Data Processing

After the CSAMT data were downloaded from the field computer system, they were examined for anisotropy and noise both visually and with software. The data were edited using the WinGLink software package in the following manner: (1) Data were evaluated to ensure that individual phase values were in the correct quadrants. (2) Data curves were smoothed using D+ (the delta function of Parker and Booker, 1996). (3) Data were checked to ensure that the rho (resistivity) values did not rise or fall steeper than 45 degrees in log-log space. The effect of this standard processing is to remove the occasional noise outliers and stack the data for the smoothest possible 1-D model fit. While data quality varied somewhat, it was acceptable. All data are reported and released in this report.

Assigning TE and TM directions

Selecting orientations for the transverse electric (TE) and transverse magnetic (TM) parts of the data is generally done with an eye to the regional structure (Vozoff, 1987). For this study, the TE orientation with electrical field oriented east would be used when operating above a north-south regional structure such as anticipated at Mount St. Helens (Weaver and others, 2012; Giampiccolo and others, 1999).

In general, AMT (including CSAMT) does not have a ready way to calculate in-phase induction orientations, as an MT sounding would, because it has no vertical magnetic reference sensor. In order to choose the prevailing strike direction and assign TE and TM, other information is required. For instance, it would require conducting galvanic vertical electrical soundings (VES) in two directions to compare the two curves or reviewing previous work, neither of which were available during the study. An MT survey across the Garibaldi Volcanic Field in the northern part of the Cascade Volcanic Arc indicated a roughly north-south crustal texture. “The induction arrows also indicate that the regional response can be characterized by a 2D resistivity model” (Jones and Dumas, 1993, p. 295). In the Washington Cascades, the motions of the Pacific, North American, and Juan De Fuca Plates control the locations of earthquakes, faults, and volcanoes. All of the most distinct features, including the subducting plate and the Washington coastline, trend roughly north-south (Weaver and others, 2012).

For an MT or AMT sounding, when the magnetic source field is parallel to the strike of the structure, this is by definition TM mode (Parker, 2010). The TE direction should thus be assigned to Zxy (XY), and TM should be assigned to Zyx (YX).