ABSTRACT
Microgravity is becoming increasingly recognized as a valuable tool for mapping out the subsurface mass redistributions that are associated with volcanic activity. Microgravity monitoring when combined with ground deformation and other geophysical surveillance techniques has been shown to be a valuable tool for understanding the processes occurring within a volcano before, during and after eruption. This chapter considers the instrumentation required for the various types of gravity survey that may be undertaken, and the suitability of microgravity surveys to a range of volcano settings. Detailed interpretations depend on the local situation, but generally it is clear that andesitic volcanoes tend to exhibit the largest microgravity changes prior to eruption, basaltic volcanoes undergo passive magma migration with only limited ground deformation and rhyolitic centres tend to behave more in the classic elastic way. For a Bouguer gravity survey, data are made comparable with the base or reference station firstly by correcting for elevation differences using the free-air gradient.
