Helicopter-borne EM Survey over Usu Volcano, Hokkaido, Japan, with a Special Reference to the Usu 2000 Eruption(＜Special Section＞The 2000 Eruption of Usu Volcano (3))

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  • 有珠火山地域における空中電磁・磁気探査 : 有珠2000年噴火に関連して(＜特集＞2000年有珠山噴火 (3))
  • 有珠火山地域における空中電磁・磁気調査--有珠2000年噴火に関連して
  • ウス カザン チイキ ニ オケル クウチュウ デンジ ジキ チョウサ ウス 2000ネン フンカ ニ カンレン シテ

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Abstract

A helicopter-borne electromagnetic survey (HEM) with simultaneous measurements of the Earth's magnetic field was conducted to better understand the subsurface structure of Usu volcano, Hokkaido, Japan, with a special reference to the 2000 eruption. The HEM system is a modified DIGHEM-V model that is operated at five frequencies ranging from 220 Hz to 137,500 Hz with five-fold frequency increments. The survey was flown in late October 2000 at an altitude of 70-100 m above terrain along north-south survey lines and east-west tie lines, spaced 100 m and 1,000 m apart, respectively. A diiferential GPS system was employed for flight path recovery. The observed electromagnetic data were processed and apparent resistivity maps were created for each five frequencies. Cross-sections of apparent resistivity profiles were also calculated from an interpolation of apparent resistivity data at five frequencies. Aeromagnetic anomalies were reduced onto a smoothed observed surface and a reduction-to-the-pole anomaly map was created. The characteristics of the distribution of apparent resistivities and magnetic anomalies were summarized as follows:

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