METHOD FOR TIME LAPSE RESERVOIR MONITORING

Abstract: A method of comparing multiple seismic survey data sets of a reservoir is provided, wherein a first seismic survey data set is taken at a first time and a second seismic survey data set is taken at a second time, to detect changes in the reservoir between the first time and the second time. Generally, the method comprises design of a crossequalization function for the second data set based on a comparison of unchanging portions of the two data sets. Also provided is a processing method for preparing each survey according to similar processing steps with information taken from each survey.

Description: METHOD FOR ΗME LAPSE RESERVOIR MONITORING BACKGROUND OF THE INVENTION This application is a continuation-in-part of US application serial no. 08/713,948, assigned to the assignee of the present invention, and including an inventor common to the inventors of the present invention. This invention relates to oil and gas reservoir management and, more specifically, to time lapse reservoir seismic signal processing. Reservoir characterization and monitoring in the oil and gas field are important parts of reservoir management and hydrocarbon production. Effective reservoir management is a major goal of energy producing companies as they try to reduce finding costs, optimize drilling locations and increase financial returns. One technique that is attempted in this endeavor is time-lapse (also known as 4D) seismic monitoring. As fluids are extracted, swept, or injected through production and recovery, changes in the effective elastic properties of the reservoir rocks occur. The ability to monitor reservoir changes as a function of time by the use of seismic methods can lead to better location of production and infill wells, the possibility of locating unswept zones, and more efficient field maintenance, thus raising the overall value of the production lease. In a two-dimensional approach, seismic monitoring has been examined in crosswell procedures. However, the repeated results have only been qualitatively compared, and two dimensional time-lapse surveys, so far, do not contain me type of information desired in modern reservoir management (for example, see Paulsson, et. al, 1994, The -Leading Edge, incorporated herein by reference). Time-lapse 3D techniques have also been attempted, but they involve complex modeling procedures and require a great deal of processing without using direct data available in the survey itself. See, e.g., U.S. Patent 4,969,130, incorporated herein by reference. One problem in time lapse processing is that many conditions change over time, not just the changes in the state of the reservoir. For example, the locations of the source and receiver in the second survey will necessarily be different from those in the first. Further, the tide in a second marine survey may be higher or lower, as may the temperature of the air and water. Likewise, the specific characteristics of the sources and receivers used in the second survey will be different. Other differences, besides changes in reservoir state also occur, such as differences in the manner in which the two surveys are processed. Thus, there is a need for a method of dealing with the two surveys whereby processing differences do not detrimentally affect the result of the comparison. For example, in gathering seismic data, a source is used to generate seismic waves which reflect from the reflectors in the earth (e.g. layer boundaries) and are received at receivers. In some cases, the source signature is a spike, although, in reality, it is not perfect. During its journey through the strata and reflectors, the signal shape is changed, and the reflection signal received at the receivers is, therefore, no longer a spike, or even close. Deconvolution is the process by which the shape of the reflection signal is "whitened" to recreate the spike shape of the data. In another example, a broad, band-limited signal is used, which is zero phase. Deconvolution is used in such a case to remove the distortions caused by the earth. In still another example, in performing the deconvolution in the frequency domain, all frequency samples are multiplied to bring them to an equal level, following the assumption that the source is a minimum phase signal, immediately rising to a peak and then dying. This is accomplished by autocorrelating the trace in the time domain multiple times, at a series of lag samples, which results in a generally symmetrical wavelet. The power spectrum of the wavelet is then analyzed, to determine what multipliers are needed at each frequency to flatten the frequency spectrum. This process is performed on a windowed basis, both along each trace, and across the record (as used herein, the term "record" refers to, alternatively for example, a common receiver record, a CMP record, a common shot record, a stacked trace record, etc.) The autocorrelation is performed on various windows, and the results are averaged to give the spectrum. From that spectrum, the operators needed to flatten the spectrum are chosen. The operators are then applied to all of the traces used in the input. Typically, the window is about 10 times the length of the operator to be generated, measured in number of samples. The deconvolution process and the design of a deconvolution operator are well known in the art, and it is not limited to the frequency domain example, above. It is also routinely performed in the time domain. See, e.g., Yilmaz, nvestigations in Geophysics, Vol. 2, Seismic Data Processing, Society of Exploration Geophysicists (1987) and references cited therein. In performing deconvolution, it is important to design a deconvolution operator dependent upon the data of the survey, in order to account for the specific source signature, and other equipment distortions that occur in the data. Therefore, the data in each survey has been adjusted through the use of a specific optimum deconvolution operator which is not applicable to other surveys. The result of this difference in the use of the separate deconvolution operators in time-lapse surveys is that structure appears in the difference records when two records are subtracted. This result is undesirable. However, to date, no one has proposed a practical solution to the problem. SUMMARY OF THE INVENTION It is an object of the present invention to address the above