ASSESSING REGIONAL SUBSURFACE VARIABILITY IN THE ATLANTIC COASTAL PLAIN

Traditional methods of assessing the regional subsurface variability of geological parameters are often qualitatively based, and boundaries between regions are subjectively placed without regard for spatial variability or the rate at which the parameters change. Traditional methods also do not allow for comparing the rate of change of one variable with that of another. To minimize the drawbacks of these traditional approaches, a new method was developed for use with geophysical log data. This new method involves characterizing each geophysical log with a high order polynomial expression, which is divided into segments based on the depth at which the polynomial’s first derivatives occur. For those segments whose boundaries (i.e. first derivatives) are identifiable in all wells and are assumed with confidence to be correlative, the polynomial for that segment is expressed by an equal number of uniformly spaced points in every well, regardless of the stratigraphic thickness of the segment. All correlative segments are averaged to establish an “ideal” segment, and the coefficient of determination (R²) is calculated in order to obtain a quantitative measure of the similarity between the “ideal” segments and the correlative segments in a given well. The coefficients of determination for all wells included in the study are mapped and contoured, and the degree of similarity between two well logs is reflected by their location relative to the contours. In addition, orientation of the contours in relation to the strike and dip of the units provides information for interpreting depositional environments and/or diagenesis.

This method for assessing the regional variability of geophysical logs has been applied to select areas of the Atlantic Coastal Plain in South Carolina. A pilot study using numerous closely-spaced wells has been completed, and a larger scale study of the Upper Cretaceous Series throughout the state is underway.