Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/28—Processing seismic data, e.g. for interpretation or for event detection
  • G01V1/36—Effecting static or dynamic corrections on records, e.g. correcting spread; Correlating seismic signals; Eliminating effects of unwanted energy
  • G01V1/364—Seismic filtering
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/28—Processing seismic data, e.g. for interpretation or for event detection
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V2210/00—Details of seismic processing or analysis
  • G01V2210/50—Corrections or adjustments related to wave propagation
  • G01V2210/57—Trace interpolation or extrapolation, e.g. for virtual receiver; Anti-aliasing for missing receivers

Definitions

• This invention relates generally to the field of seismic prospecting and, more particularly, to seismic data processing and interpretation. Specifically, the invention is a method for enhancing seismic data so that subtle geologic features are easier to identify and interpret.
• seismic prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits.
• a seismic source is used to generate a physical impulse known as a "seismic signal" that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances).
• the reflected signals (known as “seismic reflections") are detected and recorded by seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the subsurface formations.
• Seismic prospecting consists of three separate stages: data acquisition, data processing, and data interpretation.
• the success of a seismic prospecting operation depends on satisfactory completion of all three stages.
• the seismic energy recorded by each seismic receiver during the data acquisition stage is known as a "seismic data trace.”
• the raw seismic data traces are refined and enhanced so as to facilitate the data interpretation stage.
• one common method for enhancing seismic data traces is through the common-midpoint (CMP) stacking process.
• CMP common-midpoint
• the "midpoint" for a seismic data trace is the point midway between the source location and the receiver location for that trace.
• the recorded seismic data traces are sorted into common-midpoint gathers each of which contains a number of different seismic data traces having the same midpoint but different source-to-receiver offset distances.
• the seismic data traces within each CMP gather are corrected for statics (i.e., the effects of variations in elevation, weathered layer thickness and/or velocity, and reference datum) and normal moveout (i.e., the variation of traveltime with respect to source-to-receiver offset) and are then summed or "stacked" to yield a stacked data trace which is a composite of the individual seismic data traces in the CMP gather.
• the stacked data trace has a significantly improved signal-to-noise ratio compared to that of the unstacked seismic data traces in the CMP gather.
• Stacked data traces for a series of CMP locations falling along a particular survey line may be displayed side-by-side to form a stacked seismic section which simulates a zero-offset seismic section (i.e., a seismic section where every trace is the result of a coincident source and receiver).
• a stacked seismic section is a representation, in two-way seismic signal traveltime, of a vertical cross-section of the earth below the survey line in question.
• Stacked seismic sections are used in the data interpretation stage to predict subsurface structure and stratigraphy.
• the seismic data traces recorded during the data acquisition stage are "minimum-phase," or nearly so. In other words, at the instant that a seismic signal reaches a subsurface reflector, a reflected signal begins to form.
• each subsurface reflector is marked by the leading edge of a seismic pulse or "wavelet.”
• a seismic data trace represents a convolution of many overlapping reflections, it is often difficult to clearly identify the leading edge of a seismic wavelet. It would facilitate interpretation of seismic data if the subsurface reflectors -3- were marked by peaks or troughs in the data rather than by a rising or falling edge of a seismic wavelet because peaks and troughs are easier to identify. A procedure known as "zero-phase processing" is commonly used in the industry to accomplish this result.
• the minimum-phase seismic wavelet embedded in the seismic data is converted to a zero-phase wavelet.
• Zero-phase wavelets are symmetrical, and the time scale is shifted so that the center of the wavelet indicates the arrival time. In other words, the center of a zero-phase wavelet coincides with the subsurface seismic reflector that caused the reflection.
• the conversion to zero-phase is preferably performed on the individual seismic data traces within a CMP gather prior to stacking; however, the conversion may also be performed after stacking has occurred. See, e.g., Sheriff, R. E. and Geldart, L. P., Exploration Seismology, Volume 1: History, theory, & data acquisition and Volume 2: Data-processing and interpretation, sections 4.3.4, 8.1.4, and 10.6. ⁇ d, Cambridge University Press, 1982.
• the result of this process is a zero-phase seismic section in which the subsurface reflectors generally are marked by peaks and/or troughs in the stacked zero-phase data traces.
