Joint application of seismic and electromagnetic methods to coal characterisation at West Cliff Colliery, New South Wales

Abstract

Both seismic and EM measurements are used to provide information about the geometry of coal seams in advance of longwall mining. However, each method has limitations and the results may be capable of several different interpretations. An approach which is now common in such cases is the joint application of two or more independent types of measurements to help resolve ambiguity. This paper describes the application of long-offset transient electromagnetic (LOTEM) sounding in Australian coal mining at the West Cliff Colliery, New South Wales. A feasibility study was carried out before the field survey in order to obtain the optimum measurement parameters. The interpretation of LOTEM data is based on both 1-D and 2.5-D automatic inversion techniques. Using prior information from seismic surveys and well logs, it was possible to map the electrical properties in coal seams. Since the electrical properties are closely related to stress as well as water saturation in coal seams within a sedimentary pile, quantitative relationships between the changes in stress or water saturation and electrical properties can therefore be established. The results indicated that the progress of drainage and of stress advance in association with mining can be monitored by measuring the changes of resistivity in coal seams from the surface using the LOTEM method.