WO2012170091A1 - System and method for seismic data inversion - Google Patents
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- WO2012170091A1 WO2012170091A1 PCT/US2012/028504 US2012028504W WO2012170091A1 WO 2012170091 A1 WO2012170091 A1 WO 2012170091A1 US 2012028504 W US2012028504 W US 2012028504W WO 2012170091 A1 WO2012170091 A1 WO 2012170091A1
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/28—Processing seismic data, e.g. for interpretation or for event detection
- G01V1/30—Analysis
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V2210/00—Details of seismic processing or analysis
- G01V2210/40—Transforming data representation
- G01V2210/45—F-x or F-xy domain
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V2210/00—Details of seismic processing or analysis
- G01V2210/60—Analysis
- G01V2210/61—Analysis by combining or comparing a seismic data set with other data
- G01V2210/614—Synthetically generated data
Definitions
- the present invention relates generally to methods and systems for inverting seismic data to compute physical properties of the earth's subsurface, and in particular methods and systems for performing phase-only full waveform inversion to compute velocity models from seismic data.
- Subsurface exploration typically uses methods such as migration of seismic data to produce interpretable images of the earth's subsurface.
- traditional migration methods often fail to produce adequate images.
- traditional migration methods require a reasonably accurate velocity model of the subsurface; such velocity models may also be determined from the seismic data but may be very expensive in both expertise and computational cost.
- NMO velocity analysis e.g., NMO velocity analysis
- migration velocity analysis e.g., tomography
- full waveform inversion e.g., full waveform inversion, are very computationally expensive and have only recently become practical as computing power has increased.
- a computer-implemented method of determining properties of a subsurface region includes obtaining actual seismic data representative of the subsurface region and an initial earth property model for the subsurface region, performing forward modeling using the initial earth property model to create modeled seismic data with similar acquisition specifications as the actual seismic data, transforming the modeled and actual seismic data to a temporal Fourier frequency domain to create frequency domain modeled and actual seismic data wherein the frequency domain modeled and actual seismic data include an amplitude portion and a phase portion, measuring the misfit between the frequency domain modeled seismic data and frequency domain actual seismic data to produce frequency domain residual seismic data, performing phase unwrapping of the phase portion of certain observed frequency components of the frequency domain residual seismic data to create an unwrapped residual phase portion, and inverting the unwrapped residual phase portion to determine desired properties of the subsurface region of interest, wherein the inverting minimizes an objective function defined to measure the misfit.
- the method may also include phase extrapolation. Additionally, the method may perform a second inverting step using the properties determined from the first inverting step as an initial model.
- a system for determining properties of a subsurface region includes a data source, a user interface, and a processor configured to execute computer modules designed to perform the method.
- an article of manufacture for determining properties of a subsurface region includes a computer readable medium having a computer readable code embodied therein, the code being configured to execute the method.
- Figure 1 is a flowchart illustrating a method of full waveform inversion
- FIG. 1 illustrates gradient bandwidths at various frequencies
- Figure 3 illustrates a conventional full waveform inversion process beginning from a good initial earth properties model
- Figure 4 illustrates a conventional full waveform inversion process beginning from a poor initial earth properties model
- FIG. 5 is a flowchart illustrating a method in accordance with an embodiment of the invention.
- Figure 6 illustrates a method of phase unwrapping with and without a preconditioner at a very low frequency
- Figure 7 illustrates a method of phase unwrapping with and without a preconditioner at a moderately low frequency
- Figure 8 illustrates a result of an embodiment of phase-only full waveform inversion
- Figure 9 illustrates a result of another embodiment of phase-only full waveform inversion followed by conventional full waveform inversion
- Figure 10 is a flow chart illustrating another embodiment of the present invention using phase extrapolation
- Figure 11 illustrates a result of an embodiment using phase extrapolation
- Figure 12 schematically illustrates a system for performing a method in accordance with an embodiment of the invention.
- the present invention may be described and implemented in the general context of a system and computer methods to be executed by a computer.
- Such computer-executable instructions may include programs, routines, objects, components, data structures, and computer software technologies that can be used to perform particular tasks and process abstract data types.
