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Data Series 901


Southerly view into the crater of Mount St. Helens

Mount St. Helens: Controlled-Source Audio-Frequency Magnetotelluric (CSAMT) Data and Inversions


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Cover
Introduction
Data Acquisition
GPS Data
EDI Data
Data Processing
Occam Inversions
Fischer Inversions
Marquardt Inversions
References
Index page

Fischer Inversions

The singular advantage of a Fischer-type inversion is that it does not rely on a starting model. Rather, it makes direct use of the CSAMT data for information regarding layering. A typical use of the Fischer inversion is to determine how many layers can be resolved with the existing data; generally, the layers are visually obvious in the inversion (see fig. 4). These results are then used to help build a Marquardt starting model and then iterated to a true inversion (Fischer and others, 1981; Pedersen and Gharibi, 2000).

All Fischer inversions were done for the TE = XY orientation. Station MSH-1001 is identical to station MSH-1002, and station MSH-1003 is identical to station MSH-1004 (two sets of repeated measurements not duplicated here).

(NOTE: each line is a link to a PDF)

MSH-1002-FischerInversion

MSH-1003-FischerInversion

MSH-1102-FischerInversion

MSH-1103-FischerInversion

MSH-1104-FischerInversion

MSH-1105-FischerInversion

MSH-1106-FischerInversion

MSH-1107-FischerInversion

MSH-1108-FischerInversion

MSH-1109-FischerInversion

MSH-1110-FischerInversion

Example of a Fischer Inversion of the controlled-source audio-frequency magnetotelluric (CSAMT) data. A, The computer inversion of the data into a resistivity versus depth vertical profile for station MSH-1105. B, The original resistivity data and the inversion model fit for resistivity as a function of measurement frequency. C, The original phase data and the inversion model fit for phase as a function of measurement frequency.

Figure 4. Example of a Fischer Inversion of the controlled-source audio-frequency magnetotelluric (CSAMT) data. A, The computer inversion of the data into a resistivity versus depth vertical profile for station MSH-1105. B, The original resistivity data and the inversion model fit for resistivity as a function of measurement frequency. C, The original phase data and the inversion model fit for phase as a function of measurement frequency.

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