Highly productive geophysical mapping of groundwater systems in water-scare countries

Groundwater is a hidden resource, so as part of making it more visible, geophysical methods can be very useful in inferring the delineation of aquifers and/or more productive zones to target in fractured rock environments. The most commonly used techniques to assist groundwater studies or exploration are still resistivity profiles or sections known as ERT or electrical resistivity tomography and vertical electrical soundings or VES. One of the limiting factors with this technique is the scale of what surveys can be conducted, resulting in, at best, some kilometers per day. The Hydrogeophysics group of Aarhus University have developed the towed transient electromagnetic (tTEM) system as a cost-efficient tool for characterizing regional hydrological systems to depths of up to 70 m as an alternative to these more traditional methods - which is highly productive in that collection of 40- to-80-line kilometers of data per day is feasible. The system is based on the transient electromagnetic (TEM) method, which involves using a transmitter and receiver coil to measure the electrical resistivity of the subsurface. The hydrological value in electrical resistivity images stems from the ability to delineate different hydrogeological units based on their contrasting electrical properties. Consequently, 3D electrical resistivity images can infer the subsurface hydrogeology and enhance the success of installing productive boreholes. This work presents case studies from several African countries (e.g., South Africa, Zimbabwe, Ethiopia, Senegal, and Togo). It demonstrates how the tTEM method can identify reliable drinking water sources in these countries.