• zero-phase processing Another advantage of zero-phase processing is that the resulting zero-phase seismic data traces typically have better seismic resolution (i.e., the ability to distinguish two reflectors which are close together) than the seismic data traces recorded during the data acquisition stage. See Schoenberger, M., "Resolution comparison of minimum-phase and zero-phase signals," Geophysics, Vol. 39, No. 6, pp. 826-833, December 1974. Accordingly, converting the recorded seismic data traces to zero-phase data traces permits identification and interpretation of shorter geologic intervals than is possible with conventional seismic data processing. As is well known in the art, the seismic data resulting from zero-phase processing may deviate from true zero-phase by as much as 30 degrees.
• this "near-zero-phase" seismic data is generally considered to be substantially equivalent to true zero-phase seismic data. Accordingly, as used herein and in the claims, “zero-phase” will be deemed to include both true zero-phase and near-zero-phase seismic data.
• quadrature traces are used to determine instantaneous seismic attributes for use in seismic attribute analysis. As is well known in the -art, a quadrature trace is a 90-degree phase-shifted version of the recorded minimum-phase seismic data trace. It is obtained by taking the Hubert transform of the recorded trace.
• the "quadrature" concept may be extended to other types of processed data traces.
• a 90-degree phase shift may be applied to a zero-phase (or near-zero-phase) data trace to yield a "zero-phase quadrature" trace.
• Techniques for applying the 90-degree phase shift to a zero-phase data trace are well known to persons skilled in the art.
• seismic resolution may be a problem for thin geologic features.
• Many subsurface geologic features of interest to the petroleum industry are from about five to about 50 feet in thickness.
• the cycle of a seismic pulse is typically sinusoidal and from about 80 to about 800 feet in length. Because a cycle consists of both a positive phase and a negative phase, the approximate resolution of a typical seismic pulse is from about 40 to about 400 feet.
• a seismic reflection is generated each time the seismic pulse encounters an impedance boundary. When the impedance boundaries are closer together than the resolution of the seismic pulse, the seismic reflections overlap, as noted above.
• an impedance boundary of interest may appear as only a small anomaly on the seismic data trace, such as a subdued peak or a departure from sinusoidal (i.e., a bend or kink in the data). Failure to identify and interpret these anomalies can result in erroneous conclusions regarding the subsurface stratigraphy.
• Such a method should be applicable to all types of seismic data traces, including but not limited to minimum-phase, zero-phase, -5- quadrature, and zero-phase quadrature data traces. Such a method also should permit the identification and interpretation of geological features marked only by an anomaly in the data.
• the present invention satisfies this need.
• the present invention is a method for enhancing a seismic data trace comprising the steps of (i) locating all peaks and troughs on at least a portion of the seismic data trace; and (ii) enhancing the amplitude values of the peaks and troughs.
• This enhancement can be accomplished in a variety of ways. For example, the amplitudes of all peaks may be adjusted to equal a first arbitrarily selected constant amplitude, and the amplitudes of all troughs may be adjusted to equal a second arbitrarily selected constant, which may be equal to or different from the first constant.
• the amplitudes of all peaks may be made equal to the largest peak amplitude on the data trace, and the amplitudes of all troughs may be made equal to the largest trough amplitude on the data trace.
• minimum amplitudes are specified for the peaks and troughs, and all peaks and troughs having amplitudes less than the specified minimum are enhanced to the specified minimum values.
• the inventive method may be applied to any type of seismic data trace including without limitation minimum-phase seismic data traces, zero-phase seismic data traces, quadrature traces, and zero-phase quadrature traces.
• the seismic data traces are converted to curvature traces.
• the curvature traces may be generated by taking the second derivative with respect to time of the data traces, provided that the data traces have been converted to continuous interpolated data traces defined at every time point.
• the curvature traces can be approximated by calculating the negative second difference of the discretized digital data at each sample point and plotting the result.
• FIG. 1A illustrates a hypothetical zero-phase seismic data trace
• FIG. IB illustrates the same seismic data trace after application of a first embodiment of the present invention
• FIG. 2 is a zero-phase seismic section for a particular survey line
• FIG. 3 is the same seismic section as shown in FIG. 2 after enhancement of the peaks and troughs according to the present invention
• FIG. 4 illustrates the negative second difference concept
• FIG. 5 illustrates a zero-phase seismic data trace and its corresponding curvature trace
• FIG. 6 illustrates another zero-phase seismic data trace and its corresponding curvature trace
• FIGS. 7 A and 7B illustrate a series of zero-phase stacked data traces for a particular survey line and the corresponding curvature traces
• FIGS. 8 A and 8B illustrate a zero-phase stacked seismic section and its corresponding curvature section; and FIG. 9 illustrates an impedance curve derived from well log data, a zero-phase quadrature trace obtained from a location near the well, and a second derivative of the zero-phase quadrature trace obtained according to the present invention.
