2020 Volume 73 Pages 117-122
Petrophysical data, including magnetic properties of rocks are very effective for interpreting results of magnetic surveys. However, details of rock magnetic properties have been examined rarely in parallel with magnetic survey data. Whereas, apparent magnetic lows are observed in the Musadake geothermal area, eastern Hokkaido by a recent high-resolution aeromagnetic survey of JOGMEC to estimate geothermal potential of the area. In this study, we have made laboratory measurements of density, magnetic susceptibility and natural remanent magnetization (NRM) of rock samples from outcrops in the area to evaluate its application to an interpretation of the magnetic anomalies.
Block samples had been collected from three outcrops: the right bank of the Ikeshomanai River (Site A), the Shura River (Site B), and the right bank of Kutekunbetsu River (Site C) and had been shaped into cylindrical specimens. All the specimens from the Site B and some specimens from the Site C indicate high NRM intensities (≥ 1.0 A/m) with high Qn ratios (≥ 1.0) and negative NRM inclinations. Whereas, all the specimens from the Site A and some specimens from the Site C show low NRM intensities (≤ 1.0 A/m) with low Qn ratios (≤ 1.0) and positive NRM inclinations.
AC demagnetization experiments were also conducted for selected specimens from the three outcrops and all the selected specimens were found to show negative NRM inclination, suggesting its reverse magnetization as the initial magnetization. This result is consistent with the obvious low magnetic anomalies observed over the rock sampling sites and our results imply that the low magnetic anomalies are not attributed to geothermal signs, but rather to the distribution of reversely magnetized volcanic rocks. According to photomicrograph and X-ray fluorescence analyses conducted additionally, the rock sample from the Site A is seen to lack opaque minerals by alteration, causing its low magnetic properties.
On a basis of good agreements between the petrophysical property and magnetic anomaly, it is concluded that examinations of petrophysical properties of rocks are useful when interpreting magnetic survey results in the study area.
