Abstract: In this chapter, I discuss an alternative perspective on interpreting the results of joint and constrained inversions of geophysical data. Typically such inversions are performed based on inductive reasoning (i.e. we fit a limited set of observations and conclude that the resulting model is representative of the Earth). While this has seen many successes, it is less useful when, for example, the specified relationship between different physical parameters is violated in parts of the inversion domain. I argue that in these cases a hypothesis testing perspective can help to learn more about the properties of the Earth. I present joint and constrained inversion examples that show how we can use violations of the assumptions specified in the inversion to study the subsurface. In particular I focus on the combination of gravity and magnetic data with seismic constraints in the western United States. There I see that high velocity structures in the crust are associated with relatively low density anomalies, a possible indication of the presence of melt in a strong rock matrix. The concepts, however, can be applied to other types of data and other regions and offer an extra dimension of analysis to interpret the results of geophysical inversion algorithms.

In Chapter 6.1 we briefly review the main instruments used in today’s ground-based geomagnetic observations, focusing on their performances and working principles (from a user’s point of view). Next, the major measurement methods and systems currently in use will be introduced, with a focus on the latest developments in the field. In Chapter 6.2 electromagnetic (EM) methods will be discussed to study the electrical conductivity structure within Earth in a wide depth range and can be measured at the Earth’s surface by magnetometers and telluric electrodes. In Chapter 6.3 a new technique based on differences in instrument responses from ground-based magnetic measurements that extracts the frequency content of the magnetic field with periods ranging from 0.1 to 100 seconds will be discussed. This method enables the study of field line oscillations using the publicly available, worldwide database of geomagnetic observatories.