Some Aspects of Magnetic Anomalies obtained by the Aeromagnetic Survey over the Izu and Tokai Districts
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概要
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An aeromagnetic survey covering the area between latitudes 33°20' and 36°00'N and longitudes 136° and 139°15'E has been conducted by the Geographical Survey Institute in February 1968, as one of the serial surveys over the whole Japan and her vicinity. The instrument used is a proton precession magnetometer, by which the value of the total magnetic force is recorded continually on an analog recorder with the measuring accuracy of±1 γ. The measurements have been made every 6 seconds along N-S flight courses having altitude of 3, 000 m. The spacing between successive courses is about 10 km, the total measured range amounting to 7000 km. After correcting the altitude reduction and the effect of magnetization of the aircraft to the observed values, the epoch reduction has been made to the epoch of 1965.0 by using the continuous record of the total force at the Kanozan Geodetic Observatory. The isodynamic curves of the total force are drawn in the interval of 100 γ and the isanomalic chart is constructed by taking the difference of the reduced values and the computed values from the quadratic formula of the total force in terms of latitude and longitude. As the result, the following aspects of magnetic anomaly are found. 1) In general, large magnetic anomalies are found over the Fossa Magna, the Izu peninsula and her southern sea area, while over the western land and coastal area from the Fossa Magna and the Suruga bay anomalies are very small or approximately zero. 2) Three couples of magnetic lineation patterns are found over the Izu peninsula and the southern sea area. Each couple of the lineation pattern has the trend of about east-west direction and the distances between southern high and northern low in the patterns are about 7, 12 and 15 km respectively, while the distance between successive patterns is about 50 km. These lineation patterns seem to be connected in their east extention to the similar patterns which N. Fujita found previously over the southern Kanto District. It seems likely that these linear anomalies are caused by the intrusion of basaltic material along the second order tectonic lines or ridges which are parallel to the east part of the Median Tectonic Line.
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