Introduction

Interpreting seismic amplitudes requires an understanding of seismic acquisition and processing as well as modelling for describing and evaluating acoustic behaviour. Separate books have been written about each of these subjects and there is certainly more to say on these issues than can be presented here. The aim of this chapter is to provide a framework of basic information which the interpreter requires in order to start the process of seismic amplitude interpretation.

Seismic geometry

Seismic data are acquired with acoustic sources and receivers. There are numerous types of seismic geometry depending on the requirements of the survey and the environment of operation. Whether it is on land or at sea the data needed for seismic amplitude analysis typically require a number of traces for each subsurface point, effectively providing measurements across a range of angles of incidence. The marine environment provides an ideal setting for acquiring such data and a typical towed gun and streamer arrangement is illustrated in Fig. 2.1a. Each shot sends a wave of sound energy into the subsurface, and each receiver on the cable records energy that has been reflected from contrasts in acoustic hardness (or impedance) associated with geological interfaces. It is convenient to describe the path of the sound energy by rays drawn perpendicular to the seismic wavefront; this in turn clarifies the notion of the angle of incidence (θ in Fig. 2.1a). Usually, the reflections recorded on the near receivers have lower angles of incidence than those recorded on the far receivers.