CUMAS: a seafloor multi-sensor module for volcanic hazard monitoring - First long-term experiment and performance assessment

摘要

A seafloor multi-sensor module with real-time data transmission, named CUMAS (Cabled Underwater Module forAcquisition of Seismological data), has been deployed in January 2008 in the Gulf of Pozzuoli, in the Campi Flegreicaldera (southern Italy), which is one of the most active volcanic areas in the world. The sensors installed in CUMASwere selected to monitor a set of signals related to the local seismicity as well as the ground uplift and subsidence of theseafloor that are related to the bradyseismic phenomenon. In particular, together with a broad-band three-componentseismometer and a low-frequency hydrophone, a seafloor water-pressure sensor is used to assess the feasibility ofmeasurements of the slow vertical movement of the seafloor (bradyseism).Further sensors are acquired by two embedded Linux computers, namely tilt and heading sensors for the measure of theactual module orientation on the seafloor, and status sensors that monitor the state of health of the vessel (e.g., internaltemperature, power absorption, water intrusion).The underwater acquisition systems are linked to a support infrastructure, a floating buoy (elastic beacon), through anelectro-mechanical cable with an Ethernet line. The buoy provides the needed power supply thanks to batteries chargedby solar panels and a wind- generator. A Wi-Fi antenna on the buoy is used to transmit the seafloor data from the seasurface to the land acquisition centre in the city of Naples. A meteorological station is also mounted on the buoy, to allowthe correlation of the air and seafloor data.CUMAS, although based on commercial sensors, relies on an original system for the centralized management of a wideset of geophysical and physical oceanographic sensors, that handles the continuous data acquisition and real-time datatransmission.After the installation in the Gulf of Pozzuoli at about 100 m w.d., and after a test period, CUMAS uninterruptedly operatedfrom May 2008 to June 2009, thus providing continuous geophysical data to the Monitoring Center of the Campi Flegreivolcanic areas, managed by the Istituto Nazionale di Geofisica e Vulcanologia.The long-term operational performance of CUMAS is presented here, together with the first results from the analysis ofthe geophysical long time-series acquired.Examples of the acquired signals, especially geophysical data, will be presented to point out the high quality in term ofsignal-to-noise ratio. In particular, earthquake recordings obtained from the hydrophone resulted of comparable quality tothe seismic data acquired on land by the permanent network, thus demonstrating the suitability of hydrophones tomonitor the seismic activity of the caldera.