The electromagnetic field change in the ultra low-frequency (ULF) band was observed in order to investigate the precursory phenomena of crystal activity such as earthquakes, volcanic eruptions and landslides. The measurements are conducted to make clear the generation mechanism of the anomalous electromagnetic field occurring before and during these disastrous phenomena. We have constructed a portable system with a HTS-SQUID magnetometer for the observation of the ULF magnetic field. A dewar that kept liquid nitrogen for about three weeks was developed for a long recording period. The performance of the system was tested at Mt Bandai, an active volcano in Fukushima prefecture, Japan. The system worked for 36 hours. Some anomalous magnetic held changes were recorded before and after the volcanic earthquake which occurred during the field test.
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