Measuring core loss from diamond drilling - the benefits.

The data used to plan mine developments and major civil works are mostly derived from drill core logs. These provide detailed descriptions and analyses of the ground sampled from the recovered core segments, but rarely identify and characterize sections along the borehole where sample is missing.

These core loss gaps result from difficulties with the drilling process, either due to poor ground conditions, or mechanical problems, and are therefore zones of potentially high economic and geotechnical risk. For example, many ores are associated with weak altered rock that could therefore be under-sampled, plus the viability of any deposit, or civil construction, depends on the degree of support required to stabilize the ground.

Thus it is imperative to delineate these breaks and produce a ‘Core-Loss Log’ explaining the reason for each missing section. Although this may seem difficult to do, in fact it is a relatively easy task, so long as the driller adheres strictly to the definition of a drill run, being:

A drill run occurs whenever the core barrel inner-tube is extracted after an advance, however short this may be.

To explain: A full drill run occurs once the driller has advanced the borehole by a depth equivalent to the capacity of the core barrel inner-tube. To make life easy for the driller, this capacity is designed to be slightly more than one, two, or three rod-lengths worth of core – so all the driller has to do is keep an eye on the relevant rod-join as the borehole is advanced, and stop drilling once this join is drawn down level with the driller’s datum on the rig (Fig 1). At this point, a FULL DRILL RUN is achieved, and the core barrel inner tube is extracted, or ‘pulled’ to discover how much core was recovered from the advance.

Note:?A core barrel inner-tube normally has a +/- 15cm excess capacity, depending on how the inner-tube head is adjusted, to allow for an average length of stub to be picked up from the previous run.

A SHORT DRILL RUN occurs when the core barrel inner-tube is pulled after a short borehole advance. Hence, to find the new borehole depth, the distance from the relevant rod-join down to the datum – a distance that is colloquially called the “additional stick-up” – is measured and added to the “constant stick-up”, which is the fixed distance of the datum above the borehole collar (Fig 1). This combined length is then subtracted from the length of the drill string, being the distance from the cutting face of the drill bit resting on the bottom of the borehole, up to the relevant rod-join, and the result is the depth of the borehole.

Done properly, this depth is accurate to the nearest centimeter, the calculations for which are recorded in a “Stick-up Log” (Fig 2), and the new borehole depth is also written on a core block, which is placed in the core tray at the end of whatever length of core was recovered from the drill run – even if no core was recovered.

At this juncture, it is important to note that no driller stops a drill run short for a trivial reason. It could happen for quality assurance – to count the drill rods and confirm borehole depth – but the downtime involved eats into the drilling crew’s bonus, calculated on the basis of meters drilled per shift, so such interruptions are not welcomed. Drillers and their crews use all their experience to keep producing full drill runs for as long as possible and hate delays, especially those caused by equipment failure.

Thus, it is more common to find that short runs are drilled because of bad ground conditions – and here is where the story gets interesting: Drill contractors are typically held to a minimum core recovery that is nominally upwards of 95%, to avoid a substantial penalty being levied by the client, hence it falls to the drill crews to ensure no such penalty is incurred, otherwise their bonuses are also in jeopardy. So, faced with dire consequences for poor core recoveries and slow drilling rates, what does any good driller do?

The most effective course of action is simply to forget about wasting time measuring additional stick-ups until after the bad ground is traversed and/or the mechanical problem is solved. Instead, rewards are assured simply by aggregating the core recovered from short advances until sufficient is obtained to make up a full drill run, or runs. Thus the human error artificially reduces the borehole depth, so the contractor does not charge for all the meters drilled, but the shortfall is more than compensated by the forfeiture due if the true run depths were to be recorded.

Evidence of under-reported core loss is easy to find. Rod counts to confirm the drill string length, including the fully assembled core barrel outer tube length, are definitive. Also, it is exceedingly rare to find a driller has produced more core than borehole! In the author’s auditing experience, the only occasion that did happen, the core foolishly discarded from a nearby wedging operation had been slipped in to the record…

Other indicators are core recoveries that exceed the core-barrel inner-tube capacity; consecutive full drill runs with core gain; and, on some site visits, the driller’s assistant was found to have pre-marked core blocks with full drill run successive depths ahead of the drilling. Naturally, the latter finding is always defended by the crews, who vigorously assert the driller’s skills, which, as it happens, are not in doubt.

So, to return to the topic at hand: The measurement of CORE LOSS.

Given a disciplined driller, who unfailingly records a drill run every time the inner tube is extracted, irrespective of the length of the advance, or the amount of core recovered, then:

• As explained in a previous post (https://www.dhirubhai.net/pulse/depth-registration-drill-core-john-orpen ), the core recovery measured for each drill run will show either a “gain” or a “loss” caused by the variable stub lengths. See also the stick-up log calculation example, Fig. 2, below.
• The previous post also illustrates how these stubs give rise to false gaps in the core as well as illogical overlapping sections using the standard core depth registration method of measuring down from the core block depth at the top of each drill run. Hence the technique is useless for identifying runs with real core loss.
• Also shown, is how, using the StereoCore stacked depth registration method, the gaps left at the top of a drill run are solely attributable to actual core loss, the amount of which can therefore be distributed to core breaks where loss is evident – whether the break is mechanical, such as a grind, or natural, such as a cavity recognized when the broken ends of core cannot be matched across the gap for example.

Thus, the hardest part of core loss logging is how much vacant space to allocate to each break. Here, comparison with downhole geophysical logs is helpful, presupposing the depth registration of both the core and the acoustic/optical televiewer log is well correlated. Then less arbitrary estimates can be made as to how much loss can be attributed to each break, as well as the reason for the gap – if the ATV log shows an intensely fractured zone for example.

Notwithstanding these efforts, the amount of true core loss suffered in the drilling of a borehole – from the overburden / hard rock interface down to the end-of-hole – is a vitally important metric to inform the reliability of any attempt to map and model the subsurface for whatever purpose. The measures that need to be taken to ensure honest drilling in this regard currently fall short of the objective, however, and recommendations for remedying this will be put forward in the next post.

? J.L. Orpen (Resource Exploration & Development Pty. Ltd.)

Orpen, J. and Orpen, D. 2020, Error-Proofing Diamond Drilling and Drill Core Measurements: SEG Newsletter, no. 123, p. 23–34.

Figure 1. The constant and additional stick-up measurements required to calculate the borehole depth. (Orpen, J. and Orpen, D. 2020)

Figure 2. A typical driller’s stick-up log for calculating borehole depth. (Orpen, J. and Orpen, D. 2020)