- Software implementations of the present invention may be coded in different languages for application in a variety of computing platforms and environments. It will be appreciated that the scope and underlying principles of the present invention are not limited to any particular computer software technology.
- the present invention may be practiced using any one or combination of hardware and software configurations, including but not limited to a system having single and/or multiple computer processors, hand-held devices, programmable consumer electronics, mini-computers, mainframe computers, and the like.
- the invention may also be practiced in distributed computing environments where tasks are performed by servers or other processing devices that are linked through a one or more data communications network.
- program modules may be located in both local and remote computer storage media including memory storage devices.
- an article of manufacture for use with a computer processor such as a CD, pre-recorded disk or other equivalent devices, may include a computer program storage medium and program means recorded thereon for directing the computer processor to facilitate the implementation and practice of the present invention.
- Such devices and articles of manufacture also fall within the spirit and scope of the present invention.
- the invention can be implemented in numerous ways, including for example as a system (including a computer processing system), a method (including a computer implemented method), an apparatus, a computer readable medium, a computer program product, a graphical user interface, a web portal, or a data structure tangibly fixed in a computer readable memory.
- a system including a computer processing system
- a method including a computer implemented method
- an apparatus including a computer readable medium, a computer program product, a graphical user interface, a web portal, or a data structure tangibly fixed in a computer readable memory.
- Full waveform inversion is a local optimization method and therefore depends strongly on where the optimization starts.
- the initial model must generate data that is within half a wave-cycle of the observed data at the lowest usable temporal frequency. It is important to note that with the conventional approach there is no easy way to determine if the initial model meets this condition, and the optimization can easily fail with a poor initial model.
- the initial model of earth properties is used by a seismic modeling engine to generate modeled seismic data.
- modeling can be performed in either the time domain or the frequency domain (temporal Fourier transform) with no penalty, depending on various factors like the size/extent of the modeling domain and the amount of memory available.
- Large 3D surveys typically require time-domain modeling because frequency domain modeling is extremely memory intensive for large numbers of model parameters.
- frequency domain modeling is that one directly has access to both amplitude and phase, and this allows the use of "phase only" approaches that can be geared to be dominated by kinematics instead of amplitudes.
- step 14 we compute an objective function that will measure the misfit between the recorded seismic data and the modeled seismic data.
- the most widely used objective function for conventional full waveform inversion is simple least squares: the sum of the squares of the differences between the observed data and the modeled data for all sources, receivers and recorded time samples. However, this is not meant to be limiting; other objective functions can be used, including correlation, the LI norm, and hybrid or long-tailed norms.
- E is the objective function
- s are the sources
- r are the receivers
- t time
- ⁇ ⁇ ⁇ is the recorded data
- y/ mo£ is the modeled data.
- This objective function suffers from the critical flaw that seismic data is bandlimited. Differencing of bandlimited signals introduces the possibility of "cycle skipping", where the wave shapes of the modeled and observed data are similar enough to cause a small difference, but are misaligned in an absolute sense by (at least) one wave cycle. This, together with the local nature of full waveform inversion, leads to the likely possibility that the nonlinear optimization will fail and converge to a local minima rather than the global solution.
- the objective function becomes: where A 0bs ((o,r,s) is the amplitude of the observed data at receiver r, from source s, at temporal frequency ⁇ , (p 0bs (o,r,s) is the phase of the observed data, A moc i(co,r,s) is the amplitude of the modeled data, and ⁇ p m od(oo,r,s) is the phase of the modeled data.
- E ( ⁇ ) - ⁇ s ⁇ r I (p obs ( ⁇ , r, s) - (p mod ( ⁇ , r, s)
- Eqn. 3 The modeled data in Eqns. 1-3 may be generated in the time or the frequency domain.
- the objective functions of Eqns. 1-3 measure the mismatch between the observed and modeled data and are decreased at each iteration.
- the inversion may be done as a phase-only inversion in either the time or frequency domain, as long as the mismatch can be measured directly or indirectly in terms of the phase of one or more frequency components.
- a search direction is computed in step 16.
- the gradient of the objective function is used to generate a search direction for improving the model.
- the earth properties model is then iteratively perturbed along successive search directions until some satisfaction criteria are reached.
