Long-Term Monitoring with Fiber Optics Distributed Temperature Sensing at Campi Flegrei: The Campi Flegrei Deep Drilling Project.

Renato Somma,Luigi Zeni,Claudia Troise,Giuseppe De Natale,Maurizio Mirabile,Aldo Minardo,Alessandro Fedele

doi:10.3390/s19051009

Abstract

Monitoring volcanic phenomena is a key question, for both volcanological research and for civil protection purposes. This is particularly true in densely populated volcanic areas, like the Campi Flegrei caldera, which includes part of the large city of Naples (Italy). Borehole monitoring of volcanoes is the most promising way to improve classical methods of surface monitoring, although not commonly applied yet. Fiber optics technology is the most practical and suitable way to operate in such high temperature and aggressive environmental conditions. In this paper, we describe a fiber optics Distributed Temperature Sensing (DTS) sensor, which has been designed to continuously measure temperature all along a 500 m. deep well drilled in the west side of Naples (Bagnoli area), lying in the Campi Flegrei volcanic area. It has then been installed as part of the international ‘Campi Flegrei Deep Drilling Project’, and is continuously operating, giving insight on the time variation of temperature along the whole borehole depth. Such continuous monitoring of temperature can in turn indicate volcanic processes linked to magma dynamics and/or to changes in the hydrothermal system. The developed monitoring system, working at bottom temperatures higher than 100 °C, demonstrates the feasibility and effectiveness of using DTS for borehole volcanic monitoring.

Highlights

Monitoring hazardous geological processes is a fundamental tool for risk preparedness and mitigation efforts
We present here the borehole temperature sensing apparatus, belonging to the first nucleus of CFDO, as well as the results of a long‐term monitoring campaign carried out along a 500 m‐deep borehole located in Bagnoli, Campi Flegrei caldera, Italy, through a Raman scattering based Distributed Temperature Sensing (DTS) and a multi‐mode optical fiber sensor cable capable of operation up to 150 °C
Changes of temperature at different depths are a critical, difficult to measure directly, datum for understanding the possible evolution of volcanic activity

Summary

Introduction

Monitoring hazardous geological processes is a fundamental tool for risk preparedness and mitigation efforts. Opto-electronic sensors have gained much attention, since their early introduction, even in the field of geological hazard monitoring [1,2]. Distributed Optical Fiber Sensors (DOFSs) are among the most promising technologies for such monitoring purposes. A fundamental application of this opto-electronic technique, in such a research framework, is the continuous, distributed temperature monitoring along fiber optics cables. Multiple applications of such a technology can be imagined, all of them very important; for geological monitoring purposes, interesting are the application for submarine, sea-bottom monitoring [3] and for borehole monitoring. Borehole temperature monitoring can be very important in volcanic and geothermal areas, both for sensing the heating of a volcanic system, which can highlight processes of magma migration in shallow reservoirs, and for geothermal exploration studies, in which it is Sensors 2019, 19, 1009; doi:10.3390/s19051009 www.mdpi.com/journal/sensors

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