Geologic and Geotechnical logging of coal
In mining industry, geologic and geotechnical logging are of vital importance. Thousands of dollars are spent to drill just one hole to collect a few coal seams. It is possible that the drill hole may be 2000 feet deep and the total mineable coal seams only make up about 10 feet of total rocks! So, the most important part of any drilling program for coal reserve study is recovering the coal. We must have at least 95% (usually better) core recovery for the coal seam itself. In most drilling programs, the drill hole is immediately logged using geophysical tools before sealing up with bentonites. The geophysical tools produce e-logs at the field. A geologist must check the elog for seam thickness and calculate coal recovery. If the coal recovery is less than 95%, a re – drilling is required.
The next important thing in the drilling program is the geotechnical properties of the roof and floor rock immediately above and below the coal seam. Usually driller’s are supposed to box 20 feet of roof rock and 10 feet of floor rock. The core boxes can be logged at the field by a field geologist. In many cases the core boxes are stored in a warehouse or sent to the rock lab to be logged by geologists.
The coal samples are also boxed and wrapped in polythene paper or plastic to preserve the original moisture content. The coal samples must be geotechnically logged by a geologist before preparing for sampling.
All that we said above is straightforward and easy to understand. But in reality there could be lots of problems during a drilling program to geologic logging of coal and rock cores. Let’s discuss a few problems you may find as a geologist logging cores.
1. Boxes marked improperly: Driller’s are not geologists and often make mistakes keeping track of the depth of their run. So, the depths they record in the core boxes can be off by a few feet to several feet. I have seen drillers even making careless mistakes such as putting 332 instead 232 and instantly your coal seam is 100 feet away from where it actually is. A good E-log should be enough to fix the problem.
2. Missing Box: Well, I hope you never face this situation. Usually drillers use a 10 feet box to sample rock cores. They usually box immediately after each run. So, theoretically you should have something like this:
| Box No. | Depths |
| 1 | 0-10 |
| 2 | 10-20 |
| 3 | 20-30 |
and so on…
But sometimes the drillers mis-label the Box No. such as they put box no. 4 for interval 20-30 while the box no. should have been 3. So, you end up with a missing box (box 3)!! If you face a problem of a missing box, do check the thickness reported on the boxes and see if you are actually missing some rocks or you have a problem with mis – numbering.
3. Core Boxed in Reverse Order: This is again another common mistake by the drillers. You should always use E-log (whenever available) to check the rock sequence make sense and match with the E-log. If you do not have an E-log, use driller’s log to check. You will be amazed how often the drillers box the cores in reverse order.
4. End not matching between broken core units: When you are doing a core logging, consider yourself as Sherlock homes. Oh yes..you have to be a minute observer to fix the problems. The “ends of rock units not matching” arise from three situations:
- Due to core loss
- Portions of the rock not boxed by the driller
- Misplaced cores.
To start with, check if all the ends of your rocks match with each other. Otherwise you are logging incorrectly and you are stating false geotechnical characteristics of the roof and floor rocks to your client. For deep mine, many small details of the roof and floor is of great importance and it helps the mining conpany to take adequate precautions and safety measures for the miners.
5. Core full with mud and clay: This can be resolved by carefully washing the core samples before logging.
Well, I have not encountered any other problem due to the drillers!! I will talk about more problems during logging in my next write up. I will also post some pictures so that you understand the logging process better!
NOTE: I am young geologist with just one year experince in coal field. If you have any suggestions or comments, please do not hesitate to add here. You are welcome to correct me anywhere you may find me wrong.
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I am having 4 years of experience in basemetal Exploration and 1 year with coal.
One thing i suggest that please consult your driller to give a mark on the core if they have broken intensly for fitting the core in the box , usually they do. We as a geologist can identify these unnatural marks but sometime we overlook.
Other suggestion is to write the meterage on the last Core piece of the run to avoid any mismatch.