Electrical resistivity investigation of part of Mmamabula coal field area for structures that influence the subsurface placement of the coal seams

Abstract

This study focuses on using electrical resistivity tomography (ERT) to map coal seam and associated structures found within the seams in parts of the Mmamabula Coal Field- central Botswana. The coals of this area belong to the Ecca Group’ s Mmamabula Fm and Mosomane Fm. Several major faults are reported to have affected the Karoo strata in the Mmamabula area. The northern limit of the coalfields is defined by the ENE-trending Zoetfontein Fault which has a downthrow to the north of approximately 250 m. The area of study overlies the northwest-south east trending Mabuane fault. This research was undertaken in the area to determine the presence of structures (e, g faults) and how they affect subsurface placement of the coal seams in the area. Results of this study will provide relevant stakeholders with information that could be used for mine planning, mine design, environmental planning, as structural features such as faults have direct effects on coal mining. The present study was undertaken along 14 transverse lines in the area using electrical resistivity tomography survey (using the roll along-dipole dipole configuration). The results from the inversion of data obtained show a large resistivity variation within the subsurface ranging from 1.89 to 758 Ωm. From results of the resistivity the subsurface in the area was interpreted to be made of the following sedimentary units; mudstone/ shale (1.89 – 10.1 Ωm), sandstone & grit (10.1 – 20.7 Ωm), grey siltstone (28.3 – 55.1 Ωm), mudstone with thin coal (119 – 289 Ωm) and coal (289 – 758 Ωm). The subsurface placement of the coal seam as identified from the inversion models is variable; ranging from a depth 5- 11 m in the NE side and 37- 46m in the SW side. The fault throw (vertical displacement) affecting the coal seams (as deduced from the interpreted resistivity sections) is approximately 32-35m. From the models it can also be concluded that the faulting is post coal deposition as the coal beds have been displaced relative to one another.