An exploration geophysicist asked us a couple of days ago, "What is this Magnetization Vector Inversion,or MVI, that Geosoft has recently released and that is generating so much interest?" We replied that Geosoft has online information about MVI, but on later reflection realized that there is still a need for a quick overview of the physical principles driving MVI, which leads us to this blog post.
We should first appreciate that geophysical methods aim to determine physical properties of rocks within the earth, and with modelled rock properties we can make better exploration decisions. The magnetic method in particular is sensitive to the magnetization property of rocks. Some rocks are more "magnetic" than others, and it is this contrast that we are able to map and model in 3D with the magnetic methods.
The physical principle behind Magnetization Vector Inversion is that magnetic material in the Earth is affected by the presence of the Earth's geomagnetic field. Basically, the Earth's field causes a perturbation of the magnetic domains in magnetic materials, for example, in rocks containing traces of magnetite. We can picture these magnetic domains as small magnetic dipoles which, in the absence of the Earth's geomagnetic field, might be oriented more or less randomly as shown in the upper layer of Figure 1, or in a more ordered manner as shown in the lower layer. In the presence of the Earth's geomagnetic field, the magnetic domains will orient themselves in some more ordered arrangement like that shown in Figure 2. The actual ordering is controlled by a number of interacting magnetic effects, including for example, induction, remanence, anisotropy, and demagnetization, amongst others.
Figure 1: In the absence of any external magnetic field, the magnetic domains (represented by green cones) might be oriented more or less randomly. The ordering of the magnetic domains depends on conditions during the rock's formation and subsequent history
Figure 2: In the presence of the Earth's geomagnetic field (represented by the red cone) the magnetic domains shown in Figure 1 will align themselves in a more ordered fashion.
Now, in terms of Magnetization Vector Inversion in VOXI, we invert magnetic field data to recover both magnetization direction and amplitude resulting in Earth models exactly like that shown in Figure 2. That is, the output from MVI is a set of vectors giving the directions and amplitudes of any magnetic domains in the survey area.
Many geophysicists are familiar with Susceptibility Inversion so it is worthwhile comparing Susceptibility Inversion with Magnetization Vector Inversion. Susceptibility Inversion is based on the premise that, in the presence of the Earth's geomagnetic field, the magnetic domains in all rocks orient themselves in the parallel to the Earth's geomagnetic field. That is, the domains shown in Figure 1 are hypothesized to orient themselves like that shown in Figure 3. When the magnetic domains all tend to align themselves with the Earth's geomagnetic field we have a mathematically simple situation, albeit quite unrealistic in the Earth. In this special situation we can take the ratio of the amplitude of the magnetization and normalize it by the Earth's geomagnetic field to yield the "susceptibility". The net magnetization direction associated with the magnetic domains is, then by definition, the Earth's geomagnetic field direction.
Figure 3: In Susceptibility Inversion, the presence of the Earth's geomagnetic field (represented by the red cone), is assumed to cause the magnetic domains to align parallel to the external field.
Susceptibility is a useful quantity for describing the magnetic properties of a limited class of homogeneous isotropic materials, however, it is well known that the Earth is composed of complicated, heterogeneous material on a wide range of length scales. This makes it much more likely that the magnetic domains in Earth material will not be aligned with the Earth field direction and that susceptibility inversion might give a first approximation to the true Earth, but will fail in many Exploration situations. Nevertheless, because susceptibility is a well-studied and tabulated measure for homogeneous laboratory samples, in VOXI output we normalize the amplitude of the magnetization vectors by the Earth's geomagnetic field. With this normalization, the amplitude of the vectors will be equal to the corresponding susceptibility in the special case that the Earth was indeed homogeneous and isotropic, that is, if magnetic domains were aligned with the Earth's geomagnetic field.
In summary, based on the physical properties of rocks, Magnetization Vector Inversion is designed to recover both the amplitude and direction of magnetic domains and therefore provides an accurate and robust description of the magnetic properties of rock in the Earth. Experience to date demonstrates that Magnetization Vector Inversion supports effective and efficient Exploration.