Refining the Model Using the Products of Prestack Depth Migration - The Depth Gather
This chapter describes the methods by which residual moveout observations made in the depth gather, the product of prestack depth migration, may be used to refine a velocity model. It will be shown that when an earth model is correct, reflections in the depth gather will be flat. We will show how the non-flatness in the depth gather, termed ìdelaysî, can be used to infer how to refine the model so that flatter depth gathers are produced with the next migration. Two techniques are described: the one-dimensional method and the ray-based global tomographic method.
Two dimensional seismic data can be thought of as a three dimensional data entity. The third dimension is offset. Figure 7.1 shows a schematic of an unmigrated 2D line with offsets ranging from 1 to 40.
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.