空中電磁法・全磁力測定による雲仙普賢岳及び眉山の地下構造

概要

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We conducted a helicopter-borne electromagnetic (EM) and magnetic survey using Dighem IV EM system in the Unzen volcanic area in December 1992 to investigate subsurface structure of the volcano. EM data were obtained at an interval of 0.1 sec. with horizontal coplanar coil pairs at 28800, 3600 and 385 Hz, and a vertical coaxial coil pair at 900 Hz. Magnetic survey was done with the Cesium magnetometer. The standard flight altitude of the sensors was 30 m for EM survey and 45 m for Magnetic survey. Apparent resistivity map produced from EM data revealed that the area covered by the younger lava (Fugendake stage) around Fugendake and Mayuyama showed high apparent resistivity (>2000 ohm-m), while the older lava and collapse deposits showed relatively low resistivity (<1000 ohm-m). Low resistivity zones (<500 ohm-m) appeared along fault zones. We attempted to illustrate resistivity layer structures along a survey line using the Marquardt least square inversion method. Thickness of highly resistive overburden was estimated as 100 m to 200 m. Magnetic anomaly map after topographic correction showed that the high magnetization anomalies (>7 A/m) exist at younger lava domes such as Fugendake, Kunimidake and Mayuyama as well as along the Akamatudani fault, but the magnetization in the other places, even in the younger lava domes, was estimated as 5 A/m or less.
1995-09-20