Page 87 - Rock Mechanics For Underground Mining

PRESENTATION OF STRUCTURAL DATA

Downhole logging. The logging of core may be supplemented by logging of the
borehole walls with a range of downhole tools such as borehole cameras, televi-
sion cameras and geophysical tools. Optical imaging may be enhanced by the use of
digital imaging software to process the signals received from a downhole scanner.
The digitally recorded data yield high resolution colour images, enabling detailed
measurements to be made of discontinuities (e.g. Kamewada et al., 1990, Goodman,
1993) and mineralisation to be identified. However, it must be remembered that sam-
pling issues such as the orientation bias and the lack of persistence data also apply to
borehole imaging techniques.
Geophysical logging systems consist of a downhole probe or tool attached to a
multi-conductor electric cable often referred to as a wireline. Several types of geo-
physical devices may be combined to form one downhole logging tool. The most
common types of downhole geophysical logging test include seismic velocity (full
wave seismic logs), acoustic scanning, electrical resistivity, gamma-gamma and self
potential. These logs may be correlated with rock types, material strengths and moduli
(e.g. Schepers et al., 2001). The ISRM Commission (1981) has published Suggested
Methodsforthestandardgeophysicalloggingtests.Themostusefuldeviceformaking
discontinuity measurements is the acoustic scanner, seisviewer or televiewer, which
is able to provide oriented images of borehole walls at very fine resolutions (Hatherly
and Medhurst, 2000).

3.5 Presentation of structural data

3.5.1 Major features
The effective utilisation of geological data by a mining or rock mechanics engineer
requires that the engineer must first be able to understand and digest the data and to
visualise their relation to the proposed mining excavation. It is necessary, therefore,
that means be found of presenting the data so that the often complex three-dimensional
geometrical relations between excavations and structural features can be determined
and portrayed.
Major structural features such as dykes, faults, shear zones and persistent joints
may be depicted in a variety of ways. Their traces may be plotted directly on to mine
plans with the dips and dip directions marked. Alternatively, structural features may
be plotted, level by level, on transparent overlays which can be laid over mine plans
so that their influence can be assessed in developing mining layouts.
However,themosteffectivemethodofdepictingmajorstructuralfeaturesisthrough
the use of modern computer-based mine planning systems. These systems which are
used at all major mines in one form or another, permit geological and geotechnical data
to be integrated with mine planning and design systems. They are able to produce plan
or sectional views as well as three-dimensional representations which can be rotated
about chosen axes. Figure 3.20 shows such a depiction of two faults with orientations
(dip/dip direction) of 70/255 and 70/319 intersecting stope development excavations
in a bench-and-fill stoping block at the Neves Corvo mine, Portugal.

3.5.2 Joints and bedding planes
The data for joints and bedding planes differ in two significant respects from the data
for major structural features such as faults. First, they are much more numerous, giving
69