Abstract
The Takashima coal mine of the Mitsubishi Coal Mining Co., Ltd. is located at the Takashima Island, 15km to the southwest of the Nagasaki Harbour. The present mining faces are located in the offshore area 7km from the island. Annual production in the last 10 years averages to 700×103 tons of mainly metal-lurgical coal.
The mine is located in the central part of the Takashima Coal Field. The coal bearing Palaeogene outcrops on the several islands, and in the offshore area where it is covered by unconsolidated Quaternary sediments. Based on the geological information on the islands and outcrops in the surrounding shallow water area, several coal mines were operated. The Takashima coal mine is only one operated at present among them. Although the mine has attempted various offshore geological and geophysical survey in the past, the data obtained was not sufficient enough for the long term mine planning. The Tobishima area is covered by 70-80m of sea water and the Palaeogene is overlain by Quaternary sediments of 100m in thickness.
In 1981, the Takashima coal mine applied the reflection seismic survey to the virgin field to estimate the depth of coal seams, delineate structural contours of coal seams and pick up tectonic faults. The survey resulted in detecting characteristic reflectours in the Palaeogene coal bearing formation. One of these was interpreted as the reflection from the main coal seams being mined out at present, based on a correlation with the mined out area and the bore hole drilling data. The depth contour map indicated that the main coal seams lay in the large area with minable depth, a low degree of dip and a few, small displacement by faults.
However, the seismic survey was not enough to obtain the information as to individual seam thickness and minor faults. Thus an offshore drilling was carried out in 1982 by a semi-submersible oil rig to determine seam thickness and coal qualities such as ash contents and coking properties in order to meet operation planning. Drilling programme was made based on the seismic section to drill the cover rock by non-coring bit and to drill the objec-tive coal seams by coring bit to minimize the drilling cost. The prognosed depth was so accurate that the whole coal seams as well as roof and floor rocks were cored successfully without a side track drilling.
The Takashima coal mine has succeeded by these surveys to discover new and large amount of coal reserves in the structurally favourable area neighbouring the present mining area. The reserves would contribute to the efficient mining operation in the near future.