problems. It has been found that, contrary to earlier beliefs, a single deconvolution operator can be used on multiple sets of data, not only without detrimental results, but improving the quality of the processing of time-lapse comparisons of seismic surveys. Accordingly, in one aspect of the present invention, a method of deconvolution of multiple sets of seismic data from the same geographic area is provided, the method comprising: designing of a deconvolution operator dependent upon data from at least two of the sets of seismic data, wherein the at least two sets of seismic data were recorded at different times or calendar dates; applying the deconvolution operator in a deconvolution process to both of the at least two sets of data; and conducting further time-lapse processing to form a difference record. According to one embodiment of the invention, the conducting of further time- lapse processing comprises: providing a first reflection event (for example, a wavelet) in the first seismic survey data set having a corresponding second reflection event in the second seismic survey data set, wherein the first reflection event and the second reflection event represent an unchanged portion of geologic structure in or near the reservoir and wherein the first reflection event is represented by a first set of event parameters and the second reflection event is represented by a second set of event parameters. Next, an acceptance threshold difference function between the first set of event parameters and the second set of event parameters is provided. Then, a crossequalization function is determined to apply to the second set of event parameters. According to another aspect of the invention, the crossequalization function is determined such that, upon application of the crossequalization function to the second set of event parameters, a crossequalized set of event parameters is defined, and the difference between the first set of event parameters and the crossequalized set of event parameters is below the threshold difference function. Next, the crossequalization function is applied to a third reflection event, the third reflection event being related to the second data set, wherein a crossequalized third reflection event is defined, wherein the third reflection event has a corresponding fourth reflection event in the first data set, and wherein the third and fourth reflection events represent a changing portion of the reservoir. Comparison of the crossequalized third reflection event to the fourth reflection event by subtracting the crossequalized third reflection event from the fourth reflection event results in the desired information. According to a more specific example embodiment, said providing said acceptance threshold difference function comprises: iterative selection of event parameter modifications to the second set of event parameters, application of the event parameter modifications to the second set of event parameters, wherein a modified set of event parameters is defined, comparison of the modified set of event parameters to the first set of event parameters, wherein said iterative selection continues until a convergence is reached, and wherein the acceptance threshold difference function comprises the modified set of event parameters at convergence. Example event parameters comprise any combination of amplitude, phase, bandwidth, and time, or any of the foregoing individually. According to another example embodiment, the determining of the crossequalization function comprises: iterative selection of event parameter modifications to the second set of event parameters, application of the event parameter modifications to the second set of event parameters, wherein a modified set of event parameters is defined, comparison of the modified set of event parameters to the first set of event parameters, and providing an acceptance threshold difference, wherein said iterative selection continues until a comparison result from said comparison designates a difference between the first set of event parameters and the modified set of event parameters below the acceptance threshold difference . According to yet another example embodiment, said providing an acceptance threshold difference function comprises: providing a windowed trace difference between a time window of a first trace from the first seismic survey data set and a time window of a second trace from the second seismic survey data set, wherein the second trace includes reflection events corresponding to reflection events in the first trace and wherein the time window of the second trace is substantially the same as the time window of the first trace, and providing a ratio of the windowed trace difference over the time window of the first trace, and choosing the acceptance threshold difference to be less than the ratio. The time windows in both the unchanging and changing portions of the reservoir have similar spectral characteristics. For example, if the data from the reservoir has a dominant frequency of 30 Hz, the time window used should be picked from an unchanging portion of the survey having a dominant frequency as close to 30 Hz as possible. Likewise, phase changes in the reservoir and the unchanging portion should be as close as possible. It is preferred, however, to err on the side of broader time windows. For example, if the reservoir dominant frequency is 30 Hz, a window having 35 Hz is considered preferable to one of 25 Hz. Such bandwidth error of less than about 25% in frequency bandwidth is believed to yield adequate results. Best results should be seen when bandwidth error is below 10% . In still another embodiment, said providing an acceptance threshold difference function comprises: providing a windowed trace difference between a time window of the square of a first trace from the first seismic survey data set and a time window of the square of a second trace from the second seismic survey data set, wherein the second trace includes reflection events corresponding to reflection events in the first trace and wherein the time window of the second trace is substantially the same as the time window of the first trace, and providing a ratio of the windowed trace difference over the time window of the square of the first trace, choosing the acceptance threshold difference to be less than the ratio. In still another embodiment, said applying the crossequalization