212040
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Aristotle University of Thessaloniki
Faculty of Sciences
Department of Geology and Physical Geography
54006 Thessaloniki
Greece

SANTORINI ISLAND : VOLCANOLOGICAL MONITORING NETWORK

A. Continuous monitoring network

A.1. Seismological network

The seismological network consists of 7 analogical - one component - seismographs and one of 3 components. The seismographs are telemetrical of TELEDYNE type.

They are installed in such a way that cover the whole volcano island complex. One of the seismographs that was installed originally on Ios island was recently moved to Amorgos where his imputation was improved.

All analogical signals transmitted via antennas to the main station which is situated on the highest point of Thera (Profitis Ilias mountain, 500 m altitude).

The main station (observatory) is a relatively small construction that insures the antenna receivers and the computerised recording unit and analysing of the seismological data.

The recording and analysing system of the signals consists of three units : the conversion system of analogical to digital signals, the server and the terminal (workstation). The system is equipped with software designed for the recognition and analyzation of the analogical signals.

A.2. Sea-level monitoring network

The network consists of 5 sea-level meters designed, constructed and installed by researchers of Aristotle University of Thessaloniki. This is an automatic sea-level meter, radio-connected by antenna to a central unit installed in a facility room of P.M. Nomikos Conference Center, on the rim of the caldera.

The central unit consists of two quite sophisticated parts : one PC equipped with a particular card and a logical electronic circuit for decoding, and a system transmission-reception equipped with a high amplification antenna.

A.3. Continuous monitoring system of the CO₂ flux

It is a device designed by researchers of the University of Pisa (Institute of Isotopic Geochronology of CNR) that has been tested on Italian volcanoes as well.

It was installed on a specific point on Nea Kameni island (recent volcanism), near the active fumarolic activity area. The point was chosen after a series of field measurements. It records with a certain frequency and period the natural CO₂ flux from the soil, converts the measurement into a digital signal and transmits it via antenna to a specific receiver and a PC situated on the P.M. Nomikos Conference Center.

A.4. Continuous temperature monitoring network

A high precision thermocouple was installed inside a research drill-hole on Palea Kameni island. This is a system of continuous telemetric recording of temperature variation, together with the sea-level network.

B. Periodic measurements network

B.1. GPS network

Twenty-four bases on cement were constructed for the three-dimensional measurements of fixed points for the GPS receivers. The GPS stations are measured twice a year.

B.2. Geochemical and thermometrical surveillance network

On a total of 47 water points and 6 gas escape points, samples and relative studies take place as well as measurements of CO₂ flux from the soil. The chemical and isotopic analyses of the samples determined the parameters and the instrumentation of the sites that after that was utilised as stable points of detailed measurements.

Periodical field measurements of the temperature on the water points (thermal springs, drill-holes), and of the Nea Kameni fumaroles contribute on the recognition of the thermal regime of the volcanic and non volcanic formations, and the creation of a data base on the dormant stage of the volcano.

B.3. Microgravimetric surveillance network

A network of 17 stations is already installed on Santorini since 1984. The measurements of the gravimetric field are realised by gravimeters Lacoste and Romberg G-496 and G-203, and in this last period twice a year.

B.4. Conventional geodetic surveillance network

Its principal is based on the establishment of a network of reference sensible, and indicative points, of possible topographic changes. That is ensured by 2 distinct networks : the network of horizontal control, and the network of vertical control.

The horizontal control is based on the measurements of horizontal distances, using as referring stations 10 stations of the national geodetic network that is arranged perimetrically of the caldera and 1 central station on the island of Nea Kameni.

The network of vertical control connects three sea-level meters with the station of triangulation: the differences on the sea-level are estimated on the triangulation that is based on the distances, and the vertical angles measured.