Patent Publication Number: US-10782430-B2

Title: Method for seismic exploration using a multiple-inclusive source wavelet

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to and claims the benefit of priority of U.S. Provisional Application 62/616,541 filed Jan. 12, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems for exploring a subsurface geological formation, and, in particular, to methods and systems that use shot records in a joint inversion yielding a source wavelet and a reflectivity model without changing the velocity model. 
     Discussion Of The Background 
     Probing underground formations in search of hydrocarbon resources is an ongoing process driven by continually increasing worldwide demand. Seismic surveys are used for exploration, hydrocarbon reservoir field development, and production monitoring (time lapse). 
     During seismic data acquisition, sensors detect acoustic signals emerging from the geological formation following shots fired by a source. The sensors sample the signals to record series of signal amplitudes versus time (known as traces), which carry information about the geological formation&#39;s structure. The interaction of the seismic excitation generated by the source inside the geological formation includes reflections, which are refractions at boundaries between layers characterized by different seismic propagation velocities and attenuation. Note that “seismic” and “acoustic” are used interchangeably in this document while referring to mechanical waves (i.e., propagating oscillations) and, thus, to propagating mechanical energy. 
     “Shot record” is a term used for all the sensor-generated records following a shot. The seismic “illumination” of the geological formation due to the shot provides a snapshot of the formation&#39;s structure, according to the source and sensors&#39; positions. The shot record is processed to determine physical properties of the geological formation&#39;s illuminated portion; these physical properties may indicate the presence and location of hydrocarbon (i.e., oil and/or gas) reservoirs. 
     A model of velocity (i.e., velocity values inside the geological formation) may be used during the processing. Migration velocity analysis (MVA) uses migration to estimate reflectivity from the shot records and improves the velocity model by optimizing the reflectivity according to a particular criterion. MVA has the drawback of being hampered by gradient artifacts that prevent convergence. Another difficulty is that MVA may be affected by the presence of multiples. Therefore, seismic data (i.e., shot records) is usually pre-processed before applying MVA. Conventionally, pre-processing prepares data for MVA, which then obtains the velocity model and the reflectivity. However, the pre-processing is quite challenging for broadband processing, requiring extra care to preserve signal at the lower frequencies. 
     Conventional pre-processing includes separate steps of source deconvolution, source de-ghosting, sensor de-ghosting, and surface-related multiple attenuation/removal. All these steps have the potential of altering the structural information. 
     Thus, there is a need to propose methods and systems that, unlike the conventional approach, use the shot records (i.e., original, not pre-processed) during processing to extract structural information (e.g., inversions). 
     SUMMARY 
     Methods and devices according to various embodiments jointly estimate a multiple-inclusive source wavelet and a reflectivity model by performing an inversion, minimizing the difference between the shot records and modeled shots. The modeled shots include primaries, ghosts and multiples, thereby using the raw data (not pre-processed for removing the multiples) when obtaining a geological product suitable for hydrocarbon exploration in the geological formation. 
     According to an embodiment, there is a method for exploring a geological formation. The method includes obtaining shot records generated by sensors detecting signals emerging from the geological formation following shots fired by a source, and a velocity model of the geological formation. The method further includes jointly estimating a multiple-inclusive source wavelet and a reflectivity model by performing an inversion minimizing difference between the shot records and modeled shots, the modeled shots including primaries and multiples obtained by propagating up-going and down-going wavefields through the velocity model that is not updated during the inversion. The method then includes converting the shot records into a geological product suitable for hydrocarbon exploration in the geological formation using the multiple-inclusive source wavelet and the reflectivity model. 
     According to another embodiment, there is a seismic exploration apparatus configured to convert shot records acquired for a geological formation into a geological product suitable for hydrocarbon exploration in the geological formation. The apparatus includes an interface configured to receive shot records generated by sensors detecting signals emerging from the geological formation following shots fired by a source, and a velocity model. The apparatus also includes a processing module connected to the interface and configured to jointly estimate a multiple-inclusive source wavelet and a reflectivity model by performing an inversion minimizing difference between the shot records and modeled shots, the modeled shots including primaries and multiples obtained by propagating up-going and down-going wavefields through the velocity model that is not updated during the inversion, and to convert the shot records into a geological product suitable for hydrocarbon exploration in the geological formation using the multiple-inclusive source wavelet and the reflectivity model. 
     According to yet another embodiment, there is a non-transitory computer-readable medium storing executable codes which, when executed by a computer, make the computer perform a method for exploring a geological formation. The method includes obtaining shot records generated by sensors detecting signals emerging from the geological formation following shots fired by a source, and a velocity model of the geological formation. The method further includes jointly estimating a multiple-inclusive source wavelet and a reflectivity model by performing an inversion minimizing difference between the shot records and modeled shots, the modeled shots including primaries and multiples obtained by propagating up-going and down-going wavefields through the velocity model that is not updated during the inversion. The method then includes converting the shot records into a geological product suitable for hydrocarbon exploration in the geological formation using the multiple-inclusive source wavelet and the reflectivity model. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a flow diagram of a method according to an exemplary embodiment; 