function to a third reflection event in the second data set comprises convolution between the crossequalization function and the third reflection event in the second data set, said first data set comprises a trace from a seismic receiver. Alternatively, said first data set and said second data set comprise a summed set of traces from a set of seismic receivers, or CMP ("common mid-point") data. In still a further embodiment, said first data set and said second data set comprise shot data. In further alternatives, said first data set and said second data set comprise prestack data or migrated data. In many embodiments, said first data set and said second data set are subjected to equivalent prestack processes. For example, in addition to the deconvolution described above, in some embodiments the first data set uses the same designature process as the second data set, the same noise attenuation processing steps as second data set, and die same multiple attenuation processing as the second data set. Further, in many embodiments, the same DMO operator is used on first data set as on the second data set, and migration on the first data set is conducted with the same velocity field as migration on the second data set. Finally according to a further aspect of the invention, a method is provided for performing time-lapse seismic survey signal processing, the method comprising: performing a set of processing steps on the first survey; performing the set of processing steps on the second survey, wherein die set of processing steps is dependent upon a set of seismic signal parameters; choosing at least one of the set of parameters by a selection process dependent upon data from both surveys; and (b) applying the at least one of the sets of parameters in the at least one of the set of processing steps to both the first survey and the second survey. DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and for further advantages diereof, reference is made to die following Description of Example Embodiments of die Invention, taken in conjunction with the accompanying drawings, in which: Figure 1A is a plot of a first record of a survey taken at a first time, witi out deconvolution. Figure IB is a plot of the first record of Fig. 1A with deconvolution, wherein die deconvolution operator was designed dependent upon the data of die first record. Figure 1C is a plot of the raw data of Fig. 1A witii deconvolution applied, wherein die deconvolution operator was designed dependent on both die data of Fig. 1 A and another survey. Fig. 2 A is a plot of a second record of a second survey taken of me same geographic area as tiiat of Fig. 1 A, but at a different time, witiiout deconvolution. Fig. 2B is a plot of die second record of Fig. 2A, witii deconvolution, wherein the deconvolution operator was designed dependent upon die data of me second record. Fig. 2C is a plot of die second record of Fig. 2A, with deconvolution, wherein the deconvolution operator was designed dependent upon the data of bodi the record of Fig. 1 A and die record of Fig. 2A. Fig. 3A is a plot of die difference between Figs. 1 A and 2C. Fig. 3B is a plot of die difference between Figs. IB and 2B Fig. 3C is a plot of the difference between Figs. 1C and 2C It is to be noted, however, tiiat die appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for die invention may admit to otiier equally effective embodiments. It is to be noted, however, tiiat the appended drawings illustrate only typical embodiments of tiiis invention and are therefore not to be considered Umiting of its scope, for die invention may admit to otiier equally effective embodiments. DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION As used herein, "crossequalization" is die catch-all industry term for die match- filtering, amplitude scaling and static corrections necessary for time-lapse seismic monitoring. In essence, a wavelet operator or operators are estimated to shape and match me reflection data from one survey to another. Typically, operators are designed over static reflectors tiiat exclude die reservoir where meaningful changes in (pore) fluid state may occur. Theoretically, me difference between two volumes of data after crossequalization should be zero everywhere (inside and outside die design window) except where there are changes in me reservoir. All of die static or non-reservoir events should be subtracted out, leaving only changes in dynamic events (i.e., fluid replacement witiύn die pores). Whedier a change in die reservoir state is seismically detectable is determined by me reservoir's lidiology and die acoustic properties of die pore fluids (which are temperature and pressure dependent), d e type of production or recovery process, die resolution (spatial and temporal) of die data, in addition to die repeatability of the seismic system. In certain reservoirs witii gas drives, gas injection may reduce die acoustic impedance (oil being displaced by gas) sufficiently to induce a bright spot. See, e.g., Fulp and Greaves, GEOPHYSICS, 1987, incorporated herein by reference, for a bright spot generated by a fire flood. In otiiers, gas injection or water injection (water drive) may have die opposite reaction and generate a dim spot. Where steam flooding is involved, velocity pull-down or sag often is used to indicate steam fronts to determine to what extent die recovery has progressed. For example, see Lumley, et. al., SEG Expanded Abstracts, 1995, incorporated herein by reference. As aforementioned, petrophysical and seismic interpretation are required to understand what attribute is meaningful, and how much change one might observe. Processing The rudimentary data processing steps toward d e goal of obtaining spatially- correct, relative amplitude preserved data include NMO/DMO and prestack (zero-offset) migration used for die forthcoming examples. Otiier appropriate prestack procedures will occur to diose of skill in die art, including data compression and sampling techniques tiiat can be employed to reduce die computational overhead and preserve the quality of die prestack processing tiiat will occur to tiiose of skill in die art. After such processing, relative amplitude data conditioning yields prestack migrated CMP data on which crossequalization and time-lapse differencing measures are performed. Depending on die producing reservoir, according to alternative embodiments