Estimation of intrusive rocks from topographic effect removed magnetic map in Hakkoda Volcanic Area, Japan

Abstract

An understanding of the subsurface distribution of intrusive rocks is very important for geothermal exploration since they are often related to fracture zones suitable for geothermal fluid reserves while also acting as heat sources to the reserves. Concealed intrusive rocks may be estimated by analysing magnetic anomalies. However, the effect of topography must be carefully removed from the observed magnetic anomalies in volcanic areas because surface and shallow volcanic rocks produce noise blanketing deeper information. The Topographic Effect Removed magnetic map (TER magnetic map) was developed based on the magnetisation measurement of rock specimens, geologic data, and CSAMT data in the Hakkoda geothermal field located in the northern part of mainland Japan. The intrusive-shaped magnetic bodies were then extracted using the TER map. Drilling holes which hit the Quaternary intrusive rocks were not available. Therefore, we can not determine whether the intrusive-shaped bodies directly correspond to actual intrusions, although many hot springs occur around the bodies. This may confirm the hypothesis that the bodies correspond to intrusions associated with Quaternary volcanic activities in Hakkoda district.