Recommended Basic Borehole Geophysics for Hydrologic Investigations in a Fractured Bedrock Aquifer

Hydrogeologists do not always receive a great exposure to borehole geophysics within many educational institutions, just as geophysicists do not get a lot of exposure to hydrogeology. Yet an often-heard inquiry from the hydrogeologist starts with “I have a 6-inch casing stick-up, but I don’t know much else about this well…” ending with “is there anything geophysics can do for me?” The geophysical choices can be many, and the answer sometimes depends on the person answering. Using a case history from a fouled groundwater supply well, constructed in the 1940’s, this presentation will look at a classic hydrologic suite of log measurements recommended to help the hydrogeologist understand the well and aquifer. In this particular instance, the well log was long lost to history, along with the well depth, yield, and water bearing zones and lithology. The geophysical investigation was designed to identify basic well characteristics to aid well rehabilitation. The basic logging suite recommended included Temperature, Caliper, Spontaneous Potential, Single Point Resistance, Normal Resistivity (8”, 16”, 32”, and 64”), Fluid Resistivity, Natural Gamma Radiation, Heat Pulse Flow Meter, and Optical Televiewer. None of the geophysical measurements is unusual. However, the combination provides well details and insight into geology and hydrogeology penetrated by this well. This recommended basic suite of borehole geophysical measurements should be included in all fundamental hydrogeologic investigations. The presentation will review the measured results for this well and compare the information to data gathered during the well rehabilitation process.