of the invention, CMPs, AVO attributes, and/or migrated stacks from each survey are crossequalized and subtracted (differenced) to observe interpore fluid displacements. It is recommended tiiat each survey be processed as close to identically as possible (which is not normally done in time-lapse processing). Before the present invention, whatever processed data existed on workstations (witii dissimilar preconditioning procedures and algoritiims) was used, and removal of die resultant variations in data was attempted dirough crossequalization. Better crossequalizations and more meaningful seismic differences will be obtained by reprocessing existing data witii the same metiiodology and software as die recent seismic monitoring survey. Thus, it is particularly preferred to designature, deconvolve, attenuate noise and multiples with die same parameterization, survey to survey, just as it is important to use the same DMO operator and migrate die data witii die same velocity field, to obtain premier time-lapse measurements. Therefore, according to one aspect of die invention, a method is provided for performing time-lapse seismic survey signal processing, wherein a set of processing steps is performed on the first survey, die set of processing steps in performed on die second survey, and die set of processing steps is dependent upon a set of seismic signal parameters, die meti od comprising: (a) choosing at least one of the set of parameters by a selection process dependent upon data from botii surveys; and (b) applying d e at least one of die sets of parameters in the at least one of die set of processing steps to botii die first survey and die second survey. For example, it has been found tiiat improved deconvolution processing of die two surveys results from die design of a deconvolution operator dependent upon multiple sets of data. This deconvolution operator is then applied to both data sets. The result of such a design, while perhaps not optimum for either particular set, is, nevertheless, optimum for time-lapse comparison purposes. According to one example embodiment of this aspect of die invention, a method of deconvolution of multiple sets of seismic data from die same geographic area is provided, die metiiod comprising, designing of a deconvolution operator dependent upon data from at least two of the sets of seismic data, wherein die at least two sets of seismic data were recorded at different times; applying the deconvolution operator in a deconvolution process to botii of die at least two sets of data; and conducting further time-lapse processing to form a difference record. In some cases, tiiere are at least tiiree sets of seismic data, wherein each of the at least three set represents recordings from surveys conducted at different times, and wherein die designing of a deconvolution operator is dependent upon data from each of die sets of surveys. In some of tiiese cases, a further step is provided, comprising designing a second deconvolution operator dependent upon data from die first and d e tiiird sets of data, the second deconvolution operator being applied to the first set of data and die tiύrd set of data. The first set of data may have been taken before or after die second set of data witii equal effectiveness. According to a more specific example embodiment, die design of die first deconvolution operator comprises: averaging of a power spectrum for a first ensemble of traces from die first set; averaging of a power spectrum for a second ensemble of traces from die second set; averaging of the power spectrum averages of die first and the second ensemble; and designing a deconvolution operator for die first and die second survey from the averaging of the power spectrums of die first and die second ensemble. In some circumstances, die above averaging for each subsequent survey would be prohibitively expensive. Therefore, according to an even further embodiment of the invention, multiple surveys are handled by storing the averaging of die first survey, so tiiat when a later survey is made, die first survey's autocorrelations will not have to be recalculated. According to such an embodiment, the designing of the first deconvolution operator comprises: averaging of a power spectrum for a first ensemble of traces from tiie first set; inverse tfamforming die average into a time-domain representation of me average for die power spectrum of die first ensemble; storing die time-domain representation; averaging of a power spectrum for a second ensemble of traces from the second set; inverse transforming die average into a time-domain representation of the average of die power spectrum for the second ensemble; averaging of the time-domain representations of die averages for die power spectrum of die first and die second ensemble; and designing a deconvolution operator for the first and die second survey from the average of die time-domain representations of die averages for the power spectrum of die first and the second ensemble. Referring now to Fig. 1A, a specific example will be discussed. Fig. 1A shows a plot of a first raw stack (no DMO, migration, or other noise suppression used, only spherical divergence and geometry correction) of a survey taken at a first time is shown, without deconvolution. Figure IB is a plot of the first record of Fig. 1A with deconvolution, wherein the deconvolution operator was designed before die stack dependent upon die data of die first raw stack. The specific parameters were: Sample rate: 2 milliseconds Opperator Lengtii: 140 milliseconds Prediction gap: Spiking, 2 milliseconds Analysis Window: Offset dependent: for die near offsets, 300-5000 msec., for the far offsets, 3600 - 5000msec. Added white noise : 0.5% Decon apply window: 0 - 600 msec. Fig. 2A is a plot of a second raw stack of a second survey taken of die same geographic area as that of Fig. 1A, but at a different time, without deconvolution, and Fig. 2B is a plot of die second record of Fig. 2A, witii deconvolution, wherein the deconvolution operator was designed before die stack dependent upon the data of die second raw stack. Here, die specific parameters were as above. Fig. 3A is a plot of die difference in Figs. 1A and 2A. Fig. 3B is a plot of the difference between Figs. IB and 2B. Ideally, tiiere would be notiiing but noise or changes in structure showing in difference plot of Fig. 3A, if the two surveys were