         FIG. 2  is schematic diagram of a marine seismic data acquisition system; 
         FIG. 3  is a vertical slice in a velocity model; 
         FIG. 4  illustrates a source wavelet; 
         FIG. 5  is spectra of the source wavelet in  FIG. 4 ; 
         FIG. 6  an estimated velocity model obtained by a velocity modeling inversion which freezes the source wavelet while estimating the velocity model and the extended reflectivity model, according to an embodiment; 
         FIG. 7  illustrates (a) input raw shot and (b) the reconstructed shot obtained from (a) using the estimated source wavelet in  FIG. 4 , with the estimated velocity and reflectivity models illustrated in  FIG. 6 ; according to an embodiment; 
         FIG. 8  illustrates (a) input raw shot and (b) the reconstructed shot obtained from (a) using conventional methods; and 
         FIG. 9  is a schematic diagram of a seismic exploration apparatus, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology of marine seismic surveys. However, the inventive concepts to be discussed next are not limited to marine seismic surveys, but they may also be applied for land surveys and surveys with electromagnetic waves. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     An alternative to conventional pre-processing is to include in a forward modeling of the source wavelet (without ghost) the source and sensor ghosts and the multiples in order to perform an inversion using raw data (i.e., the shot records).  FIG. 1  is a flow diagram of a method  100  for exploring a geological formation according to an exemplary embodiment. 
     Method  100  first includes obtaining shot records and a velocity model of the geological formation at  110 . The shot records are generated by sensors detecting signals emerging from the geological formation following shots fired by a source. Some details related to step  110  are discussed next, before presenting the other steps of method  100 . 
       FIG. 2  illustrates a marine seismic data acquisition system that acquires such shot records. In  FIG. 2 , vessel  210  tows plural seismic sensors  212 , which are disposed along a flexible cable  214  (typically several kilometers long) and configured to detect seismic signals emerging from the explored geological formation. Those skilled in the art use the term “streamer” (labeled  216 ) for the cable and the corresponding sensors. A vessel usually tows plural streamers at predetermined cross-line intervals (cross-line being a direction perpendicular to the towing direction), with the streamers forming a spread in the horizontal (xy) plane. Streamer  216  is towed at a substantially constant depth z 1  relative to the water surface  218 . However, streamers may be towed at a slant (i.e., to form a constant angle) with respect to the water surface, or may have a curved profile as described, for example, in U.S. Pat. No. 8,593,904, the entire content of which is incorporated herein by reference. Each streamer is normally equipped with compasses, acoustic pingers and depth sensors that give continuous location information about heading, position and depth. Furthermore, each streamer is typically equipped with attached units  217  (only one labeled) known as “birds” that control the cross-line position and depth of that streamer (control operations often referred to as streamer steering). Although  FIG. 2  illustrates marine data acquisition, the current inventive concept can be applied to land as well, for example, for interbed multiples. 
     Vessel  210  (or another vessel) may also tow seismic source  220  configured to generate acoustic waves  222   a.  Acoustic waves  222   a  propagate downward and penetrate the seafloor  224 . For simplicity,  FIG. 2  shows only two paths  222   a  corresponding to the source-emitted acoustic waves. When encountering a layer interface  226  (the interface separating layers inside into which the acoustic waves propagate with different velocities), the acoustic waves are at least partially reflected. Reflected acoustic waves  222   b  and  222   c  propagate upward. Reflected acoustic wave  222   b  is received by one of sensors  212 , while reflected wave  222   c  passes by the sensors and is reflected back at the water surface  218  (i.e., the interface between the water and air serving as a quasi-perfect reflector to the acoustic waves). Wave  222   d,  which is wave  222   c &#39;s reflection at the water surface, travels downward and is then also detected by the sensors. 
     A velocity model means velocity values are attributed to a three-dimensional (3D) representation of the explored underground formation. For example,  FIG. 3  represents a vertical slice in a velocity model where different nuances of gray correspond to different velocity values. 
     Returning now to  FIG. 1 , method  100  then includes estimating a multiple-inclusive source wavelet and a reflectivity model by performing an inversion minimizing the difference between the shot records and modeled shots at  120 . Here, “multiple-inclusive” is a label signaling that the methods of this embodiment use the raw data including multiples. The modeled shots include direct arrivals, primaries, ghosts and multiples obtained by propagating up-going and down-going wavefields through the velocity model, which is not updated during this inversion. Computing the modeled shots may use one-way wave-equation propagation, and may include:
         modeling the source with the wavelet, building an initial down-going wavefield with the source ghost,   propagating the down-going wavefield through the velocity model,   obtaining an up-going wavefield with the reflectivity model,   reflecting the up-going field at the water surface (or another surface boundary), and   generating components of the model shot by adding the up-going field and the other down-going wavefield at locations of the sensors.       

