Quantitative Stratigraphic Correlations
Where thick lithologlc units exist in the subsurface, quantitative approaches to stratigraphic correlations using the tools and methods described can be conceived. By thick units, we mean those whose boundaries are demarked by reflection events having sufficient separation in time to be clearly resolvable. In such a case, the necessary calibarations between arrival time and depth and between arrival time difference and thickness are established using velocity information from nearby well log measurements. If the lithologlc unit is sufficiently well defined and thick it may even be possible to develop the velocity Information from the seismic data itself (see Taner and Koehler (1969) for a basic discussion of seismic velocity determination, and Neidell and Taner (1971) for refined considerations pertaining to such determinations). The barrier bar sand described in the context of using models aptly illustrates a unit of this type.
For normal seismic data, thick lithologic units typically encompass 20 m or more depending on depth of burial, regional velocity variation with depth, and specific characteristic of the effective seismic wavelet. This value is, in fact, very much in line with our usual view of the thickness resolution inherent in our seismic data (see Sheriff, 1976). Because many exploration situations are concerned with beds having less than 20-m thickness, the matter of their resolution in quantitative terms is far from academic. Hence, we must consider in analytical terms the thin-bed stratigraphic resolution potential inherent in seismic data.
The exposition of wavelet-processing concepts and practices gave an indication that the
Figures & Tables
Stratigraphic interpretation of seismic data requires that the seismic information be expressed in geological terms. The strictly geological view of the earth is developed from surface observations, guiding principles of geological evolution and subsurface information from bore holes. Seismic measurements on the other hand, define in some sense the subsurface geometry and give estimates of the acoustic impedance which is related to the rock velocities and densities. This publication contains eight chapters on: relating seismic data to stratigraphy; information content and resolution potential of seismic data; processing for preservation of seismic amplitudes; the role of seismic wavelets and wavelet processing; contributions of geoseismic modeling; qualitative stratigraphic correlations; quantitative stratigraphic correlations; and conversion of seismic data to log-like displays.