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Integration of Borehole Radar and Acoustic Televiewer Data in Geotechnical Boreholes – A Case Study of the Use of Downhole Geophysical Data in Mitigating Risk to a new Mining Shaft Development
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 11th SAGA Biennial Technical Meeting and Exhibition, Sep 2009, cp-241-00033
Abstract
Anglo Platinum is evaluating a new underground platinum mine in the Bushveld Complex, South Africa. Major capital investment is required to sink twin shafts to access the platinum reefs from ~700 m depth. Shaft geotechnical boreholes were drilled along the two proposed shafts’ axes and were extensively logged with a suite of geophysical tools to detect potentially hazardous structures that may influence shaft sinking operations. This paper reports the integration of borehole radar (BHR) and acoustic televiewer (ATV) data down the proposed main and ventilation shaft positions. BHR was used in reflection mode in the boreholes to delineate any near-vertical structures close to the intended shaft positions. The drawback of current slim-line BHR probes is that they are omni-directional; hence it is not possible to determine the absolute orientation of radar reflectors in relation to the borehole. The ATV is used to map structures that intersect the boreholes by providing a high-resolution, oriented acoustic scan of the borehole sidewall. Borehole radar reflectors that intersect the borehole can be depth-correlated with structures identified by the ATV. In this way, BHR reflectors can be oriented in space. BHR identified a sub-vertical structure running sub-parallel and less than 15 m from the main shaft borehole, over a depth range of ~250 m. The structure was linked with a low-strength lamprophyre dyke that could pose a risk to the stability of a vertical excavation in close proximity to the structure. Based on the geophysical interpretation, the shaft positions were moved 30 m in a direction away from the indicated structure. Data captured in new pilot geotechnical boreholes revealed more suitable ground conditions. The sub-vertical dyke was identified in BHR images, but at a safe distance away from the new shaft sites.