     The up and down propagation may be repeated N times, N being the multiple order (which may be user-defined and varied). Mathematically, each modeled trace m,(t) is therefore the sum of 2N+1 components (dw 0 (t) being the direct arrival): 
     
       
         
           
             
               
                 
                   
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     Since the shot modeling takes into account the source wavelet, the reflectivity, the ghosts and the multiples, the inversion can in principle perform pre-processing, velocity estimation and imaging steps. The inversion uses a cost function and calculates a gradient of the cost function with respect to parameters used in computing the modeled shot. The gradient may be used to iteratively update one or more of the source wavelet, the velocity model, and the reflectivity model. However, when all three terms (i.e., the source wavelet, the velocity model and the reflectivity) are estimated jointly, the inversion starting from crude initial estimates may fail to converge. 
     The inversion uses an initial velocity model. This model may be very crude, for example, considering the water velocity or linear gradient from the water-bottom constant. The inversion jointly estimates the source wavelet and an unconstrained extended reflectivity using, for example, a cost function that minimizes misfit between data (i.e., shot records) and modeled shots: 
                     C   ⁡     (     r   ,   v     )       =           1   2     ⁡     [       G   ⁢     (   v   )     ⁢   r     -     d   0       ]       *     ⁡     [         G   ⁡     (   v   )       ⁢   r     -     d   0       ]               (   2   )               
with do the recorded data, v the velocity model, r the reflectivity, and G(v) the modeling operator that produces the modeled data d with the linear modeling equation d=G(v)r.
 
