Open Cut » Geology
Before analysing geophysical data, it is essential to smooth the raw data. However, insufficient smoothing makes interpretation extremely difficult if not impossible due to noise obscuring meaningful data, and too much smoothing causes the loss of significant geological information. To ascertain, the ideal level of smoothing, five senior coal geologists were supplied with gamma and density data smoothed at different levels and asked, according to their experience, to pick the ideal level of smoothing and from their responses an ideal level of smoothing is recommended. To assist this, a statistic known as the relative average difference between consecutive readings has been developed to describe the degree of smoothing.
Generally, no interpretation is performed on density logs above the base of casing as the casing affects the density log and any coal in this interval would likely be unsuitable for sale. To facilitate data analysis of multiple holes, a methodology has been developed to automatically pick the base of casing from the caliper log.
The resolution of downhole density logs is limited due to them measuring the density of the material between the tool's source and detector rather than at a particular point. In general, this is only an issue when trying to determine the thickness of coal seams and partings whose thicknesses are thinner than the source - detector spacing. One of the objectives of this study was to see if a mathematical relationship could be established to correct the interpreted boundaries to better match the actual boundaries for these beds. Unfortunately, even though a number of hypotheses for doing this were tested, none were successful.
Downhole geophysical logs recorded on blast holes are used slightly differently to those for exploration holes. Generally, exploration holes will also have a geologist's log of the core or chips and the geologist's main task is to adjust the boundary depths in their geological logs to their geophysical logs whereas for blast holes the geologist must derive a simplified geology log entirely from the geophysical logs. The main items that this log must include are:
- Location of mineable coal seams for protection from blasting;
- Location of harder units such as conglomerates and siderites that may be more difficult to blast;
- Location and geophysical signature of mineable coal seams, non-mineable coal seams and carbonaceous zones for correlation purposes with other blast holes and nearby exploration holes.
A methodology has been developed and demonstrated for generating the lithology of blast holes directly from the geophysics. This entails finding the lithological boundaries of coals from the density log as in the previous work but then doing an interpretation of the lithology of the unit between the boundaries based on the average density and natural gamma values recorded for the lithology.