MXPA96003026A

MXPA96003026A - Sismi signal exploration and processing method

Info

Publication number: MXPA96003026A
Application number: MXPA/A/1996/003026A
Authority: MX
Inventors: S Bahorich Michael; L Farmer Steven
Original assignee: Amoco Corporation
Priority date: 1994-12-12
Filing date: 1996-07-26
Publication date: 1997-06-01

Abstract

The present invention relates to a method for the exploration of hydrocarbons, which comprises the steps of: obtaining a set of traces of distributed seismic signals on a pre-determined three-dimensional volume of the earth, dividing the three-dimensional volume into a plurality of vertically stacked and generally spaced partitions, dividing each of the partitions into a plurality of cells that have portions of at least three seismic traces located there, measuring the cross-correlation between a pair of traces that lie in a plane of traces that are find in another vertical plane to obtain a value of transversal line, combine the value in line and the value of transversal line to obtain a value of coherence for each of the cells, and visualize the values of coherenc

Description

METHOD OF EXPLORING AND PROCESSING SEISMIC SIGNALS Technical Field This invention relates to the general object of seismic exploration and, in particular, to methods for identifying structural and stratigraphic characteristics in three dimensions. BACKGROUND OF THE INVENTION Normally, 2-D seismic data is acquired along lines (see lines 10 and 11 in Figure 1) which consist of geophonic arrays in the ground or transverse lines of hydrophilic current propagation in the submarine zone. . Geophones and hydrophones act as sensors to receive energy that is transmitted to the bottom and reflected back to the surface from underground rock interfaces 12. Energy is usually provided on land by vibrosysic vehicles that transmit impulses by shaking the bottom at pre-determined intervals and frequencies. -determined on the surface. In the submarine zone, sources of pneumatic guns are normally used. Subtle changes in the energy returned to the surfaces often reflect variations in the stratigraphic, structural and fluid contents of the deposits. In 3-D seismic, the principle is similar, although the lines and matrices are spaced more closely (see Figures 1 and 2) to provide more detailed underground coverage. With this high density coverage, it is necessary to record, store and process extremely large volumes of digital data, before being able to make the final interpretation. The processing requires complex computer and software resources to improve the signal received from the underground zone and to silence the accompanying noise that masks the signal. Once the data is processed, geophysical personnel compile and interpret the 3-D seismic information in the form of a 3-D cube (see Figure 4), which effectively represents a visualization of underground features. Using the data cube, information can be displayed in several ways. Maps of horizontal time partitions can be made at selected depths (see Figure 5). Using a workstation with a computer, an interpreter can cut through the field to investigate reservoir sources in different horizons. Partitions or sections can also be made in any direction using seismic or well data. Time maps can be converted to depth to provide a structural interpretation at a specific level. Three-dimensional seismic (3-D) is widely used throughout the world to provide a more detailed structural and stratigraphic image of underground deposits. Acceptance of 3-D seismic has accelerated over the past five years based on a proven trajectory record that continues to grow. The 3-D payment has been measured by estimates of increased reserves, cost savings from the more accurate positioning of delineation and development wells, improved reservoir characterization leading to better simulation models and the ability to predict more accurately future opportunities and problems during the production history of a field. More importantly, 3-D seismic has also been used as an exploration tool to reduce the risk of drilling in structurally complex areas and to predict reservoir quality in unperforated areas. Although seismic inspections and interpreters have become good, improvements are needed. In particular, the seismic data have been acquired and processed traditionally with the purpose of forming images of seismic reflections. Changes in stratigraphy are often difficult to detect in traditional seismic displays due to the limited amount of information that stratigraphic features present in a cross-sectional view. Although such views provide an opportunity to see a much larger portion of these characteristics, it is difficult to identify fault surfaces within a 3-D volume where no fault reflections have been recorded. More important, it is not known that seismic data have been acquired or used with the purpose of forming images of seismic discontinuities instead of seismic reflections. SUMMARY OF THE INVENTION In accordance with the present invention, a method for the exploration of hydrocarbons is described. The method comprises the steps of: obtaining a set of trcces of seismic signals distributed over a predetermined three-dimensional volume of the earth; dividing the three-dimensional volume into a plurality of vertically stacked and generally spaced horizontal partitions; dividing each of the partitions into a plurality of cells arranged in rows and columns extending laterally and having portions of at least three seismic traces extending generally in the vertical direction located there; measure across each of the cells the cross-correlation between a pair of traces that are in a vertical plane to