     The data is correctly modeled because the unconstrained extended reflectivity compensates for the errors in the velocity model and takes into account the AVAeffects. This inversion may be seen as corresponding to pre-processing because once the modeled traces are obtained using the above equation for m i (t), the 2N+1 components of this model, which are models of the ghosts and of the multiples, can be used for de-ghosting and multiple attenuation. The velocity model is not updated (i.e., it is “frozen”) during this first inversion. 
     Returning yet again to  FIG. 1 , method  100  finally includes, at  130 , converting the shot records into a geological product suitable for hydrocarbon exploration in the geological formation using the multiple-inclusive source wavelet and the reflectivity model obtained at  120 . Here, the term “geological product” refers to any geological aspects used for an exploration-related decision. For example, a geological product may be changes in a volume including an oil and gas reservoir, presence and direction of fractures, identification of layers&#39; type so as to select a drilling location and/or design a drilling path, etc. 
     In one embodiment, step  130  includes a second inversion, which jointly estimates the velocity model and a constrained extended reflectivity, using the wavelet which is frozen during this second inversion. The cost function used during this second inversion includes a reflectivity focusing term, in addition to the data misfit term in the first inversion: 
                     C   ⁡     (     r   ,   v     )       =             1   2     ⁡     [         G   ⁡     (   v   )       ⁢   r     -     d   0       ]       *     ⁡     [         G   ⁡     (   v   )       ⁢   r     -     d   0       ]       +       σ   2     ⁢          Ar        2                 (   3   )               
where σ is a weight, and A is a matrix that extracts the extended part of the reflectivity. The two terms of this cost function are a data misfit term and a reflectivity focusing term.
 
     This second inversion is similar to a migration velocity analysis (MVA) technique in which the velocity model is adjusted in order to focus the reflectivity. The MVA may utilize an extended reflectivity space. 
     Since the extended reflectivity space used in such an MVA technique allows the data misfit to be small from the first iteration, a second-order Gauss-Newton scheme can be used. The Gauss-Newton scheme involves simultaneously updating the velocity and reflectivity: 
                     [           r     n   +   1                 v     n   +   1             ]     =       [           r   n               v   n           ]     -       λ   n     ⁢       H   n     -   1       ⁡     (       r   n     ,     v   n       )       ⁢       g   n     ⁡     (       r   n     ,     v   n       )                   (   4   )               
using the gradient g n  and an approximate Hessian H n  of the cost function in the reflectivity model r n  and velocity model v n :
 
                       g   n     ⁡     (       r   n     ,     v   n       )       =       [         ∂   C       ∂     r   n   *             ∂   C       ∂     v   n   T           ]     =         [           G   *                 ∂       (     Gr   n     )     *         ∂     v   n   T               ]     ⁢   e     +     σ   ⁡     [             A   *     ⁢     Ar   n               0         ]                   (   5   )                 H   ⁡     (       r   n     ,     v   n       )       =         [           G   *                 ∂     (     Gr   n     )         ∂     v   n   T               ]     ⁡     [         G           ∂     (     Gr   n     )         ∂     v   n               ]       +     σ   ⁡     [             A   *     ⁢   A         0           0       0         ]                 (   6   )               
Here, G is the velocity dependent Green-function matrix that applied to the extended reflectivity yields the data.
 
