Tony Huang

3D angle gathers from reverse time migration

Reverse time migration (RTM), based on directly solving the two-way wave equation, provides a natural way to deal with large lateral velocity variations and imposes no dip limitations on the seismic images. It has become a standard migration tool for subsalt imaging in the Gulf of Mexico. In addition, the adoption of wide-azimuth (WAZ) data acquisition provides better subsurface illumination and more azimuthal information, which significantly aids imaging beneath complex overburdens. Because WAZ data are richer in the propagation directions of the waves recorded, RTM and WAZ acquisition go hand-in-hand.

The benefit of TTI tomography for dual azimuth data in Gulf of Mexico

Over the past decade, the majority of deepwater blocks in the Gulf of Mexico have been covered multiple times with seismic data from narrow-azimuth, towed-streamer acquisition (NAZ). In complex subsalt areas, each NAZ dataset provides unique subsurface illumination benefits. Multiple-azimuth data are now frequently integrated to provide extended subsurface coverage and for better imaging of complex subsalt structures. Multiple-azimuth seismic data, with shot and receiver locations covering a large portion of the two dimensional surface, present a new challenge for deriving a single velocity model that satisfies both datasets. In exploration and development work in the deepwater Gulf of Mexico, there has been an increasing demand to incorporate anisotropy in prestack depth imaging workflows. Incorporating anisotropy improves image quality and well/seismic misties. While most pre-stack depth imaging involves vertical transverse isotropy (VTI) anisotropy, transverse isotropy with tilted symmetry axis (TTI) is generally overlooked. Shale layering near steeply-dipping salt flanks can cause TTI anisotropy issues. In such a case, ignoring the tilted symmetry of salt flank bedding causes image blurring and mispositioning of the salt flank structure. Velocity variation with azimuth is observed in an orthogonal dual-azimuth streamer dataset, as well as wide-azimuth data in the deepwater Gulf of Mexico. The paper presents a study to build a single TTI anisotropy model for pre-stack depth imaging of dual-azimuth data in the deepwater Gulf of Mexico to yield an anisotropy model that flattens gathers for all azimuths, as well as improves focusing and spatial positioning of steeply-dipping salt flanks.

Subsalt imaging using TTI reverse time migration

TTI depth imaging technology is routinely applied to image structures that lie beneath dipping, anisotropic overburden in the Canadian Foothills (Vestrum and Vermeulen, 2004), the North Sea (Hawkins et al., 2002), and offshore West Africa (Ball, 1995). Until now, ray-based migration algorithms served as the only choice for TTI imaging because upgrading ray-based imaging algorithms for TTI is straightforward and incurs minor additional computational cost. Unlike ray-based algorithms, TTI wave-based algorithms are difficult to formulate, and their implementations are often unstable and computationally intensive. Unfortunately, ray-based algorithms perform poorly in comparison to wave-based algorithms in imaging structures beneath a complex overburden such as the salt canopy in the Gulf of Mexico. However, the use of TTI imaging in this area is impeded by the lack of appropriate check shots and offset VSPs that can constrain anisotropic parameters.

The Application of RTM 3D Gathers for Wide Azimuth Data in Garden Banks, Gulf of Mexico

Over the past few years, the availability of wide azimuth (WAZ) data, in conjunction with reverse time migration (RTM), has drastically increased our capability to image complex subsalt geology. One of the remaining challenges is to generate accurate RTM 3D common-image gathers (CIGs) that retain the rich azimuth information inherent in WAZ data while honoring anisotropy. The immediate benefits of RTM 3D CIGs include improved tomographic velocity inversion and reservoir attribute analysis for data containing a wide range of azimuth information. A method to output RTM 3D CIGs that contain both subsurface azimuth and reflection angle information was recently developed by Xu, et al. (2010). In this abstract, we will demonstrate the benefits of using the new technique to generate RTM 3D CIGs for a WAZ dataset in Garden Banks, Gulf of Mexico.

Enhanced Tomography Resolution By a Fat Ray Technique

Summary We propose a tomography algorithm to enhance the resolution of 3D velocity model building. Our algorithm uses a fat ray instead of the conventional asymptotic ray. Compared to conventional ray tomography, the resulting tomography matrix is far less sparse. Thus, while the new matrix may still be ill-conditioned, much less model regularization is required to produce a well-posed system. In turn this should lead to a higher resolution tomographic update. A subsalt velocity analysis test on the Sigsbee 2a model shows promising result; we also apply the fat ray technique to well-tie tomography that estimates anisotropy parameters.

Anisotropy Estimation For Prestack Depth Imaging - A Tomographic Approach

In areas where well data (sonic logs, check shots et al) are available, we are able to get accurate vertical velocity by calibrating seismic migration velocity with check shot velocity. We present a method to accurately determine anisotropic parameters in transversely isotropic media without the weak anisotropy assumption. We test the algorithm on synthetic data and present a work flow to determine anisotropic parameters for prestack depth imaging in the Gulf of Mexico.

Unlocking the Potential of WAZ data at the Tonga Discovery with TTI Reverse Time Migration

Over the past decade, deep water Tertiary subsalt structural plays in the Gulf of Mexico have attracted a lot of attention from exploration companies. However, accurately imaging these reservoirs has been very challenging since they are very deep and obscured by thick salt. Fortunately, the availability of wide-azimuth data and the development of reverse time migration (RTM) have significantly increased our ability to image subsalt. Still, there is room for improvement. One area currently under development is the incorporation of anisotropy in subsalt imaging, especially tilted transverse isotropy (TTI). Wide-azimuth data contains more abundant azimuthal information than either narrow-azimuth or multiple narrow-azimuth datasets. This additional azimuthal information can be used to further constrain the estimation of TTI parameters.

Migration Artifacts And Velocity Analysis

Kirchhoff depth migration is conventionally used for large 3D marine surveys. It has advantages for both fast track imaging and velocity model building. Conventional model building requires a depth-migrated common-image gather (CIG) (Al-Yahya, 1989) in which the reflectors are migrated multiple times using a multi-channel input dataset. If the prestack migration uses the correct velocity the seismic events will focus properly in the CIG regardless of any redundant axes (offset, reflection angle etc.). Otherwise, the seismic event will appear at a different depth. The difference in depths, the so-called residual moveout, provides information to update the velocity model. Generally, the moveouts are automatically picked from coherent seismic events in CIGs and this process provides the basis for an iterative tomography algorithm for complex velocity model building.

The Benefit of TTI Reverse Time Migration for Subsalt Imaging, Gulf of Mexico

In reverse time migration (RTM), the wave propagation is based on directly solving the two-way wave equation. Thus RTM provides a natural way to deal with large lateral velocity variation and imposes no dip limitations on the images, and it becomes a standard migration tool for subsalt imaging in Gulf of Mexico. On the other hand, recent advances in wide azimuth (WAZ) and multi-azimuth (MAZ) data acquisition provide more azimuth information, which offers better opportunities for estimation of anisotropy parameters, and improves images under steeply-dipping anisotropy overburdens. This requires RTM to handle general anisotropy media, especially tilted transverse isotropy (TTI) to achieve a significant improvement in image clarity. We demonstrate in this paper, particularly in the Gulf of Mexico, using TTI reverse time migration can produce better subsalt images.