Journal of the Japan Society of Engineering Geology

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The magnetic susceptibility of rocks is correlated with the type, amount, and grain size of ferromagnetic minerals in measured part. The analysis is performed by a non-destructively measurement using handy device. Therefore, magnetic susceptibility is expected to be effective to recognize the geological characteristics of rocks. However, when the measurement target has multiple geological characteristics, effects by these characteristics may overlap on the values of magnetic susceptibility. If this problem could be overcome, it would contribute to the works of collecting geological information, but there are few examples of such related studies. In this study, we used continuous drill core samples from a deep borehole at the crustal section of the Yakuno Ophiolite in northern part of the Oshima Peninsula. We systematically measured the magnetic susceptibility of the core and verified how multiple geological characteristics can be understood from the magnetic susceptibility. The data of magnetic susceptibility analysis were correlated with the results of petrographic observations, mineral mode composition analysis, X-ray CT photography, and portable XRF measurements. It became clear that the influence of lithological boundaries and altered parts in basaltic rocks is reflected in the centimeter-scale changes in the magnetic susceptibility, and that the chemical composition of basaltic rocks is reflected in the meter to hundred meter-scale changes in the magnetic susceptibility. The discussed method for the multiple geological characteristics by focusing on the values and scale of the changes of magnetic susceptibility is expected to be applied to field surveys and core descriptions at variety of sites.

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