Submarine electrical prospecting at the Takasima Coal Field, Japan

Abstract

1. One of conspicuous features of coal industry in Japan, is the development of submarine mining. The output from submarine fields amounted to about 13% (6.1 millions metric tons) of the total coal production of Japan in fiscal year 1951. On the other hand, the reserves of the submarine fields are assumed to exceed 24% of overall Japanese coal reserves. It is very obvious from these figures that the coal industry will require much more effort of geologists to discover new prospecting methods in order to explore submarine fields.
2. The Takasima coal field, situated in western Kyûsyû, off Nagasaki harbour, consists of a few small islands and occupies an area, measuring about 15km north-south by 10 km east-west. The coal is of prime quality for the manufacture of gas with a heating value of 7800 calories and containing 8% ash. The Mitsubishi Mining Co. is operating two pits in this coal field (Hutago and Hasima) under the name of Takasima Colliery.
3. Since 1949, electrical prospecting of sea floor has been carried out with the cooperation of Dr. J. Iwatu of Osaka Municipal University. The basic feature of the method used is the continuous measurement recorded on oscillograph film of spontaneous polarization and apparent resistivity as electrodes are towed along by steam boat. The circuit usually adopted, is the configuration by Dr. Iwatu which can express on one curve both SP and ρ, by the distance from neutral line to the curve and by the amplitude of oscillation, respectively. ρ is usually not effective for detection of geological features, due to the low resistivity of sea water except where coal seams and coal bearing formations are present. The recording of SP and ρ on separate curves, using a specified commutator and the adoption of slow period vibrator, however, makes it possible to detect ρ effectively. To locate the position and course of boat, two sextants are used. The current of 30-70 mA is supplied from dry cells (45 volt) and passed through a commutator to the electrodes. Echo sounding records are made for each traverse in order to avoid misunderstanding anomalies on curves, caused by the boundary between rocky shoals and flat bottom covered by recent deposits.