Grounded electrical-source airborne transient electromagnetic (GREATEM) survey of Mount Bandai, north-eastern Japan

Airborneelectromagnetics(AEM)isausefultoolforinvestigatingvolcanicstructuresbecauseitcansurveylarge and inaccessible areas. Disadvantages include lower accuracy and limited depth of investigation. The Grounded Electrical SourceAirborneTransientElectromagnetic(GREATEM)surveysystemwasdevelopedtoincreasethedepthofinvestigation possibleusingAEM.ThemethodwastestedinasurveyatMountBandaiinnorth-easternJapan.MountBandaiisanandesitic stratovolcanothatrises1819mabovesealevel.AneruptioninJuly1888leftahoof-shapedcollapsedwallinitsnortherncrater andavalanchedebrisatitsbase.PrevioussurveysofMountBandaiallowforcomparisonsofdataonitsstructureandcollapse mechanism as obtained by GREATEM and other geophysical methods. The results show resistive structures in recent volcanicconesandconductivestructuresinthecollapsed-craterarea.Conductiveareasaroundthecollapsedwallcorrespond to an alteration zone resulting from hydrothermal activity, supporting the contention that a major cause of the collapse associated with the1888 eruption was hydrothermal alteration that structurally weakened the interior of the volcanic edifice.