Patent Publication Number: US-8990053-B2

Title: Method of wavelet estimation and multiple prediction in full wavefield inversion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application No. 61/470,237 filed on Mar. 31, 2011, entitled Method of Wavelet Estimation and Multiple Prediction in Full Wavefield Inversion, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field of geophysical prospecting and, more particularly to seismic data processing. Specifically, the invention is a method of wavelet estimation and multiple prediction in full wavefield inversion of seismic data. 
     BACKGROUND OF THE INVENTION 
     Full waveform inversion (“FWI”) is a method of inverting seismic data to infer earth subsurface properties that affect seismic wave propagation. Its forward modeling engine utilizes finite difference or other computational methods to model propagation of acoustic or elastic seismic waves through the earth subsurface model. FWI seeks the optimal subsurface model such that simulated seismic waveforms match field recorded seismic waveforms at receiver locations. The theory of FWI was initially developed by Tarantola (1989). Research and applications of FWI in exploration geophysics have been very active in the past decade, thanks to the dramatic increase of computing power. 
     It is well known that simulated waveforms depend linearly on the input source wavelet when linear acoustic or elastic wave equations are used to model seismic wave propagation. In fact, accurate estimation of source wavelet plays a critical role in FWI. Delprat-Jannaud and Lailly (2005) pointed that accurate wavelet measurements appear to be a major challenge for a sound reconstruction of impedance profiles in FWI. They noticed that a small error in the source wavelet leads to a strong disturbance in the deeper part of the inverted model due to the mismatch of multiple reflections. They concluded that “the classical approach for estimating the wavelet by minimizing the energy of the primary reflection waveform is not likely to provide the required accuracy except for very special cases”. 
     Indeed, inversion of primary reflections without well control faces a fundamental non-uniqueness in estimating the wavelet. For example, larger reflection events can be caused by larger impedance contrasts or a stronger source. Similar ambiguity exits for wavelet phase and power spectrum. Well data are commonly used to constrain wavelet strength and phase. But well logs are not always available, especially in an early exploration setting, or in shallow subsurface cases. 
     There have been extensive studies of wavelet estimation in the geophysical literature. In particular, inversion of wavelet signature for FWI was discussed by Wang et al. (2009) and the references therein. However, these methods all implicitly rely on direct arrivals or refracted waves for wavelet estimation. Because these transmitted modes propagate along mostly horizontal ray paths, they are influenced by effects (e.g. radiation pattern, complex interaction with the free surface) not affecting the near-vertical reflection ray paths. Such effects are often difficult to describe accurately and to simulate. Hence, there is a need to estimate the wavelet for the vertically-propagated energy, and this is particularly relevant for reflection-dominated applications (e.g. deep-water acquisition, imaging of deeper targets). 
     Multiples are considered to be noise in traditional seismic processing since they often contaminate primary reflections and make interpretation more difficult. On the other hand, it is known that multiples may also be useful for constraining subsurface properties and the seismic source wavelet. Verschuur et al. (1989, 1992) proposed a method of surface-related multiple elimination (SRME), by which wavelet estimation can be performed along with multiple elimination. The principle of SRME has been extended to an inversion scheme by G. J. A. van Groenestijn et al. (2009) to reconstruct the missing near-offset primaries and the wavelet. However it is unclear whether the optimal wavelet for SRME is also optimal for FWI. Papers such as van Groenestijn and Verschuur perform their multiple modeling and wavelet estimation in the time domain, i.e., data-driven without a subsurface model. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention is a computer-implemented seismic processing method comprising generating and simultaneously optimizing a source wavelet and subsurface model to an extendable depth, wherein simulated waveforms of both primary reflections and multiple reflections are generated from the source wavelet and the subsurface model in the depth domain and are then compared for a match to waveforms as recorded at seismic receiver locations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention and its advantages will be better understood by referring to the following detailed description and the attached drawings in which: 
         FIG. 1  is a flowchart showing basic steps in the present inventive method; 
         FIG. 2  shows a diagrammatic illustration of multiple reflections in seismic acquisition with source S 1  and receiver R 1 ; 
         FIG. 3  shows a shot gather of deep water seismic data, showing shallow primaries arriving before the multiples of interest, which are water-surface related multiples; 
         FIG. 4  shows an inverted shallow subsurface model from the shallow primaries in  FIG. 3 ; 
         FIG. 5  shows a recorded shot gather, a simulated shot gather based on the inverted shallow subsurface model in  FIG. 4 , and their data residual; 
         FIG. 6  shows recorded data having both deep primaries and multiples, simulated multiples based on the shallow subsurface model in  FIG. 4 , and the subtraction of the simulated multiples from the recorded data; 
         FIG. 7  shows blow-up of  FIG. 6  (3.8 s to 4.5 s) to clearly demonstrate quality of multiple prediction, with the frame showing data selected to perform wavelet estimation; 
         FIG. 8  shows multiple modeling with 30-deg phase rotation applied to the optimal wavelet: (from left) recorded data, simulated multiples based on rotated wavelet and inverted subsurface model, and their residual; 
         FIG. 9  shows results of multiple modeling based on a wavelet with an incorrect power spectrum (rich in low frequency compared with the optimal wavelet): (from left) recorded data, simulated multiples based on low frequency wavelet, and their residual; 
         FIG. 10  shows a power spectrum of recorded multiples and that of simulated multiples, used to design a shaping filter of the present invention; 
         FIG. 11  shows power spectrum of recorded multiples, and that of simulated multiples after applying the shaping filter to the wavelet used in  FIGS. 9 and 10 ; 
         FIG. 12  illustrates how a decision made in step  103  of  FIG. 1  is likely changed in step  106 ; and 
         FIG. 13  illustrates recursion of the present inventive method from shallow to deep. 
     