otiierwise identical and tiiere had been no change in die state of the reservoir. However, it can be seen tiiat unchanged structure appears in the difference plot 3B. Fig. 1C is a plot of the first stack of Fig. 1A, with pre-stack deconvolution, wherein the deconvolution operator was designed dependent upon die data of die first record and the second stack. Fig. 2C is a plot of die second stack of Fig. 2A, witii prestack deconvolution, wherein die deconvolution operator was designed dependent upon die data of the first record and the second record. The specific parameters were, for botii Figs. 1C and 2C, the same as for Fig. 1A. Fig. 3C is a plot of the difference between die plots of Figs 1C and 2C. Notice the lack of unchanged structure. In designing d e deconvolution filter used some embodiments, the first (i.e. "base") dataset is processed, wherein die average of die trace autocorrelation functions within each ensemble (i.e. shot, receiver, CMP, etc.) is computed in die f-x domain, to estimate the power spectrum: i = 0 P(i, k ) = (1 / N )∑ P (i, k ), i = 1 , . . . O * = ι where: N is the number of traces in the x direction, Q is die nyquist frequency index, P(i, k) is die power spectrum estimation at frequency index and trace number k, and i is the time-lapse index. Next, an inverse FFT is performed and the autocorrelation function traces for each ensemble are saved. The same power spectrum estimation is performed on a dataset taken at a different time (i.e. a "monitor" dataset) to compute: i = 1 P(i, k) where i = 1 is the first time lapse monitor survey. Next, tiie all die time lapse autocorrelation average functions are averaged to generate updated autocorrelation functions, according to die following formula: M p i A. BM = / ;. ι M i = 1 , .. -M where M is die number of time lapses. The resulting autocorrelation traces are used to design convolution filters which are applied to all die time lapse surveys. Other deconvolution filter design processes (e.g., single window deconvolution, time varying deconvolution, surface consistent deconvolution, and common domain deconvolution (shot, receiver, midpoint, offset)) will occur to tiiose of skill in d e art, which are dependent upon both sets of data, which do not depart from die spirit of the present invention. Further, like die deconvolution operation described above, tiiere are other processing steps which benefit under one aspect of die present invention by designing the process parameters dependent upon die data from each set, rather tiian the date from one set. Examples include: crossequalization (discussed in more detail below), f-k filtering, τ-p and Radon domain filtering, static calculations, and multiple removal in FK, t-p, and Radon domains. There are always variations in actual acquisition, no matter how well operations are planned or executed. And, in an ideal world die processing would also be identical, because the acquisition would be replicated from base survey to monitor survey. But that is not realistic. Thus, intersurvey acquisition variations occur with many systems tiiat are useful witii the present invention (for example, fixed installation systems, OBS systems, and streamer systems). These variations could be caused by featiiering, cable drift, shooting azimutii, source variation, sea state conditions, etc., which can result in processing variations between surveys. Crossequalization According to one example embodiment of die invention, there are four "correction" elements the crossequalization function used. They are timing corrections, RMS energy balancing, bandwidtii normalizing, and phase matching. Each element essentially budds a transfer or impulse response function between the two surveys, or alternatively, between two daces in each survey. The resultant, crossequalized trace (I XEQ ) is calculated in die following manner: txEQ = t * f ( £ rm_ n p w . ) (1) where t is the input trace, * connotes convolution, and s^., rms, -,., m ^ . and p^. are crossequalization elements (in impulse response f) corresponding to time, amplitude, magnitude, and phase, respectively. According to alternative embodiments of the invention, impulse response functions are computed trace-by-trace, in a surface consistent manner, or globally from the base-line survey (base) and the monitor survey (repeat). According to the present example embodiment, a monitor survey is transformed (crossequalized) to look like a base survey, and die crossequalization impulse response functions are designed by horizon-specific time gates that exclude die reservoir zone where change is anticipated. The effects of each component dtrough simple models and example data from a time-lapse three-dimensional project will now be explained. Crossequalization Elements Time corrections (s ^ ..) Besides rough seas in a storm and current conditions that may cause streamer feathering, tidal and temperature changes are also in play. These changes may not be significant when recording, processing or interpreting seismic data for a single 3D survey. However, when several 3D surveys are involved for time-lapse monitoring, there are potential intersurvey sea state variations. Seasonal and shorter-period (storm related) variations in temperature and salinity can also cause intersurvey differences between time reflectors. The degree of variation is a function of the severity of die change between the temperature and salinity profiles and the depth of these variations in die water column. For example, surveys taken in die East China sea show tiiat over a 10 year period large scale, seasonal variations are conspicuous, and tiiey are sufficient to cause a temporal intersurvey variation of two milliseconds (TWT) between reference reflectors during seasonal extremes (if acquired in January and June, per se), where water deptiis are equal to or greater than 100 m. As for tidal changes, an extreme example would be to shoot seismic data in the Bay of Fundy, between the Canadian Adantic provinces of New Brunswick and Nova Scotia. Here, the tidal changes are approximately 15 m tiiat is equivalent to 20 ms two- way time (assuming a typical water velocity of 1500 m/s). If one survey was acquired at low tide and another at high tide (assuming very short survey periods), seismic differencing of the datasets without a time correction would not yield satisfactory results. In general, the errors increase with increasing