obtain an in-line value and measure the cross-correlation between another pair of traces that is in another vertical plane to obtain a cross-line value which are estimates of the inflection of time in an on-line direction and in a cross-line direction; combine the on-line value and the cross-line value to obtain a coherence value for each of the cells; and visualize the coherence values of the cells through at least one horizontal partition. This technique is particularly well suited to interpret fault planes within a 3-D seismic volume and to detect subtle 3-D stratigraphic features. This is because seismic partitions cut by a fault line usually have a different seismic character than traces on each side of the fault. The trace similarity measurement (ie, consistency or 3-D continuity) throughout a time partition reveals features of low coherence along these fault lines. Such coherence values can reveal critical underground details that are not easily apparent in traditional seismic sections. In addition, by calculating the coherence along a series of time partitions, these failure features identify planes or surfaces of failure. Numerous other advantages and features of the present invention will be readily apparent from the following detailed description of the invention, the embodiments described herein, from the claims, and from the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates an arrangement of geophones for obtaining 3-D seismic data from the subsoil the earth for processing in accordance with the present invention. Figure 2 is a plan view of the arrangement shown in Figure 1. Figure 3 is a representation of the seismic traces that are in a plane passing through a row of geophones shown in Figure 2. Figure 4 is a graphic representation of the information obtained from the 3-D seismic data processing. Figure 5 is a graphical representation of a horizontal time slice of 3-D seismic data processed according to the prior art; and Figure 6 is a graphical representation of a horizontal time partition of 3-D seismic data processed in accordance with the present invention. DETAILED DESCRIPTION Although this invention is capable of execution in many different ways, it is shown in the drawings and a specific embodiment of the invention will be described in detail here. However, it should be understood that the present disclosure should be considered only as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment or algorithm described in this manner. The first stage is to obtain a set of seismic data in the form of traces of seismic signals on a three-dimensional volume of the earth. The methods by which such data is obtained and reduced to digital form for processing as 3-D seismic data are well known to those skilled in the art. The next stage is to generate a "discontinuity cube". This is done by applying a coherence algorithm to the 3-D seismic data. This algorithm can take many forms. Whatever the form, its function is to compare the similarity of nearby regions of seismic data within seismic volume 3-D. If a trace segment is similar to its neighbors (for example, in the on-line and cross-line directions), it is assigned a low discontinuity value; if a trace segment is not similar to its neighbors, it is assigned a high discontinuity value.

Figure 2 is a plan view of a 3-D seismic volume portion. To measure discontinuity, a trace segment at a point A is compared to adjacent trace segments B and C. One way to calculate trace similarity is described below. The mean zero cross-correlation delayed in the in-line direction (x-direction) between trace u (t, x, y) and u (t, x + dx, y) with a delay time of "tlag" msec. is defined as: k = + w u (t + k, x, y) u (t + k + tlag, x + dx, y) 7rx (t, tlag) = k = -v ^ V aa ((tt ,, xx ,,; y) a (t, x + dx, y) where: k = + w a (t, x, y) =) u2 (t + k, x, y) k = -w k = + wa (t, x + dx, y) =) u2 (t + k, x-dx, y) k = -w are autocorrelations used to normalize the cross-correlation, and where w + w is the length in msec of the correlation window. It is important to select w large enough, so that the zero-mean hypothesis is valid. Other methods of normalization can be used, (for example, product of the energies of the traces, etc.). In particular, cross-correlation is a method of combining two waveforms to measure the similarities of waveforms. Autocorrelation is a method of combining a waveform with itself. See Sheriff's "Encyclopedic Dictionary of Exploration Geophysics," Society of Exploration Geophysicists, Tulsa, Oklahoma. The mean zero cross-correlation delayed in the cross-line (y-direction) between trace u (t, x, y) and u (t, x, y + dy) with a delay time of tlag msec. is defined as: k = + w \ u (t + k, x, y) u. { t + k + tlag, x, y + dy) px (t, tlag) = k = -w a (t, x, y) a (t, x, y + dy) where: k = + w a (t, x, y + dy) = u (t + k, x, y + dy) k = -w The direction of the apparent time inflection in the directions x and y is estimated to be that lag (ie, tlagx and tlagy) that has the highest cross-correlation (that is, the most positive). These values are px (t, tlagx) and try (t, tlagy). Given the apparent inflections (in msec / trace), it is an easy calculation (but not necessarily accurate when dealing with noisy data) to obtain inflection and azimuthal inflection. More importantly, the concept of cross-correlation extends to two dimensions by taking the geometric mean between the classic one-dimensional cross-correlations: pxy (t, tlagx, tlagy) = and go x (t, tlagx) and p (t, tlagy) This value (or attribute) serves as a fairly robust estimate of signal discontinuity within geological formations as well as signal discontinuities through faults and erosive nonconformities.

