Practical imaging of complex geological structures using seismic prestack depth migration

Zhu, Jinming (1997) Practical imaging of complex geological structures using seismic prestack depth migration. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (11MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

This thesis develops innovative procedures to address problems in imaging multichannel reflection seismic data in regions of complex geology. Conventional common midpoint (CMP) based processing fails to produce adequate Earth images for complex geological structures with both vertical and lateral heterogeneities. This failure is due to the breakdown of assumptions such as common midpoint stacking and exploding reflector models. In these cases, seismic prestack depth migration is necessary since it can produce an accurate subsurface image - provided that a good estimate of the low wavenumber component of the velocity model is available. Two powerful prestack depth migration techniques are developed through the integral and finite-difference solutions of the wave equation. -- I first develop a new, robust, and accurate traveltime calculation method which is essentially a wavefront tracing procedure. This is implemented as a combination of a finite-difference solution of the eikonal equation, an excitation of Huygens' secondary sources, and an application of Fennat's principle. This method is very general and can be directly applied to compute first arrival traveltimes of incident plane waves. These traveltimes are extensively used by the Kirchhoff integral method to determine the integral surface, and also by the reverse-time migration to determine imaging conditions. -- The prestack Kirchhoff integral migration of shot profiles which is developed using the WKBJ approximation to the Green's function is simply a summation of amplitudes of differential traces along an integral surface with amplitudes being modulated by certain geometrical functions. I demonstrate that this summation scheme along a general integral surface is the mathematically more rigorous extension of the summation scheme along diffraction surfaces and of the superposition scheme of aplanatic surfaces. With the utilization of efficient traveltime computations, the integral depth migration is very computationally effective. It can be easily used to perform target-oriented imaging tasks by migrating selective shots and traces. -- In contrast to the Kirchhoff method, reverse-time migration is based on a direct solution of the wave equation by approximating the differential terms of the wave equation with finite differences. It is theoretically more accurate than the Kirchhoff method since it attempts to solve the wave equation without a high frequency approximation. In addition to such attractions as implicit static corrections and coherent noise elimination based on velocity information, I find that there exist self-healing mechanisms of the wavefield due to constructive interference during the reverse-time propagation of the unaliased wavefield. The self-healing ability of waves thus provides the basis of migrating sparsely and irregularly sampled unaliased recordings relative to a fine finite-difference grid without prior interpolation of missing traces. This is particularly valuable in migrating unaliased shot records with a gridded velocity model as fine as common depth point (CDP) bins with no explicit trace interpolation. As in the integral method, I implement the reverse-time migration directly from topography using the actual source and receiver positions. -- Considering the nature of imaging in geologically complex areas, I view the geophysicists' goal of obtaining an accurate Earth image as an iterative interpretive imaging procedure. This procedure consists of an initial velocity model building followed by iterative prestack depth migration, geological interpretation and velocity analysis. I formulate a very general prestack depth migration velocity analysis method with illustrations of both simple and complex examples. The evaluation of the performances of both the integral and the reverse-time migrations, especially through extensive application examples of both methods to geologically complex areas, demonstrates that the Kirchhoff integral scheme should be a better candidate for iterative imaging from the cost effectiveness viewpoint. Nevertheless, reverse-time migration is a valuable complement to Kirchhoff migration, since it can possibly produce a more accurate image of the Earth during the final imaging iterations. In this study, I extensively compare Kirchhoff and reverse-time migration procedures both on model data and an Alberta foothills real data set provided by Husky Oil Inc.

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