QUANTIFYING CORRELATION FOR THREE-DIMENSIONAL GEOLOGIC MAPPING
In this study, indicator geostatistics and transition probability geostatistics are applied to nearly 300 km of drilling data derived from over 11,000 private well drilling logs and over 200 high-quality well logs from monitoring well installation. Analyses are focused on a 12,000-km2 region in central Minnesota in a setting dominated by the late-Wisconsinan Rainy, Superior, and Wadena lobes.
The drilling data are categorized according to the logged sediment descriptions. In one scheme, the materials are separated according to presumed high or low hydraulic conductivity (K) for use in binary indicator geostatistical and transition probability geostatistical analyses. In another, the number of categories is expanded to five on the basis of combined K and depositional setting information for use in a separate transition probability analysis.
The data are separated into different geomorphological settings associated with various depositional environments of several ice advances. Vertical variograms are very well supported in all geomorphological areas, while lateral variograms vary from well supported to random scatter. The ratio of vertical to lateral range varies but has an average value of ten. Results for the numerous geomorphological settings indicate overlapping geostatistical ranges, sills, and vertical lens thicknesses. A lack of stationarity was observed, consistent with a fundamental complexity of glacial depositional and erosional processes. Correlation generally varied as much between geographically distinct zones of like geomorphology as it did between zones of different geomorphology. High-resolution data associated with monitoring well installation typically deviated from the private well data; this is attributed to site-specific geology and detailed logging of thin units. Overall results underscore the difficulty in correlative assumptions in glaciated regions.