Geochemical and geophysical monitoring in tectonically active areas of the Po Valley (Northern Italy). Case histories linked to gas emission structures

Abstract

In some tectonically active areas of the Po river plain the ground surface is locally affected by collapse phenomena linked to gas escapes. In order to better identify these «pockmarks-featured» structures whose occurrence seems to be proof of an eruptive seepage at a high flow rate, between 1996 and 1998 the main geochemical trends in 15 selected waterwells were investigated and geophysical prospections and levelling surveys were performed. In the wells, whose depths range from about 5 to 70 m, some geochemical anomalies have been detected suggesting mixing phenomena among fresh shallow waters and brackish deep-seated waters. Refraction seismic data and seismic tomography highligted the first 50 m and revealed possibly tectonic disturbance at a depth of about 30-35 m. Topographic survey allowed to recognize ground deformations consisting of some steps generated along straight lines. Data collected about local earthquakes occurred in the study area should indicate a relation between seismic events and the anomalous behaviour observed in the geochemical parameters detected in the monitored wells. Thus a possible relation between all the observed phenomena and geodynamic factors, probably linked to actual evolution of the apenninic chain structural arc, can be inferred.