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

Airborne electromagnetics (AEM) is a useful tool for investigating volcanic structures because it can survey large
and inaccessible areas. Disadvantages include lower accuracy and limited depth of investigation. The Grounded Electrical
Source Airborne Transient Electromagnetic(GREATEM)survey system was developed to increase the depth of investigation
possible usingAEM.The method was tested in a survey at Mount Bandai in north-eastern Japan. Mount Bandai is an andesitic
stratovolcano that rises 1819mabove sea level.Aneruption in July 1888 left a hoof-shaped collapsed wall in its northern crater
and avalanche debris at its base. Previous surveys of Mount Bandai allow for comparisons of data on its structure and collapse
mechanism as obtained by GREATEM and other geophysical methods. The results show resistive structures in recent
volcanic cones and conductive structures in the collapsed-crater area. Conductive areas around the collapsed wall correspond
to an alteration zone resulting from hydrothermal activity, supporting the contention that a major cause of the collapse
associated with the 1888 eruption was hydrothermal alteration that structurally weakened the interior of the volcanic edifice.