Initial Results Using Natural Magnetulluric (MT) field Measurements to Determine the Ice Thickness in the Summit Caldera of Mt. Wrangell Volcano, Alaska.

We report the initial results of a magnetulluric (MT) field survey made in June 2005 & 2006 on the ice surface in the summit caldera (4200 m) of Mt. Wrangell Volcano, Alaska. The purpose was to determine the ice thickness of Mt. Wrangell's summit caldera with on-site passive measurements of VLF/ELF signals. Measurements were made using a Zonge GDP 32II, over a frequency range of roughly 1Hz 8 kHz. Included also are VLF field measurements of existing 24kHz US Navy signals made using a Geonics EM-16R in June 2004 & 2006. In 2006, measurements were coordinated with operation of the High Frequency Active Auroral Research Program's (HAARP) RF transmitter to test whether ionospheric ELF waves, potentially excited by HAARP, could be measured at the summit of Mt. Wrangell. This work was done in coordination with the 2004- 06 Mt. Wrangell international ice coring and research efforts (Hokkaido University, Japan, Russian Academy of Science, Kamchatka, and the University of Alaska). Though estimated to be as much as 1000m thick, the thickness of the ice in the center of Mt. Wrangell's caldera is not known. Efforts over the past 45 years to determine the ice thickness have met with numerous difficulties, and none have been conclusive in the center region. Previous radio echo sounding efforts: Motyka & Benson, 1976(surface,); Clark & Benson, 1989 (airborne) and Kanamori & Shairaiwa, 2003 (surface) gave reasonable results near the rim of the caldera but were inconclusive in the center region where the depth is expected to be maximum In summer 2004 a single radio-echo measurement in the center by Kanamori gave a depth of roughly 750 meters. Seismic measurements in 2004 by Lüthi &Truffer suggest an ice thickness of roughly 700- 900 meters though data interpretation was problematic. The 2005 Resistivity vs. Depth results from a site in the center of the caldera, using a smoothed 1-d inverse model (Zonge) show a significant change in resistivity at a depth of between 700-800 meters, possibly indicating the bottom of the ice. Analysis of the same data set using a Bostick model shows a resistivity break at a slightly shallower depth 600 700 meters. This effort is part of a pilot project investigating whether the HAARP RF transmitter located in Gakona, Alaska can effectively generate ionospheric VLF/ELF signals that can be used as probe waves to determine the thickness of the ice and potentially deeper structure in the caldera of Mt. Wrangell Volcano, Alaska. This work is the exploratory phase of the project determining whether VLF/ELF signals can be successfully used to measure the ice thickness in the center of Mt. Wrangell's summit caldera.