Wiktor Weibull

Automatic Migration Velocity Analysis Using Reverse Time Migration

SUMMARY The objective of this paper is to describe an automatic velocity analysis method based on Reverse Time Migration and Differential Semblance Optimization. The velocity analysis is based on the solution of a nonlinear least squares problem aiming at the focusing of offset domain common image point gathers constructed by Reverse Time Migration. Because the method is based on the solution of the two-way wave equation, it can deal with strong and sharp velocity contrasts both in a stable and accurate manner. It is therefore expected that this method will help improve seismic imaging over complex geological settings. We illustrate the method with a simple synthetic 2D seismic example.

Automatic Shear-wave Velocity Analysis with Elastic Reverse Time Migration

Converted-wave has the potential to be a positive complement of the compressional imaging in some special cases. However, converted wave imaging is more difficult because the need of estimating the background velocity of S-wave. By applying differential semblance to measure the focusing error in the imaging domain, we automatically estimate the background S-wave velocity using reverse-time migration with a given estimated P-wave velocity. Compared to the P-wave, the S-wave velocity estimation via PS imaging gather does not need gradient from the source side. A simple synthetic result demonstrates the feasibility of the proposed method.

Implications of the Born Approximation For WEMVA - a Systematic Approach

Wave equation migration velocity analysis is an automatic iterative method for estimating seismic velocities from migrated images. The problem of obtaining velocity updates from images is currently solved by first expressing the object function perturbation as a linear function of the the model perturbations. And then solving the linear system using adjoint methods. Here we carefully analyse the errors associated with the linearization in the velocity analysis framework. The goal is to be able to quatify approximately what are the necessary conditions for a sucessful velocity inversion.

3D Vector Imaging of Converted Waves for Fractured Reservoirs

Summary The fractured system widely exists in the earth media. It causes anisotropy on the seismic wave scale. Passing through such media, a shear wave splits into two mutually orthogonal waves with different propagation velocities. Unlike the well-behaved qP mode, the qS1- and qS2-wave do not consistently polarize as a function of the propagation direction. Therefore, continuous polarization directions of qSV and qSH mode are used for accurate wave mode decoupling. Combined with vector imaging condition, the proposed method properly utilizes two converted shear waves in elastic reverse time migration (ERTM), without polarity reversal. Sensitivity analysis of wave mode decoupling demonstrates the potential of converted wave imaging in analyzing fracture attribute. Synthetic examples demonstrate the validity of this approach.

Elastic Reverse-time Migration and Velocity Analysis Using Multicomponent Geophone Data

In this paper, we deal with the problem of P-P and P-S imaging and velocity analysis using multicomponent geophone data and elastic reverse-time migration. The case of elastic reverse-time migration, and that of elastic imaging in general is more complicated than the acoustic counterparts by the existence of P- and S-wave modes. This requires an imaging condition able to distinguish apart the different scattering events. Another important requirement is the ability to build accurate P-wave (vp) and S-wave (vs) velocity models. The difficulties associated with obtaining good quality images of P-S reflections results in that, even if multicomponent data is available, it is seldom that the horizontal components are actually used for subsurface imaging, analysis and interpretation. Here, we present imaging conditions capable of imaging P-P and P-S scattering events using all geophone components. The migrated images are then used in an automatic stack-power maximisation procedure to estimate optimal vp and vs migration velocity models. We test the methodology in 2D synthetic and field datasets.

Source Wavefield Reconstruction for Large-scale 3D Elastic Full-waveform Inversion

In elastic full-waveform inversion, the medium parameters are updated iteratively by incrementing them with the derivatives of the misfit functional with respect to each of the medium parameters. The efficient implementation of the derivative computations require large amount of computer memory storage. The large requirements in terms of storage is one the main barriers for the application of elastic full-waveform inversion to large scale 3D problems. In this paper, we propose and test a strategy based on reverse-time wavefield reconstruction using the Kirchhoff integral that effectively reduces the storage requirements, at the cost of a factor of two increase in the computational runtime.

Reverse Time Migration Velocity Analysis - A Real Field Data Example

Depth migration by reverse time migration requires the knowledge of a smooth approximation to the seismic velocity field. This background velocity model can be estimated by wave equation migration velocity analysis (WEMVA), an automatic process based on minimizing the errors in the kinematics of the depth migrated image. In this paper we present a WEMVA method where we use a combination of semblance and differential semblance to measure the errors in the positioning of reverse time migrated images. The errors are then turned into velocity updates by a gradient based optimization scheme. We apply the method to a 2D line extracted from a 3D marine survey acquired over the Snorre field in the North Sea. The resulting WEMVA velocities obtained from the field data are compared to the background velocities obtained by traveltime tomography, and also to well logs.