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Best or Preferred RC Sampling Method (7 replies)
This is an excellent question and something which I have wondered about and studied briefly. I have no definite answers, but here is my contribution to the discussion.
The influence of Style 2 is less where you are sampling bulk commodities where mineralization where the length of the intersection is greater than the length of the rod. At the end of each rod you know that the sample has been completed at a certain depth, so any mistakes in depth reporting within the rod are not so important. Also, bulk commodities are less susceptible to contamination (although this is not entirely true, as sampling the deleterious elements is also important).
However, Style 2 may cause a significant problem if the project is attempting to identify narrow precious or base metals mineralization (low concentrations). As you rightly point out contamination from one sample to another and poor depth reporting, both of which may cause a problem.
My opinion is that Style 1 is preferable, but let's face facts; when you turn your back it is possible that the drillers might not do what you ask. My opinion is that you should identify the risks and quantify the impact where-ever possible.
The impact of Style 2 can be quantified using down-hole variograms, twinning (this only works subject to a number of caveats) but also by measuring the percentage of recovery of RC cuttings. These methods can also be used to assess bias due to segregation of minerals within the RC sampling method.
Using the above methods and the resource estimate, you can perform some calculations regarding the amount of financial risk the company is exposed to. In some circumstances, the risk might significantly impact the company's viability.
The risk describedhere is considered by many mine planners to be contributing to non quantifiable geological uncertainty. Many operating mines battle with reconciliation problems between resource estimates and production, if companies quantified this risk could mining become more predictable?
Style 1 would be my preferred method. Not only for confidence in the sample interval and quality but it also give the samplers more timed to deal with changing the sample bags. However I doubt the drillers would allow the reduced productivity unless it is negotiated up-front and included in the contract.
You raise two issues. Firstly the sample interval and secondly contamination of the sample!
With the sample interval the critical factor is the age of the drilling rig. An old rig may only have flagging tape tied on to the pull down chain and so the sample interval may indeed vary. Whereas a modern rig may have a digital read out of the hole depth and so the sample interval should not vary.
With regards to contamination from the bit to the sampling device, as long as the hammer, rods and sample hose are well maintained i.e. no loss of rings in rods and no kinks or holes in the sample hose contamination should be minimal. However in the sampling device either riffle or cone splitter the design, age and whether the device is well maintained determine the level of contamination. Design wise the good sampling devices usually have two doors separating the cyclone from the sample device. This separates the samples and providing the sampling device is cleaned between sample intervals/rods either by air blast/shaking/hammering/scraping contamination should be minimized.
Finally to answer your question I would have a modern drilling rig with a well maintained sampling device either cone or riffle. I would also make sure my geo/fieldies understood how the sampling device works and would have them at the rig to check that the sampling is occurring correctly.
I agree with your statement, a new drill rig being operated correctly is the ideal solution. There are probably some exploration programmes where you have good interaction between driller, geologist and field assistant. However, I have seen many programmes where this is not the case, which exposes companies to a great deal of uncontrolled risk. My suggestion is that the situation is dealt with like this:
Quality Assurance: Train all participants well before the program starts. Tell them to use Style 1 and train them in all concepts mentioned earlier in comment.
Quality Control: Measure the quality of sampling using down-hole variograms, twinning and measuring the percentage of recovery of RC cuttings. Most importantly, the results of the QC must be passed onto the field staff.
The method I outline here minimizes the chance that the company puts money into a bad project and accounts for the normal human fallibility.
Thanks for the discussion everyone. Please keep commenting, I am keen to know everyone's opinion and experience.
To manage the drilling practices I prefer to include in each drilling contract a series of attachments as living documents but contractually binding. One attachment is the Conduct of Drilling; not instructing the driller how to pull leavers but setting out the QC required. The conduct of drilling attachment is a technical standard and not confidential so that it can be given to the driller and supervising geologist (probably the most inexperienced within each team) so that they can all “sing from the same hymn book”. Too often drillers do not see the contract or at best the supervisor focuses on the unit rates.
The conduct of drilling can be drafted with the input from the drilling contractor management drawing on their experience when negotiating the contract. Agree close cooperation with the driller is essential to deliver the best result. Meaningless terms such as “best practice” or “standard industry practice” I avoid inserting in contracts as there does not appear to be any written standards to refer to. I have not seen in drilling contractor’s policies or procedures actual drilling practice standard described, at best.
