USING THE MAGNETIC METHOD DEVELOPING GROUNDWATER RESOURCES IN FRACTURED CRYSTALLINE BEDROCK SOUTHSIDE VIRGINIA – A CASE STUDY

Geophysical surveys are often used to explore and screen large areas for minerals, oil and natural gas but less frequently for water. When large quantities of water are required, finding methods for locating groundwater reserves helps lower development costs by reducing the extent of drilling needed. Finding groundwater in large areas in fractured bedrock terrains can be costly and time consuming. Reducing the time required to survey large areas at a screening level is critical to zeroing in on target areas and achieving success at the lowest possible cost.

With improvements in geophysical equipment, positioning and data logging technology, ground-deployed magnetic methods are now adaptable to covering large areas. Magnetic measurements are effective in delineating subsurface anomalies that may reflect enhanced porosity in fractured crystalline bedrock terrains. Magnetometers equipped with GPS receivers and automated data loggers greatly increase the rate and efficiency at which data are collected and processed thus making investigations across large areas more feasible and cost effective.

A large-scale total field magnetic survey was conducted to identify groundwater resources in the fractured crystalline bedrock aquifer beneath the Fall Zone of south central Virginia. A geophysical screening method was needed to identify bedrock structures such as faults or lithologic contacts associated with fracturing and secondary porosity. A total field magnetometer geophysical survey was conducted utilizing a sled-mounted magnetometer with smart antenna GPS unit. Field data were post-processed to generate total field and first derivative contour maps. Total field maps were useful for identifying large-scale trends in the magnetic field while first derivative maps were useful in accentuating steep gradients reflective of geologic boundaries and possibly enhanced porosity.

Several large-scale, linear NW-SE-oriented anomalies evident in the survey area appeared to reflect fault-related structures previously mapped. Test wells drilled at selected target sites along these features yielded in excess of 12 gpm and in some cases > 100 gpm in 66% of the attempts. This demonstrates that magnetic data provide a useful cost effective tool in identifying groundwater resources in fractured crystalline bedrock.