The Audio-Magnetotelluric Method (AMT)

The Audio-magnetotelluric (AMT) method uses natural electromagnetic (EM) fields to investigate the electrical conductivity structure of the earth. Energy for the natural source EM frequencies higher than about one hertz are generated by thunderstorms worldwide. The energy discharged by lightning radiates EM fields that propagate great distances. AMT fields at the surface of the earth behave like plane waves, with most of the energy reflected, however, a small amount of energy propagates vertically downward into the earth. The amplitude, phase, and directional relationships between electric (E) and magnetic (H or B) fields on the surface depend on the distribution of electrical conductivity in the subsurface (Vozoff, 1991).

 The Culpeper Basin audio-magnetotelluric data sets of this report were compiled in support of the Bedrock Regional Aquifer Systematic Study (BRASS) project, the Geologic Mapping Program, and the Water Resources Virginia District Office of the USGS. AMT surveys are typically collected as a profile across a structure of interest. The AMT sets were sited to cover diabase dike bounded compartments, diabase confining sills and lopoliths, water-saturated sediments within the compartments, and the basins' western-most bounding fault. All of the sounding sites in this survey used the Geometrics EH-4 receiver, augmented, when needed, by a portable transmitter for the middle frequencies (450-4,500 Hz) when the natural signal was below the detection limit of the instrument.

The Culpeper Basin AMT profiles were constructed from soundings collected during 2000, 2001, and 2002. The data from these surveys are generally of good quality but some stations are noisy due to wide-spread cultural noise.  The design (station spacing, line direction, and number of sites)  varies between survey areas depending on field conditions and the type and orientation of the geologic target.  An index maps and data table gives an overview of the survey sites and summarizes their locations.  The resulting sections have data sets that can be downloaded. The sections are based upon a Bostick (1977) inversion of the resistivities and frequencies coupled with the slope computed from the phase calculated by the Hilbert transform. The Bostick transform provides a quick way to transform frequency domain apparent resistivity data into a resistivity versus depth sounding. The Hilbert-transform uses the relationship between the apparent resistivity and its phase in degrees.

These sections are interim products. Considerable editing and modeling of the data was undertaken, and continues. Smoothing of the sections was necessary to reduce noise caused by cultural effects near some of the sites.  Vertical resolution is best in the near surface and falls off the deeper the sounding. Horizontal resolution is controlled by station spacing. In some areas, cultural activities, roads, and structures limit how close the stations can be sited.

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

Contact: Herbert A. Pierce
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