All this being up front the drilling rates negotiated reflect the QC and fair to the contractor and no reason the contractor should not deliver. If a dispute arises the parties (upper management, most experienced of eachparty) can refer to the attachment, modify by mutual agreement, as there may be technological or other limitations so that the revised instructions can be re-issued to the driller and supervising geologist. I think this will give a modicum of protection from the driller running feral when you “turn your back” and reinforces to the supervising geologist what is considered material to the project to monitor. If the driller continues to flout the intent of the contracted Conduct of Drilling, then the only option is to terminate the contract, if it is an option that is and may not if rigs are hard to source. If the contract is terminated by non-performance then a clause to waver paying demobilization costs may be a deterrent.
It appears we are all in agreement style 1 is more appropriate than style 2 which coincides with my experience, but agree it can be difficult to enforce all the time and hence my desire to embody the Conduct of Drilling in an attachment in the drilling agreement as part of QA management. A CP can form their own judgment on the conduct of the drilling in the context of the circumstances.
Unfortunately bad drilling practices such as style 2 are brought into a project. I did recently raise my concerns with a drilling manager about style 2 which he was promoting that it minimizes contamination. I regarded the drilling manager as being very experienced and having worked for some major drilling companies, includinginternational. As style 2 is more common (more profitable for the contractor) I thought I would seek further opinion in case I was missing something.
It is important as you have pointed out to weigh each RC sample at the drill rig as a proxy for sample recovery and weights of sample delivered to the laboratory and focus duplicate field samples in the mineralized zone to indicate the samples submitted to the laboratory are representative of the interval sampled. I use that information for defending my position with drillers.
If anyone has drafted similar conduct of drilling practices I would not mind sharing.
I'm a stickler for quality, or at least pretend to, but you have to take practicalities into account?
If you are doing a resource drillout for coarse, NV gold then you are likely to know roughly where you're in and where you're out. When you hit more interesting zones you could start to pay more attention. If you're drilling bulks then I wouldn’t recon it makes a noticeable difference, although i take the point on the deleterious elements. Also perhaps more exploration orientated holes I wouldn’t bother as much as my budget is likely tight-arsed and the more meters I get for my initial holes without compromising too much on quality the better.
The nature of the beast is also just simply that the ideal situation doesn’t exist. Currently, you'd be lucky to find a rig to begin with, let alone one with all the digital and sampling bells and whistles. There aren't many places in the world where they have all those beautiful sampling systems fitted etc as well.
Sure, you can probably spend a whole heap of time on downhole statistics etc but considering all the other things that affect your variance, does this really have an effect? I doubt it. I spend as much time with drillers and rig geos as i can and make sure that they clean their tools, take due care etc and that has always given me great results. To be honest, if you really investigate the errors introduced at each step, what we're talking about here will not affect it by even less of a %.
And at the end of the day guys, as geos we do all we can to put as much detail and care into it and an engineer will come along, suck his thumb and say: "i reckon we model this on $XXXX/ounce".
Both of these suggested configurations are at least 15 years out of date, particularly the first one. You may be interested in the AusIMM's August Conference "Sampling 2012" in Perth. There's no reason at all for unrepresentative RC samples today and both of these suggested "styles" will unfortunately give you unrepresentative samples.
There are two styles of collecting RC samples, fitted with or without a rig mounted sample splitter.
Style 1: at the end of each meter sampled, the penetration of the hammer is halted and the hammer not lifted off from the bottom of the hole until the sample dust has been cleared from the cyclone and sample bag removed. Then the hammer can be raised from the bottom of the hole to clear any material that may have fallen down outside the drill string. Once the hammer has been lowered to the base of the hole and the hammer fired one or two times the sample bag can be attached to the cyclone and the cycle repeated.
Style 2: much favored by drillers is to signal to the sampler the down-hole interval to be sampled has been reached and remove the sample bag and attach another sample bag as drilling continues and the cycle continues until a rod change.
If the second style is performed the driller maintains any sample outside the drill-string remains in suspension, eliminating contaminating the sample.
My view is the second style results in undefined variable sample interval lengths, and geostatistics is predicated on a constant sample support. Also the lag of sample flow from the bit faces to the sampling device, which becomes longer with the greater depth drilled, causes contamination from the previous sample.Has anyone a view on the preferred method and why?