated with that portion of the seismic trace  48  which is disposed inside the window  52  below the fractured rock zone  52  (hereinafter referred to as “Below”) is illustrated in FIG.  5 . The frequency spectrum “Below” of  FIG. 5  can be generated by using the Fast Fourier Transform or a ‘Cosine Correlation Transform’. One example of the use of the Fast Fourier Transform is illustrated in U.S. Pat. No. 5,870,691 to Partyka et al, the disclosure of which has already been incorporated by reference into this specification. In addition, one example of the use of the ‘Cosine Correlation Transform’, is disclosed in U.S. patent application Ser. No. 10/017,565, filed Dec. 14, 2001, entitled “Seismic signal processing method and apparatus for generating correlation spectral volumes to determine geologic features”, the disclosure of which has already been incorporated by reference into this specification. 
   Referring to  FIGS. 6 and 10 , a frequency spectrum is illustrated, where the frequency spectra of  FIG. 4  (i.e, ‘Above’) is superimposed over the frequency spectra of  FIG. 5  (i.e., ‘Below’). In the frequency spectrum of  FIG. 6 , select a low frequency ‘Low’ and a high frequency ‘High’ along the ‘x’ frequency axis. Using the ‘Low’ frequency in  FIG. 6 , locate an amplitude on the ‘y’ amplitude axis of the ‘Above’ frequency spectra, ‘Fa(low)’, and locate an amplitude on the ‘y’ amplitude axis of the ‘Below’ frequency spectra, ‘Fb(low)’. Using the ‘High’ frequency in  FIG. 6 , locate an amplitude on the ‘y’ amplitude axis of the ‘Above’ frequency spectra, ‘Fa(high)’, and locate an amplitude on the ‘y’ amplitude axis of the ‘Below’ frequency spectra, ‘Fb(high)’. Now, as noted in  FIG. 6 , six different values or measurements have been defined, as follows: (1) Low, (2) High, (3) Fa(low), (4) Fb(low), (5) Fa(high), and (6) Fb(high). Each of these six values or measurements will be used in an algorithm to be described below with reference to FIG.  7 . 
   Referring to  FIG. 7 , define the value ‘F high’ as follows:
 
 F  high= Fa (high)/ Fb (high)
 
   Define a value ‘F low’ as follows:
 
 F  low= Fa (low)/ Fb (low)
 
   From the values ‘F high’ and ‘F low’, define an attribute hereinafter called the “t* attribute”, as follows:
 
 t *=[ln ( F  high)−ln ( F  low)]/(High−Low)
 
   Referring back to  FIG. 3 , the t* attribute can be defined as follows: recalling that the seismic trace  48  has a particular frequency before the trace  48  propagates through the layer of fractured rock  34 , the t* attribute represents an indication of how much that frequency (of the seismic trace  48 ) shifts or changes when the seismic trace  48  propagates through the layer of fractured rock  34  in FIG.  3 . 
   Referring to  FIG. 8 , recalling that the seismic trace  48  of  FIG. 3  intersected the horizon  42  at a location on the horizon which is defined by the (x, y) coordinates (x 1 , y 1 ), a ‘map of the fractured zone’ can be plotted. On the ‘map’, plot the above defined ‘t*’ attribute on the ‘map’ at the same coordinate location (x 1 , y 1 ). Recall that the seismic trace  48  intersected the horizon  42  at coordinate location (x 1 , y 1 ). Then, assign a unique color to the ‘t*’ attribute which is plotted on the map, the unique color corresponding directly to the t* attribute value plotted on the map. For each t* attribute value plotted on the map, assign a corresponding and possibly different and unique color to each t* attribute. As a result, a user can see the color on the map and associate the color on the map to a unique t* attribute value. 
   Referring to  FIG. 11 , the above process plotted the t* attribute on the ‘map’ using a single seismic trace  48 . Repeat the above process for all the other seismic traces which are recorded by the geophone  44  representative of the reflected sound vibrations  40   a  of FIG.  2 . When the above process is repeated for all the other seismic traces, the “map of the Fractured Zone” of  FIG. 11  will be the result. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.