Culpeper Basin, Virginia: Audio-magnetotelluric Data Processing
The assembly of the individual Audio-magnetotelluric survey areas around the Culpeper Basin, Virginia was completed in several steps
General Data Processing steps
For this study we used the Geometrics EH-4 Audio-magnetotelluric (AMT) system with a controlled source to fill in the middle frequencies (450-4,500 Hz) where natural signal strength is often below the detection limits of the instrument. This instrument uses lighting or atmospheric disturbances (sferics) as an energy source. For some AMT soundings, atmospheric disturbances were proximal to the sites making the signals strong enough that the controlled source was not needed.
Electric and Magnetic Field
The AMT impedance tensor (Z) contains four complex components which relate the measured electric (E) and magnetic (H) fields:
The impedances are computed from spectra collected in the field using a local H field reference.
where R* is the reference field.
From the impedances, computing the apparent resistivities and phases uses the four components of the impedance tensor (Zxx, Zxy, Zyx, Zyy). Apparent resistivities r(f) and the corresponding phase f(f) are computed using:
where f is the frequency of interest.
Bostick depth transforms are calculated for each frequency using:
where the Hilbert transform relationship between the apparent resistivity r and its phase f in degrees translated into the first quadrant and clipped to the range where f must be greater than or equal to 0 and less than or equal to 90º.
The Morven and Selma cross-sections use the rotationally invariant impedances derived from the full tensor. The arithmetic impedance average is computed using the following formula:
The Dulles cross-section uses the data from the maximized (rotated) xy direction and the two Rapidan cross-sections use transverse magnetic orientation.
A more complete explanation of theory, instruments, processing, and interpretation can be found in Vozoff, 1987, 1991 and Spies and Frischknecht, 1987.
|| Culpeper Basin AMT || USGS - Eastern Earth Surface Processes Team || USGS - Geology || USGS ||
Contact: Herbert A. Pierce
U.S. Department of the Interior, U.S. Geological Survey
Maintained by Publishing Services
Last modified: 23:00:37 Sat 12 Jan 2013
Privacy statement | General disclaimer | Accessibility