# Data¶

In this section we present a summary about the different types of magnetic data. Different assumptions are made depending on the type of instrument used during acquisition.

## Magnetic field data¶

As demonstrated in Fig. 40, the magnetic field measured above the surface is a vector quantity. The magnetic field data measured at any location contains the signal from both the source ($$\mathbf{B}_0$$), as well as the response ($$\mathbf{B}_A$$) from magnetized material:

$\mathbf{B} = \mathbf{B}_0 + \mathbf{B}_A\;.$

In ideal cases, magnetic surveys would measure all three components of the field (fluxgate magnetometer). The magnetic field anomaly, the quantity of interest, is readily available by simple subtraction of the inducing field such that:

$\mathbf{B}_A = \mathbf{B} - \mathbf{B}_0 \;.$

The acquisition of three-components data remains challenging however. The orientation of each components needs to corrected in real-time in order to compensate for sensors rotation. Most surveys measure instead the total strength of the field, or Total Magnetic Intensity data:

$|\mathbf{B}| = |\mathbf{B}_0 + \mathbf{B}_A| \;.$

Since we do not know the direction of $$\mathbf{B}_A$$ we assume that the anomalous field is mostly induced and that it’s direction aligns with the Earth’s inducing field $$\mathbf{B}_0$$. This allows us to approximate the Total Magnetic Anomaly datum:

$B_T = \mathbf{B}_A \cdot \mathbf{\hat B}_0 \;,$

This assumption holds as long as $$\mathbf{\hat B}_0 \gg \mathbf{\hat B}_A$$, which is valid in most cases considering the strength of the Earth’s field.