In part one of this series of blog posts we looked at the features of StereoCore™ PhotoLog which allow you to easily depth register structures and lithology contacts. In part two, we looked at StereoCore™ PhotoLog depth modes and the reasons for having different modes for measuring depth. In this final blog post on depth, we will go over the reasoning behind the “Stacked” depth mode in StereoCore™ PhotoLog.
In any borehole, we trivially know that the total core loss and the total core recovered added together must be equal to the final depth of the borehole. If we want to be pedantic we can mention that in drilling the last run we may leave a core stub of unknown length behind.
There is one very important thing to note about the driller’s depth recorded on the core block. All of the rock encountered up until that core block must have come from a depth less than that recorded on the core block. Let’s say we have a depth of 30 m recorded on the core block for Run 10. That means that no matter what, all of the rock in the runs up to and including Run 10 must have come from a depth less than 30 metres, since the driller’s depth is the depth of the cutting face of the drill bit at the end of the run.
The Stacking-from-the-bottom algorithm
Let’s picture the borehole as vertical (see Figure 1). We’re going to take all the core from the core trays and dump it into the hole, in order, from the last core tray to the first, but we’re going to do it in a clever way. Now we know the depth of the hole because we have the recorded depth of the last core block (for now we assume that there was no core stub on the last run). Let’s take all the core from the last run (let’s call it Run n) and tip it into the hole. It fills up the hole to depth X.
As you can see from the diagram, if the TCR recorded for Run n is greater than the advance, then Run n has a core gain. Since we can’t possibly have extracted more core from the borehole than the advance, that core must be due to a core stub from the previous run.
Thus when we stack Run n-1’s core, we must start from the depth X, and not from the recorded end depth for Run n-1.
Again referring to the diagram, if the TCR from Run n-1 is insufficient to reach Run n-2’s end depth, then since we know that all the core in Run n-2 came from above the core block, we must start stacking Run n-2’s core from Run n-2’s recorded end depth, and record a core loss for Run n-1.
The Standard method for assigning depth to some feature measured in the core, is to measure from the core block above to the feature (some people measure from the core block below and back to the feature, but it’s ultimately subject to the same problems). The problem with this is discussed at length in the previous blog post in this series, namely that when a core stub is present some sections of the borehole apparently have duplicate depths.
How can we do better? Let’s again look at the end result once we’ve finished stacking the core from the bottom of the borehole (see Figure 2).
What we have is a “tube” containing core and gaps corresponding to core loss. Let’s assume for the moment that the core loss is in all the right places; if there is a feature located at a particular position in the core then we can easily assign a depth to it and that depth will be unique. Also, one can prove that the depth won’t be “too far out” from the true depth of that feature in the borehole.
The issue that we’re going to have though, is that it’s hardly straightforward to actually measure a depth using this technique, as it requires one to start from the bottom of the borehole and work backwards, hardly practical for logging under time pressure.
This is where StereoCore™ PhotoLog comes in. As mentioned in Part 2 of this series of blog posts, StereoCore™ PhotoLog calculates depths of features itself, using the core blocks and segment lines marked by the user on the core tray photographs. It is a simple matter then to switch the program across to using the appropriate “Stacked” depth mode to use the Stacking-from-the-bottom algorithm.
The role of spacers
There is one issue which needs to be resolved before we can be totally confident in our depths obtained by the Stacking-from-the-bottom method. As it stands, the core loss is allocated to the start of each run where it occurs. This may not actually be the case (for example, drilling through a cavity). The StereoCore™ PhotoLog answer to this problem is to allow the user to place “spacers” in the core tray photograph indicating where the core loss actually occurred (if the geologist can identify the place) and allowing the geologist to estimate how much. The “Unassigned core loss” column on the Runs tab of the Data Input screen allows the geologist to see the maximum core loss which he can assign to a particular spacer. Once spacers have placed then the depths are adjusted accordingly, as shown in Figure 3 below. Note that it is not necessary to place spacers if one cannot confidently assign the core loss to a particular place in the run, just that the option is available.
I started writing a whole section here on how we adjust RQD measurements for each run when using the Stacking algorithm, but it’s actually quite an involved topic, so I think I’m going to end this blog post here, and promise yet another update on RQD in the near future.
Dave (Member of the StereoCore™ PhotoLog Team)
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