     The deconvolution of the gradient tends to suppress gradient artifacts. 
     In one embodiment, a third inversion may be performed to estimate an enhanced, unconstrained extended reflectivity while freezing the wavelet and the velocity model. Accurate angle gathers can be produced from this final reflectivity. This third inversion, which may be considered as corresponding to imaging, also uses the raw shot records and ghosts and multiples modeling. Therefore, ghosts and multiples are used for the imaging, instead of being treated as noise. 
     As previously mentioned,  FIG. 3  illustrates an exemplary crude initial velocity model adapted for use with exemplary raw shot records characterizing an exemplary 2D line acquired using a variable-depth streamer acquisition. In the acquisition, the sensors used were restricted to the first 3 km of the streamer, and the maximum frequency used was 80 Hz. The exemplary crude initial velocity is a linear gradient from the water bottom. The present inventive concepts seem similarly pertinent for 3D raw shot records.  FIG. 4  illustrates a multiple-inclusive source wavelet obtained from an inversion that keeps the velocity model (e.g., the crude initial velocity model illustrated in  FIG. 3 ) unchanged and estimates the source wavelet jointly with an extended reflectivity model. The vertical axis is the amplitude, which has the same scale as the data. If the data is correctly scaled to a given pressure unit, the source wavelet scale is the same unit. In practice, the data may have an arbitrary scale. 
       FIG. 5  illustrates the spectrum of the estimated source wavelet in  FIG. 4 . The exemplary estimated source wavelet has an accurate time zero, scale and polarity. The time zero is the start of time scale in  FIG. 4 , which time does not coincide with the negative peak of the wavelet. 
       FIG. 6  exemplarily illustrates an estimated velocity model obtained by a velocity modeling inversion which freezes the estimated source wavelet (e.g., the source wavelet in  FIG. 5 ) while estimating the velocity model and the extended reflectivity model. The velocity modeling inversion uses only a portion of the data up to 40 Hz. 
     Further, the reflectivity model is enhanced by performing a reflectivity modeling inversion in which the source wavelet (e.g., as illustrated in  FIG. 4 ) and the velocity model (e.g., as illustrated in  FIG. 6 ) are unchanged (i.e., frozen). The reflectivity modeling inversion used all the data (i.e., up to 80 Hz). The portion labeled  610  on  FIG. 6  exemplarily illustrates angle gathers. 
       FIG. 7  illustrates (a) input raw shot with a bandpass of [2.5 Hz-80 Hz] and (b) reconstructed shot obtained using the estimated source wavelet in  FIG. 4 , with the estimated velocity and reflectivity models illustrated in  FIG. 6 . The modeling includes source wavelet, source and sensor ghosts, and water-surface multiples. The two shots are very similar, with a correlation over 0.96, which validates the present approach. 
       FIG. 8  illustrates (a) input raw shot of  FIG. 7( a )  conventionally processed with source deconvolution, source de-ghosting, sensor de-ghosting, and surface-related multiple attenuation, and (b) a corresponding reconstructed shot obtained using a Dirac wavelet as the source, the estimated velocity and reflectivity and a modeling without ghost or multiple modeling. The low frequency content obtained with the above-described methods is enhanced. 
     The above-discussed methods may be implemented in a computing device  900  as illustrated in  FIG. 9 . Hardware, firmware, software or a combination thereof may be used to perform the various steps and operations described herein. 
     Computing device  900  suitable for performing the activities described in the exemplary embodiments includes a server  901  with a central processor (CPU)  902  that may be coupled to a random access memory (RAM)  904  and to a read-only memory (ROM)  906 . ROM  906  may be replaced by other types of storage media to store programs, such as programmable ROM (PROM), erasable PROM (EPROM), etc. Processor  902  may communicate with other internal and external components through input/output (I/O) circuitry  908  and bussing  910  to provide control signals and the like. Processor  902  carries out a variety of functions as are known in the art, as dictated by software and/or firmware instructions. 
     Server  901  may also include one or more data storage devices, including hard drives  912 , CD-ROM drives  914  and other hardware capable of reading and/or storing information, such as DVD, etc. In one embodiment, software for carrying out the above-discussed steps may be stored and distributed on a CD-ROM or DVD  916 , a USB storage device  918  or other forms of media capable of portably storing information. These storage media may be inserted into, and read by, devices such as CD-ROM drive  914 , disk drive  912 , etc. Server  901  may be coupled to a display  920 , which may be any type of known display or presentation screen, such as LCD, plasma display, cathode ray tube (CRT), etc. A user input interface  922  is provided, including one or more user interface mechanisms such as a mouse, keyboard, microphone, touchpad, touch screen, voice-recognition system, etc. 
     Server  901  may be coupled to other devices, such as sources, sensors, etc. The server may be part of a larger network configuration as in a global area network (GAN) such as the Internet  928 , which allows ultimate connection to various computing devices. 
     The disclosed exemplary embodiments provide methods devices for exploring a geological formation that use raw data in inversions and perform a joint inversion to estimate a source wavelet and reflectivity while keeping the velocity model unchanged. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.