Reservoir Structure and Hydraulic Properties of the Campi Flegrei Geothermal System Inferred by Audiomagnetotelluric, Geochemical, and Seismicity Study

Abstract

The Campi Flegrei caldera is a large volcanic complex lying in the Campanian Plain, Southern Italy. During its history the caldera experienced episodes of bradyseism and intense swarm seismicity. The mechanism leading to unrest episodes is still debated, and great efforts are ongoing to improve the knowledge of this structure and its evolution due to the high volcanic risk in such a densely populated area. Here we present a resistivity model from a two‐dimensional inversion of audiomagnetotelluric data acquired along an approximately 5.6‐km long profile crosscutting the Solfatara‐Pisciarelli district and the Agnano plain. The resistivity model shows (1) very low resistivity values confined in the first 500 m of depth both in correspondence of the Solfatara‐Pisciarelli districts and the Agnano depression; (2) a resistive plume that extends underneath the Solfatara crater down to 2,000‐ to 3,000‐m depth, and (3) an adjoining relative conductive unit eastward. We discuss the resistivity structures in a multidisciplinary framework integrating inedited geochemical and seismological observations with existing surface geology and subsurface information. The Solfatara‐Pisciarelli district and the Agnano plain, both being expression of intense hydrothermal activity, show different characteristics. Below the Solfatara‐Pisciarelli area, the shallow conductive zone is interpreted as a faulted clay cap that overlies a highly active vapor‐dominated reservoir characterized by a convective mechanism. Conversely, below the Agnano plain, a liquid phase seems to prevail in the reservoir. The spatiotemporal variations of seismicity imply a combined action of preexisting tectonic lineaments and fluid interaction between the gas/steam reservoir and the outflow zone.

Feasibility to Use Continuous Magnetotelluric Observations for Monitoring Hydrothermal Activity. Numerical Modeling Applied to Campi Flegrei Volcanic System (Southern Italy)

“…Since no significant variations (i.e., greater than 10% error) have been observed for the MT phases, in the following the MT response will be shown just for the apparent resistivity and, in particular, for the three central MT stations (i.e., 3, 4, and 5 in Figure 3), where the performed numerical simulations found the largest variations in temperature and gas saturation. In this regard, we note that the corresponding variations in the MT signals are found within a 500 m radius from the center of the hypothesized source and this result fits well with the inverted resistivity model retrieved by Siniscalchi et al (2019) through AMT soundings along a profile crosscutting the Solfatara crater. Finally, we point out that the apparent resistivity curves shown in this work refer to the TM mode, since it is more sensitive than the TE mode to the marked lateral variations observed for both scenarios in the temperature and gas saturation distributions.…”

“…4) has been validated by comparing the resistivity estimates retrieved from the petrophysical model to existing resistivity data on CF area. Interestingly, the chosen relationship allowed us to predict along the central portion of the analyzed profile (red line in Figure 3) the occurrence of: (i) a shallow region (i.e., above 300 m below ground level) characterized by relatively high resistivity values, which well match the results by Gresse et al (2017), (ii) a highest conductivity zone below 1000 m depth, which is in very good agreement with the resistivity behavior recently found by Siniscalchi et al (2019) from 2D inversion of audiomagnetotelluric (AMT) data.…”

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

“…The energy released here by degassing is nowadays much higher than the energy released within the whole caldera through other processes such as thermal conduction, earthquakes, and ground deformation. The crater, with a diameter of about 600 m, has been investigated by diverse multidisciplinary campaigns, that include active seismic (Bruno et al, 2007(Bruno et al, , 2017Serra et al, 2016;Gammaldi et al, 2018;Scala et al, 2019), magnetotelluric (Siniscalchi et al, 2019), and geochemical (Chiodini et al, 2001) surveys and resistivity surveys (Byrdina et al, 2014;Gresse et al, 2017). The velocity tomographic images (Serra et al, 2016;De Landro et al, 2017, Gammaldi et al, 2018) reveal a complex structure with an alternation of water-rich and gas-rich rocks.…”

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

“…The energy released here by degassing is nowadays much higher than the energy released within the whole caldera through other processes such as thermal conduction, earthquakes, and ground deformation. The crater, with a diameter of about 600 m, has been investigated by diverse multidisciplinary campaigns, that include active seismic (Bruno et al, 2007Serra et al, 2016;Gammaldi et al, 2018;Scala et al, 2019), magnetotelluric (Siniscalchi et al, 2019), and geochemical (Chiodini et al, 2001) surveys and resistivity surveys (Byrdina et al, 2014;Gresse et al, 2017). The velocity tomographic images ( Serra et al, 2016;De Landro et al, 2017, Gammaldi et al, 2018) reveal a complex structure with an alternation of water-rich and gas-rich rocks.…”