A dissertation submitted to the Faculty of Engineering and the Built Environment,
University of the Witwatersrand, in fulfilment of the requirements for the degree of
Masters in Engineering.
13st October 2015 / This study examines the history of underground face sampling since the earliest
writers documented their findings regarding the method. An historical review of the
errors associated with sampling is followed by a discussion of findings from a
number of underground investigations. The thesis examines how errors introduced
during and after sample extraction are responsible for error and bias. A Simulated
Chip Sample Model was created and used to evaluate the nature of sampling errors
and bias. Learning’s from this model include an understanding of the effects of
sample extraction on variations in grade and precision. The effects of sample shape
on error were considered and the aspect ratio of the sample was found to be a
critical factor in minimising error It was found that if the homogeneity of the ore
material decrease the error increase proportionally, despite the extraction error
remaining constant. During extraction the material will be biased and the effect of
using accumulation values will amplify the error and bias in samples; the error and
bias in this case is secondary and has nothing to do with the extraction error. The
historical ideas about error being a consequence of the nugget effect have been
shown to be false; variability between samples in this case is a direct result of poor
sample extraction rather than the occurrence of large gold grains in the ore.
The thesis also identified sources of sampling bias and two distinct types, namely the
soft-reef bias and waste-discard bias were identified. The perception that soft-reef
bias is the main contributor to the deterioration of the MCF was found not to be the
case. This finding is based on a comparison between chip sampling and perfect
sampling, in the form of so-called “coffin samples”, that indicated there is no bias
between these sample types. Other possibilities for sample error and bias were
investigated and found to be related to human preference for selecting material that
had to be discarded during or immediately after the sample had been taken. This
was confirmed by a survey of 70 samplers with different experience levels and from
different mines, who indicated the same preferences when selecting material they
chose to discard from the sample material collected. The waste-discard bias is a
better contender for introducing bias serious enough to affect the MCF because it
occurs at each and every sample site, unlike the soft-reef bias. This type of bias was
shown to mimic the soft-reef bias using the Simulated Chip Sample Model. Chip sampling has been the simplest and at the same time most misunderstood
sampling method there has ever been, but it has stood the test of time and is shown
to be without a meaningful replacement. Despite the appearance of poor extraction
compared to other methods, chip sampling is an acceptable technique when one
understands and eliminates biases during and after extraction.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/20174 |
| Date | 05 April 2016 |
| Creators | Fourie, Andries |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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