- the calculation of the search direction becomes more clear if we treat the modeled data as the action of a nonlinear seismic modeling operator on the earth property model.
- the operator being nonlinear means that a linear change in velocity does not necessarily result in a linear change in the modeled data.
- Eqn. 5 shows that the derivatives used to update the earth property model depend very importantly on the modeling operator, the derivatives of the modeling operator with respect to velocity, and the current seismic data residual.
- a f is the adjoint (conjugate transpose) of the linear operator A.
- a f is the adjoint (conjugate transpose) of the linear operator A.
- a f is the adjoint (conjugate transpose) of the linear operator A.
- the nonlinear operator TV we have the nonlinear operator TV, and we need the adjoint of the linearized operator in order to compute a gradient.
- L for the linearized operator
- L f for the adjoint of the linearized operator.
- the operator L maps a vector of velocity perturbations into a vector of wavefield perturbations
- the adjoint operator L f maps a vector of wavefield perturbations into a vector of velocity perturbations (Eqn. 8).
- Step 14 is performed and, if the difference between the modeled seismic data and the recorded seismic data is large, steps 16 and 18 are also performed and looped back to step 12, until the difference at step 14 is sufficiently small or the number of loops or iterations reaches a predefined number.
- full waveform inversion is a local optimization method, which means it is sensitive to where the nonlinear evolution starts. If the initial model is far from the true model, local approaches fail. This problem impacts all local methods, including Newton and quasi-Newton methods.
- it is absolutely critical to obtain a good starting model. In general, there are no obvious ways to determine quantitatively if a given starting model will converge to the true global minimum.
- Another serious limitation of conventional full waveform inversion is the bandwidth limitation. There is a direct relationship between the temporal bandwidth of data used to generate a gradient (search direction) and the spatial bandwidth of the gradient obtained by evaluation of Eqn. 5.
- the initial velocity model in panel 40 is constant and is set to be water velocity. This is far from the true velocity model in panel 48.
- Panels 41-47 show the result of conventional full waveform inversion at 8 successive frequencies: 1, 3, 5, 7, 9, 11, and 13 Hz. While the uppermost part of the model is accurately recovered, the deeper parts have converged to a local minimum that is very far from the true solution.
- conventional full waveform inversion must have a good initial earth properties model to converge to the correct solution.
- the present invention sets an arbitrary initial earth model such as, by way of example and not limitation, setting the entire initial model to be water velocity of 1500 m/s.
- This initial model is used to generate modeled seismic data at step 51.
- the forward modeling of the modeled seismic data may be done in the time domain or in the frequency domain by any of the many known forward modeling algorithms, such as finite difference modeling. If the forward modeling is done in the time domain, it may then be transformed to the frequency domain.
- recorded seismic data is obtained and in step 53 it is transformed into the frequency domain.
- a residual phase may be computed at step 54, which is the difference between the phase portions of the modeled and recorded seismic data.
- the residual phase is phase unwrapped. It is also possible to unwrap the phase of the modeled seismic data and the recorded seismic data separately. The unwrapped phases may then be used to compute an unwrapped residual phase.
- Phase unwrapping ensures that all appropriate multiples of 2 ⁇ have been included in the phase portion of the data, meaning that the phase is continuous rather than jumping by 2 ⁇ .
- the new method uses a particular type of left preconditioning that de-weights the influence of large phase jumps. Either the observed phase and modeled phase may be unwrapped individually or their difference, the residual phase, may be unwrapped. The latter is preferred since the phase differences between adjacent data points will be smaller.
- the procedure we use for phase unwrapping is inspired by a fundamental theorem of vector calculus, also called the Helmholtz Decomposition.
- the Helmholtz Decomposition can be used to decompose a vector field into a curl-free component and a divergence-free component. We are interested in the curl-free component only, so we do not require a precise Helmholtz decomposition.
- the curl-free component is the gradient of a scalar potential, and is a conservative field.
- a conservative field is a vector field for which line integrals between arbitrary points are path independent. We identify unwrapped residual phase with the scalar potential whose gradient is the conservative field of a Helmholtz decomposition.
- this user-defined positive power a may be set to 2.5.
