02 October 2015

Aerial Metal Detecting - Understanding Magnetic Maps

Anybody who has looked at the magnetics basemap on the TasGov ListMap will realise the map shows almost no relationship to topographical features or to google satellite maps. Unfortunately, ListMap does not give the public a way of easily overlaying a magnetics map with familiar geographical landmarks. (if anyone can do this let me know)
It was only after using graphics software to overlay topographic features onto a magnetics map that I was able to begin to understand these maps. I used a temporary layer to line-up the magnetics map with a topographic map. I then deleted the temporary layer. The magnetics map extends into the ocean all around Tasmania but here I cut-out the top left corner in order to accurately scale the topographic overlay.

The map. The white blob is the alumina smelter at Bell Bay. At first I thought the large NNW-pointing magnetic formation was Cabbage Tree Hill but that hill is a lot smaller and to the right of the impressive magnetic feature. The town of Beaconsfield is to the right of this smaller hill. The large magnetic feature is actually an iron/chromite/nickel ultramafic intrusion called the Barnes Hill deposit. On a hand-held metal detector all this ferrous metal would probably be ignored. For exactly this reason magnetic information is often discounted in the search for gold deposits but there appears to be a close relationship.
Apparently Tasmanian gold deposits were originally formed by Cambrium ultramafic intrusions. These intrusions either caused or flowed into fracture faults at the top of the intrusion. Due to subsequent folding and dipping these faults are now often adjacent to the original intrusions. Gold bearing intrusions are always associated with granodiorite in NE Tasmania. Interestingly, the Tasmania Reef is approximately at a 90 degree angle to the Barnes Hill formation. Most of the magnetic formations appear to follow the Earths magnetic field.
I believe it is best to use magnetic information at a local level to help understand local geology. If you would like a magnetic map of a particular area of Tasmania email me. My address is at the bottom of this page.
Disclaimer. I'm not a geologist but enjoy using graphics software to expand my geological knowledge. I'm happy to accept any technical criticism of this article.

I've discovered that highly mineralised deposits like Barnes Hill are fairly toxic to trees and plants all over the world. My observation is this is true in the Beaconsfield area. The trees look quite stunted in this particular area. I have even seen a failed tree plantation in the whitish area above. With a bit of basic geological research they may not have wasted money on that particular venture.

3 comments:

Ryan Perro said...

Are you able to use this in areas outside of where you are? I'm looking to have this done in Maine, USA. Thanks! Feel free to shoot me an e-mail

Anonymous said...

Hi Ryan, This looks like a basemap you could use:
http://pubs.usgs.gov/of/2004/1258/
The towns, roads etc map goes over the top, scaled accurately and made partly transparent.
Any graphic designer could do it in half an hour.
Cheers

Brendan Midson said...

wow bro, you know your shit! what sort of detector are you running? keen to meep up some time for a chat and someone to find some yella with!