Fig.3:
Distribution of the magnetic observation points. Dot marks indicate the observation points. KNZ = Kano-zan Geodetic Observatory.
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2.Magnetic Data Collection
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2.1 Magnetic surveys Fig.3: Distribution of the magnetic observation points. Dot marks indicate the observation points. KNZ = Kano-zan Geodetic Observatory. |
| The magnetic surveys on ground were carried out from 1985 through 1989. The measurements were made with a portable proton precession magnetometer Geometrics G-816 with a sensor height of 1.6 meters from the ground. Observation points were distributed from the southern end of the Boso Peninsula to 35°09'N (Fig. 3). We placed the observation points at intervals of 500 m. In some places where we found bare rock outcrops of the ophiolite, the points were picked up at 100 m interval. Of 1,212 observations made, 403 were run between midnight and dawn to avoid field disturbances from electric railway activities near the survey area (Morijiri, 1988; Fujiwara, 1990). Magnetic data along a ship track in Tokyo Bay were obtained during the DELP 1987 Ogasawara cruise in November 1987 (Isezaki et al., 1989). The magnetic total intensity anomaly was calculated by subtracting International Geomagnetic Reference Field (IGRF) 1985 model (IAGA, 1985) from the observed data. Diurnal variation of the geomagnetic field was corrected by using total geomagnetic intensity data measured every minute at the Kano-zan Geodetic Observatory, Geographical Survey Institute of Japan (139°57.5'E, 35°15.7'N) which was the nearest geomagnetic station and was located 7-34 km from the survey area. An upward continuation filter (Kato, 1987) was applied to eliminate anomalies due to artificial materials and to reduce topographic effects on the magnetic anomaly. |
| Sea-surface surveys of magnetics as well as swath bathymetry and gravity to the southeast of the Boso Peninsula were conducted aboard the R/V Yokosuka during the cruise YK98-02 Leg 1 in 1998. Total intensity and three-component geomagnetic field data were collected in this survey. The survey ship tracks trend in an ENE-WSW direction (Fig. 3). The interval of the northern tracks is about 4 km, and that of the southern tracks is about 6 km (Fujiwara et al., 1998). Magnetic total intensity and vector anomalies were calculated by subtracting the IGRF 1995 (IAGA, 1995) from the data. The processing of the magnetic three-component data was accomplished using the method of Isezaki (1986). Correction of diurnal variation and upward continuation were not applied for this sea-surface magnetic data. |
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