On the Investigation of the Buried Hills in the Jôban Coal-fields(1)

To Calculate the Amount of Closure and Investigate the Buried Hills in the Pit

Abstract

The Pre-Tertiary Basement underlying the coal-bearing strata, Hanakawa Formation developed in the Joban Coal-fields, often produces buried hills in the overlying beds. The development of workable coal seams have been greatly influenced by the existence of buried hills, namely, the main coal seams all tend to become thinner towards the hill and thin out completely over the top of the hill. These above mentioned events are economically important.
The following two methods were devised by the author to investigate the buried hills:
(1) From the porosity and density of drill core, the amount of closure reflected in the buried hill is approximately calculated by the following equation.
If a constant stress works perpendicular to the section of a cylinder composed of a homogeneous, porous medium, the equation may be expressed as H1/H2=V1/V2=(1-P2)/(1-P1), where H1. H2, V1. V2, P1. P2 denote respectively the height, volume, porosity before and after the stress is applied.
The author believes that these values can outline the area of investigation on the geological-map of this district.
(2) From the following abnormal sedimentation of the coal-bearing strata at the approach of the buried hill in the pit, the author has also tried to get a useful clue to fine it. The following was noted:
(i) The distance between two coal seams decreases from 6m to 0.5m in 300m of horizontal distance, the distance between two partings in the lower main coal seams decreases also from 0.5m to 0.1m for the same horizontal distance. Thus, each coal seams decreases in thickness from its lower part and abruptly or gradually thins out at.the top.
(ii) As the buried hill are mainly composed of amphibole schist and amphibolite, the surrounding coal-bearing strata are dominantly greenish schistose mudstone and contain many large schist fragments and calcite, derived from the schist.
(iii) The cyclothems developed in the coal-bearing strata change from complete ones to incomplete ones towards the buried hills.