Abstract: 

In this presentation we wish to give a brief overview of the applicability of so-called cosmic rays as a passive source for both 3D or 4D bulk-density mapping of the subsurface.  Naturally occurring cosmic radiation is a well-known, penetrating nuclear probe that, analogously to X-rays, can be used to infer a density map of the subsurface by using suitably designed and well positioned nuclear detectors. In most practical scenarios, however, one must deal with the fact that this flux, while free, is not constant in time nor in space, and that the measured data-sets are typically incomplete, i.e. they cannot be easily inverted to provide a voxel by voxel density map. This has typically limited the application space to the detection of gross features, such as large voids in ancient pyramids.  Nonetheless, when more properly viewed as an aid to benchmark geological models or to monitor dynamic density or saturation changes in the underground, cosmic-ray density measurements appear to be well suited for a number of practical applications in mining, civil engineering as well as oil production. Historically, another practical obstacle has to do with a lack of detector configurations suitable for borehole deployment, however we will show that some are indeed possible at a relatively low cost by integrating off-the-shelf components. The necessary trade-offs between detector and installation costs, observation time and spatial/temporal resolution will be highlighted and results from a few exemplary test-case studies will be presented.