Model Building Concepts
In this chapter the variety of techniques and approaches to building velocity models are discussed. The main factors which distinguish seismic velocity estimation techniques are described and their relative advantages contrasted. In particular the special characteristics of model-based approaches are emphasized.
Velocity model building is the single most important factor determining the success of imaging projects. This is underscored when one considers the results of imaging experiments on synthetic data sets. In these experiments the velocity field is known and so errors and artifacts are attributable only to the limitations of the imaging approach. From these experiments it is generally found, with some important exceptions, that most prestack depth imaging algorithms are able to image most structures without errors. In light of this result, we may infer that, given adequate signal content, the success of prestack depth imaging depends mainly on the quality of the velocity model. With this outlook the bulk of this volume is dedicated to model building issues. Other factors which influence imaging success are discussed in Chapter 6.
Figures & Tables
Model-Based Depth Imaging
In this chapter the benefits of depth imaging are reviewed. The distinction between depth conversion through depth imaging and map-based depth conversion is made. The four principle geophysical advantages are defined. Traditional barriers to depth imaging are described.
Figure 1.1 is a flow chart which makes the distinction between traditional map-based depth conversion and image-based depth conversion. Both procedures begin with CMP gathers and end with depth maps. Map-based depth conversion relies on time imaging to define the structural framework. Velocity variation is expressed in the form of layer interval velocity maps, and time-depth conversion is performed through grid operations. In contrast, image-based depth conversion utilizes depth imaging to define the structural framework. The depth conversion is part of the imaging process and velocity information is obtained in the imaging process itself.
The key difference between the techniques is first, in image-based depth conversion, the structural interpretation is made utilizing the superior imaging capability of depth imaging, and second, that depth imaging is, in itself, a strong velocity estimation tool.
Before reviewing the geophysical basis for the expectation that depth images will be superior it is worth examining some data examples that show, from a geologic standpoint, the additional structural information gained. Figures 1.2 to 1:12 are time and depth image comparisons from a variety of structural settings.
Salt examples - Figure 1.2 is from a salt sill in the deepwater Gulf of Mexico. The time image shows a complex arrangement of reflectors in the subsalt section. In the depth image the subsalt section is shown to be planar and relatively unbroken (there remains a single shadow zone below the nose of the sill) with a slight dip away from the salt sill. The base of salt reflections show alternations of steep and moderate dip perhaps related to changes in sedimentation rate versus speed of salt sill advancement. The development of this depth image is reviewed in the case history by Egozi in this volume.
Figure 1.3 depicts a salt diapir from the transition zone of the Gulf Coast. Exploration in this area often relies on an accurate estimate of the position of the salt sediment interface. Where the diapir flank becomes steep the time image shows little information. The depth image renders the interface continuously; where it is vertical and even where it is overhung.
Figure 1.4 depicts a salt-cored anticline in the Southern North Sea. The high amplitude reflection near the base of the time image is a salt weld along which the upper Permian salt has been evacuated.