Velocity tomography using property scans

Abstract

Method for building a subsurface model of velocity or other elastic property from seismic reflection data using tomography. The method uses velocity scans to pick a focusing velocity model at each image point (40). The focusing velocities are used to pick depth errors from tables (60) generated using a tomographic inversion matrix (30) and a suite of different velocity models (10). The depth errors are then reconstructed at each image point from the velocity scans based on the difference between the base velocity model and the most coherent velocity from the scan (70). The reconstructed depth errors are used to compute the velocity model update (80).

Description

FIELD OF THE INVENTION

Technological Field

This disclosure relates generally to the field of geophysical prospecting and, more particularly, to seismic data processing. Specifically, this disclosure concerns a method for building a subsurface velocity model from seismic reflection data using tomography, where the velocity model will be used in subsequent processing of the seismic data to prospect for hydrocarbons.

Background of the Invention

Tomography may be defined as a method for finding the velocity and reflectivity distribution from a multitude of observations using combinations of source and receiver locations. (Encyclopedic Dictionary of Applied Geophysics, 4^th Ed., R. E. Sheriff) Reflection tomography uses data from a seismic survey in which both sources and receivers were placed on the surface. In reflection tomography, migrated gathers (offset or angle gathers) are used for updating the velocity model, and the flatness of the gathers, which is measured by the depth differences of the same reflection event in the different traces of a gather, provides information whether the migration velocity model is correct or not. Typically, there is no depth difference of the same reflection event in all traces of each gather, when the migration velocity model is correct. Those depth differences are also called residual depth errors (“RDE”) because they describe the relative depths errors of the same reflection event in different traces of a gather. The velocity model may then be perturbed, with the objective of reducing the RDE, and the process is repeated iteratively to optimize the model.

The term velocity model or physical property model as used herein refers to an array of numbers, typically a 3-D array, where each number, which may be called a model parameter, is a value of velocity or another physical property in a cell, where a subsurface region has been conceptually divided into discrete cells for computational purposes.

Successful implementation of reflection tomography for velocity model building requires reliable measurement of the residual depth errors in a migrated subsurface image. (Migration, or imaging, is a data processing technique that moves subsurface reflectors to their correct locations.) Direct measurement of RDE is difficult in complex imaging areas, such as sub-salt. Velocity scanning provides an alternative way to update velocity model in complex imaging areas. A velocity scan, or velocity panel, may be defined as a display of the coherency when various normal moveouts, implying various velocities, are assumed. (Sheriff, op. cit.) The coherency may be judged, for example, according to which velocity model images a reflection point most nearly to the same depth, i.e. the flattest. Published methods to use velocity scanning in this way include the following.

Jiao, et al. (2006) proposed a1D vertical updating method. This method updates the velocity model from scans by using a formula based on a 1D assumption. A drawback of this method is inaccurate formulation for complex structures where the 1D assumption is invalid.

Wang, et al. (2006) disclosed a 3D kinematic demigration/remigration updating method. This method converts velocity scans into RDE by kinematic demigration and remigration. The estimated RDE will be used for model updating by tomography. A drawback of this method is that kinematic demigration/remigration is a complicated process in which stability and accuracy are difficult to achieve.

SUMMARY OF THE INVENTION

In one embodiment, the invention is a method for a scientific method for transforming seismic data into a subsurface physical property model, comprising constructing the subsurface physical property model by performing tomographic inversion of the seismic data, using a computer, with residual depth errors reconstructed using property scanning, wherein the residual depth errors are reconstructed using the following relationship at each imaging point DA (v_m−v_f) where A is a matrix built from ray tracing, v_m is a base migration model of the property, v_f is a model of the property as picked from a property scan of migrated seismic data, and deviation operator D is defined by its operation on an arbitrary n-dimensional vector a={a_j},

${\left(\mathrm{Da}\right)}_j=a_j-\sum _i\frac{a_i}{n}$ where indices i and j denote different source-receiver offsets among a total of n offsets present in the migrated seismic data.

