Interpretation of subsurface geology commonly depends on correlation of geophysical logs. In its simplest form, the process of correlation involves visual matching of similar characteristics on several logs. In regions where strata do not thicken or thin, correlation simply involves shifting one of the logs relative to the other until common characteristics are aligned. Variations in strata thickness, however, complicate the procedure and emphasize a need for computer analysis.

Automatic correlation of logs by computer techniques depends on utilizing the mathematical concept of a cross-correlation function. Position of the maximum value of the function identifies the amount of shift. Correlation of logs with varying thicknesses of strata involves a 2-step process. First, 1 log is resampled mathematically at an expanded or stretched interval, and then the stretched log is used in the cross-correlation process.

Models simulating geophysical logs demonstrate computer capability to compute shift and stretch. Computer output consists of a plotted reproduction of 2 logs with correlation lines connecting similar strata in each log. Applications to real well data in Indiana support visual correlations suggested by subsurface stratigraphers. Copies of the computer program can be obtained from the Indiana Geological Survey.

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