    
    
     The invention will be described in connection with example embodiments. However, to the extent that the following detailed description is specific to a particular embodiment or a particular use of the invention, this is intended to be illustrative only, and is not to be construed as limiting the scope of the invention. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the scope of the invention, as defined by the appended claims. Persons skilled in the technical field will readily recognize that in practical applications of the present inventive method, it must be performed on a suitably programmed computer. 
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The present invention is a wavelet estimation method for FWI applications that makes use of both the primary and multiple reflections in the data. The present inventive method uses the FWI algorithm to generate a subsurface model from primary reflections. The model is then used to simulate multiples. The wavelet is subsequently modified such that the simulated multiples closely match the true recorded multiples. 
     In addition to providing an accurate wavelet estimate, this method may provide some benefits as a multiple elimination strategy. In particular, it has no need for dense cross-line sampling to model multiples in 3D: once a model of the shallow subsurface has been built with FWI, modeling multiples is straightforward for any acquisition configuration. Also, because the method is model-based and does not rely on primary/multiple moveout separability, it naturally offers good protection of primaries. 
     Following the  FIG. 1  flow chart showing basic steps in one embodiment of this invention, deep water seismic data is used as an example to demonstrate the inventive method. Attention is primarily directed to wavelet estimation and modeling of water-surface related multiples, but persons skilled in the technical field will readily understand that a similar approach can be applied to other types multiples, an example of which is shown in  FIG. 2 . 
       FIG. 2  shows a diagrammatic illustration of multiple reflections in seismic acquisition with source S 1  and receiver R 1 . Multiples are considered as signals reflected more than once between any subsurface boundary Ai and any subsurface boundary Bi in the figure. In practice, at least one strong reflector is present to make an interesting scenario. Assume A 1  in the figure is a strong reflector, which could be the water-air interface in marine data or air-earth interface in land acquisition, or salt boundary, or any high-contrast subsurface. The multiples of interest are now defined as signals reflected more than once between A 1  and any of the Bi&#39;s. The term shallow primaries means signals arriving before the first arrival of the multiples of interest. Accordingly, deep primaries are defined as primary reflections arriving after the first arrival of the multiples of interest. 
     In step  101  of  FIG. 1 , a shallow subsurface model is inverted from shallow primaries.  FIG. 3  shows one shot-gather of this deep water seismic data set. In this case, the shallow primaries  7  refer to arrivals before 3.7 s since the water-surface related multiple arrives after 3.7 s. We are able to identify water-surface related multiples because the earliest arrival of these multiples should be 2× water bottom reflection time, by definition. That is approximately 3.7 s. (Time is shown on the vertical scale in  FIG. 3 , increasing downward.) The multiples of interest are water-surface related multiples. Relating back to  FIG. 2 , A 1  is now the water-air interface and B 1  represents the water-bottom. The “shallow primaries” used for inverting a shallow subsurface model are in frame  7 , which arrive before water-bottom multiples  8 . 
     A P-wave velocity model in the depth domain is now constructed from the shallow primaries referred to above, using a standard velocity model building tool. Applying acoustic FWI to the shallow primary gives the shallow acoustic impedance in  FIG. 4 , which is in the depth-cdp domain. Thus,  FIG. 4  shows the inverted shallow subsurface model from “shallow primaries”  7  in  FIG. 3 . The water-surface location is denoted by  9  and water-bottom is denoted by  10 . Notice that elastic FWI or more sophisticated physics could be applied in this step to capture waveforms more accurately up to longer offsets. 
       FIG. 5  shows recorded shot-gather  11  (which is just region  7  from  FIG. 3  with the time scale expanded), a simulated shot-gather  12  based on subsurface model in  FIG. 4 , and the difference between  11  and  12  is shown in  13 . The inverted shallow earth model in  FIG. 4  explains the measured data very well, as evidenced by the small data residual  13 . Notice that surface-related multiples are simulated by having the air-water interface in the model, although they are not present before 3.7 s, and hence are not shown in  FIG. 5 . 
     In step  102 , multiple reflections are simulated, i.e. modeled, using the shallow subsurface velocity model obtained by inversion in step  101  and an assumed seismic wavelet. By extending the simulation time to 5.5 s, we are able to simulate water-surface related multiples based on the subsurface model in  FIG. 4 . In  FIG. 6 ,  14  shows recorded seismic data in a 1 km common offset domain, which includes both deep primaries and water-surface related multiples generated from shallow sub-surfaces;  15  shows simulated water-surface related multiples based on the shallow subsurface model in  FIGS. 4 ; and  16  is a subtraction of  15  from  14 , and so  16  are the estimated deep primaries. 
     In step  103 , the simulated multiples are compared to the recorded data within a selected window to determine the degree of mismatch.  FIG. 7  shows a blow-up of  FIG. 6  (3.8 s to 4.5 s) to clearly demonstrate the quality of the multiple prediction. The window  17  was chosen in the recorded data, trying to minimize overlapping between multiple and primary reflections. The corresponding window in the simulated multiples is denoted by  18 . Data in the windows  17  and  18  are selected to perform wavelet estimation. The objective in the wavelet estimation is to minimize differences between recorded data  17  and simulated multiples  18 , since the window was chosen to avoid containing strong deep primaries in the data. The difference resulting from an optimal wavelet is shown in  19 . 
     If the match between simulated multiples and recorded data in step  103  is not satisfactory, the wavelet used to simulate the multiples may be adjusted in step  104  by correcting any one or more of the three wavelet properties that go into estimating the wavelet, i.e. wavelet amplitude, phase and power spectrum. Simulated multiples in  18  will not match recorded multiples in  17  unless optimal wavelet estimation is achieved.  19  shows the difference between  17  and  18  when the optimal wavelet estimation is used to generate the sub-surface model in  FIG. 4 . The three wavelet adjustments are discussed next in more detail. 
     Wavelet amplitude. The amplitude of the primary reflection p is determined by the source wavelet S, source (receiver) ghost G src (G src ), subsurface reflectivity R i  and geometric spreading factor L p :
 
