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

1. Raw field data are processed in the field into three file types. X-type files are the cross power files, Y-type files are the time series data, and Z-type files are the impedance files. All data were processed through a two stage coherency cutoff. In stage one data with coherencies less that 0.3 were rejected; in stage two, data with coherencies less than 0.5 were rejected.
2. Data bases were constructed from the original Audio-magnetotelluric survey impedance data based upon the areal extent of the soundings.
3. MT parameters are computed then phase curves were edited to insure that spurious data points are within their respective first and third quadrants.
4. A spline function was applied to the resistivity and phase curves to smooth them frequencies with anomalous values (generated by equipment malfunction or noise) inactivated as necessary.
5. The curves were then interpolated.
6. Bostick (1977) inversions were calculated for the individual sites and  lines were then plotted as electrical sections.
7. The soundings are grouped into site sets and plotted as electrical sections then, after inspection, repotted using a minimum curvature algorithm.
8. The original station data are always available for inspection or further processing as are the interpolated data.
9. The sections are converted from level (depth reference) to drape on the Digital Elevation Model topography.
10. Sections may then, in general, be further gridded and smoothed prior to presentation.

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:

                                                                                           (1)

The impedances are computed from spectra collected in the field using a local H field reference.

                                                                        (2)

                                                                         (3)

                                                                        (4)

                                                                        (5)

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:

                                                                                                                      (6)

                                                              ,                                                (7)

where f is the frequency of interest. 

Bostick depth transforms are calculated for each frequency using:

                                                                                                          (8)

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.

                                                                                                                  (9)

and                                                                                                       (10)

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:

                                                                                         (11)

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
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