- examples of phase unwrapping with and without the preconditioner can be seen for data at .5 Hz in Figure 6 and at 1.5 Hz in Figure 7.
- Figure 6 and Figure 7 show the wrapped phase in panel A, the unwrapped phase without use of a preconditioner in panel B, and the unwrapped phase with a left diagonal preconditioner in panel C.
- the result without the preconditioner has erroneously changed the phase in the areas indicated by D and E, indicating that as the frequency gets higher, the preconditioning is necessary to obtain a good result.
- phase unwrapping does not require integration or the specification of boundary conditions in order to obtain unwrapped phase from the principal value of the gradient of wrapped phase.
- phase unwrapping may be used in a nonlinear line search where the search direction for velocity update has been pre-determined.
- a conventional objective function is used, but data whose residual phase magnitude exceeds ⁇ is excluded. This implies that the line search is only sensitive to data that is not cycle skipped.
- the objective function for the nonlinear line search is replaced with the least squares sum of the unwrapped residual phase. This means that the line search will correctly handle cycle skipped data.
- unwrapped residual phase ((p res ) as shown in equation 14.
- unwrapped residual phase could be used as an objective function for stochastic or Bayesian inversion in order to correctly handle cycle skipped data.
- phase unwrapping with a preconditioner has been explained in terms of preparing seismic data for inversion, this is not meant to be limiting.
- unwrapped seismic data may be useful in other processing flows such as horizon flattening, homomorphic deconvolution, refraction statics, and residual alignment; and that other types of data, such as synthetic aperture radar, could benefit from this method of phase unwrapping with a preconditioner.
- step 55 computes an objective function measuring the misfit between the phases of the recorded data and the modeled seismic data.
- this objective function might be Eqn. 3.
- the continuation approach is application of homotopy to smoothing regularization for nonlinear optimization. Homotopy here means starting with large magnitude for smoothing regularization and gradually decreasing the magnitude of the smoothing regularization over the course of the nonlinear evolution.
- Smoothing regularization can implemented by adding rows to the linear system to penalize roughness in the model that is optimized.
- the continuation approach may use analytic derivatives of polynomials representing slowness.
- a change of basis to smooth functions for example radial basis functions, also works.
- Other possibilities include but are not limited to the spatial Fourier basis with a right preconditioner that scales with wavenumber, and 1 st or 2 nd numerical derivatives, either centered or not.
- roughness penalties may be applied by application of 1 st forward numerical differences to pixelized models.
- can be obtained, for example, using the power method.
- the phase-only full waveform inversion performed in the present invention may also include more accurately solving the linear problems at each iteration. If, at each successive linearization, we solve the Gauss-Newton problem to obtain the model update, rather than employ the combination of steepest descent and a line search, we get an improved result.
- Figure 8 shows the result of an embodiment of the present invention, a phase-only full waveform inversion using phase unwrapping with a left preconditioner, continuation approach, and solving the successive linear problems.
- Panel 80 is the initial model, which is a constant 1500 m/s (water velocity). This is the same initial model that was shown in Figure 4 panel 40.
- Panel 88 in Figure 8 shows the true velocity model.
- Panels 81-87 show successive nonlinear iterations at 1 Hz, beginning from the initial model.
- Panel 81 shows that, after one iteration, accurate long spatial wavelengths are present in the inverted model and they are refined as the iterations progress through panels 82-87. Seven nonlinear iterations allow the recovery of the missing long spatial wavelengths of velocity not possible using the conventional approach, as seen in Figure 4.
- the model generated by the phase-only full waveform inversion may be used as an initial model for conventional full waveform inversion.
- the initial model for the conventional full waveform inversion in panel 90 is the model generated by 7 iterations of phase-only full waveform inversion in Figure 8, panel 87.
- Performing 5 iterations of conventional full waveform inversion (panels 91-95) at 2.5 Hz results in an inverted model (panel 95) that is very comparable to the true velocity model in panel 96.
- FIG. 10 illustrates yet another embodiment of the present invention.
- the phase-only full waveform inversion flow is shown as method 1000.
- the steps are the same as those of method 500 in Figure 5 with the addition of step 1007, phase extrapolation, after the phase unwrapping step 1006.