Computer program The following is a FORTRAN 77 program to perform these calculations: Given a trace "x" from a seismic amplitude volume 3-D, and its two neighboring traces "y" (in the on-line direction) and "z" (in the cross-line direction), the COH subroutine calculates an output trace "rho" that contains coherence coefficients using an execution window cross-correlation algorithm, where: "mins" and "maxs" are the minimum and maximum sample rates for all four traces; "inwinl" is the length of the window in samples; "nlags" specifies the number of delays (relative to time shifts) to make each side of "0" in the cross-correlation; and "sr" is the interval between samples in ms.

In each sample, the CROSS subroutine calculates a series of normalized cross-correlation coefficients, returning the maximum coefficients for each direction in "rhol" and "rho2". The time offset in which the maximum coefficients occur is returned in "tshfl" and "tshf2"; these times are not used. The COH subroutine is called repeatedly, once for each trace in the input seismic amplitude volume, to produce a new 3-D data volume or "coherency cube" containing coherence coefficients. subroutine coh (x, y, z, rho, ins, maxs, iwinl, nglas, sr) real x (mins: maxs), and (mins: maxs), z (mins: maxs) real rho (mins: maxs) ihwin = iwinl / 2 do j = mins + ihwin, axs-ihwin k = j- ihwin cali cross (x (k), iwinl, and (k), iwinl, nlags, sr, rhol, tshfl) cali cross (x (k), iwinl, z (k), iwinl, nlags, sr, rho2, tshf2) rho (J) = sqrt (rhol * rho2) enddo return end subroutine cross (x, nx, and, ny, lags, sr, peak, tshift) real x (0: nx-l), y (0: ny-1), sr, peak, tshift parameter (maxlags = 128) real g (-maxlags: + maxlags) double precision xx, yy nlags = max (0, min (lags, maxlags)) tshift = 0..0 peak = 0.0 xx = 0.0 yy = 0.0 ks = 0 do ix = 0, nx-l xx = x (ix) ** 2 + xx enddo if (xx.eq.0.0) return do iy = 0, ny-l and yy = y (iy) ** 2 + yy enddo if (yy.eq.0.0) return do is = -nlags, + nlags g (is) = 0.0 do it = O, nx-1 if (it-is .ge.O) then if (it-is .le. ny-l) then g (is) = g (is) + x (it) * y (it-is) endif endif if (abs (peak) .lt. abs (g (is))) then peak = g (is) ks = is endif enddo tshift = ks * sr peak == peak / sqrt (xx * yy) return end Landmark and GeoQuest interpretation workstations, for example, can be used to visualize and interpret faults and stratigraphic features by loading the cube of discontinuity as a seismic volume. The visualization software (for example, SeisCube of Landmarks software) can be used to cut quickly through the discontinuity volume to help in the understanding of complex fault relationships. Discontinuity displays can reduce the interpretation cycle time when they are used to select which seismic lines should be interpreted, allowing the interpreter to work around faults and sparse data areas. In addition, subtle stratigraphic features and complex faults that do not readily manifest in traditional seismic visualizations can be quickly identified and interpreted. Figures 5 and 6 are side-by-side comparisons of the same seismic information displayed and processed conventionally and in accordance with the present invention. Fault lines are easily seen in Figure 6. Consistency maps have been made in several 3-D inspections. At depths of reasonable data quality, approximately 90% of failures can be easily identified. Faults were identified in coherence maps that were very subtle in seismic sections, but clearly present on coherence maps due to the strength of the method and the perspective of the map of the fault figures. Since the coherence maps can be executed on uninterpreted time partitions, the present invention offers a means to greatly accelerate the mapping of the structural framework and to reveal details of the relationships of the faults that would otherwise only be interpreted. through tedious selection of faults. Specific examples 2-D seismic coherence maps were generated along selected horizons and slate diapirs were clearly identified in the submarine zone in Nigeria. In the submarine zone of the Gulf of Mexico, the technique easily identified diapiric structures. Over several coherence time partitions considerable details of stratigraphic features were visualized, such as abandoned river channels, mud flows, and underwater canyons. In seismic sections, these characteristics were sometimes manifested, but, in some cases, they were not identifiable even with thorough examination.

