Some thoughts on Passive Seismic aka Ambient Noise Tomography...

Some thoughts on Passive Seismic aka Ambient Noise Tomography (ANT)

Passive Seismic would appear to hold massive potential for TG6 but to understand that potential one needs to have some idea what it is and how it works. The following is pieced together primarily from a couple of longer presentation then Q&A videos. Passive seismic is reliant on accurate measurement of the seismic waves, very accurate timing of those waves at different geographic points and massive use of computing power. Developments over the last few years have improved at least two of these (Computing and timing crystals) lowering the cost and improving the accuracy around what is possible. One video was mentioning the cost of studies and they were sub A$100k so this is cost effective if it provides useful information.

Seismic waves are created by many natural and man made events. Among the better one's are nearby drilling, blasting, heavy trains and heavy trucks like ore road trains. Natural events include waves crashing onto the coastline and earthquakes. While big earthquakes are rare in Australia, there are smaller earthquakes and the shockwave from distant earthquakes.

The passive system works by setting down an array of sensors. When a shock wave is recorded, the relative time it was recorded at different sites tells you the direction it came from. Knowing this, you can then pretend the first sensor it passes through triggered the seismic wave and then the next sensor receives this signal sent by the first. Through a lot of maths, passive seismic signals are being used as if the first sensor was sending an active seismic signal. The cost is however massively cheaper than active seismic.

Several other key points about passive seismic:
>The seismic wave frequency provides an indication of what's happening at different depths. Low frequency provides an indication of deeper structures. Higher frequency seismic tells you about nearer to the surface. You can therefore get a sense as to horizontal slices by changing the measurement frequency (assuming waves of that frequency exist to be measured)
>Waves travel through different materials at different speeds. Typically the denser the material, the faster the seismic wave travels (I'm not sure if there are exceptions)
>The more sensors in the array, the more "pairs" that exist creating a potentially better image
>Passive seismic can work in that critical 50m-200m zone which has robust open pit potential
>The work Core lithium did with Fleet Space indicated that their Pegmatites had a sufficiently different density from surrounding rocks to be identified
>The combination of lots of surface location pairs and different frequencies enables the construction of a 3D sub-surface map
>The more different directions that the passive seismic signals come in from, the better the 3D map
>Thicker pegmatites are going to have more ability to be identified than thin pegmatites

Finniss region companies using ANT are reporting velocities in the low 3000's range so its the P or pressure waves that they are measuring. A 300m apart sensor will have the wave taking about 1/10th of a second to pass between the two points. Accurate time measurement is essential. At Finniss, the pegmatite had a lower velocity than the surrounding host rock.