 p =( S*G   src   G   rec )· R   i   /L   p .  (1)
 
Similarly, the amplitude of the multiple reflection m is determined by the free-surface reflectivity R fs  and the reflectivity of multiple reflection locations. For instance, a “pegleg” multiple is related to the water-bottom reflectivity R wb  by:
 
 m =( S*G   src   *G   rec )· R   i   ·R   wb   ·R   fs   /L   m .  (2)
 
Dividing (2) by (1) produces the following relation between relative multiple/primary amplitude ratio and wavelet amplitude, assuming perfect reflection at the water-surface (R fs =1):
 
                     m   p     ∝     R   wb     ∝     1   S             (   3   )               
where L p  and L m  are fixed relative to R wb  and are hence treated as proportionality constants. The second proportionality in (3) comes from equation (1) which, where the subsurface reflector is the water bottom, can be written as
 
               R   wb     ∝     1   S           
where the remaining terms from equation (1) make up a proportionality constant independent of S. Given that the primary reflections in the field data were matched by the inverted model using a wavelet of strength S at step  101 , equation (3) then indicates that
 
             m   ∝       1   S     .           
Thus, if the simulated multiples are too large or too small compared to the multiples in the recorded data, this can be adjusted by increasing or decreasing the source wavelet&#39;s strength proportionately. In other words, the geophysical meaning of equation (3) is that, the relative water-bottom multiple/primary amplitude ratio constrains the water-bottom reflectivity. Knowledge of the water-bottom reflectivity allows us then to estimate the wavelet amplitude by matching the amplitude of the water-bottom primary reflection.
 
