The thing that really defines the direction of a plane in 3D space is the pole of the plane, which is a 3D vector normal to the plane. Structures in drill core which intersect the core axis have a characteristic elliptical shape. It's a bit of a pain to directly measure the plane pole, so most goniometers measure instead an (α, β) angle pair, which can then be converted into a plane pole with a simple calculation.

An α angle is measured relative to the core axis, and the β angle is measured as a sort of "azimuth" relative to the orientation reference line (defining the intersection of the geographic vertical plane along the length core – top-side or bottom-side). Different goniometers use different conventions for defining their own specific α or β angles. Why are we introducing our very own convention? Because the conventions currently in use are difficult to visualize intuitively - once you get the hang of it it's very easy to visualize a structure with a particular StereoCore™ (α,β) or to estimate what the StereoCore™ (α,β) of a structure that you're measuring should be. It is also easy to convert between the StereoCore™ convention and other goniometer measurement conventions.

Without further ado, referring to Figure 1, the (α,β) angle pair is measured for either end of the long axis of the structure ellipse. This means there are two (α,β) measurements for any one structure. The α angle is measured as the angle between the structure plane and the core axis, from 0 degrees uphole to 180 degrees downhole, and the β angle is measured clockwise from the reference line to the nose of the ellipse. Note that a structure with α angle of 90 has no defined β angle since there is no long axis of the ellipse – the plane pole is parallel to the core axis.

Although StereoCore™ Photolog 2.0 reports StereoCore™ (α,β) measurements for every structure, internally the program always uses the plane pole for structure calculations. Also, in order to keep things super simple, only one (α,β) pair is reported - the one with α < 90. This means that the reported (α,β) pair always refers to the "uphole nose" of the ellipse.

When you use a tool for measurements, you typically want to know how much you can trust the measurements to be a true reflection of the real world. When using StereoCore™ PhotoLog for example, if it reports an alpha angle of 56 degrees for a structure, is that in fact true?

When one wants to check the precision and accuracy of a measurement tool, one way of doing this is by doing a bench test. What you do is take a set of measurements using your tool, and you compare them to their actual real world values, or at least, to the closest approximation to their real world values that you possess. The more measurements the better, obviously, as that lets you have confidence that your test results are not a fluke. Also you've got to be careful to test the tool thoroughly - to put it through it's paces. It's all very well if your ruler measures lengths of 1 m very very well, but fails on measurements less than 20 cm.

If your tool's measurements compare well to the known real-world measurements then you're home and dry. If not, you have to find the reason why not - this may lead to improvements in your tool or better guidelines for its use.

How we bench test StereoCore™ PhotoLog

When we first decided to bench test StereoCore™ PhotoLog we had a bit of a problem in that we'd need lots of measurements of lengths and angles in drill core, and we'd need to know that those measurements were accurate, and then we'd need to take photographs of the same core and measure the lengths and angles of the same structures and core segments using StereoCore™ PhotoLog. My major problem is with the goniometers and tape measures that people use to make these measurements - I think they're not accurate enough for testing purposes. 

If I saw a discrepancy between StereoCore™ PhotoLog results and the manually measured results there'd be a temptation (as the guy who wrote the software) to dismiss the manually measured results. It's bad because I can't be sure if the error is due to 
StereoCore™ PhotoLog.

We have actually done this kind of bench test for the first version of StereoCore™ PhotoLog using data we collected from one of the mine projects, and it is important to check that one's measurements do correspond to actual measurements in the real world, but in order to get reference measurements of sufficient accuracy and in order to get enough of them, we resorted to the following sneaky trick: we make photographs of core with known lengths and angles. We do this by generating 3D models of core and structures on a computer. I'm not the most hectic graphics programmer around, otherwise the synthetic photos would have really come out looking awesome, but I just wrote what's called a wireframe engine - and even that was basic for a wireframe engine - which could take pictures of my 3D models which I created.

I've been doing this kind of bench testing for a while, since about midway through development of StereoCore™ PhotoLog. So I've discovered a few more sneaky tricks. Firstly, you as the bench tester mustn't know the lengths and angles you're trying to measure, otherwise you subconsciously fiddle with the results - you can't help wanting them to be more accurate, so instead of for example placing the structure as best you can and matching the curve on the photograph, you place it and fiddle with it until the reported alpha and beta angles match closely to the ones which you know should be there. Secondly randomizing the photographs is the name of the game. The idea is that you want data that matches closely to the kind of data that one would encounter in a real world drill core. So in the real world people take photos from odd angles, and they don't always align the reference line to the top of the core. The structure angles must also be random - this falls under the heading "testing the tool thoroughly" - if I only test a small range of structure angles and the tool works well to measure them, I haven't really checked that it works well in all situations.

I'm wrapping up the first bench test right now, the program has been performing beyond what I expected actually - it's pretty darned accurate. I will post the results shortly :)

Have yourselves a very merry festive season :) 

Regards - Dave (Member of the StereoCore PhotoLog Team)

StereoCore™ PhotoLog 1.0 has been used on several core logging projects ranging from the Philippines to Botswana and Papua New Guinea and has performed to a consistently high standard. The time has come for it to be retired from service. 

StereoCore™ PhotoLog 1.0 will cease to be supported by Ground Modelling Technologies Ltd from 5th January 2015, the date of release of our new and updated software, StereoCore™ 2.0.

Information for customers currently using StereoCore™ PhotoLog 1.0

Customers who wish to continue using StereoCore™ PhotoLog 1.0 may continue to do so, and everything will continue to work as before, but we recommend using StereoCore™ PhotoLog 2.0 for all new core logging projects. For customers who wish to be able to access their archived projects made using StereoCore™ PhotoLog 1.0, the software will be available on request.

StereoCore™ PhotoLog 2.0 has had a complete rewrite of many core aspects of the functionality based on lessons learned from StereoCore™ PhotoLog 1.0 and user feedback. Because of this StereoCore™ PhotoLog 2.0 projects are incompatible with StereoCore™ PhotoLog 1.0. We recommend that existing StereoCore™ PhotoLog 1.0 projects are archived, data exported to your mine database, and that further logging is done with StereoCore™ PhotoLog 2.0.