ESTIMATING THE DISTRIBUTION OF HYDRAULIC PROPERTIES USING RESISTIVITY MODELS DERIVED FROM AIRBORNE TIME DOMAIN ELECTROMAGNETIC DATA

摘要

The Ramotswa transboundary aquifer is shared between South Africa and Botswana. An extended drought throughout the region has increased the need to understand this shared water resource. An airborne electromagnetic (AEM) survey was conducted to generate new information about the aquifer in order to support water security and facilitate its sustainable management between the two countries. Using the AEM survey results, the three-dimensional spatial distribution of the dolomite aquifer was interpreted and estimates of the aquifer's distribution and range of porosity and hydraulic conductivity were made. Porosities throughout the dolomite were estimated using a modified version or Archie's Law specific to carbonates. Transmissivity values were previously calculated in the Ramotswa Well Field, from both constant rate and step pumping tests, and were used in conjunction with resistivity data, extrapolated over the well field, to calculate the relationship between hydraulic conductivity and resistivity for the aquifer. Minimal hydraulic data was collected on the ground within the study area, presenting a challenge for relating the electrical resistivity data to hydraulic properties, especially over a large and heterogeneous aquifer. The distribution of hydraulic conductivity in the aquifer was geoelectrically estimated using a generalized log-log linear electrical-hydraulic correlation function, the slope of which is dependent on the geologic and geochemical environment.