Resistivity anomalies of Izu-Oshima volcano revealed by airborne VLF-EM survey.

Abstract

Airborne VLF-EM and ground VLF-MT surveys were carried out to study the resistivity structure of Izu-Oshima volcano after its fissure eruptions in November, 1986. In both the airborne and ground surveys, we employed electromagnetic induction techniques with VLF radio waves (Very Low Frequency, 17.4kHz in our case) as the primary source field.Airborne VLF-EM surveys measure the variations in the vertical component of the magnetic field which are coherent to those of the horizontal component, and map a quantity that corresponds to the magnetic transfer function derived from the ratio of the vertical to the horizontal component. Ground VLF-MT surveys, on the other hand, measure the horizontal electric field simultaneously with the horizontal magnetic field and compute the apparent resistivity.Airborne VLF-EM surveys of Izu-Oshima volcano revealed that there are clear trends of alignment in the resistivity anomalies in many cases. A remarkable anomaly belt has been found to run along the fissures through which fountain eruptions occurred in November. Many of the anomaly belts are related to geological structures due to activity in historic times. However, there are some anomalies which do not clearly correlate with historical volcanism.The results of the ground VLF-MT measurements along three survey lines are consistent with the airborne survey, confirming the existence of low resistivity anomalies.