A TECHNIQUE FOR MODELING GEOPHYSICAL LOGS AND ITS APPLICATION TO THE COASTAL PLAIN OF SOUTH CAROLINA

One technique for modeling a geophysical log response at any undrilled location within a study area relies on the relationship between statistical moments (e.g., mean, standard deviation, skewness and kurtosis) determined from log data of existing wells and the polynomial equation that would characterize that geophysical log. When statistical moments are calculated for a specific geophysical log in a number of wells, and when these moments are mapped, the value of these moments can be predicted for any location in the study area; using these predicted moments, the polynomial expression that models the geophysical log at that location can be generated.

This approach to modeling geophysical logs at any location within a study area was applied to the gamma-ray and long-normal resistivity logs in the Upper Cretaceous Series of the Coastal Plain in South Carolina. Logs were modeled at closely spaced intervals (<3 miles), and these logs were used to construct a series of strike- and dip-oriented cross sections. Additionally, one strike-oriented cross section and one dip-oriented cross section were constructed at an interval of approximately one mile between predicted well logs. All cross sections reveal that both hydrostratigraphic and lithostratigraphic units change in a nonlinear manner between existing well locations. The nonlinear changes illustrated by these cross sections suggest: 1) the presence of a subtle subsurface dip reversal in the coastal region of South Carolina; 2) the possibility of faulting in the Florence and Sumter Counties; and 3) the thickness and continuity (and quality?) of both aquifers and confining units is highly variable throughout the Coastal Plain.