Imaging the density profile of a volcano interior with cosmic-ray muon radiography combined with classical gravimetry

Abstract

Cosmic-ray muon radiography has the potential to reveal the density structure of gigantic objects. It utilizes the strong penetration ability of high-energy muons. By measuring the number of muons that travel through a target object, the average density can be calculated along the muon path. Since muons travel in straight paths through matter, specially designed detectors can generate density maps with higher spatial resolution than those obtained with conventional geophysical methods. However, this technique has a few notable limitations in that it can only be applied to near-surface structures above the muon sensor and strongly depends on the characteristics of the local topography. This is due to the fact that almost all cosmic-ray muons arrive only from the upper hemisphere. Geological structures, e.g. volcanoes, that allow for muon detectors to be placed on a slope directly below the point of interest are thus the best candidates for this technique. The drawback of muon radiography that only the horizontally integrated density above the sensor is measured with a time resolution larger than several weeks may be partly remedied by combining its results with gravity data, as they are both sensitive to target density while complementary to each other in several aspects. An example of such a combination is presented: real-time monitoring of magma head height in a volcano conduit.