HR: 0800h
AN: NS31A-0153    [Abstracts]
TI: Helicopter Electromagnetic and Magnetic Surveys over Volcanoes – Resolution Analysis
AU: * Deszcz-Pan, M
EM: maryla@usgs.gov
AF: Maria Deszcz-Pan, Denver Federal Center, MS 964, Denver, CO 80225, United States
AU: Finn, C A
EM: cfinn@usgs.gov
AF: Maria Deszcz-Pan, Denver Federal Center, MS 964, Denver, CO 80225, United States
AU: Anderson, E
EM: ericanderson@usgs.gov
AF: Maria Deszcz-Pan, Denver Federal Center, MS 964, Denver, CO 80225, United States
AB: Helicopter electromagnetic (HEM) and magnetic surveys over Mt. Adams, Mt. Baker and Mt. Rainier volcanoes were conducted to map altered zones to aid in volcano hazards assessments. As the three volcanoes are covered with ice and have highly magnetic rocks with electrical resistivities spanning several orders of magnitude, inversion of electromagnetic (EM) data to meaningful resistivity values included magnetic susceptibility and dielectric properties of rocks in addition to the standard resistivity vs. thickness parameters. The sensitivity of the HEM data to the approximate conditions on the volcano was examined in order to constrain the inversion. The responses of simple two-layer electrical property models representing the resistivity structure of the volcano were calculated over the full range of HEM frequencies. In all models the top layer resistivity of 106 ohm-m represented dry altered rocks, fresh volcanic rocks or ice. The top layer thickness, bottom layer resistivity, magnetic susceptibility and dielectric permittivity of both layers were varied and the HEM system ppm response was compared to the detectability threshold set at 5 ppm. The HEM system ppm response was modeled at 50 m above ground which was close to the average Mt. Adams survey elevation. The calculations showed that ppm response for a 10 ohm-m bottom layer, representing wet altered rocks, is above 5 ppm up to depths of 200 m indicating that the top of the low resistivity layer could be detected up to this depth. The results showed that the basement resistivities above 1000 ohm-m below 30 m overburden will not be well resolved. The effect of magnetic susceptibility and dielectric permittivity on the measured Hz fields was evaluated for the case of a 30 m thick top layer overlying a 1000 ohm-m bottom layer – a model chosen because it is close to the resolution limit of the system. The susceptibility of 0.025 SI strongly influences the in-phase component by lowering its value by a few ppm along the whole spectrum of frequencies, but has less influence on quadrature response. The in- phase and quadrature of the Hz component calculated for varying relative dielectric permittivity showed that for this resistive model only the highest frequencies, above 10 kHz, are influenced by the dielectric properties of the model, and only for the case of high water permittivity.
DE: 0794 Instruments and techniques
DE: 0910 Data processing
DE: 0925 Magnetic and electrical methods (5109)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting