A 3-D acquisition geometry should be designed such that at the end of the acquisition and processing sequence the desired signal can be reliably interpreted and the noise is suppressed as much as possible. This chapter focuses on noise suppression.
The main types of noise are multiples and low-velocity noise such as ground roll and scattered energy. How much low-velocity noise can be suppressed depends on the choice of field arrays, the stack response (implicitly also on fold) and on various processing steps. One of the reasons to select a wide orthogonal geometry is that it allows tackling low-velocity noise by filtering in the shot as well as in the receiver domain. The total amount of multiple suppression depends on the stack response (implicitly also on range of offsets) and on the success of multiple elimination programs, but not on field arrays. At present there is no clear theory on how much noise can be removed in processing. As a consequence, the required noise suppression by field arrays and stacking is relatively unknown, and, to a large extent, the choice of field arrays and fold is dependent on experience.
In this chapter, the effect of field arrays on low-velocity noise and of the stack response on low-velocity noise and multiples is discussed. This chapter begins with a discussion of the properties of the low-velocity noise as essential knowledge for the optimal choice of field arrays (linear or areal, shot and/or receiver arrays). Another very useful piece of knowledge would be a quantitative assessment of the amount of noise (ground roll and scattered energy) relative to the desired primary energy.
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.