Experimental Study of Acoustic Emissions from Boiling as a Possible Source of Geothermal Ground Noise

Abstract

Acoustic emissions produced by single collapsing steam bubbles were studied in a laboratory simulation of local (subcooled) aquifer boiling. Local boiling was simulated by injecting steam at low mass flux into a pressure vessel filled with water. Direct contact condensation of the steam produced a train of individual collapsing steam bubbles which generated strong acoustic pulses. These pulses were measured with a hydrophone submerged in the pressure vessel. The dependence of the bubble pulse acoustic spectrum on the degree of water subcooling (or equivalent, collapse pressure) and bubble size was investigated. Within the limited range of pressures, and bubble size tested, bubble diameter, most of the free field acoustic energy generated fell in the kilohertz frequency range. A preliminary comparison of the results with an earlier developed theoretical model for boiling generated seismic emissions indicates that model is probably adequate of making order of magnitude estimates of expected seismic output from aquifer boiling, after a correction suggested by the experimental results has been made.