CHARACTERIZING THIN HORIZONTAL BEDROCK DISCONTINUITIES USING BOREHOLE GEOPHYSICAL LOGGING AND HYDRAULIC TESTING

Thin, near-horizontal, high-transmissivity discontinuities in relatively undeformed sedimentary bedrock sequences can exert profound influences on hydrogeology. Such thin (less than 10 cm) discontinuities include bedding planes, more permeable units, fractures, and fracture zones. These features can be horizontally extensive on the scale of kilometers and can dominate local and regional groundwater flow systems. In particular, such thin but horizontally extensive features can control head distributions, the transport of solutes, the locations of springs and seeps, and the contributing areas for wells. These thin features can be difficult to recognize during bedrock drilling.

Borehole geophysical investigations offer an efficient and cost-effective method to determine the presence, location, and hydraulic properties (head, hydraulic conductivity) of these near-horizontal features. Borehole geophysics has now become an essential part of the hydrogeologist's toolbox, particularly in bedrock settings where drilling costs can be high. At the Wisconsin Geological and Natural History Survey we routinely acquire suites of geophysical logs both from existing water supply wells and from research wells. A typical suite of logs includes caliper, natural gamma, single-point and normal resistivity, fluid temperature, fluid conductivity, and borehole flow, using either the heat-pulse or spinner flowmeter. In typical sedimentary units found in Wisconsin (dolomite, sandstone, shale), the gamma and resistivity tools provide excellent stratigraphic correlation, while the caliper tool helps locate potential discontinuities. Fluid temperature and conductivity logs can be used to identify hydraulically active discontinuities. The heat-pulse flowmeter provides quantitative measures of vertical borehole flow, and can be used with inversion techniques to determine the head and transmissivity of individual discontinuities. Examples of such applications in Wisconsin include the delineations of regionally-significant fracture zones in Silurian dolomite, the discovery of significant horizontal discontinuities within a shale aquitard, and the determination of contributing areas for springs and flowing wells in sandstone. In each of these projects borehole geophysical data was an essential part of the hydrogeologic analyses.