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 flow chart showing basic steps in one embodiment of the present inventive method.

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.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Velocity tomography relies on reliable measurement of RDE, which may not be available in complex imaging areas. In the present invention, a formula is derived that allows one to directly reconstruct RDE from velocity scans. The reconstructed RDE can be imported into conventional tomographic inversion work flow, so that the model will be updated through velocity tomographic inversion. The derivation of the formula is based on the fact that a migrated gather at a reflection point (also called image point) is flat when the depth image is focused at this location, i.e. when the image was migrated using the velocity model that best focuses the image at this location. In other words, for each image point, a velocity scan is constructed, and the scan displays coherency, at that image point, for minimum RDE or maximum stacking power, for different migration images (gathers or stacked section), each representing a different velocity model.

A formula, for which the derivation is given in the Appendix, to reconstruct RDE from velocity scans can be expressed at each imaging point as:Dz_m=DA(v_m−v_f)  (1)where Dz_m is the reconstructed RDE, z_m is migrated depth, D is an operator that may be called the deviation operator, A is a matrix that consists of the derivatives of the imaged depths with respect to parameters of the velocity model, v_m is a vector containing the base migration velocity model parameters, and v_f is a vector containing the parameters of the focusing velocity model picked from property scanning, i.e. from coherency comparison of two or more velocity models. The elements of the matrix A will depend on the model that is used to image the data. A velocity model parameter is the value of wave propagation velocity at a particular cell in the discrete model, typically in 3-D space. Base velocity refers to an initial or current velocity model, and the focusing velocity refers to the velocity model that gives the most coherent result in the velocity scan that is focused on the particular image point.

The validity of equation (1) is based on an assumption of a small perturbation. Thus, the difference between the base migration velocity vm and the velocity of picked from the velocity scan should be small, for example <10%. In other words, for the velocity scan, the user selects velocity models that differ from each other by less than some preselected tolerance.

For an n-dimensional vector a={a_j}, Da is defined by

$\begin{array}{cc}{\left(\mathrm{Da}\right)}_j=a_j-\sum _i\frac{a_i}{n}& \left(2\right)\end{array}$ where j is the offset index and the sum is over the number n of different offsets present in the data. Thus, equation (2) defines the operator D, i.e. Dz measures the difference for the imaged depths of the same reflection event at different offsets—in other words, a measure of how much error exists in the migration velocity model. The matrix A will be computed in a conventional tomographic inversion work flow; see, for example, Liu—Ref [2], which reference is incorporated herein in all jurisdictions that allow it. The deviation of equation (1) uses the fact of Dz_f=0. After the RDE is reconstructed in equation (1) for each image point, an equation is formed to solve for a corresponding velocity model update, Δv:(DA)Δv=Dz_m,  (3)which equation must be solved by numerical methods, using a computer of course for any practical application. Invariably, no one model in the velocity panel will best focus every image point, and therefore equation (3) represents the synthesis of all the different focusing velocities to make the best update to the entire base velocity model. Equation (3) is the same velocity model update equation that is typically used in conventional tomography. The difference in the case of the present invention is that Dz_m is given by equation (1).

Compared to a conventional tomographic inversion work flow, which estimates Dzm from migrated gathers that are generated using the base migration velocity, the present inventive method obtains a more reliable estimation of Dzm in complex imaging areas, and, therefore, is a more effective way for velocity updating.

