The Electrical Resistivity Structure of the  Dulles Compartment

With moderate total dissolved solids (electrolytes), ground water becomes a relatively good electrical conductor. Ground water-charged fractures are thus expected to make rocks at depth potentially more conductive than dry rocks in the near-surface. Thus, the AMT surveys we have conducted across Dulles International Airport to determine the electrical resistivity structure of the Dulles compartment are expected to reflect the contrasts in resistivity between the relatively resistive diabase and the more conductive Balls Bluff siltstone. Starting on the route  606 dike, and proceeding due south across the thermally-metamorphosed margins and into the Balls Bluff Siltstone, (parallel to the main N-S runway), this AMT survey was terminated on the north-central interior of the Centreville lopolith. Within this Dulles survey, six one-dimensional variable frequency soundings of resistivity  have been conducted to depths of  800 meters. By connecting these one-dimensional survey results and interpolating between vertical soundings, integrated two-dimensional vertical cross-sections may be constructed of the spatial variations in resistivity in the shallow crust beneath Dulles.

The Dulles electrical section illustrates the two-dimensional electrical resistivity structure of the Dulles compartment to a depth of 800 meters. The horizontal extent of the survey is ~15,500 meters. Individual one-dimensional survey lines are shown as crosses, that extend to depth vertically. 1-D inversion sites 001 and 002 are virtually coincident in this portrayal, due to their close proximity (and thus plot as a single data series). The uppermost sections of the electrical cross-section have the lowest resistive values of 6.77-60.6 Ohm-M and reflect the effects of weathering. In the central portion of the figure, yellows and greens cover the resistivity range ~94 to ~543 Ohm-M, and are associated with water-charged fractures in the Balls Bluff siltstone. Along the left-hand- side, the  orange-to-orange-red of the dike and its' thermally metamorphosed margins are associated with relatively resistive values of ~843 to ~1309 Ohm-M, from a depth of 800 meters to the near-surface. Along the upper right-hand-side, the Centreville diabase lopolith and its' thermal aureole is seen in yellow-to-orange, with relatively resistive values [~543 to ~1309 Ohm-M]. The shallow expression of the Centreville lopolith is evident from the ~300-400 meter depth extent of the resistivity structure. Beneath the lopolith, somewhat conductive portions of the Balls Bluff (yellows, with values in the 350-543 Ohm-M range may be seen).

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

Contact: Herbert A. Pierce
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