Monitoring geophysical phenomena
The onset of the new Millennium, in late 1999 - early 2000, found remote sensing scientists, geophysicists, geodesists, and engineers equipped with powerful new tools for measuring crustal deformation via Earth Observation. The growing flow of satellite data, along with the development of innovative algorithms and processing chains, have allowed the systematic mapping of surface deformation, pertinent to earthquakes, volcanic eruptions, landslides and ground subsidence occurring from manmade activities, leading to the enhancement of our knowledge and understanding of the manifestation of several geophysical phenomena and the processes that govern them.
Radar interferometry has highlighted the value of remote geodetic measurements for estimating ground displacement patterns with an unprecedented spatial coverage and accuracy. Interferometry is based on the simple idea that by sensing or viewing the same object twice, in separate times, one can identify the changes that the observed object has undergone between these two distinct time instants. The radar transmits successive pulses to the Earth from a distance, from two slightly different locations and at different times, and collects the backscattered echoes, leading to an interference pattern, analogues to classic physics experiments. This pattern has an invaluable geodetic measurement potential.
Initially, radar interferometry was applied to measure deformation that was inflicted after abrupt catastrophic events, like an earthquake, or rapidly deforming calderas. In time, new techniques emerged, which exploit time-series of satellite observations to generate maps depicting surface displacement rates. The Persistent Scatter Interferometry concept is based on the detection of stable targets that do not change their scattering characteristics over time and remain coherent under all imaging geometries. Hence, the methodologies have evolved to such a level that radar interferometry can be used as if a very dense network of precise ground-based geodetic instruments was deployed.
These techniques have been applied at NOA for several geophysical phenomena, and has been supported by the integration of in-situ observations from the NOANET GNSS network and the ENIGMA magnetometer network.
The ENIGMA network is used in an attempt to address the issue of whether earthquakes (EQ) are predictable by studying electromagnetic (EM) emissions as one of the most promising potential pre-seismic transients. To this end we are building an array of three ultra-low frequency EM recording sites in central and southern Greece in order to understand EQ physics and to detect pre-seismic transients if they exist. Our EM sensors include fluxgate magnetometers to detect tiny changes in Earth’s magnetic field, and electrodes to detect tiny electric currents inside the Earth. Our sensors are collocated with existing seismological stations, so that any anomalies observed on these can be cross-checked against more-conventional EQ detection instruments: strain meters and seismographs.