Gavdos Project - Introduction

Why Gavdos

Eastern Mediterranean is a natural laboratory for applied geophysics. This is reflected in the large number of international programs, covered the area, with projects at all scales over the past two decades. WEGENER, for example, has collaborated with the Continental Drilling Project and the Dynamics of the Solid Earth Program, in establishing networks for geophysical research. In the past, Crete was visited by transportable laser systems, and was part of GPS campaigns in the area.

As the international community has moved towards the concept of permanent arrays and away from expensive, hard to move systems (like the SLR and VLBI (Very Long Baseline Interferometry), the region has been left void of any positioning systems.

Continuous monitoring at tide-gauge sites to establish a global network of sea level observing system has gained importance lately though efforts from the Intergovernmental Oceanographic Commission (IOC), GLOSS, and the IGS. Other projects such as the SELF-I and SELF-II (Sea Level Fluctuations), and a WEGENER study, aimed at the Mediterranean and Black Sea regions during 1992-1995 visited tide-gauges by GPS periodically.

Intense seismicity in this area around the facility and large tectonic motions require that characterisation of the deformation needs to be done with dense local networks, operating permanently and continuously. Also the Gavdos location is close to the Cretan Arc straits and considering the evidence of East Mediterranean transients occurring in the past, a sea level station on the island will serve as a long-term climate monitoring station. These requirements become an absolute necessity when the ultimate goal is the monitoring of sea level and the calibration of altimeter missions.

The GAVDOS project contributes directly in this vein through monitoring of the tectonic signals at the existing tide gauges in the area and the establishment of a new absolute sea-level monitoring site at Gavdos. The site at Gavdos is situated under a satellite crossover point of the T/P and Jason-1 missions (see Fig. 1, 2 and 3). The GAVDOS facility will not only create a permanent observatory on the fringes of the Hellenic trench, it will also expand its utility to sea-level monitoring and the altimeter calibration of a number of current and future oceanographic missions (e.g. T/P, GFO, Jason-1, Envisat). The establishment of such a well-instrumented facility at such an appropriate location does not exist at any place on the globe.

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Figure 1. The location of Gavdos isle in the Eastern Mediterranean Sea

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Figure 2 & 3 . TOPEX/POSEIDON and JASON-1 groundtracks over Crete and Gavdos

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Figure 4. TOPEX/POSEIDON and Jason-1 groundtracks cross-over point

Also, the joint determination of gravity and SST is novel. It represents applications of a methodology developed under a FP4 EU project AGMASCO (Airborne Geoid Mapping System for Coastal Oceanography) combined with Swiss experience (ETHZ) in ultra-high precision airborne sea-surface mapping.

The use of transponders in the frame of an interdisciplinary station (with WVR (Water Vapour Radiometers), radio sondes, etc.) is an innovation. Transponders, collocated with tide gauges, are the only means to determine the altimeter calibration constant using altimeter height transfer from a well defined terrestrial surface to the mean sea-surface. This method has only been used marginally during the calibration phase of ERS-2. Only in one experiment, the height of an oil-platform in the North Sea was determined. The same situation now exists on Gavdos Island. All other experiments were carried out on mainland.