delay, and broader bandwidth wavelets obtain larger error differences at shorter delays. RMS energy balancing (rms^). In a perfect time-lapse seismic monitoring experiment, the same seismic crew would acquire each monitor survey with the exact same equipment, under d e exact (sea state) conditions that the original baseline survey was acquired with. However, acquisition differences do occur, even under the best controlled situations. For example, scaling the data to equivalent RMS levels is important, especially if it (die RMS energy) alters significantly between surveys. In one example, from approximately 100 CMPs in a time-lapse 3D dataset acquired by su"eamer vessels approximately 18 months apart, acquisition geometry and instrumentation were nearly identical, with the exception that one survey was acquired in the summer months and the other in the winter. Analysis of the average amplitude of die seismic data witi in discrete 200 ms windows shows tiiat tiiere is amplitude bias between the base survey and the monitor survey throughout die profile. After application of a smoothed, amplitude-scaling crossequalization element within each 200 ms time gate, it was seen that the disparities between die base and monitor surveys are smaller after crossequalization. Without question, variations such as these need to be corrected before time-lapse seismic difference sections/surveys can be interpreted. Bandwidtii Normalization ( ^.). Equating bandwidtiis is also part of seismic differencing. If two seismic volumes are to be subtracted, it would be best to do so witii each volume having equivalent spectra. Crossequalization (if robust enough), should correct one spectrum to match another, but if die crossequalization is not optimized, discrepancies appear. Larger errors occur for large mismatches in bandwidtii. Tests indicate that residual reflector energy (i.e. energy left in die difference section/survey that, ideally, should be removed) ranges from 7% to 40% of input amplitude for center frequency differences of 2.5 to 12.5 Hz, respectively (for Ricker wavelets with center frequencies incrementally separated by 2.5 Hz subtracted from a baseline 30 Hz Ricker wavelet). A maximum center frequency spread of 12.5 Hz is reasonable for most vintage 3D surveys, but larger bandwidtii difference occurs in some cases, and, in tiiose cases, the residual reflector energy is larger. Therefore, according to embodiments processing these surveys, crossequalization normalizes the bandwidtii differences to reduce tiiese potential seismic differencing errors. Phase matching (p, ^ ). Finally, errors are also associated with phase mismatches. For example, for a 30 Hz Ricker wavelet, phase-rotated by a constant 5, 10, 15, 30, 60 and 90 degrees from zero-phase, residual reflector energy caused by phase differences can be as large as 20% of die input for phase mismatches as small as 15 degrees. Further, the human eye can have difficulty seeing phase differences of 15 degrees or less, so it is significant tiiat the crossequalization operator correct for small phase mismatches as well as large ones. Phase difference errors are unaffected by bandwidth differences. It has been found tiiat residual reflector errors of 20% can occur for wavelets out of phase by 15 degrees. Seismic Differencing The ultimate goal in time-lapse seismic monitoring is to see changes that infer interpore fluid movement or die absence of interpore fluid movement between calendar dates. Currently this is obtained by subtracting a monitor survey from tiie base survey after rectifying the data dirough crossequalization. Differenced data witii reduced amounts of residual reflector energy will have a higher probabihty of identifying fluid movement than those with higher residual reflector energy. To characterize the effectiveness of crossequalization, we show several difference plots with various component crossequalization applications using a North Sea time-lapse experiment. Migrated inline stacks of the base and monitor survey traversing a non-reservoir area are presented in Figures 4 and 5, respectively, and a prominent series of strong static reflectors is visible between 2000 and 2600 ms. For tiiis example a design window of 1900 ms to 2700 ms is used. An 800 ms operator was created and applied from the design window, and a bandpass filter of 3/8-35V55 Hz was applied to botii datasets before crossequalization. Since these are static reflectors (i.e., tiiey are reflection events representing an unchanged portion of the geologic structure in or near tiie reservoir), subtraction of the base and monitor survey should ideally result in very little residual reflector energy, since botii surveys were acquired identically. However, die difference section obtained witiiout any crossequalization, showed tiiat subtraction did not. The residual reflector energy is significant. With these large amounts of residual reflector energy left in tiiis section, identifying any fluid movement (elsewhere) in reservoir portions of the volume would be difficult. Comparison of a difference section obtained witiiout any crossequalization with one in which only tiie amplitude spectrum was crossequalized, one in which only the phase spectrum was crossequalized, and one in which botii were crossequalized, shows that phase corrections are a major component of the total crossequalization operation, which reiterates the synthetic modeling results. The above processing is performed on a massively parallel processor platform (e.g. IBM SP2, Intel Paragon) and software compatible with such hardware (e.g. PGS Tensor Inc. 's CUBE MANAGER™ software). Suggested Readings Two published case histories on time-lapse seismic monitoring are Greaves and Fulp (Geophysics, 1987), incorporated herein by reference, who monitor a fire-flood in west Texas, and Johnstad, Uden and Dunlop (First Break, 1993), also incorporated herein by reference, monitoring gas injection in the Oseberg Field located in the Norwegian sector of the North Sea. Another good paper involving a steam flood in die Duri Field in Indonesia was presented by Lumley, et. al. at die 1995 SEG meeting in Houston, which can be through the SEG. The above description is given by way of example only and otiier embodiments will occur to ti ose of skill in the art without departing from die scope of die invention which is defined by the claims below and equivalents thereof .

