Reflectorless Prisms

27 October 2010

I had a good look at the reflectorless results on site and I have discovered an interesting phenomonen that is affecting the results. Autoslope records the new position of a prism after each recording so that it searches for the prism in that updated location the next time. This is so that prisms in fast moving environments continue to be tracked when they move and do not eventually move outside the instruments searching footprint. However what I am seeing with reflectorless measurements is that the position is drifting away from its original position at an accelerated rate. It creates a trend that looks like real movement but it is not real movement at all because real prisms in a similar area are only moving very slightly. Also some reflectorless prisms are drifting away more than others.

What is causing this is the angled surface of the rock that is being measured to. Imagine shining a torch against a wall on an angle and you would see a small circle of light close to the torch but the circle would become larger and less bright as it moves away from the torch. If you were to shine the torch at the wall from directly in front (right angle) the circle of light would be nice and evenly round. Reflectorless measurements use light in the same way so what is happenning is the theoretical position of the prism is moving away slightly each time the prism is read and its position is updated.

So how do we fix this??? Well the first thing I would like you to do is stop reading the reflectorless points you have in the system already as I don’t want them to keep moving until they are lost or not positioned well. In the meantime we will look at modifying the way Autoslope handles reflectorless positions so that the position is not updated each time it is measured. We should get much better repeatability as a result. This might take a bit of work so please patient while we work through it but certainly the potential of the reflectorless monitoring is there when the right conditions are in place. I think this will prove to be a great tool once we sort out the repositioning effect. It is the first time it has been used in a real mining environment so we didn’t know how it would work out but initial results look really promising guys.

Clint van der Loon