Using geophysical logs and Coupled Markov-chain method to map hydraulic property zones

A numerical model was required to evaluate groundwater resources in the southern New Castle County, Delaware. The major deposits in Delaware Coastal Plain sediments are a seaward-dipping wedge of alternating layers of sand, silt and clay. The accurate distribution of these materials is very hard, if not impossible, to be obtained based on limited drilling boreholes. To simplify the model, we defined four hydraulic property zones based on sand percentage: Poor (<25%), Normal (25%-50%), Good (50%-75%) and Very good (>75%). The objective of this project is to map the distribution of the hydraulic property zones. Geophysical logs have been widely used for many years to interpret/predict subsurface lithology. In this study, we compiled various geophysical logs (normal gamma, spontaneous-potential and normal-resisitivity logs) recorded from 98 boreholes to determine the sand percentage. And then a coupled Markov-Chain method was applied to generate the distribution of the hydraulic property zones conditional on calculated sand percentages.