Release of geogenic gases as a signal of deformation in rock

摘要

Helium and argon are represented by known amounts in air. Helium is 5.2 ppm by volume in the atmosphere and primarily the result of the natural radioactive decay of heavy radioactive elements. Argon is the third most abundant gas in the Earth's atmosphere, 9340 ppm; radiogenic argon-40, is derived from the decay of potassium-40 in the Earth's crust. The isotopic signature of noble gases found in rocks is vastly different than that of the atmosphere which is contributed by a variety of sources. Geogenic noble gases are contained in most crustal rock at inter and intra granular sites, their release during natural and man-made stress and strain changes represents a signal of deformation. When rock is subjected to stress conditions exceeding about half its yield strength, micro-cracks begin to form. As rock deformation progresses a fracture network evolves, releasing trapped noble gases and changing the transport properties to gas migration. Thus, changes in gas emanation and noble gas composition from rocks could be used to infer changes in stress-state and deformation. An experimental system we developed combines triaxial rock deformation and mass spectrometry to measure noble gas flow real-time during deformation. Geogenic noble gases are released during triaxial deformation and that release is related to volume strain and acoustic emissions. The noble gas release then represents a signal of deformation during its stages of development. Gases released depend on initial gas content, pore structure and its evolution, and amount of deformation imposed. Noble gas release is stress/strain history dependent and pressure and strain rate dependent. Sensing of gases released related to both earthquakes and volcanic activity has resulted in anomalies detected for these natural processes. We propose using this deformation signal as a tool to detect subterranean deformation (fracture).