Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-04-30
2004-11-16
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06820010
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of geophysical prospecting and, more particularly, to a method of imaging the subsurface of the earth in depth using compressional-wave, shear-wave and mode-converted wave data.
BACKGROUND OF INVENTION
Seismic data acquired in the field is always recorded in time due to the nature of the acquisition process. For seismic exploration using both compressional and mode-converted waves, it is a common practice to deploy a large number of multicomponent geophones on the surface of the earth and to record the vertical and horizontal vibrations of the earth at each geophone location to obtain a collection of seismic traces. When the vibrations are caused by a seismic source activated at a known time and location, the recorded data can be processed using methods well known in the art to produce an image of the subsurface. The image thus produced is commonly interpreted by geophysicists to detect the presence of hydrocarbons.
The geophones sensitive to vertical vibration modes record mostly compressional (P) waves while the geophones sensitive to horizontal modes record mostly shear (S) waves. The shear wave energy may be due to mode-conversion, i.e., when some compressional energy is converted to shear wave energy at a subsurface layer boundary or other impedance contrast. This type of wave is also referred to as a PS wave, signifying the conversion from P-wave to S-wave. Downgoing P-wave energy reflected as an upgoing P-wave and then recorded is referred to as a PP reflection.
Seismic data are interpreted to provide subsurface information of geological structure and conditions prior to drilling for minerals such as oil and gas. Seismic data are often displayed as “time sections” where the vertical scale is linear in arrival time, i.e., an ordinary seismic section. The seismic data interpreted on a time section is converted to depth before drilling for hydrocarbons commences. Depth conversion of time section data requires an accurate velocity model. Another approach is to depth migrate the seismic data before stacking in order to properly position the subsurface geological entities such as faults, channels, etc. The use of prestack depth migration has proliferated in recent years as the computing power has increased and the relative cost has decreased. Processing of mode-converted data is computationally expensive. Depth migration is more attractive than time migration because the common conversion point (CCP) varies with depth due to the asymmetric ray path of the downgoing compressional wave and the upgoing shear wave. The CCP is the common reflecting point where mode conversion from P- to S-waves or vice-versa occur for a dataset. Reciprocity (the conversion point being the same when source and receiver are interchanged) does not apply because of the asymmetric raypath, making lime migration processing more difficult. Thus the processing of mode-converted data in time is much more difficult and can be less accurate than the processing of compressional wave data. However, the depth migration of seismic data requires an accurate velocity model.
The velocity model for depth migration of compressional wave data can be obtained by one of several techniques discussed in the literature (e.g., Kosloff et al. (1996), Meek et al. (1998), Schultz and Canales (1997), and Jones et al. (1998). The migration of mode-converted data also requires knowledge of the shear wave velocity field. D'Agosto and Michelena (1997) developed a tomographic method to obtain P and S wave velocities from mode-converted data.
Mikhailov and Frasier (2000), Bale et al. (1998), and Shoshitaishvili et al. (2000) have discussed the depth migration of mode-converted data. Their methods assume that the depth model and corresponding P-wave velocities are known from the analysis of the P-wave data. The methods describe using the residual moveout of mode-converted waves to update the velocity model and are fairly complex and computationally expensive.
There is a need for a method of updating the S-wave velocity model that is very simple and quickly leads to an acceptable solution for the final velocity model for depth migration of mode-converted data. The present invention satisfies this need.
SUMMARY OF THE INVENTION
The present invention provides for a method of processing mode converted seismic data. The method comprises acquiring seismic data and determining a P-wave velocity model for the seismic data. An S-wave velocity model is determined from the P-wave velocity model. The mode converted seismic data are depth migrated forming CIP mode converted seismic data gathers. Corrected S-wave velocities are determined from near-offset data of the CIP gathers which corrects the depth migrated mode converted seismic data to the P-wave velocity model depth. A correction factor is provided and the iterating with the method quickly converges to a solution.
It is an objective of this invention to provide an efficient method of updating the shear wave velocity model for prestack depth migration of mode-converted data. First, a seismic dataset (P-wave) is processed and prestack depth migrated using a number of different methods. A velocity model for P-waves is obtained which is used to calculate the travel-time tables for the downgoing P-waves. This model provides an initial velocity model for S-waves that is used to calculate the travel-time tables for the upgoing S-waves. The travel-time tables calculated from these steps are then used to depth migrate the mode-converted data. The common image point (CIP) depth gathers generated by depth migration are then examined to determine if the S-velocity of a horizon is too low or too high with respect to the S-velocity that flattens the seismic events on the CIP gathers. The depth of the near-offset on the CIP mode-converted image gather for a given horizon is compared to its depth on the velocity model for P-waves at the corresponding surface location. The discrepancy in depth is then used to update the S-wave velocity. The mode-converted data is depth migrated again with the updated S-wave velocity model and the CIP gathers are examined. Generally, this procedure leads to a satisfactory velocity model. However, another iteration of this process may be necessary sometimes to flatten the seismic event on the CIP gathers.
REFERENCES:
patent: 4839869 (1989-06-01), Corcoran
patent: 6128580 (2000-10-01), Thomsen
patent: 6212477 (2001-04-01), Zhu et al.
patent: 2003/0021184 (2003-01-01), Zhang
Richard Bale et al.;Prestack Depth Migration of Converted Wave Data in Anisotropic Media,1998 SEG Expanded Abstracts, 3 Figs.
Ian F. Jones et al.;3-Prestack Depth Migration and Velocity Model Building,The Leading Edge, Jul. 1998, pp. 897-906, 11 Figs.
Dan Kosloff et al.;Velocity and Interface Depth Determination by Tomography of Depth Migrated Gathers,Geophysics, vol. 61, No. 5 (Sep.-Oct. 1996), pp. 1511-1523, 18 Figs.
Robert A. Meek et al.,3D Interval Velocity/Depth Model Building in the Southern Gas Basin UK, 3D Versus 2D Model Building,1998 SEG Expanded Abstracts, 9 Figs.
Oleg Mikhailov et al.;Converted Wave Imaging: Theory for Residual Velocity Analysis and Examples of Pre-Stack Depth Migration of Alba 4C Data.
Phil Schultz et al.;Seismic Velocity Model Building: CE in Dallas 2 November,The Leading Edge, Jul., 1997, 3 Figs.
Elena Shoshitaishvili et al.;Pre-stack Depth Migration Workflow for Converted Waves.
Claudio D'Agosto et al.;Tomographic Estimation of Compressional and Shear Wave Velocities from P-S Converted Waves,ST 15.7, pp. 1885-1888, 6 Figs.
Meek Robert A.
Sahai Surinder K.
ConocoPhillips Company
Madan Mossman & Sriram P.C.
McElheny Jr. Donald
LandOfFree
Method for determining shear-wave velocity model for depth... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for determining shear-wave velocity model for depth..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for determining shear-wave velocity model for depth... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3276483