Abstract
Study of the electric self-potential (SP) and the magnetic field was conducted at the explosion seismic experiment in Ooyama, Toyama Prefecture. The explosion was induced by 450 kg gun powder placed at a depth of 32.2-75 m. The SP was measured by four electrodes arranged in a line crossing the explosion point, where the distance of each electrode from the explosion ranged from 10 to 100 m. Induced SP changes consisted of two portions, namely, fluctuations with shorter periods (∼10 ms) and fluctuations with longer periods (∼50 ms). Regarding the latter, the amplitude was in proportion to the span length of electrode couples and the fluctuations arrived faster at the electrode couples nearer to the explosion. Further, the increment of ground level in SP was observed in each electrode couple after the explosion, which was concordant with the distribution of apparent resistivity underground studied by the survey. The magnetic field measured by the fluxgate magnetometer at the site 35 m north from the explosion also showed change of a few nT after the explosion. This may have been caused by change of magnetic properties underground.
