Information about conditions at a volcano vent can be extracted from observations of atmospheric shock waves. An improved understanding of the burst phenomena that generate shock waves and the dependence upon volcano shape and eruption properties may lead to new methods to access information on the eruption intensity as well as generate the initial conditions required in a buoyancy predictive models of the gas/ash plume dispersion. This EaGER project is a one-year feasibility study that will investigate experimental simulation of a volcanic explosion using a unique shock tube apparatus.
Use of atmospheric shock propagation to gauge explosive power and eruption temperature is new and untested. If successful, atmospheric shock propagation has the potential to radically improve the accuracy of measuring explosive power. At the conclusion of the project we anticipate being able to definitively address concerns regarding the feasibility of a shock tube experiment to scale volcanic explosions. The theory developed will enable more precise prediction of explosive power derived from atmospheric shock wave propagation. The proposed research is interdisciplinary and represents a new collaboration between geophysics and mechanical engineering at Michigan Tech. It includes an important opportunity for the postdoctoral investigator to establish his career. The project will address an area of high risk: the feasibility of scaling volcanic eruption with a shock tube experiment.
This project investigated the generation and propagation of atmospheric shockwaves induced by explosive volcanoes. At present there are only limited theoretical studies and a limited number of field measurements of volcanic-induced atmospheric shock waves. Yet a wealth of information about conditions at the vent during the initial stage of an explosive eruption can be extracted from shockwave observations. Thus, the scope of this project was to develop an experimental model of the burst phenomena that generate shockwaves and the dependence on the volcano eruption properties specially the total energy released to the atmosphere. An improved understanding of these phenomena may lead to new methods for accessing information on the eruption intensity and eventual plume high based on the observations of the shockwave in real time. An open-end shock tube facility was developed using a modified Split-Hopkinson pressure bar test gun to generate spherical shock waves as shown in in Figure 1. The open-end shock tube was equipped with a high speed camera utilize a Photron APX-RS imager capable of 3000 fps full-field up to 250,000 fps reduce field in a shadowgraph array and a high response, high amplitude pressure transducer from which the spherical shock waves imaged and the pressure fluctuations were recorded, Figure 2. The shock tube experiment conditions were scaled to simulate the potential conditions occurring during an explosive volcanic eruption. From the experimental observation, the shock wave propagation speed and pressure decay versus distance from the beginning of the expansion were measured and compared to similar predictions from the strong shock wave theory (Figures 3 and 4). This unique set of measurement allows calculating a correlation between pressure and distance for weak shock waves which can be directly compared with the infrasound measurements recordings on explosive volcanic eruptions. From the comparison of the experimental observations of expanding weak shock waves with the prediction from the strong shock wave, we concluded that despite the differences, the strong shock wave theory may still be used as first approach for modeling weak shock waves induced by explosive volcanoes. However, an improved modeling scaling for weak shock waves is needed. The strong shock wave theory was applied to field measurements of historical explosive eruptions to calculate the energy released during the eruptions. Two different methods were used to calculate the energy released based on the available field measurements, infrasound or video recordings. From the infrasound measurements, the pressure at the vent of the volcano was obtained by extrapolation. The energy released was calculated using the strong shock wave correlation between detonation energy and pressure (pressure at the vent). Another option to calculate the energy released was by calculating the shock wave speed from the video recordings. Using a proper imaging processing, the shock wave position for every corded frame can be obtained and converted into speed, Figure 5. The energy released can be now calculated using the correlation between the strong shock wave speed and detonation energy. A good correlation was found between the calculation of the energy released by the eruption applying the strong shock wave theory the infrasound or video recordings and the standard method to calculate the energy released based on plume height as shown in Table 1 of Figure 6. The good correlation between both methods of estimating the released energy implies that it is possible, with a more detailed study, to estimate plume heights based on the strong shock wave theory as an early warning system. The understanding of weak shock waves induced by volcanic eruptions can improve warning times for large volcanic plumes, which have the potential to be much more dangerous. A rapid assessment of eruption plume height is critical for aircraft safety and is not readily available at remote volcanoes monitored only by sparse seismic networks and satellite imaging. The modeling developed in this project can allow for rapid calculations the plume height based on infrasound measurements, rather than observed by satellite, airborne, or ground-based observations once the plume has already ascended. The improved understanding of weak shock characteristics, have also direct impact on volcanic eruption hazard analysis. Shock waves have been documented to cause damage to structures within 5-10 km of volcanoes and must be better understood in order for an accurate assessment of their hazard.