FIG. 1 is a flow chart showing basic steps in one example embodiment of the present inventive method. It is not shown in the drawings, but a seismic data set is an input quantity required for steps 20 and 30. The seismic data may consist of, for example, common-offset gathers or common-shot gathers. In step 10, a suite of velocity models is selected, being mindful of the small perturbation assumption. In step 30, a base model is selected, typically one of the velocity models from step 10. (The term “model” as used in this illustrative example embodiment refers to a velocity model, but in the case of an anisotropic medium, this can be a model of a component of velocity, for example the vertical component or the horizontal component, or a model of any one or more of the anisotropy parameters, or the term can refer to any other property of the medium that affects a kinematic property of propagation of acoustic waves, for example the position of horizons that define a region in a velocity model.) The base model is the starting model for the iterative tomography inversion process. Also, the matrix A for the tomographic inversion is generated—see ref [2] for details. The matrix A operating on a velocity model will predict the depth of each image point according to that velocity model. In step 50, tables of the depth errors are generated for a plurality of image points (preferably all, but at least enough to make an image) for each model selected in step 10. The depth error for each image point is the difference in migrated depth as migrated by a velocity model from step 10 as compared to when migrated by the base migration model. A table of depth errors for a particular image point will thus show a value of depth error for each of various migration velocity models at that image point.

In step 20, migrated images are formed from the seismic data using each of the velocity models selected in step 10. In step 40, at each image point, a focusing velocity model is selected, i.e. the velocity model (picked from among the velocity models 10) that maximizes coherency. For example, each trace in a common-image gather (a gather of traces that have a common image point but different offsets) after migration using any one of the velocity models 10 will image a particular reflection point at a somewhat different depth. The selection in step 40 may be performed by picking the migration velocity model that generates the flattest (same depth) migrated gathers at the particular image point. The migrated gathers may, for example, be offset gathers generated by Kirchhoff or beam migration, or angle gathers generated by shot beam migration or wave equation based migration. As an alternative, data representing different offsets may be stacked (summed), and then migrated, and the picking might choose the best migrated stack response, e.g. the best stacked images as generated by Kirchhoff, beam, or wave equation based migration. It should be noted that the method used for picking a velocity model from velocity scan panels is not within the scope of the present inventive method. Any picking method may be used for purposes of the present invention.

In step 50, the depth error tables are generated. This is done by multiplying the tomographic inversion matrix A from step 30 by the difference between the base migration velocity model and, in turn, each of the suite of velocity models selected in step 10, where the velocity models are expressed as vectors.

In step 60, a depth error is obtained for each image point from the tables from step 50, by picking the error corresponding to the focusing velocity model. Then, in step 70, the depth errors from step 60 are reconstructed using the present inventive method, by applying equation (1) or some equivalent expression.

In step 80, the velocity model update is obtained by numerically solving equation (3) or an equivalent expression for Δv. The right-hand side of equation (3) is the reconstructed residual depth error from step 70, and the matrix A, which is composed of the gradients of imaged depth with respect to the model parameters, comes from step 30.

It may be noted that conventional tomographic inversion using measured RDE's can be represented just by step 30, a simplified step 70 specifying direct measurement of RDE, then steps 80 and 90. Steps 10-40 are common to previous attempts to use velocity scans to update the velocity model.

In an alternative embodiment of the invention, instead of using only the reconstructed residual depth errors in step 80, step 80 may be performed using the reconstructed residual depth errors combined with residual depth errors generated using the base migration velocity model.

The accuracy of the updated model can be improved by iterating the process (step 90), i.e. returning to the steps 10 and 20, and using the updated model as the base model in step 30. If the suite of velocity models 10 from the previous iteration is considered suitable to use again, the next iteration may skip steps 10, 20, and 40, and begin with step 30 and proceed to 50, 60 and beyond. Preferably, however, the suite of velocity models should be regenerated in step 10 using the new base model, for example with scaling factors.

Software for executing the present inventive method on a computer can be developed by adapting existing tomographic inversion software to incorporate the present inventive method for reconstructing residual depth errors. Existing software will, for example, generate the matrix A and solve an equation similar to equation (3), and perform other computational steps needed in tomographic inversion. The tomographic inversion used in the present inventive method may be ray-based or wave-based.

The foregoing description 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 by the appended claims.

