The Ground Water Characterization Program operates, in cooperation with the U.S. Geologic Survey, a geophysical logging truck that staff geologists use to characterize individual wells throughout the Commonwealth. The truck is equipped with instruments that analyze electro-magnetic induction, fluid resistivity, formation resistivity, formation gamma-ray emission, water flow through discrete intervals of the well, and shape of the borehole. In addition the truck has tools on-board that will produce oriented imagery of the borehole.
The truck is used differently in the various regions due to the differing types of geology in which wells are constructed. In the ground water management area of the Coastal Plain, where nearly horizontal unconsolidated units produce discrete water-bearing and confining units each unit tends to produce a distinctive signal on either the gamma-ray, resistivity, or induction logs that helps to determine what regulated aquifer system the well is obtaining water from. The geophysical logging truck is also used in the Coastal Plain to verify that a permitted ground-water user is obtaining water only from the water-bearing unit that they are permitted.
In the fractured “hard-rock” terrains of Piedmont, Blue-Ridge, Valley, Ridge, and Plateau, where no ground water management area currently exists, wells are characterized using acoustic televiewer, electromagnetic flow, color pan/tilt bore-hole camera, and caliper instruments. Unlike the Coastal Plain where water flows around grains of aquifer material, ground water in central and western Virginia flows through a tectonic fabric (e.g. fractures, bedding planes, folds, faults, and voids) that is superimposed on the landscape to varying degrees. Instruments that can image the bore-hole such as the acoustic televiewer and down-hole camera aid the geologist in mapping the tectonic fabric of the region that water exploits as it moves through the system.
Figure 1: GWCP Logging Truck at Fauquier County State Observation Well 215.
Figure 2: Inside the GWCP Geophysical Logging Truck.
Above Left: Screenshot from Borehole Video of former public supply well showing small-scale conduit flow in limestone (Depth 174.6 feet below surface), Natural Tunnel State Park, Scott County, Virginia.
Above Right: Screenshot from Borehole Video of active municipal supply well showing large-scale conduit flow in Cambrian Elbrook Formation, Rockingham County, Virginia.