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Tech-Log™ · 3D radar · underground
3D off-hole imaging
from a three-hole fan
Three 300 m holes from one collar found the UG2 reef but not the fault the model predicted. Borehole radar in each hole imaged reflectors 35 to 40 m into the rock; oriented in 3D and interpreted jointly, they yielded the UG2 and a parallel lower reflector as surfaces, and from those the fault's location and throw.
The question
Increased certainty in a previously modelled fault location was required before underground mining operations could advance. Underground operations extracting chromitite reefs in the Bushveld Igneous Complex often face geomorphological complexity from syngenetic slumping and subsequent brittle deformation.
Three sub-horizontal boreholes of about 300 m were drilled ahead of development from the same collar in the UG2 hanging wall, intersecting the UG2 some distance along each hole and terminating in the footwall. Core logging captured the UG2 intersection in each hole but failed to locate the modelled fault. Uncertainty in its location, orientation and throw remained.
Joint 3D interpretation of the three radargrams produced a high-certainty interpretation of the reef and the fault the drill holes never intersected, de-risking the advance at comparatively low additional cost and time.
- SettingThree sub-horizontal ~300 m holes from one collar, UG2 hanging wall to footwall
- ToolBorehole radar, 250 MHz, imaging 35 to 40 m into the formation
- ReflectorsUG2 (upper) and a parallel lower reflector, possibly UG1
- OutputTriangulated reef surfaces, fault location and throw between holes
What we did
A suite of downhole tools including borehole radar was deployed in each hole. Of the tools deployed, only the radar can image reflectors away from the borehole; here it imaged reflectors some 35 to 40 m into the formation.
Knowing the regional dip of the UG2, the 2D radargrams were oriented in 3D space and attached along each borehole path. A second set of radargrams projected at 180 degrees to the first allowed continuous interpretation of any reflector along its entire length. In all three holes the radar reflectors could be attributed to the UG2 (upper reflector) and a lower reflector parallel to it, possibly the UG1.
With the radargrams oriented in 3D, triangulated surfaces were created for the UG2 and the lower sub-parallel reflector, making it possible to infer the location and throw of the fault between two of the holes. The result was presented as a UG2 elevation map with the interpreted fault and a fault-throw cross-section.
What it changed
Conventional core drilling often under-samples the near-mine volume, elevating the technical risk of advancing underground excavations. Borehole radar's ability to map reflectors away from the hole de-risks advances by locating geological and structural features in the off-hole rock mass, maximising the value of boreholes with comparatively low additional cost and time. Jointly interpreting multiple closely spaced radargrams in 3D reveals off-hole features that are not directly apparent as reflections in the individual radargrams.
- Fault located and its throw estimated without a single hole intersecting it
- UG2 and a parallel lower reflector modelled as surfaces 35 to 40 m off-hole
- Near-mine volume characterised from holes already drilled
- Joint 3D interpretation revealed what single radargrams could not
Evolved from ground penetrating radar, borehole radar operates at about 250 MHz. Signals radiate outward from the tool into the formation and reflect at contrasts in dielectric permittivity, usually changes in lithology and brittle fault structures. Tools deploy into holes of about 34 mm diameter by hand, anchored winch or on the end of drill rods.