This is the first known method of revealing fault plans within a 3-D volume, where no fault reflections have been recorded. Faults are often critical to the accumulation of oil. A fault can form a seal by cutting a structural or stratigraphic characteristic, so oil is trapped against the fault. On the other hand, if the plane of the fault contains debris that has not been cemented, it can form a conduit for fluids. This may allow the hydrocarbons to travel over the plane of the fault within the feature and be trapped in it or escape from the feature by moving over the plane of the fault outside it. Thus, fault lines can predict flow patterns in a reservoir and communication between injector and production wells, for example. Seismic discontinuities can also provide the necessary link to allow the prediction of reservoirs between wells and establish the continuity of reservoirs and flow patterns through a field. The mapping of the coherence map of 3-D seismic is an extremely powerful and efficient tool for mapping the structure and stratigraphy. The new method is particularly sensitive to the common causes of lateral variations in the small wave (ie, displacement of the fault or stratigraphic variations). This 3-D method analyzes a time partition or a horizon-based interval and measures the maximum of normalized cross-correlation in the on-line and cross-line directions. Additional analysis The discontinuity cube will clearly highlight fault planes as zones of high discontinuity. However, these areas may not be clear in areas of low signal-to-noise ratio. One method of improving these fault zones involves the application of a "medium plane operator". Faults in the subsoil of the earth are generally expressed as planes or surfaces. In the case of a curved fault surface, a series of small smooth planes can be used to approximate the surface of the fault. In accordance with this aspect of the invention, a small flat operator is used to improve (ie, to "filter") the identification of subtle stratigraphic characteristics. First, a small region of seismic data is selected around a central value. This region can be formed from a plurality of cells used to form the "coherence cube". Then a small fault plane is mathematically inserted into the region, and the mean value of the points within the plane is calculated for the dip and azimuth that is best aligned with the zone of high discontinuity. This mean value is then assigned to the central value of a new matrix. Next, the data region is shifted (for example, a line) and the process is repeated until each point in the previously determined discontinuity cube has been analyzed as a central value. The final result is a completely new discontinuity cube with improved fault planes and noise and stratigraphic characteristics (ie, non-flat features) attenuated. These stratigraphic characteristics can be separated by subtracting the new discontinuity cube from the old discontinuity cube without the application of the flat filter. From the above description it will be noted that numerous variations, alternatives and modifications will become apparent to those skilled in the art. Accordingly, this description should be construed as illustrative only and is intended to teach those skilled in the art how to carry out the invention. Other algorithms can be used to measure the similarity of nearby regions of seismic data or to generate the "discontinuity cube". In addition, equivalent calculations can replace those illustrated and described. Also certain features of the invention can be used independently of other features of the invention. For example, stratigraphic features have generally been identified over time partitions where dips were low; and consequently, the time window captured a narrow stratigraphic section. In areas of higher dip, the method should work on selected horizons. Therefore, as a tool for tracing stratigraphic maps, there is good reason to think that maps can be drawn with new levels of detail compared to the previous, although this may require drawing the horizon of interest. As another example, although coherence mapping maps are by themselves very valuable tools for mapping, when used in combination with mapping of amplitude recognition and dip maps, there is a promise of technological milestone in the effectiveness of mapping for the Gulf of Mexico or similar basins with 3-D seismic. It is believed that the detailed mapping of structure and stratigraphy maps will be accelerated by the mapping of a map view and less by the traditional section line by line. Interpretation in a "recognition" data map view offers a significant improvement in the quality and quantity of interpretation. Thus, it will be appreciated that various modifications, alternatives, variations and changes may be made without departing from the spirit and scope of the invention as defined in the appended claims. Naturally, it is intended that the appended claims cover all these implied modifications within the scope of the claims.

