Depth measurements in StereoCore™ PhotoLog 2.0 Part 2

In my previous post I looked at depth registering core using core blocks and the facilities provided by StereoCore™ PhotoLog 2.0 in order to do that. Today I want to look at depth modes.

A typical drill rig extracts core by the following process:

• The diamond drill bit rotates, cutting the rock in front of it.
• The drill advances and because the drill bit is hollow, the drilled portion of the rock is pushed through the drill bit and into the core barrel, which sits behind the bit.
• When the core barrel is full, drilling is halted and the core barrel is pulled to the surface using the Wireline.
• The core is extracted and placed into a core tray.
• The driller writes the depth (to the cutting face of the drill bit) on a core block and places it in the tray, marking the end of the run.

So far so good. The driller can also record some statistics (see Figure 1) in his log for each run

• Start depth
• End depth
• Advance
• Total core recovered (including rubble)
• Solid core recovered
• Core loss

Core loss is the advance minus the total core recovered. Core loss may occur due to grinding (the core is ground away by the drill) or due to simply drilling through a cavity in the ground. A negative core loss corresponds to a core gain, something which can occur under circumstances outlined later in this article.

Figure 1: Run statistics

Sometimes a stub of drilled core may be left in the borehole when the core barrel is pulled out as a result of the way the drilling process works. When a large stub is left a driller can sometimes "fish" for it by attempting to lower the non-rotating drill bit over it and push it into the barrel again, but small stubs get drilled out and added to the next run. Thus if a core stub is left in the hole on run number 10 then run number 10 will have a smaller TCR (total core recovered) and thus a larger core loss recorded than in should have actually had, and if run number 11 consists entirely of solid core it may have a core gain as a result of picking up the stub from run number 10.

To see how this works (Figure 2) assume that we start run 10 at depth 27m and advance 3m to 30m. We pull out the core and we recover 2.8m, and we write “30m” on the core block which we place in the core tray. Now the core loss recorded for run 10 is 0.2m. We drill out run 11 and only advance 2.8m to 32.8m, but we recover 3m of core – the core gain is thus 0.2m (alternatively one can say that the core loss is -0.2m) which includes the stub picked up from run 10.

Figure 2: Apparent core loss and gain due to a core stub

The kind of core loss/gain in the above example is not real, it’s just an artefact of the drilling process, but it does make it harder to do proper depth registration, especially when there is natural core loss to account for as well. The question becomes “How do we tell what is true core loss (i.e. due to causes such as grinding or cavities in the rock) and what is apparent core loss (due to the aforementioned problem that part of one run left behind in the hole may be picked up in the next run)?”

Sometimes people try to get fancy with moving core blocks around after the fact but that’s an approach fraught with danger. Every time you move the core block you are erasing information about how the drilling proceeded – even worse, you are making your raw data tell lies. I personally think it far better to leave the core blocks as they are in the core tray and correct the depths by calculation afterwards, you don’t have to take my word for it though. Because this post is getting long I am skipping explaining the mechanics how we correct depths in StereoCore™ PhotoLog in favour of explaining what depth modes are and how to use them, but there will be another blog post where I will explain the thinking behind how “Stacked” depths are calculated.

Depth modes in StereoCore™ PhotoLog 2.0

If you open a project in StereoCore™ PhotoLog and click on Project->Options in the menu at the top of the screen, you will see that there are two possible depth modes for the project, “Standard” and “Stacked”.

StereoCore™ PhotoLog is unique (as far as we know) among core logging data capture tools in that depth is a calculated quantity. In manual core logging, one measures the depth of e.g. a structure by running a tape measure from the core block above down to the structure. This in StereoCore™ PhotoLog corresponds to the “Standard” depth mode, and is subject to errors such as the possibility of depths being duplicated – see Figure 3 for details. It is useful though, because one can easily compare a StereoCore™ PhotoLog generated structure log with a manual log.

Figure 3: Duplicate depths when using Standard depth logging

The other depth mode is “Stacked”, and in this depth mode we apply a more sophisticated algorithm to depth calculations in order to account for apparent core loss.

Spacers

If you look at the Runs tab in the StereoCore™ PhotoLog Data Input screen you will see a column for “Unassigned Loss” and “Assigned Loss”. For each run, the stacked depth algorithm calculates the maximum actual core loss which could have occurred in that run, and this is initially the number in the Unassigned Loss column. The logger can place a “spacer” at the point in the run where he or she thinks that the loss occurred, and can fill in the core loss at that point, which will adjust the depths of structures, segments and lithology contacts accordingly. When spacers have been placed and core loss assigned to them, then the “Assigned Loss” column in the Runs tab shows the total assigned loss for the run.

Spacers will work in both Stacked depth mode and Standard depth mode, but really they only make sense in Stacked depth mode, as Standard depth mode does not take apparent core loss into account and therefore it would be very difficult to assign a correct core loss for each run when working in Standard depth mode.

In summary:

• Draw segment lines, place structure and lithology markers on your core tray images and use descriptors to describe them when logging in the core shed.
• Change the project depth mode to “Standard” and export if you want to compare the StereoCore™ PhotoLog logs with manual logs.
• Place spacers when in "Stacked" depth mode to indicate where you think core loss occurred.
• Export in "Stacked" depth mode to obtain depths which take into account apparent core loss.