Scientific Objectives and Field Experiment Strategy

The Audio-magnetotelluric (AMT) experiments are part of a broader program of geophysical and geological research aimed at understanding the fracture flow hydrology of the major Mesozoic basins of eastern North America. In brief, we seek to understand the three-dimensional structure of the basin and the geometric interrelations between the primary basin rock types. Of particular interest are permeable rocks that store significant  volumes of ground water, which are transmissive, and that comprise important aquifers. Examples of such high storage capacity rocks are the Balls Bluff Siltstone and the Poolesville member of the Manassas Sandstone. Equally important however, are the relatively impermeable rocks which have the potential of acting as internal 'no-flow' boundaries controlling the rates, directions and extents of fluid migration, as well as the internal storage. A prime example of such impermeable rocks, as discussed above, are the massive diabase bodies that lace the Culpeper basin throughout.

One group of AMT experiments was designed to explore the geometric interrelationships between the sedimentary rocks and their diabase neighbors--and hence constrain the structural context of the compartmentalization process. Traverses and soundings through the Dulles International Airport, as well as across the Rapidan diabase lopolith, were selected to explore this "permeability contrast" parameter set.

The deep structure of the basin-bounding normal faults along the Culpeper basin's western margin has been a matter of conjecture and speculation (Lee and Froelich, 1989).  Understanding this structure would be of considerable help in constraining numerical models of flow and recharge from the bordering western highlands. A second set of AMT experiments was therefore mounted from the summit region of Catoctin mountain in crystalline Proterozoic rocks, eastwards toward U.S. Route 15, in Mesozoic conglomerates.

|| Culpeper Basin AMT || USGS - Eastern Earth Surface Processes Team || USGS - Geology || USGS ||

Contact: Herbert A. Pierce
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