Guidelines for design of “land-type” 3-D geometry
In this chapter the symmetric sampling criteria are expanded into guidelines for parameter selection for the survey geometry.
Often, geophysicists dealing with the design of 3-D seismic surveys concentrate on the properties of the bin: offset distribution, azimuth mix, midpoint scatter. In my approach, even more emphasis is put on the spatial properties of a geometry across the bins. These spatial aspects are so important because most seismic processing programs operate in some spatial domain, i.e., combine neighboring traces into new output traces, and because it is the spatial behavior of the 3-D seismic volume which the interpreter has to translate into maps.
These guidelines start with a brief description of the knowledge base, which has to be built to allow a satisfactory choice of all parameters. The first choice to be made is the type of geometry. In general, orthogonal geometry is the geometry of choice for land data acquisition and for marine data acquisition in combination with ocean-bottom cables. Yet, other geometries may also be selected, and a short review outlines pros and cons of various geometries that may be chosen.
This chapter focuses on orthogonal geometry. If 3-D symmetric sampling is taken as a starting point, the choice of parameters for this geometry is simplified considerably. Instead of having to decide on the shot interval and on the receiver interval, a decision need only be made as to the sampling interval. Similarly, the maximum inline and maximum crossline offsets can be made equal. It is also recommended to see what the consequences are of making the shot-line interval and the receiverline interval the same.
Figures & Tables
3-D Seismic Survey Design
Three-dimensional (3-D) seismic surveys have become a major tool in the exploration and exploitation of hydrocarbons. The first few 3-D seismic surveys were acquired in the late 1970s, but it took until the early 1990s before they gained general acceptance throughout the industry. Until then, the subsurface was being mapped using two-dimensional (2-D) seismic surveys.
Theories on the best way of sampling 2-D seismic lines were not published until the late 1980s, notably by Anstey, Ongkiehong and Askin, and Vermeer. These theories were all based on the insight that offset forms a third dimension, for which sampling rules must be given.
The design of the first 3-D surveys was severely limited by what technology could offer. Gradually, the number of channels that could be used increased, leading to discussions on what constitutes a good 3-D acquisition geometry. The general philosophy was to expand lessons learned from 2-D acquisition to 3-D. This approach led to much emphasis on the properties of the CMP gather (or bin), because good sampling of offsets in a CMP gather was the main criterion in 2-D design. Three-D design programs were developed that concentrated mainly on analysis of bin attributes and, in particular, on offset sampling (regularity, effective fold, azimuth distribution, etc.).
This conventional approach to 3-D survey design is limited by an incomplete understanding of the differing properties of the many geometries that can be used in 3-D seismic surveys. In particular, the sampling requirements for optimal prestack imaging were not properly taken into account. This book addresses these problems and provides a new methodology for the design of 3-D seismic surveys.
The approach used in this book is the same as employed in my Seismic Wavefield Sampling, a book on 2-D seismic survey design published in 1990: Before the sampling problem can be addressed, it is essential to develop a good understanding of the continuous wavefield to be sampled. In 2-D acquisition, only a 3-D wavefield has to be studied, consisting of temporal coordinate t, and two spatial coordinates: shot coordinate xs, and receiver coordinate xr. In 3-D acquisition, the prestack wavefield is 5-D with two extra spatial coordinates, shot coordinate ys, and receiver coordinate yr.
In practice, not all four spatial coordinates of the prestack wavefield can be properly sampled (proper sampling is defined as a sampling technique which allows the faithful reconstruction of the underlying continuous wavefield). Instead, it is possible to define three-dimensional subsets of the 5-D prestack wavefield which can be properly sampled. In fact, the 2-D seismic line is but one example of such 3-D subsets.