Overview

This chapter uses a geometrical-optics approach to derive the basic relationships between traveltime and the locations of reflecting/refracting interfaces; most structural interpretation relies on such an approach.

The accurate interpretation of reflection data requires a knowledge of the velocity at all points along the reflection paths. However, even if we had such a detailed knowledge of the velocity, the calculations would be tedious; often we assume a simple distribution of velocity that is close enough to give useable results. The simplest assumption, which is made in §4.1, is that the velocity is constant between the surface and the reflecting bed. Although this assumption is rarely even approximately true, it leads to simple formulas that give answers that are within the required accuracy in many instances.

The basic problem in reflection seismic surveying is to determine the position of a bed that gives rise to a reflection on a seismic record. In general, this is a problem in three dimensions. However, the dip is often very gentle and the direction of profiling is frequently nearly along either the direction of dip or the direction of strike. In such cases, a two-dimensional solution is generally used. The arrival time-versusoffset relation for a plane reflector and constant velocity is hyperbolic. The distance to the reflector can be found from the reflection arrival time at the source point if the velocity is known.