Claims

CLAIMS 1. A method for the exploration of hydrocarbons, comprising the steps of: a) obtaining a set of traces of seismic signals distributed over a predetermined three-dimensional volume of the earth; b) dividing the three-dimensional volume into a plurality of horizontally stacked and generally spaced horizontal partitions and dividing at least one of said partitions into a plurality of cells that are arranged in laterally extending rows and columns, each having said cells portions of at least three seismic traces located within, said portions of said traces generally extending in vertical direction through said cells, and a first trace and a second trace being disposed in said cell in a generally vertical plane and a third trace and said first trace being disposed in said cell in another generally vertical plane that is generally at right angles to said vertical plane; c) measure through each of said cells the cross-correlation between said traces that are in a vertical plane to obtain an on-line value and the cross-correlation between said traces that are in said other vertical plane to obtain a value of transverse line which are estimates of the inflection of time in an on-line direction and in a cross-line direction; d) combining the on-line value and the transverse line value to obtain a coherence value for each of the cells; and e) displaying said coherence values of the cells through at least one of said horizontal partitions.
2. The method of claim 1, wherein step (b) is performed on each of said horizontal partitions; and where in step (e) said coherence values are displayed on successive horizontal partitions.
The method of claim 1, wherein step (c) includes the step of normalizing each in-line value and each value in a transverse line.
4. The method of claim 3, wherein said normalization step includes the step of obtaining the product of the energies of each pair of traces.
5. The method of claim 3, wherein the auto-correlation of said traces that are in one of said vertical planes and the self-correlation of said traces that are in the other vertical plane are obtained to normalize said correlations in the address in line and in the cross-line direction.
The method of claim 1, wherein the lid (c) comprises the steps of calculating the delayed zero mean cross-correlation in said in-line direction, and calculating the delayed zero mean cross-correlation in said transverse line direction.
The method of claim 6, wherein the etcipa (d) comprises the steps of identifying the most positive value of. said delayed zero mean cross-correlation in said in-line direction, and identifying the most positive value of said delayed zero mean cross-correlation in the transverse line direction.
The method of claim 7, wherein the etcipa (d) comprises the step of computing the geometric mean between said two more positive values.
The method of claim 1, wherein in step (a) said set of seismic signal traces comprises a plurality of amplitude traces versus horizontal coordinate and seismic data time.
The method of claim 1, wherein in step (a) said signal traces are formatted dicjitally.
The method of claim 1, wherein each of said horizontal partitions of step (b) extends over approximately 100 milliseconds.
12. A method of locating underground features, faults and contours, comprising the steps of: a) obtaining 3-D seismic data covering a pre-determined volume of the earth; b) dividing said volume into a matrix of relatively small three-dimensional cells, where each of said cells is characterized by at least three vertically separated and generally vertical seismic traces located there; c) measuring in each of said cells the coherence / similarity of said at least three traces relative to two pre-determined directions; and d) visualizing said coherence / similarity of said cells to form a two-dimensional map of subterranean characteristics.
The method of claim 12, wherein in step (c) said pre-determined directions are mutually perpendicular; and where said coherence / similarity of said cells is measured as a function of cross-correlation between two traces in one direction and read cross-correlation between two traces in a direction that is perpendicular to said one direction.
14. The method of claim 13, wherein said coherence similarity of said cells is measured as a function of the maximum cross-correlation in each of said two directions.
The method of claim 14, wherein said coherence./impair is proportional to the geometric mean of said two maximum cross correlations.
16. The method of claim 12, further including the step of: e) visualizing the coherences / similarities of the successive horizontal matrices vertically separated from three-dimensional cells to identify unalterable characteristics of space and time.