     Wavelet phase. To demonstrate how wavelet phase may be corrected in the present inventive method, we applied a 30-deg phase rotation to the optimal wavelet, and repeated the FWI process of step  101  in  FIG. 1  to build a new shallow earth model which is different from the model in  FIG. 4 . However, simulated primaries match measured primaries equally well in both of the two models. (Not shown in the drawings.) While inversion is not able to detect a wavelet phase rotation based on primary reflections only, multiples are used to retrieve wavelet phase rotation.  FIG. 8  shows that simulated water-bottom multiple  21  cannot match recorded water-bottom multiple  20 , as evidenced by data residual in  22 . (This may be compared to  FIG. 6  which shows corresponding results for the optimal wavelet.) If we apply phase rotation operator Φ to the simulated water bottom multiples in  21  until the best match with measured water bottom multiple in  20  is achieved, we are able to retrieve the phase rotation of the wavelet (30-deg rotation in this case) by applying Φ to the wavelet. In this step, we have chosen the water-bottom multiple as the event to design phase rotation operator Φ. In general, we can pick any local events with good separation of multiple and primary. 
     Wavelet Power Spectrum. 
     To demonstrate how the wavelet&#39;s power spectrum, i.e. the absolute value of the coefficients of a Fourier expansion of the wavelet&#39;s waveform, can affect the agreement between simulated and measured multiples, a zero-phase shaping filter was applied to the optimal wavelet to suppress its high frequency energy. By then conducting steps  101  and  102  in  FIG. 1 , the result is now simulated multiples with a power spectrum  27  compared with the power spectrum of the recorded multiples  26 , as shown in  FIG. 10 . It is clear that the simulated multiples have richer high frequency energy than the recorded multiples. This is expected as can be seen by the following. The inverted earth power spectrum E=P/S, where P and S are the power spectrum of primaries and wavelet, respectively. Since first order multiples are primaries reflected one more time by earth, the power spectrum of multiples is M=E*P=P 2 /S. Since P is always matched by FWI on primaries, richer low frequencies in S results in richer high frequencies in M. As further evidence that the simulated multiples have richer high frequency energy than the recorded multiples,  FIG. 9  shows that the multiple residual  25  of simulated multiples  24  and recorded multiples  23  contains high frequency energy. If this were the case at step  103  in the course of applying the present inventive method, we may design in step  104  a zero-phase shaping filter H that transforms power spectrum  26  to  27 . The same shaping filter is then applied to the wavelet to get an optimal wavelet. As a result, after applying H to the wavelet used in  FIGS. 9 and 10 , the power spectrum of simulated multiples  29  is closer to the power spectrum of recorded multiples  28  in  FIG. 11 . (Curve  28  would have been identical to curve  26  if the same traces had been chosen to calculate power spectrum in each case.) 
     Upon finishing step  104  and cycling through step  101   a  second time, we have now the optimal wavelet and shallow subsurface model which predict the multiples of interest in step  102 . By subtracting simulated multiples from recorded data in step  105 , we extend primary reflections from “shallow” to “deep”. Performing FWI on the deep primaries with the optimal wavelet generates a deep subsurface model to match the deep primaries. 
     In step  106 , the comparison in step  103  is repeated. In other words, multiple reflections are simulated again, but now using the model resulting from step  105 , and these multiple predictions are compared to the recorded data in the selected window.  FIG. 12  illustrates how the decision in step  103  may be changed after having the deep primaries inverted in step  105 . If no strong primaries appear in the recorded data window during step  103 , the wavelet estimated without inversion of deep primaries will be the final wavelet. This is the case in the field data study used as an example to illustrate the method here. In the case of strong deep primaries intersecting or overlapping with multiples in the selected window, it will now be possible to separate primary from multiple after step  105 . Therefore, wavelet estimation in step  104  needs to be repeated, along with steps  101 - 106 , in order to match simulated multiples with the multiples separated from deep primaries. 
     By repeating the process above, if necessary, the present inventive method will generate the optimal wavelet and subsurface model such that simulated primaries (shallow and deep), and multiples match recorded data. Notice that the concept of “shallow/deep primaries” and “multiples of interest” are defined recursively, i.e., “deep primary” will become “shallow primary” after “multiples of interest” are modeled and separated, and the multiples that are “of interest” will be the strong multiples in the depth layer to which the model is being extended in the recursive sequence.  FIG. 13  illustrates this recursion.  131  shows the deep primary (lower - - - curve) masked by a multiple (solid line) generated from the shallow primary (upper - - - line). With the multiple thus revealed, it can be removed as shown in  132 , where a multiple appearing at a greater depth can now be modeled, and so on until all primaries are inverted ( 133 ). Thus, the whole process can be repeated until a final wavelet and subsurface model ( 107 ) are achieved. 
     The foregoing patent application is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims. 
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