- Steps 1001, 1002, 1003, 1004, 1005, 1006, 1008, 1009, and 1010 are performed in the same manner as steps 50, 51 , 52, 53, 54, 55, 56, 57, and 58, respectively.
- Step 1007 is a phase extrapolation step which may be used to extrapolate the unwrapped phase to lower frequencies than exist in the recorded seismic data.
- the phase is extrapolated to lower frequencies than those observed and conventionally usable.
- Conventionally usable frequencies are typically greater than 2 Hz. This is done by linearization of the unwrapped phase as a function of frequency and may be applied to the observed phase, the modeled phase, or the residual phase. The extrapolated data is then inverted using some objective function defined to measure phase mismatch. The method is applicable for any case when the phase is linear in frequency.
- Figure 1 1 illustrates the result of one embodiment of a phase extrapolation method.
- Panel 1 10 is the initial model, in this case constant water velocity of 1500 m/s and panel 121 is the true velocity model.
- Panels 1 1 1-1 15 are phase extrapolation inversion from 2.5 Hz to 0.1 , 0.2, 0.3, 0.4, and 0.5 Hz, respectively.
- Panels 1 16-120 are conventional inversion at frequencies 2.5, 4.5, 6.5, 8.5, and 10.5 Hz continuing from the phase extrapolation result in panel 1 15.
- phase extrapolated data By way of example and not limitation, synthetic aperture radar (SAR) data may be obtained, phase unwrapped using a preconditioner, and phase extrapolated prior to
- SAR synthetic aperture radar
- data that has been phase unwrapped using a preconditioner and phase extrapolated may then be used to evaluate a cost function.
- a cost function For example, the use of unwrapped phase to compute an objective function for stochastic or Bayesian optimization, with the advantage that the cost function would correctly handle cycle-skipped data.
- a system 1200 for performing the method is schematically illustrated in Figure 12.
- the system includes a data storage device or memory 130.
- the data storage device 130 contains recorded data and may contain an initial model.
- the recorded data may be made available to a processor 131, such as a programmable general purpose computer.
- the processor 131 is configured to execute an initial model module 132 to create an initial model if necessary or to receive the initial model from the data storage 130.
- the processor 131 is also configured to execute the domain transform module 133 for transforming recorded and optionally modeled data into the frequency domain, the data modeling module 134 for forward modeling data based on the initial and updated models, the phase preparation module 135 for phase unwrapping with a preconditioner and optionally phase extrapolating the recorded data, the objective function module 136 for computing the objective function that compares the modeled data with the phase unwrapped recorded data, the search direction module 137 for determining the search direction, and the model update module 138 for updating the model.
- the processor 131 is also configured to execute modules 134, 135, 136, 137, and 138 repeatedly until the result from the objective function module 136 meets user requirements or a maximum number of iterations is reached.
- the processor 131 may include interface components such as a user interface 139, which may include both a display and user input devices, and is used to implement the above-described transforms in accordance with embodiments of the invention.
- the user interface may be used both to display data and processed data products and to allow the user to select among options for implementing aspects of the method.
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EP12796543.2A EP2718744A4 (en) | 2011-06-08 | 2012-03-09 | System and method for seismic data inversion |
CA2819023A CA2819023A1 (en) | 2011-06-08 | 2012-03-09 | System and method for seismic data inversion |
AU2012266874A AU2012266874B2 (en) | 2011-06-08 | 2012-03-09 | System and method for seismic data inversion |
CN2012800042111A CN103270430A (en) | 2011-06-08 | 2012-03-09 | System and method for seismic data inversion |
EA201391460A EA201391460A1 (en) | 2011-06-08 | 2012-03-09 | SYSTEM AND METHOD FOR INVERSING SEISMIC DATA |
BR112013010536A BR112013010536A2 (en) | 2011-06-08 | 2012-03-09 | system and method for seismic data inversion |
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CN103270430A (en) | 2013-08-28 |
CA2819023A1 (en) | 2012-12-13 |
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EP2718744A4 (en) | 2016-04-13 |
AU2012266874A1 (en) | 2013-04-04 |
US9075159B2 (en) | 2015-07-07 |
US20120314538A1 (en) | 2012-12-13 |
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