17. In seismic operation, where 3-D seismic data comprising reflected seismic energy is recorded as a function of time to produce a series of seismic traces and where a computer is used that is adapted to process such seismic traces, an article of manufacturing comprising: a means that is readable by a computer and which instructs said computer to perform a process comprising the steps of: (a) accessing the 3-D seismic data over a predetermined volume; (b) comparing the similarity of nearby regions of said 3-D seismic data of said volume: (i) dividing said volume into at least one horizontal partition and dividing at least one partition into a plurality of cells that are arranged in rows and laterally extending columns, each of said cells having portions of at least three seismic traces extending therethrough, including a first trace and a second trace that are in a plane and a third trace that is -tra with said first trace in another plane that is generally at right angles to said plane; (ii) measuring through each of said cells the coherence of said traces that are in one of said planes to obtain a first coherence value and measure the coherence of said traces that are in said other plane to obtain a second coherence value; (iii) combining said first coherence value that is representative of the coherence of said seismic traces in said cell; and (iv) storing said coherence value of each cell of at least one horizontal partition in a form that allows visualizing said combined coherence values as a map of seismic attributes.
18. The article of manufacture of claim 17, wherein said means carries instructions for said computer to perform step (ii) determining the transversal correlation between said first trace and said second trace, and determining the transversal correlation between said first trace and said third trace .
The article of manufacture of claim 18, wherein said means carries instructions for said computer to perform step (ii) determining the delayed zero mean cross-correlation between said first trace and said second trace, and determining the delayed zero mean cross-correlation between said first trace and said third trace.
20. The article of manufacture of claim 19, wherein said means carries instructions for said computer to perform step (iii) identifying the most positive of each of said zero cross-correlations.
21. The article of manufacture of claim 20, wherein said means carries instructions for the computer to perform step (iii) further determining the geometric mean of said two zero positive retarded cross-correlations.
22. A method of seismic exploration, comprising the steps of: a) obtaining a set of 3-D seismic data comprising traces of seismic signals distributed over a volume of the earth; b) dividing said volume into separate horizontal partitions and dividing each partition into cells arranged in rows and columns that extend laterally, each of said cells having at least three seismic traces that are generally in two generally vertical planes and mutually perpendi -culars; c) calculating in each cell the cross-correlation between said traces that are in a vertical plane to obtain an on-line value and the cross-correlation between said traces that are in said other vertical plane to obtain a transverse line value; d) calculate a coherence value for each of said cells, said coherence value being representative of. an average of the most positive value of said cross-correlation in said on-line direction and the most positive value of said cross-correlation in the cross-line direction; and e) visualizing said coherence values of said cells through at least one of said horizontal partitions.
23. An apparatus, comprising: a) registered means readable by a computer and carrying instructions for a process comprising the steps of: (1) accessing data comprising traces of seismic signals distributed over a predetermined three-dimensional volume of the earth; (2) arranging said three-dimensional volume in a plurality of vertically stacked horizontal partitions and arranging at least one of said partitions in a plurality of cells that are arranged in laterally extending rows and columns, each of said cells having portions of at least three seismic traces located there; each of said portions of said traces extending generally through said cells, and a first trace and a second trace being found in said cell in a plane and a third trace being found and said first trace in said cell in another plane that is generally in an angle with respect to said previous plane; (3) calculate in each of said cells the cross-correlation between said traces that are in a plane to obtain a value in-line and calculate the cross-correlation between said traces that are in said other plane to obtain a value of cross line; and (4) combining said in-line value and said transverse line value to obtain a coherence value for each of said cells.
The apparatus of claim 23, wherein step (3) further includes the steps of: obtaining the auto-correlation of said traces that lie in a plane; and obtaining the auto-correlation of said traces that are in said other plane to normalize said cross-correlations in said in-line direction and in said transverse line direction.
25. The apparatus of claim 23, wherein step (4) comprises the steps of calculating the delayed zero mean cross-correlation in said in-line direction; and calculating the delayed zero mean cross-correlation in said in-line direction.
26. The apparatus of claim 25, wherein step (4) comprises the steps of: identifying the most positive value of said delayed zero mean cross correlation in said in-line direction; and identify the. most positive value of said delayed zero mean cross correlation in said transverse line direction.
27. The apparatus of claim 26, wherein step (4) comprises the step of calculating an average of said two more positive values.