• the present invention is a method for enhancing seismic data to make subtle geologic features easier to identify and interpret.
• the inventive method is applied to the individual stacked seismic data traces of a seismic section.
• the method may also be used to enhance seismic data traces prior to stacking.
• the method of the present invention is preferably implemented using a suitably programmed digital computer. Persons skilled in the art could easily develop computer software for practicing the inventive method based on the teachings set forth herein.
• the inventive method may be used to enhance any type of seismic data trace.
• the following detailed description will be directed toward use of the method to enhance zero-phase seismic data traces. Further, the following description will be based on implementation of the invention in the time domain. However, persons skilled in the art will understand that the invention may also be used to enhance other types of seismic data (such as minimum-phase, quadrature, or zero-phase quadrature data) and may be implemented in other data domains, such as the frequency domain, without departing from the true spirit and scope of the invention.
• the invention comprises directly enhancing the amplitudes of the peaks and troughs on zero-phase data traces so that the peaks and troughs are easier to identify.
• this enhancement may be applied to either prestack or poststack data traces.
• FIGS. 1A and IB This embodiment of the invention is illustrated in FIGS. 1A and IB.
• FIG. 1A illustrates a hypothetical zero-phase seismic data trace 10 (either prestack or poststack) having four peaks 12a - 12d and four troughs 14a - 14d.
• the amplitudes of peaks 12a and 12b are quite small compared to those of peaks 12c and 12d.
• FIG. IB illustrates the same data trace 10' after application of the present -8- invention.
• the amplitude of each of the peaks 12a' - 12d' has been adjusted to equal a constant value x
• the amplitude of each of the troughs 14a' - 14d' has been adjusted to equal a constant value y, which may or may not be equal to x.
• peaks and troughs in the hypothetical trace 10 (FIG. 1A) having small amplitudes have been enhanced to permit easy identification.
• the value of x is equal to the maximum peak amplitude of the unenhanced peaks 12a - 12d (FIG. 1A) and the value of y is equal to the maximum trough amplitude of the unenhanced troughs 14a - 14d
• FIG. 1A the amplitude of peaks 12a, 12b, and 12c would be enhanced to be equal to the amplitude of peak 12d, and the amplitude of troughs 14b, 14c, and 14d would be enhanced to be equal to the amplitude of trough 14a.
• minimum amplitude values for the peaks and troughs are specified, and all peaks and troughs having amplitudes less than the specified minimums are identified and enhanced to the specified minimum values.
• Other methods for enhancing the amplitude values of the peaks and troughs will be apparent to persons skilled in the art.
• the enhancement could be based on the local phase of the zero-phase data trace.
• the concept of local phase is reached by comparing the trace to the cosine function, i.e., local phase is zero at peaks, ⁇ at troughs, and ⁇ /2 or 3 ⁇ /2 at inflection points.
• the cosine of local phase is 1 at peaks, -1 at troughs, and 0 at inflection points.
• each peak on the zero-phase data trace would be assigned an amplitude value of 1 in the enhanced trace
• each trough would be assigned any amplitude value of-1
• each inflection point would be assigned an amplitude value of 0.
• FIGS. 2 and 3 illustrate application of the invention to an actual data set.
• FIG. 2 is a zero-phase seismic section for a particular survey line prior to enhancement.
• FIG. 3 shows the same zero-phase seismic section after enhancement of the peaks and troughs in the manner described above with respect to FIGS. 1A and IB.
• FIG. 3 shows the remnants of an ancient stream channel (reference numeral 17) that is not visible in FIG. 2 (reference number 15).
• the seismic data traces are enhanced by converting them to "curvature” traces. These curvature traces are then used to construct "curvature” sections for use in the data interpretation process.
• curvature is a measure of the concavity or convexity of an arc; i.e., it is the inverse of the radius of curvature of the arc (i.e., the radius of an inscribed circle). Sharp turns have larger curvatures than blunt turns because a circle inscribed in a sharp turn will have a smaller radius (and, therefore, a larger curvature) than a circle inscribed in a blunt turn. With respect to a seismic data trace, curvature is a measure of the rate of bending in the trace as a function of two-way seismic signal traveltime. Curvature may be used to enhance peaks and troughs in seismic data traces, and sometimes kinks or bends as well.
