No abstract

This paper suggests a scheme for compensating the effects that the near‐surface stratification, variable from spread to spread, produces on both the character and the timing of the seismic traces. For this purpose, accurate near‐surface velocity information is mandatory. This scheme should greatly reduce the correlation difficulties so frequently encountered in many areas. It may also be used to enhance the resolving power of the seismic reflection technique. The approach presented here is based on the rather restrictive assumptions of normal incidence, parallel equispaced plant reflectors, and noiseless conditions.

After more than twenty years of field practice, the most widely used signal in the Vibroseis technique remains the original so-called linear sweep. To many, it seems that substantial improvement should be expected from taking advantage of more sophisticated signal design techniques.The purpose of this paper is to present a tutorial survey of the basic concepts involved in signal design, tying together the known features of the linear sweep with those of the more general type.Several particular types of nonlinear sweeps are analyzed and the results of a field test seem to confirm the conclusions drawn from the conceptual analysis.After a brief consideration of still another type of nonlinear signals, a particular variety of pseudorandom sweeps, the author concludes by giving his views of what might be the role of nonlinear signal design in the future development of the Vibroseis technique.

Continuous velocity logs may be approximated by a series of zones in which the velocity is a linear function of the depth. The reflection response of a series of transition layers may be calculated from an iterative type formula, developed in this paper, which is well suited to digital computer use. This solution takes into account multiple reflections between layers. The reflection output for any input wave shape may be calculated. In this paper a Gram Charlier series pulse having a spectrum peaked at 40 cps is used throughout to facilitate comparison of results. The dependence of the reflection response of single and double layers on frequency and the reflections for the standard input pulse are illustrated. It is shown that 1. Symmetrical double transition layers give an appreciable reflection output even for a base thickness as low as 10 ft. 2. The upper layers of a multilayer group may influence considerably the reflection character from the lower layers.