Claims: What is claimed is: 1. A metiiod of comparing multiple seismic survey data sets of a reservoir, wherein a first seismic survey data set is taken at a first time and a second seismic survey data set is taken at a second time, to detect changes in die reservoir between the first time and die second time, the method comprising: providing a first reflection event in the first seismic survey data set having a corresponding second reflection event in the second seismic survey data set, wherein die first reflection event and the second reflection event represent an unchanged portion of geologic structure in or near tiie reservoir and wherein the first reflection event is represented by a first set of event parameters and die second reflection event is represented by a second set of event parameters; providing an acceptance threshold difference function between die first set of event parameters and die second set of event parameters; determining a crossequalization function to apply to the second set of event parameters, the crossequalization function being characterized in tiiat, upon application of the crossequalization function to the second set of event parameters, whereby a crossequalized set of event parameters is defined, and die difference between the first set of event parameters and the crossequalized set of event parameters is below the threshold difference function; applying the crossequalization function to a third reflection event, the third reflection event being related to the second data set, wherein a crossequalized third reflection event is defined, wherein the third reflection event has a corresponding fourth reflection event in the first data set, and wherein the third and fourth reflection events represent a changing portion of the reservoir; comparing tiie crossequalized tiiird reflection event to the fourth reflection event by subtracting the crossequalized tiiird reflection event from die fourth reflection event.. 2. A method as in claim 1 wherein said providing said acceptance ti reshold difference function comprises: iterative selection of event parameter modifications to the second set of event parameters, application of the event parameter modifications to d e second set of event parameters, wherein a modified set of event parameters is defined, comparison of the modified set of event parameters to die first set of event parameters, wherein said iterative selection continues until a convergence is reached, and wherein the acceptance threshold difference function comprises the modified set of event parameters at convergence. 3. A metiiod as in claim 2 wherein one of said event parameters comprise amplitude. 4. A metiiod as in claim 2 wherein one of said event parameters comprise phase. 5. A metiiod as in claim 2 wherein one of said event parameters comprise bandwidtii. 6. A metiiod as in claim 2 wherein one of said event parameters comprise time. 7. A method as in claim 2 wherein said event parameters comprise amplitude, phase, bandwidtii, and time. 8. A method as in claim 1 wherein said deteπnining a crossequalization function comprises: iterative selection of event parameter modifications to the second set of event parameters, application of the event parameter modifications to die second set of event parameters, wherein a modified set of event parameters is defined, comparison of the modified set of event parameters to die first set of event parameters, and providing an acceptance tiireshold difference, wherein said iterative selection continues until a comparison result from said comparison designates a difference between die first set of event parameters and die modified set of event parameters below the acceptance threshold difference. 9. A metiiod as in claim 1 wherein said providing an acceptance threshold difference function comprises: providing a windowed trace difference between a time window of a first trace from die first seismic survey data set and a time window of a second trace from the second seismic survey data set, wherein die second trace includes reflection events corresponding to reflection events in the first trace and wherein die time window of the second trace is substantially the same as the time window of die first trace, and providing a ratio of the windowed trace difference over the time window of the first trace, choosing die acceptance tiireshold difference to be less tiian die ratio. 10. A method as in claim 9 wherein die difference in bandwidtii of the time window of die first trace and tiie time window of the second trace is less tiian about 25% . 11. A metiiod as in claim 10 wherein the difference in widtii of the time window of die second trace and d e bandwidtii of die time window of the first trace is less than about 10%. 12. A method as in claim 1 wherein said providing an acceptance threshold difference function comprises: providing a windowed trace difference between a time window of die square of a first trace from die first seismic survey data set and a time window of the square of a second trace from the second seismic survey data set, wherein die second trace includes reflection events corresponding to reflection events in the first trace and wherein die time window of die second trace is substantially the same as the time window of d e first trace, and providing a ratio of the windowed trace difference over die time window of die square of die first trace, choosing die acceptance tiireshold difference to be less tiian the ratio. 13. A method as in claim 7 wherein the time window has a length of at least about two reflection events. 14. A method as in claim 13 wherein the time window has a lengtii of at least about five reflection events. 15. A method as in claim 1 wherein said applying the crossequalization function to a third reflection event in the second data set comprises convolution between the crossequalization function and the third reflection event in tiie second data set. 16. A method as in claim 1 wherein said first data set comprises a trace from a seismic receiver. 17. A method as in claim 1 wherein said first data set and said second data set comprise a summed set of traces from a set of seismic receivers. 18. A method as in claim 1 wherein said first data set and said second data set comprise a summed set of traces from a set of borehole receivers. 19. A method as in claim 1 wherein said first data set and said second data set comprise prestack data. 20. A metiiod as in claim 19 wherein said prestack data comprises CMP data. 21. A method as in claim 19 wherein said prestack data comprises shot data. 22. A method as in claim 19 wherein said prestack data comprises migrated data. 23. A metiiod as in claim 1 wherein said first data set and said second data set are subjected to equivalent prestack processes. 