28. In a computer adapted to receive 3-D seismic data and having a screen to represent seismic data 3-D processor, an article of manufacture comprising: a) a medium that is readable by the computer and that carries instructions so that the computer performs a process comprising, the steps of: (1) arranging the 3-D data in a plurality of cells that are arranged in rows and columns that extend laterally, each of said cells having portions of at least three traces seismic located there, including a first trace and a second trace that lie in a plane and that include a third trace that meets said first trace in another plane that is at right angles to the previous plane; (2) calculate in said cells representations of the cross-correlation between said traces that are in a plane and calculate representations of the cross-correlation between said traces that are in said other plane; and (3) combining said representations of the cross-correlation between said traces that are in a plane and said representations of the cross-correlation between said traces that are in said other plane to obtain a coherence value for each of said cells.
29. The article of manufacture of claim 28, wherein said means carries instructions for the computer to perform step (3) by combining a representation of the maximum cross-correlation in one plane and a representation of the maximum cross-correlation in said other flat .
30. The article of manufacture of claim 29, wherein said means carries instructions for the computer to combine said representations of the maximum cross-correlation in one plane and the maximum cross-correlation in said other plane by calculating a representation of an average of said two maximum cross-correlations.
31. In a computer that has 3-D seismic data stored in it that covers a pre-determined volume of the earth, a device that is readable by the computer and that carries instructions to perform a process comprising the steps of: (1 ) digitally classifying said data into a matrix of relatively small three-dimensional cells, where each of said cells is characterized by at least three laterally separated and generally vertical seismic traces located there; (2) calculate in each of said cells a coherence value from said at least three traces with respect to two pre-determined directions; and (3) storing said coherence values of said cells so that the computer visualizes a two-dimensional map of subterranean characteristics represented by said coherence values.
The device of claim 31, wherein in step (2) said two pre-determined directions are mutually perpendicular, and where each coherence value is calculated as a function of the cross-correlation between two traces in one of said two directions mutually perpendicular and the cross-correlation between two traces in the other of said two mutually perpendicular directions "
33. The device of claim 32, wherein said coherence value of step (2) is calculated as a function of the maximum cross-correlation in one direction and the maximum cross-correlation in that other direction.
34. The device of claim 33, wherein said coherence value of. stage (2) is a function of the geometric mean of said two maximum cross correlations.
35. A method for prospecting hydrocarbon deposits, comprising the steps of: a) obtaining 3-D seismic data on a pre-determined three-dimensional volume of the earth; b) using a computer and a program for said computer that instructs said computer to perform the following steps: (1) read said data and divide said volume into a matrix of relatively small three-dimensional cells, where each of said cells have at least three laterally separated seismic traces located there; and (2) calculating said coherence values in each of said cells; and c) using said computer to display said coherence values; and d) using said visualization to identify geological features and said locations that are indicative of the location of an oil or gas field.
36. The method of claim 35, further including the step of: e) drilling a well at a location identified in step (d).
37. The method of claim 35, wherein step (2) is performed: (i) measuring the cross-correlation between a pair of traces relative to a vertical plane to obtain an in-line value and measuring the cross-correlation between another pair of traces relative to another vertical plane to obtain a transverse line value; Y (ii) combining said on-line value and said transverse line value to obtain a coherence value for said cell.
38. The method of claim 37, wherein step (ii) includes the steps of: identifying a maximum in-line cross-correlation and a maximum cross-line cross-correlation; and combining said maximum cross correlations.
39. The method of claim 38, wherein said maximum cross correlations are combined by calculating their mean.
40. A device, comprising: a) a record that is readable by a computer and that carries instructions for a process comprising the steps of: (1) reading the representative data of traces of seismic signals distributed over a three-dimensional volume pre-determined of the earth; (2) classifying said signal traces by dividing said three-dimensional volume into a plurality of relatively thin cells that are arranged in rows and columns that extend laterally, each of said cells having portions of at least three similar traces located therein, which they include a first trace and a second trace that are in a plane and a third trace that meets said first trace in another plane that is at an angle with respect to the previous plane; (3) measure in each of said cells the cross-correlation between said traces that are in a plane to obtain an on-line value and the cross-correlation between said traces that are in said other plane to obtain a transverse line value; and (4) combining said in-line value and said transverse line value to obtain a coherence value for each of said cells.
