Have you ever wondered that we can categorized rock based on its magnetism? Actually I never thought of that, until I stumbled upon a lovely course of Environmental Measuring and Monitoring. It was really hard and foreign for me. Even I am not sure whether I will use the gained knowledge from this course in the future or not. It is too much of geology things. Okay, let’s stop with my personal story. Let’s go back to rock magnetism again!
So, rocks have magnetic minerals, though it is only a small fraction in the rock. But, this small portion determines the magnetic properties and the magnetization of the rock as a whole. We can say that the magnetic properties of rocks arise from the magnetic properties of the constituent mineral grains and crystals.
How much is the magnetism value? The magnetic properties within a rock type can be quite variable (Carmichael, 1989) depending on chemical inhomogenity, depositional and/or crystallization, and post-deformational conditions. Just googling in the internet about susceptibility value of rocks in SI unit and you will tables and graphs you need. This is one example by Hunt, Moskowitz and Banerjee (1995).
But, somehow, we can also get negative susceptibility value. Why? Negative values indicates diamagnetism (only in very pure materials, since diamagnetic moments are nearly always swamped by any paramagnetism present).
What is the things that influence the magnetic susceptibility? The values of magnetic susceptibility depend on the grain size, the presence of minute crystal lattice, such as dislocations, lattice vacancies, impurities, etc., and an amount of iron ore in a sample.
Some notes. The susceptibility of a rock usually depends on its magnetite content. Sediments and acid igneous rocks have small susceptibilities whereas basalts, dolerites, gabbros and serpentinites are usually strongly magnetic. Weathering generally reduces susceptibility because magnetite is oxidised to hematite, but some laterites are magnetic because of the presence of maghemite and remanently magnetised hematite.
What is the purpose of doing susceptibility measurements? Magnetic susceptibility analyses found various applications in different fields of geophysics including geophysical prospecting, mineral exploration, palaeomagnetism, archaeology, rock magnetism and environmental magnetism.
We can also observe the areal distribution of formation magnetic anomalies. Linear anomalies thought to be the expression of fracture zones and non-linear anomalies attributable to local structural relief.
How to make the measurements? We use the equipment just like in the picture. Make sure to put the coil high up, so that the background is not disturbing, and the value in the machine is zero. We get the susceptibility value in SI unit.
How to do analysis from field measurement data? We collect enough measurement points data which we can make statistical and spatial analysis from it. Don’t forget to note the GPS data of each measurement. Then, we start to work with GIS software, in which we import the data as points data with XY coordinate and its susceptibility value. We can do statistical analysis with histogram. We can analyse the minimum, maximum, mean, and standard deviation value. We can also see whether there is outliers or anomalies in the data. Concerning spatial analysis, we can do it with Kriging interpolation which create a smooth surface by interpolate the points data values. Lastly, we can categorize the rock and mineral type based on its susceptibility value.
Bedrock to be analysed in the field:
Limitation of field measurement? It would be best to do sampling in systematic way. For example, one point per grid. Random sampling will bring limitation as it won’t give data is some grids and so not give the best representation. Especially, if the surveyor doesn’t have enough experience and he end up skipping the potential targeted mineral area.