Page 563 - Rock Mechanics For Underground Mining

MONITORING SYSTEMS

them to the engineer. Although the nature of the underground mining environment
may mean that relatively simple and direct methods of collecting data are used in the
first instance, the computer-based storage, processing, management and reporting of
data is now a common feature of most mine-site monitoring systems.
In order that the monitoring system should fulfil its intended function economically
and reliably, it should satisfy a number of requirements:
(a) easy installation, if necessary under adverse conditions;
(b) adequate sensitivity, accuracy and reproducibility of measurements;
(c) robustness and suitable protection to ensure durability for the required period of
operation;
(d) ease of reading and immediate availability of the data to the engineer;
(e) negligible mutual interference with mining operations.
The terms accuracy, error, precision and sensitivity as applied to measuring devices,
require careful definition.
The stated accuracy of an instrument indicates the deviation of the output, or
reading, from a known input. Accuracy is usually expressed as a percentage of the
full-scale reading. For example, a 10 MPa pressure gauge having an accuracy of 1%
would be accurate to within ± 100 kPa over the entire range of the gauge.
The error is the difference between an observed or calculated value and the true
value; errors may be either systematic or random.
The precision is a measure of the ability of the instrument to reproduce a certain
reading. It may be defined as the closeness of approach of each of a number of similar
measurements to the arithmetic mean. Precision and accuracy are different concepts.
Accuracy requires precision and an absence of bias, whereas precision implies a close
grouping of results whether they are accurate or not.
The sensitivity of an instrument is variously defined as the ratio of the movement on
the read-out unit to the change in the measured variable causing the output, the input to
output ratio (the inverse of the previous ratio), or the smallest unit of the measurement
detectable by the instrument. Sensitivity as defined in the third way may depend on the
readability of the read-out unit (the closeness with which the scale may be read), and
the least count (the smallest difference between two indications that can be detected
on the read-out scale).

18.2.2 Modes of operation
The modes of operation of the sensing, transmission and read-out systems used in
monitoring devices may be mechanical, optical, hydraulic or electrical.
Mechanical systems often provide the simplest, cheapest and most reliable meth-
ods of detection, transmission and read-out. Mechanical movement detectors use a
steel rod or tape, fixed to the rock at one end, and in contact with a dial gauge or elec-
trical measuring system at the other. The main disadvantage of mechanical systems
is that they do not lend themselves to remote reading or to continuous recording. Me-
chanical convergence and displacement measuring systems are described in sections
18.2.3 and 18.2.4.
Opticalsystemsincludingelectro-optical(EDM)andlaser-basedsystems,areused
in conventional, precise and photogrammetric surveying methods of establishing ex-
cavation profiles, measuring movements of excavation boundaries, and recording nat-
ural and mining-induced fractures. These methods are also widely used in monitoring
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