Exact knowledge of subcrops location of coal seam under thick soil is quite necessary for the effective trenching exploration. In this study, the writers attempted the application of resistivity method to such purpose, and obtained good results.
In laboratory the resistivities of various rock specimens of the Bibai coal-bearing formation and the Noborikawa coal-bearing formation were measured by using the directcurrent Wheatstone Bridge of S. T. 1038 type. The sequence of resistivity values was coal, dark-gray coaly-shale, sandstone, and shale in descending order.
In the field performances on the Bibai coal-bearing formation at the right bank of Shimo-Utashinai, river, Shimazu Potentiometer of E. P. type and pulser of E. P. S. type were used, and for horizontal detection four-electrodes method of the Wenner type is applied.
Comparatively high resistivities were observed in coaly parts, and comparatively low resistivities in sandstone and sandy-shale parts, and their differences were clearly recognized.
Values of apparent resitivity of coal seam, coaly-shale and sandstone were revealed as peaks in, the resistivity curve, and their figures are almost order of 10⁴Ω-cm. Quality of coal had no influence on the height of peaks. Subcrops under surface soil up to 3m in depth can be prospected by this method except the case under soil of sandstone debris. Fault zones can be easily determined by the negative peak in the resistivity curve.