41. The device of claim 40, wherein the computer includes means for displaying said coherence values of said cells.
42. The device of claim 40, wherein step (4) comprises the steps of calculating the delayed zero mean cross-correlation in said in-line direction; and calculating the delayed zero mean cross-correlation in said transverse line direction.
43. The device of claim 42, wherein step (4) comprises the steps of identifying the most positive value of said mid-wax delayed cross-correlation in said in-line direction; and identify the. most positive value of said delayed zero mean cross-correlation in the cross-line direction.
44. The device of claim 43, wherein step (4) comprises the step of calculating a mean between said two more positive values.
45. A method for locating subterranean features, faults and contours, which includes the means of: a) obtaining seismic data that covers a pre-depleted volume of land; b) dividing said volume into a matrix of relatively small three-dimensional cells, where each of said cells is characterized by at least three laterally separated and generally vertical seismic traces therein; c) measuring in each of said cells the cross-correlation between two traces in one direction and the cross-correlation between two traces in a direction that is perpendicular to said previous direction; and d) displaying a representation of said cross-correlation between two traces in one direction and said cross-correlation between two traces in a direction that is perpendicular to said previous direction in the form of a two-dimensional map.
46. The method of claim 45, wherein said representation of step (d) is a function of the maximum cross-correlation in each of said two directions.
47. The method of claim 45, wherein said representation of step (d) is a function of the geometric mean of said two cross correlations.
48. A method for prospecting hydrocarbon deposits, where the 3-D seismic data is obtained on a pre-determined three-dimensional volume of the earth, where a computer reads the data and divides the volume into a matrix of relatively small three-dimensional cells, where each cell has at least three laterally separated seismic traces located there, where the computer is used to transform the data into a visualization of seismic attributes, where the computer is used to make a map of seismic attributes, and where the map is used to identify underground features commonly associated with the entrapment and storage of hydrocarbons, characterized by: (1) calculating in each of the cells a coherence value for said seismic traces; and (2) visualize the coherence value of each cell that lies between two planes within volume 3-D.
49. The method of claim 48, wherein step (1) is performed: (i) by measuring in each cell the cross-correlation between a pair of traces with respect to a vertical plane to obtain an on-line value and measuring the correlation crossed between another pair of traces with respect to another vertical plane to obtain a value of transversal line; and (ii) combining said on-line value and said transverse line value to obtain a coherence value for said cell.
50. The method of claim 49, wherein each cell contains a plurality of traces in each vertical plane; where stage (i) is performed for all traces in each vertical plane; and wherein step (ii) comprises the steps of: identifying a maximum in-line cross-correlation and a maximum cross-line cross-correlation; and combining said cross correlations in maximum line and maximum transverse line.
51. A seismic map prepared by a process, comprising the steps of: (1) accessing UJ through a computer? data set comprising traces of seismic signals distributed over a pre-determined three-dimensional volume of the earth; (2) dividing said three-dimensional volume into a plurality of vertically stacked partitions and dividing at least one of said partitions into a plurality of cells that. they are arranged in rows and columns that extend laterally, each of said cells having portions of at least three seismic traces located there, each of said portions of said traces extending generally through said cells, and being a first trace and a second trace in each cell in a plane and finding a third trace and said first trace in said cell in another plane that is generally at an angle with respect to the previous plane; (3) calculate through each cell the cross-correlation between said traces that are in a plane to obtain an on-line value and calculate the cross-correlation between said traces that are in said other plane to obtain a transverse line value; (4) combining said on-line value and said cross-line value to obtain a coherence value for each cell; and (5) displaying said coherence values of said cells through at least one of said partitions.
52. The seismic map of claim 51, wherein prior to step (5) said coherence values of said cells are stored digitally in a memory; and where step (5) is performed by printing said coherence values in the form of an image representative of the subsoil.
53. The seismic map of claim 51, wherein step (3) comprises the steps of: calculating the delayed zero mean cross-correlation in said in-line direction; and calculating the delayed zero mean cross-correlation in said transverse line direction.
54. The seismic map of claim 53, wherein step (4) comprises the steps of: identifying the most positive velocity of said delayed zero mean cross-correlation in said in-line direction; identify the most positive value of said delayed zero mean cross-correlation in the cross-line direction; and combining said two more positive values.
55. The seismic map of claim 54, wherein in step (4) said two more positive values are combined by calculating their geometric mean.