Building the Initial Model (II) Defining Layer Velocity
Layer stripping methods establish first the velocity and then the structure of each layer sequentially from top down. Chapter 4 described the procedures for defining layer structure. In this chapter we describe the techniques for defining layer velocity from moveout in the time gather. Chapter 7 describes the techniques for deriving layer velocity from the depth gather.
Moveout forms the basis for deriving velocities from the time gather. Two families of techniques are used: Dix-based and model-based. Dix-based methods assume an unstructured subsurface of flat layers. Because of this Dix-based interval velocities can be calculated globally rather than in a layer stripping manner. By global I mean that the entire stack of layers may be analyzed at once in a vertical column. In contrast, the model-based techniques presented in this section derive interval velocities in the context of a structured subsurface and work in a layer-stripping mode. In Chapter 7 global model-based techniques are presented which make use of the initial model as a starting point for creating a refined model.
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.