Soil gas measurements as an indicator of volcanic activity at Popocatepetl, Mexico

It is well known that anomalously high levels of soil gases such as radon, carbon dioxide and helium can be measured over faults or fractures. This ty pe of measurement is a useful tool to determine the structure of many types of geological environment, including volcanic edifices. It is important th at other controlling factors are considered, principally changes in the met eorological conditions, since they can have a strong influence on the conce ntrations measured. In the case of volcanoes, additional information can be obtained on mass gas movement and magmatic resurgence. A temporal variatio n in the concentration of soil gases has been observed as a precursor to bo th earthquakes and volcanic activity. However, few long-term studies have b een carried out. In the case of volcanoes, measurements are taken at a dist ance from the active crater, making it a much less hazardous form of monito ring than the direct sampling of fumaroles. Recently Popocatepetl has comme nced a nem active phase with several explosive events producing ash falls a t large distances from the volcano. Measurements of SO2 and CO2 flux have s hown a large variation. For a better understanding of the processes occurri ng within the volcano, as well as its structure, further data of different types is required. This study mill include an extensive program of soil gas measurements, including radon, carbon dioxide, methane and helium. A compa rison will be made with seismic and groundwater and ash geochemical data. O ne goal will be an improved understanding of the mechanism whereby seismic events influence the concentration of soil gases, which is not currently we ll understood. Here the preliminary results are presented.