Techno-advantages of Acoustic Televiewer Logging on Shaft Boreholes

Geophysical borehole logging of two shaft boreholes separated by 300m was used to complement geological logging of un-oriented borehole core in a geotechnical investigation aimed at mapping rock strengths and stability in advance of shaft-sinking operators. High-angle (>60º) dip magnitude fracture systems interpreted from acoustic televiewer imagery were ubiquitous throughout both holes and showed large variations around a mean strike orientation of NW-SE, in line with borehole “breakout” events also mapped from the televiewer. Fracture depths correlates well with those mapped from geological core logging. A unique, 150m wide zone of anomalously high fracture frequency density in borehole A has been confirmed from ongoing shaft-sinking as reflecting “disturbed” ground, and points to the presence of a sub-vertical structural feature intersecting the borehole. Fracture orientations over the “disturbed” ground differ considerably from those above and below this zone. Ancillary geophysical data (density) does not uniquely define the extent of the highly fractured zone. Elsewhere, shallow temperature anomalies in both boreholes possibly reflect intersections of the same aquifer