24. A metiiod as in claim 1 wherein the first data set is subjected to die same designature process as die second data set. 25. A method as in claim 1 wherein tiie first data set is subjected to die same deconvolution process as the second data set. 26. A metiiod as in claim 1 wherein die first data set is subjected to die same noise attenuation processing steps as second data set. 27. A method as in claim 1 wherein the first data set is subjected to die same multiple attenuation processing as the second data set. 28. A metiiod as in claim 1 wherein die same DMO operator is used on first data set as on die second data set. 29. A metiiod as in claim 1 wherein migration on the first data set is conducted witii the same velocity field as migration on the second data set. 30. A method as in claim 1 wherein migration on the first data set is conducted witii the same migration operator as migration on the second data set. 31. A metiiod as in claim 1 wherein filtering on die first data set is conducted with d e same filter as filtering on d e second data set. 32. A metiiod of deconvolution of multiple sets of seismic data from die same geographic area is provided, tiie method comprising: designing of a deconvolution operator dependent upon data from at least two of the sets of seismic data, wherein the at least two sets of seismic data were recorded at different times; applying the deconvolution operator in a deconvolution process to both of die at least two sets of data; and conducting further time-lapse processing to form a difference record. 33. A metiiod as in claim 32, wherein die at least two of the sets of data comprises at least three sets of seismic data, wherein each of die at least three sets represents recordings from surveys conducted at different times, and wherein the designing of a deconvolution operator is dependent upon data from each of tiie sets of surveys. 34. A method as in claim 32, wherein die at least two of ti e sets of data comprises at least tiiree sets of seismic data, wherein each of die at least three sets represents recordings from surveys conducted at different times, and wherein die designing comprises designing a first deconvolution operator dependent upon data from a first set of data and a second set of data, die first deconvolution operator being applied to the first set of data and die second set of data, and further comprising: designing a second deconvolution operator dependent upon data from die first and a third set of data, die second deconvolution operator being applied to the first set of data and the third set of data. 35. A method as in claim 34, wherein die first set of data represents recordings from a survey taken before a survey represented by die second or tiiird sets of data. 36. A metiiod as in claim 34, wherein the designing of die first deconvolution operator comprises: averaging of a power spectrum for a first ensemble of traces from the first set; inverse transforming the average into a time-domain representation of the average for the power spectrum of the first ensemble; storing the time-domain representation; averaging of a power spectrum for a second ensemble of traces from the second set; inverse transforming die average into a time-domain representation of the average of die power spectrum for the second ensemble; averaging of the time-domain representations of the averages for the power spectrum of the first and d e second ensemble; and designing a deconvolution operator for the first and the second survey from the average of the time-domain representations of the averages for the power spectrum of the first and the second ensemble. 37. A method as in claim 32, wherein die designing of the a deconvolution operator comprises: averaging of a power spectrum for a first ensemble of traces from the first set; averaging of a power spectrum for a second ensemble of traces from the second set; averaging of the power spectrum averages of the first and die second ensemble; and designing a deconvolution operator for tiie first and the second survey from die averaging of the power spectrums of the first and the second ensemble. 38. A method for performing time-lapse seismic survey signal processing, the method comprising: performing a set of processing steps on the first survey; performing the set of processing steps on tiie second survey; wherein the set of processing steps is dependent upon a set of seismic signal parameters; choosing at least one of the set of parameters by a selection process dependent upon data from botii surveys; and applying die at least one of the sets of parameters in the at least one of die set of processing steps to both the first survey and the second survey. 39. A method as in claim 38 wherein the at least one of the sets of parameters comprises a deconvolution operator. 40. A method as in claim 39 further comprising: designing of a deconvolution operator dependent upon data from at least two of the sets of seismic data, wherein the at least two sets of seismic data were recorded at different times; applying the deconvolution operator in a deconvolution process to both of the at least two sets of data; and forming a difference record dependent upon die applying. 41. A method as in claim 40, wherein the at least two of the sets of data comprises at least three sets of seismic data, wherein each of the at least three sets represents recordings from surveys conducted at different times, and wherein die designing of a deconvolution operator is dependent upon data from each of the sets of surveys. 42. A method as in claim 40, wherein the at least two of the sets of data comprises at least tiiree sets of seismic data, wherein each of die at least tiiree set represents recordings from surveys conducted at different times, and wherein the designing comprises designing a first deconvolution operator dependent upon data from the a first set of data and a second set of data, die first deconvolution operator being applied to the first set of data and die second set of data, and further comprising: designing a second deconvolution operator dependent upon data from the first and a tiiird set of data, die second deconvolution operator being applied to the first set of data and die third set of data. 43. A method as in claim 42, wherein die first set of data represents recordings from a survey taken before a survey represented by the second or third sets of data. 44. A metiiod as in claim 42, wherein the designing of the first deconvolution operator comprises: averaging of a power spectrum for a first ensemble of traces from the first set; inverse transforπύng the average into a time-domain representation of the average for the power spectrum of the first ensemble; storing the time-domain representation; averaging of a power spectrum for a second ensemble of traces from the second set; inverse trc_nsforming the average into a time-domain representation of die average of die power spectrum for the second ensemble; averaging of the time-domain representations of the averages for the power spectrum of the first and die second ensemble; and designing a deconvolution operator for the first and die second survey from the average of die time-domain representations of the averages for the power spectrum of the first and die second ensemble. 45. A method as in claim 40, wherein the designing of die a deconvolution operator comprises: averaging of a power spectrum for a first ensemble of traces from the first set; averaging of a power spectrum for a second ensemble of traces from the second set; averaging of the power spectrum averages of the first and the second ensemble; and designing a deconvolution operator for the first and the second survey from d e averaging of die power spectrums of the first and the second ensemble.

Inventors: ALTAN MEHMET SUAT, ROSS CHRISTOPHER PHILIP, ALTAN, MEHMET, SUAT, ROSS, CHRISTOPHER, PHILIP
Applicants: ALTAN MEHMET S, PGS TENSOR INC, ROSS CHRISTOPHER P, ALTAN, MEHMET, SUAT, PGS TENSOR, INC., ROSS, CHRISTOPHER, PHILIP
Application year: 1997
Main classification: G01V1
IPC codes: G01V1