| Abstract |
Phreatic explosion craters such as those recognized in the Larderello area, or the recently reconstructed history of repeated paroxysmal events in The Geysers, plus as yet unconfirmed evidence of phreatic explosion crater (oral comm. J. Hulen), point out a continuous set of states of nature ranging from stable steady to changing states and beyond, to paroxysmal events up to mechanical destabilization of the crust (large explosive eruptions) in natural systems from geothermal (hydrothermal) to strictly speaking volcanic ones. A class of conceptual models that appear to aptly describe states of nature intervening between mechanically stable steady states and fully unstable states is presented in this paper. Attention is focused on analytical rather than numerical models, i.e., on representing the basic features under as general as possible conditions rather than defining details of particular cases, considering at once as broad as possible classes of models, rather than going after detailed modelling of some particular system. This choice fits in with the requirements of the Bayesian approach in the presence of no or limited information on the specific system under consideration, re principle of simplicity. Transfer of mass and/or energy (as work and thermal energy) from deep in the Earth to its surface requires some build up of energy at depth, which can bring the considered natural system to instability and to sustain changes by internal forces. The theory shows that energy build up in the subsurface (in the form of thermal energy, fluid pressure, volume increase and related stress etc build up) can give place to heat-stress (and for that matter, porosity-permeability etc) solitary waves that travel to the surface. In particular, the pressure gradient generated by the wave is as more destabilizing as closer to the surface and to gaseous state is the involved fluid. Waves and related pressure gradient profiles range from peak to step-like (from "shock waves" to "solitons") depending on whether the energy build up is peak or step-like, and so carry with other information on the state of the system at depth, hence about subsurface conditions and their change. To any extent that solitary waves are forerunners of change towards unstable mechanical conditions their theory is relevant to forecasting volcanic explosions. With regards to geothermal systems, solitary waves may signal underground energy build up, e.g., shed light on energy transfer to the system from its thermodynamic Universe. More generally concerning observable information, it hints to some of many ways that monitoring both apparent and subtle mass and energy surface emission may contribute to the understanding of what goes on in the subsurface. Closer discussion of such a subject is premature as long as the practice remains limited to considering only well data and systematic investigation of the distribution of surface emissions is hardly carried on. |