土壌表層の攪乱場所を特定するための初磁化率の応用

抄録

 We tested magnetic susceptibility (MS) as a forensic search tool to detect the disturbed area on soil surface. A 250-cm-long survey line was assessed on the campus ground of the Japan Coast Guard Academy in this study. A hole of 10 cm length×20 cm width×7 cm depth was dug on the survey line and backfilled using soil mixture. MS was measured using a magnetometer at 10 cm intervals along the line, and the peaks were observed in the disturbed area. A soil core sample of 7 cm in length was collected near the line. The grain size of the core was classified as silty sand with granite granules that may have originated from the parent rock. The ratio of granule composition increased with soil depth. MS, anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), and magnetic hysteresis curves were measured using the dried soil samples. The values indicated a peak at the 7 cm depth, where large number of granite granules were present. This implied that MS, ARM, and IRM peaks were caused by the granite granules. The granules were moved to the disturbed soil surface and may have contributed to high MS values. Thermomagnetic analyses revealed the presence of magnetite, maghemite, and hematite in the soil. The magnetic hysteresis parameters showed that the magnetic grain size also increased with burial depth and was consistent with the observed grain size distribution. The values of the disturbed area were located at the center of all the samples on a Day plot, implying that the grains of all soil layers were contained in the disturbed area. The MS peak could be recognized one year later although the hole was covered by weeds. The location of the disturbed area could be detected using a MS survey.