Badgingarra

Magnetic survey (one of the many types of geophysics) basics with a real personal example. Have seen some outrageous magnetic targets and claims posted by some ASX listed companies and thought I'd better do a little post on the basics! You'll notice that when a discovery is made with the aid of magnetics (Julimar, etc.) a million look alike magnetic targets suddenly appear from other companies.😄 One thing I've noticed whilst working in the industry is that there is a big difference between a geologist and a geophysicist and usually, there is a big breakdown in communication between the two. Firstly, magnetic surveys aren't some magic tool used by exploration companies to directly detect orebodies. It's rare that an orebody will be detected directly using magnetics alone. Although several orebodies have been discovered directly with magnetics, its important to understand their limitations. They are a very important tool for mapping geology and structures (faults, etc.). There are some cases where it can detect an orebody directly. The following (with a few exceptions) are the main ferromagnetic minerals that are likely to stick out: Magnetite (Iron ore) Ilmenite (Mineral Sands) Pyrrhotite (Massive Sulfide Deposits) So if say a carbonatite (ferro-carbonatite) is rich in iron (magnetite), it could standout from the background. Same goes for a massive sulfide deposit rich in Pyrrhotite, or a mineral sands deposit rich in ilmenite. Olympic Dam is a good example (see below image). Given it's an IOCG (Iron Oxide Copper Gold), the magnetite within it allowed it to be detected. LCT pegmatites on the other hand (one example of many) can't be directly detected with magnetics in most cases. Some key concepts: TMI - Total Magnetic intensity A magnetometer records a value in Nanoteslas (nT). This Value is known as the TMI. Generally surveys are airborne and are flown in lines. The line spaces are important as it dictates how defined the survey will be. Government data is generally flown at 400m spaces. Private companies may decrease this to 50m-100m across their tenement to paint a more detailed picture, depending on what their goals are. Once the raw data is collected by the magnetometer, a number of corrections are made. I've listed some of them below: RTP - Reduced to Pole Very important correction applied to the data. Notice on the below picture that the magnetic field lines intersect the Earth’s surface at different angles depending on the location? At the equator its 0 degrees. At the magnetic north pole its 90 degrees. So an RTP correction essentially moves the anomaly as if its vertical (90 degrees, like you're at the magnetic north pole as the name suggests). If you don't undertake this correction, your anomaly may appear in 2D view to a different location to where it actually is. You could drill off the target and miss. You'll notice RTP TMI written on the Olympic dam magnetics below. Regional field correction - The earth's magnetic field varies greatly depending on the location. This needs to be accounted for. Diurnal correction - The earths magnetic field varies throughout the day. This change needs to be accounted for. Flight height correction - The earths magnetic field varies with height. This also needs to be accounted for. The height of the airplane needs to be taken into account. Artifact corrections - Things such as houses, sheds, etc. need to be removed from the survey as they may give a false anomaly. Once the data has been corrected, it can then be tweaked further to suit your purpose. 1VD - First Vertical Derivative. This takes a derivative of the of the magnetic data. Without digging into the mathematics, it basically makes shallower magnetic sources more pronounced and masks deeper sources. It looks at the changes in intensity, rather than the absolute highs and lows of the intensity. Short and longwave filtering. Again, without getting too bogged down into the maths, you can process your data to remove the short and long wavelengths. Shallower sources generally give off short wavelengths and deeper sources generally give off long wavelengths. At @NicholsPoint , we made a mineral sands discovery through the use of magnetics. The first step was to do an RTP transformation on the data. Then, we filtered out the longer wavelengths as to remove the deeper sources (mineral sands deposits are generally found quite close to the surface). The results were surprisingly good. As you can see on the left side of the second image, that's the unprocessed RTP data. The right side is processed data removing the long wavelengths. The deposit sticks out like a sore thumb (highlighted within blue box). In summary these are the very basics of magnetic surveys. It gets a whole lot more complex when you start performing things such as depth to magnetic sources (Euler Deconvolution, etc.), 3D modelling, etc.. They are a very important tool in mineral exploration. Orebodies have been detected directly using them, but its important to understand their limitations! Thanks for reading.