Depth measurements in StereoCore™ PhotoLog 2.0
The primary way of measuring depth when logging core is traditionally using depths marked on the core block by the driller. Usually what happens is that people take the nearest core block and measure distance along the core using a tape measure. In this way structures or other features of interest can be referenced to a depth.
A worthwhile question to ask is "How are the depths on the core blocks determined?" After all, the core block depth is taken as more or less absolute when doing logging. It turns out to be pretty simply defined as the depth from the collar of the borehole (assumed to be 0) to the cutting face of the drill bit.
This can be determined to a high degree of precision by the driller because he knows the number of drill rods that are in the hole. There is a bit of a business involving stick-ups and driller's datum lines which for brevity I'm going to ignore - it's the driller's responsibility to put an accurate depth on the core block taking into account any necessary correction factors and we know that (in principle at least) it is possible to get such accurate depths.
Once we have accurate depths recorded on the core blocks we can then look at some statistics for each run of core. One can record the TCR (Total Core Recovery), the SCR (Solid Core Recovery), RQDm (Rock Quality Designation in metres) and the Advance (Difference between the depth at the start of the run and at the end of the run). RQDm is a bit of a strange one, it is defined somewhat arbitrarily as the total length in metres of the solid core fragments longer that 10 cm. Sometimes it is defined as the total length in metres of the solid core fragments longer than the diameter of the core.
StereoCore™ PhotoLog is ideally suited for these sorts of measurements because all of them can be automatically calculated for each run, provided the user has marked the core appropriately - for example, marking the rubble sections and placing the core block markers.
The StereoCore™ PhotoLog depth registration process
StereoCore™ PhotoLog depth registration is designed to be quick and easy. Full details are available in the manual as to how to do it, but briefly:
This blog post is getting quite long so I'm going to end it here, but in the next post I will go through the details of the two depth measurement modes which one can use in StereoCore™ PhotoLog, standard depth mode, which corresponds to the depths one would measure with a tape measure when logging manually, and stacked depth mode, where StereoCore™ PhotoLog attempts to correct the measured depths to account for some common depth problems.