GNGTS 2014 - Atti del 33° Convegno Nazionale

Geothermal fluids monitoring by time lapse electrical resistivity tomography: the Pisciarelli distribute degassing testsite (Campi Flegrei - Southern Italy A. Fedele 1 , M.G. Di Giuseppe 1 , A. Troiano 1 , R. Somma 1 , R. Esposito 2 , T. Caputo 1 , D. Patella 3,1 , C. Troise 1 , G. De Natale 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Sez. Osservatorio Vesuviano, Napoli, Italy 2 Federal University of Ceará, Fortaleza, Brazil 3 Dipartimento di Fisica, Università Degli Studi ‘Federico II’, Napoli, Italy Introduction. The Pisciarelli area (Campi Flegrei) is characterised by an active distribute degassing site , which is affected by near- surface secondary processes of seasonal character that look as if they mask the deeper signals related to the temperature-pressure changes occurring in the shallow feeding hydrothermal system. Starting from 2003, the Pisciarelli field has experienced an evident increase of activity, which has been marked by a sequence of temperature peaks of the ephemeral fumaroles well above the average background temperature of 95 °C, each lasting up to half a year until early 2011, and exceptionally about one year, from mid-2011 to mid-2012, the last recorded peak. Furthermore, a nearly linear trend of the peak temperatures, from about 97 °C up to around 112 °C, has been recorded from 2003 up to date. The increase of degassing activity has also been marked by the opening of new vigorous vents and enlargement of degassing pools, also accompanied by intense local seismic activity and ground uplift. In order to evaluate the fluids dynamics in the shallow hydrothermal systems and to interpret its eventual periodicity, we perform DC electrical tomography at fixed period of time. The repeated Electrical Resistivity Tomography (ERT) should allow us to identify the main end member sources such as the meteoric component, and/or the eventual evolution of the deep fluids. ERT recently is receiving an increasingly interest in environmental and hydrogeological studies to well suited to 2-D and 3-D field data acquisition and interpretation, and can be adapted to various scales. Time-lapse ERT consists in performing an identical ERT survey several times in the same place, to characterize different hydrological/geothermal process during time. Time-lapse ERT can also be used to monitor changes in electrical resistivity linked to groundwater flows, because they create variations in water content and/or water conductivity. Many applications are discussed in literature (e.g.,White, 1994; Daily et al. , 1995; Barker and Moore, 1998; Ramirez and Daily, 2001; Carter, 2002; Slater et al. , 2002; Singha and Gorelick, 2005; Cassiani et al. , 2006; Swarzenski et al. , 2006; de Franco et al. , 2009). However all these experiments are devoted to the use of the ERT for tracer tests or in contaminant hydrology and are characterized by a short monitoring period due to the complexity and problems of long-time instrument operational maintenance. In this work �� ������� � ����� ����������� ���������� �� ���� ����� ���������� ����������� we propose a first geophysical monitoring by time lapse electrical resistivity in a distributed degassing site. A long period (about 12 months) time lapse monitoring will allow us to understand the behaviour of hydrothermal fluids in the lower 20-m of the phreatic aquifer and will supply fundamental evidences on the possible seasonal resistivity fluctuations or whether the resistivity changes are indicative of an increase in volcanic gases present in the hydrothermal system. Campi Flegrei geological background. Located in the Campanian region (South Italy), the Phlegraean Volcanic District (PVD) is a densely populated active volcanic area, including the Campi Flegrei (CF) caldera, the islands of Procida and Ischia, plus a number of submerged volcanoes. Volcanological, geophysical and geochemical evidences (De Vita et al. , 1998, 1999) support the hypothesis that remnants of the magma source feeding the two large eruptive events of Campanian Ignimbrite (37 ky BP) and Neapolitan Yellow Tuff (14.9 ky BP) are involved in more recent volcanic episodes (e.g. Agnano-Monte Spina eruption, 4 ky BP). Volcanic risk GNGTS 2014 S essione 1.3 211