RESEARCH ARTICLE Electrical resistivity tomography imaging of the near-surface structure of the Solfatara crater, Campi Flegrei (Naples, Italy) M. G. Di Giuseppe & A. Troiano & A. Fedele & T. Caputo & D. Patella & C. Troise & G. De Natale Received: 7 July 2014 /Accepted: 7 February 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract We describe the results from an electrical resistivity tomography (ERT) survey performed inside the Solfatara cra- ter, located in the central part of the Campi Flegrei (CF) com- posite caldera. The Solfatara volcano represents the most ac- tive zone within the CF area, in terms of hydrothermal mani- festations and local seismicity. Eight dipole-dipole ERT lines have been measured with the aim of deducing a 3D resistivity model for the upper 80 m beneath the Solfatara. The results have allowed classification of the shallow structure below the crater into a low-resistivity (LR) class, up to about 4 Ωm, an intermediate resistivity (IR) class, from 5 Ωm up to 50 Ωm, and a high-resistivity (HR) class, from 60 Ωm onward. In order to solve the ambiguities arising in the interpretation of the nature of these bodies, a comparison has been done be- tween the 3D ERT model and the CO 2 flux, soil temperature, and gravity maps over the same area. By combining all of these parameters, the whole LR body has been ascribed to a water-dominated geothermal basin and the HR body to a steam/gas-dominated reservoir. Finally, the IR class has been interpreted as a widespread background situation with inter- mediate character, where volatiles and condensates can coex- ist in the same volumes at variable percentages, coherently with the resistivity variation within this class. Since fluid dy- namics in the Solfatara crater change rapidly, ERT surveys repeated in the future are expected to be of great help in mon- itoring possible pre-eruptive changes, as well as in better fol- lowing evolution of the local geothermal system. Keywords Campi Flegrei . Solfatara crater . Near-surface structure . 3D resistivity tomography Introduction The Campi Flegrei (CF) caldera (Fig. 1) was formed by huge eruptions 39,000 and 15,000 years ago (Rosi and Sbrana 1987). Vertical ground movements with rates from centime- ters to meters per year characterize the dynamics of this area even during quiescent periods (Dvorak and Mastrolorenzo 1991). Since 1969, the area has been in a new phase of uplift after several centuries of subsidence dating back to 1538, when the last eruption occurred in the area (Di Vito et al. 1987). The most recent episodes of intense ground deforma- tion are the two unrests of 1970–1972 and 1982–1984, caus- ing a cumulative maximum uplift of over 3.5 m, accompanied by intense seismicity. All of the recent literature on the inter- pretation of such uplift episodes points out the driving role that fluid dynamics and related phase transformation processes have in the area, especially where hydrothermal manifesta- tions are most evident (e.g., De Natale et al. 1991, 2006; Chiodini et al. 2003; Todesco et al. 2003; Troiano et al. 2011). The Solfatara crater (Fig. 1) represents the most active zone within the CF caldera. Its activity has long been considered a direct indicator of the volcanic dynamics taking place in the whole caldera. The crater has in fact been the site of intense hydrothermal activity since Greek times, and currently ex- hibits impressive degassing manifestations. A direct relation- ship has always been observed between increases in hydro- thermal activity in the Solfatara area and ground uplift in the CF caldera. As is well known, geophysics has a prominent role in vol- canology. The geophysical mapping of the Solfatara structure is a crucial step for improving our ability to forecast pre- eruptive scenarios, as well as for estimating its geothermal Editorial responsibility: T. Nishimura M. G. Di Giuseppe (*) : A. Troiano : A. Fedele : T. Caputo : D. Patella : C. Troise : G. De Natale Istituto Nazionale di Geofisica e Vulcanologia (INGV), Osservatorio Vesuviano, Naples, Italy e-mail: mariagiulia.digiuseppe@ingv.it D. Patella Department of Physics, University Federico II, Naples, Italy Bull Volcanol (2015) 77:27 DOI 10.1007/s00445-015-0910-6