Appendix (Derivation of Equation 1)

Reflection tomographic inversion may be described byAΔv=Δz  (A-1)where A is a matrix built from ray tracing,Δv=v_m−v_t Δz=z_m−z_t v_m is the migration velocity, v_t is the true velocity, z_m is migration depth, corresponding to v_m, and z_t is the true depth corresponding to v_t.To eliminate z_t, the deviation operator, D, may be applied over offset index on equation (A-1):(DA)Δv=Dz_m  (A-2)where Dz_t=0 Is used. Deviation of a n-dimensional vector, z={z₁}, is defined by

$\begin{array}{cc}{\left(\mathrm{Dz}\right)}_i\equiv z_i-\frac{1}{n}\sum _j{z}_j& \left(A\text{-}3\right)\end{array}$ Equation (A-2) is used to update velocity from RDE in conventional tomographic inversion. A modified version of equation (A-1) isA(v_m−v_f)=z_m−z_f  (A4)Where v_f is focusing velocity picked from velocity scans, and z_f is focused migration depth corresponding to v_f. Applying the deviation operator, D, to equation (A-4) yields(DA)(v_m−v_f)=Dz_m  (A-5)where Dz_f=0 Is used. Q.E.D.

It should be noted that the validity of equations (A-1) and (A-4) depends upon a linearized approximation, which assumes a small perturbation between velocity models.

REFERENCES

• 1. Jiao, J., Lowrey, D., Willis, J., and Solano, D., “An improved methodology for sub-salt velocity analysis,” SEG Expanded Abstracts, 3105-3109 (2006).
• 2. Liu, Z., “An analytical approach to migration velocity analysis,” Geophysics 62, 1238-1249 (1997).
• 3. Wang, B., “A 3D subsalt tomography based on wave-equation migration-perturbation scans,” Geophysics 71, T129-T135 (2006).

Claims

1. A scientific method for transforming seismic data into a subsurface physical property model, comprising: constructing the subsurface physical property model by performing tomographic inversion of the seismic data, using a computer, with residual depth errors reconstructed using property scanning;wherein the physical property is one of velocity, a vector component of velocity, one or more anisotropy parameters, and any other property of a medium that affects a kinematic property of propagation of acoustic waves,wherein the residual depth errors are reconstructed using the following relationship at each imaging point DA(vm-vf)where A is a matrix built from ray tracing, vm is a base migration model of the property, vf is a model of the property as picked from a property scan of migrated seismic data, and deviation operator D is defined by its operation on an arbitrary n-dimensional vector a={aj}:

2. The tomographic method of claim 1, wherein the migrated seismic data are formed into gathers of traces with a common image point but different offsets.

3. The tomographic method of claim 1, wherein matrix A comprises derivatives of imaged depths with respect to model parameters.

4. The method of claim 1, further comprising updating the base migration model with the update and repeating the method for at least one iteration.

5. The method of claim 1, wherein the suite of property models are generated based on the base migration model.

6. The method of claim 5, wherein the property models in the suite of property models do not differ from the base migration model by more than a pre-selected tolerance.

7. The method of claim 1, wherein the focusing property is picked based on flatness of the image points.

8. The method of claim 7, wherein the migrated data are formed into offset gathers generated by Kirchhoff or beam migration, or angle gathers generated by shot beam migration or wave equation based migration.

9. The method of claim 1, wherein the tables of depth errors are generated by multiplying the matrix A by the difference between the base migration model and a property model from the suite to simulate depth errors in the plurality of image points, and repeating for each property model in the suite, wherein each model is expressed as a vector.

10. The method of claim 1, wherein the tomographic inversion is ray-based tomographic inversion or wave-based tomographic inversion.

11. The method of claim 1, further comprising using the physical property model constructed from tomographic inversion for prospecting or producing hydrocarbons.

12. A non-transitory computer usable medium having a computer readable program code embodied therein, said computer readable program code adapted to be executed to implement a scientific method for transforming seismic data into a subsurface physical property model, comprising: constructing the subsurface physical property model by performing tomographic inversion of the seismic data, with residual depth errors reconstructed using property scanning;wherein the residual depth errors are reconstructed using the following relationship at each imaging point DA(vm-vf)