Direct measurements of the Earth's magnetic field are continually made around the world at magnetic observatories, and are obtained at all kinds of exotic locations from various oceanographic, land, aircraft and even satelitte surveys. All of this activity has caused some people to wonder "why all the effort?". There are two obvious reasons that stand out to me. In the study of the Earth's history, very few physical quantities have left any record that can be measured today. The study of seismology and the Earth's gravity field have revealed many secrets about the Earth's interior, but there is no timeline in which to examine how these measurements may have, or are, changing with time. Measurements that can be examined through time are extremely important to make inferences about what the Earth may have been like in the past.
But the major reason magnetic measurements are recorded with such intensity is not so holostic. Scientists run magnetic surveys to attempt to model what the structure is like beneath the ground. When a person goes out into the field and measures the magnetic field, what they actually measure is a combination of the Earth's background magnetic field and variations that are being caused by near-surface changes in the magnetic properties of the underlying rocks. Although most rock-forming minerals are effectively non-magnetic, many rock types contain sufficient magnetic minerals to produce significant magnetic anomalies. Also, man-made objects (iron bearing) frequently generate large magnetic anomalies. Magnetic surveying can thus be used for a broad range of applications, from small-scale engineering or archaeological surveys, to large-scale surveys to investigate regional geological structure.
