THE SIGNIFICANCE OF HYDRAULIC AND THERMAL GRADIENTS IN BOREHOLES
In over a decade of collection of digital borehole logs throughout Wisconsin we have commonly found that sparse but important hydrogeologic discontinuities such as vertical or horizontal fractures or karst conduits dominate flow to wells in sedimentary and crystalline rocks. Such observations suggest that our traditional conceptual models of uniform aquifers and aquitards dominated by homogeneous porous-media processes are often incorrect. Instead, a small number of discrete features can contribute most of the transmissivity measured in a well or borehole. Geophysical logs detect these features as sources or sinks of ambient or stressed borehole flow, as temperature anomalies, and/or as steep gradients in electrical conductivity of the water column. When combined with borehole imaging tools such as the optical borehole imager (OBI) and acoustic borehole imager (ABI), geophysical logs present a very detailed and complete picture of hydrogeologic conditions inside a borehole.
The two critical measurements needed for understanding borehole hydraulics are the distribution of total hydraulic head and the distribution of flows along the borehole. Sharp changes in head can indicate an aquitard that separates two aquifers, while sharp changes in flow often indicate zones of high transmissivity related to fractures or fracture zones. In some cases these features can be correlated from well to well over significant distances. Discontinuities in the head profile sometimes indicates perched conditions, identifying an unsaturated zone separating two water table aquifers.
Placing each set of borehole data into its proper context within regional or local groundwater flow systems is essential for correct interpretation of these borehole phenomena. In turn, understanding the distribution of hydraulic discontinuities in the subsurface can be critical for appropriately addressing and responding to larger-scale hydrogeologic issues.