• Curvature also has a sign. Where a trace is a concave left, curvature is positive, regardless of whether the concavity is located on the positive or negative side of zero amplitude. Hence, even relative peaks located on the negative side of zero amplitude show up as positive peaks on a curvature trace.
• curvature is defined by the second derivative with respect to time of the function.
• digital seismic data actually comprises a series of discrete samples (typically at 2 or 4 millisecond intervals) of the amplitude of the seismic reflection.
• Computing a true second derivative for such discretized data requires spline-fitting or some other approximation to obtain a continuous interpolated trace defined at every time point.
• Methods for creating such an interpolated trace are well known to persons skilled in the art and, accordingly, will not be described herein. Care should be exercised in creating the interpolated trace to avoid potential aliasing problems.
• the second derivative with respect to time of the interpolated trace is then computed to obtain a curvature trace.
• trace curvature may be approximated by the negative second difference (- ⁇ 2 ) of the data, which is actually a measure of numeric -10- acceleration. If a,, a ⁇ , and a 3 are successive sample amplitudes on a seismic data trace, then the negative second difference at sample ⁇ is defined by the following equation:
• the "negative" second difference is used in order to compensate for a 180° phase rotation (polarity reversal) resulting from the second difference calculation.
• Negative second difference may be used to enhance subtle features (e.g., bends or kinks) in the data, as well as peaks and troughs, so that their true continuity can be identified.
• FIG. 4 shows five data samples of a seismic data trace 16, which may be any type of seismic data trace. TABLE 1 below gives the time and amplitude values for each of the five data samples, as well as the negative second difference (calculated according to the above formula) for the middle three data samples.
• the negative second difference 18 is also plotted on FIG. 4. It can be seen from FIG. 4 that negative second difference can be used to highlight subtle features of seismic character, as well as peaks and troughs.
• the bend in trace 16 at 3150 milliseconds may be a muted expression of an impedance boimdary. It may also be noise.
• the negative second difference calculation can be used to enhance this feature so that it can be identified and traced laterally through the curvature section, but its meaning requires careful interpretation.
• FIGS. 5 and 6 illustrate the results of the negative second difference calculation for two longer trace segments.
• FIG. 5 shows seismic data trace 20, which may be any type of seismic data trace, and its related curvature trace 22. Trace 20 -11- contains a number of relative peaks (e.g., relative peaks 24 and 26) located on the negative side of zero amplitude. On curvature trace 22, the corresponding peaks 28 and 30 are located on the positive side of zero amplitude.
• FIG. 6 shows a seismic data trace 32 and its corresponding curvature trace 34. It can be seen that the peaks and troughs of the two traces generally track each other, with the curvature trace being more sensitive.
• FIGS. 7A, 7B, 8A, and 8B further illustrate the utility of curvature traces.
• FIG. 7A shows 23 zero-phase stacked data traces for a particular survey line
• FIG. 7B shows the corresponding curvature traces.
• box 36 shows a peak event that seems to split.
• box 38 of FIG. 7B it can be clearly seen that there are actually two separate events.
• the peaks within area 40 are difficult to follow, while the same peaks on the corresponding curvature traces (area 42 of FIG. 7B) are quite easy to identify and follow laterally.
• FIG. 8A is the conventional stacked seismic section
• FIG. 8B is the corresponding curvature section. Sequence boundaries and other geologically significant features are much easier to identify and interpret in the curvature section. The ability to identify such features is critical to sequence stratigraphy. Changing the gain and making other changes in the conventional stacked seismic section did not bring these features forward.
• FIG. 9 illustrates another application of the present invention.
• the left panel shows an impedance curve 44 which was derived from well log data obtained from a well.
• the right panel shows a zero-phase quadrature data trace 46 for a location near the well in question.
• the center panel shows a curvature trace 48 obtained from zero-phase quadrature data trace 46 using the negative second difference calculation described above. Note that the curvature trace 48 mimics the well log impedance curve 44 much better than does the zero-phase quadrature data trace 46.
• a curvature trace may be generated by transforming a seismic data trace to the frequency domain to obtain an amplitude spectrum (i.e. a plot of amplitude versus frequency), multiplying each point on the amplitude spectrum by the associated frequency squared ( ⁇ 2 ), and then inverse transforming the result back to the time domain.
• amplitude spectrum i.e. a plot of amplitude versus frequency
• ⁇ 2 the associated frequency squared

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• 1999
  • 1999-02-09 AU AU25930/99A patent/AU2593099A/en not_active Abandoned
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