CUMAS
CUMAS (Cabled Underwater Module for Acquisition of Seismological data) is an underwater module equipped with different types of sensors that have been developed for the acquisition of geophysical signals for geohazard monitoring in volcanic areas. It was conceived for the specific application in the Campi Flegrei caldera (southern Italy), where the main features of the present volcanic activity consist of slow soil movements (bradyseism) accompanied by intense and shallow seismic activity.
Project coordinated by Giovanni Iannaccone (Research Director, INGV-Osservatorio Vesuviano).
Staff included:
- Sergio Guardato (Technologist, INGV-Osservatorio Vesuviano);
- Maurizio Vassallo (Researcher, Physics Dept., University of Naples );
- Laura Beranzoli (Technology Director, INGV-Roma2).
The aim of CUMAS is to:
- continuously acquire geophysical and oceanographic data on the seafloor according to a single time reference, and to store them on hard disk;
- transmit the acquired data in real time to an acquisition centre on land;
- receive commands from the land centre and modify its acquisition configuration accordingly.
The sensors installed in the module comprise: a broad-band seismometer and hydrophone to record local earthquakes related to volcanic activity and artificial explosions; and physical oceanographic sensors for the long-term monitoring of the water current system, which are also useful for seafloor seismic microtremor studies. A further task consists of testing the feasibility of the use of water pressure measurements at the seafloor to detect changes in the water column pressure that are potentially related to bradyseismic activity.
The module is connected via an electromechanical cable to a buoy (elastic beacon) equipped with an electrical supply and a Wi-Fi communication system to ensure the transfer of the seafloor data to the land acquisition centre in Naples.
CUMAS has been developed for the system to be installed within the sea sector of the Campi Flegrei caldera, at 97 m depth, from 2 to 3 km from shore and about 4 km from the land acquisition centre.
Technical characteristics of the buoy
The elastic beacon is a semi-rigid structure with a metallic pole of 20 m in height that is fixed to the sea bottom by an antitorsion steel cable to a ballast of 17 tons. The pole is kept vertical by a plastic submerged float pear shape. The buoy is equipped with a two square platforms mounted at 2 and 4 m above the level of the sea, which are equipped with ten batteries that are recharged by six solar panels (110 W each). On the space of the square tower, there is a 400-W aeolic generator (Air-X Marine). A meteorological station is also mounted on the tower, providing local meteorological measurements (e.g., barometric pressure, wind velocity and direction, rain-meter, thermometer), to allow the correlation of the air and seafloor data. A 19-inch enclosure for the rack-mountable electronic equipment contains an 802.11g wireless WLAN bridge (Cisco Aironet 1300 series) equipped with an omni-directional antenna, for the data transmission to the land, and other added equipment, including the RS-485 interface with the Q330 GPS receiver and antenna, an embedded CPU, an interface board connected to the CPU with electronic switches used to enable/ disable the power to the overall set-up and to the instrumentation, an Ethernet 10/100 Mbps hub, and a DC/DC power converter.
CUMAS - Cabled Underwater Module for Acquisition of Seismological data
SENSORS
The following sensors have been installed within the CUMAS’ structure:
- seismic sensor;
- hydrophone;
- current meter;
- pressure gauge;
- status and control sensors.
Seismic sensor
This comprises a three-component, broad-band seismometer, model Güralp CMG-40T OBS (30 s, 50 Hz), housed in a 16 cm glass sphere with an auto-leveling platform. It is equipped with a ±30° microprocessor-controlled leveling system, with an internal three-axis magnetometer for orientation measurements and the automatic calibration and tilt compensation system.
Hydrophone
A broad-band hydrophone, model SQ03 Sensor Technology (1 Hz ÷ 65 kHz), equipped with an integrated pre-amplifier with 40-dB gain. Our interest is concentrated around the acquisition of signals of very low frequencies (1-10 Hz) detected by the hydrophone.
Current meter
A single-point, three-component acoustic wave/current meter, model 3D-ACM Falmouth Scientific, Inc. The current meter ensures the acoustic measurement of vector-averaged current speeds and directions. It is also capable of high-accuracy temperature measurements (resolution, 0.01 °C), with an ASCII serial data output via an RS-232 interface.
Pressure gauge
The pressure gauge is a Series 8000 Paroscientific, Inc., 8CDP200-I model, characterised by:
- An accuracy better than 0.01% FS;
- 10-8 resolution (resolution/sample time depends on the integration time);
- low power.
Status and control sensors
These include tilt and heading sensors, for the measurement of the real module attitude on the seafloor, and status sensors for the monitoring of the internal status of the vessel (e.g., internal temperature, power absorption, water intrusion).
DATA ACQUISITION
Data from the broad-band seismic sensor and from the hydrophone are acquired by a Q330 Kinemetrics Quanterra digitiser at a rate of 100 sps, which is equipped with an external hard disk (Baler PB14F, 20GB) as the local mass storage. The digitiser is installed inside the vessell. The Q330 datalogger has an integrated GPS receiver, for correct synchronization of the internal clock, and an external GPS antenna with a 5-m cable. With our system configuration, due to the more than 100-m distance between the GPS antenna installed on the buoy and the Q330 positioned in the vessel on the seabed, this set-up could not be used. Therefore, it was necessary to design two electronic boards (Guardato and Iannaccone, 2007) that allow the external use of the same remotely mounted GPS antenna–receiver combination of the Q330 datalogger (unplugged from this), via an RS-485 interface, allowing a much greater distance between the antenna and the recorder.
The digital output from the sensors (current meter, pressure gauge, and status and control sensors) are acquired by an embedded computer (MOXA UC-7408) equipped with Linux OS, via an RS-232 port. This embedded CPU features eight RS-232/422/485 serial ports, an eight-channel digital input, an eight-channel digital output, dual 10/100 Mbps ethernet ports, and USB, PCMCIA and CompactFlash interfaces.
Example of data recording
Data coming from CUMAS are now fully integrated in the geophysical monitoring system managed by INGV-Osservatorio Vesuviano. On the 29th of April, at around 06:00 in the morning, the Italian Navy caused the detonation of some WW2 explosives, some few hundred meters away from the CUMAS module. The recordings of that explosion by the three components of the seismic sensor and by the hydrophone are shown in the figure.
