E. Lagios

Orthophoto generation using IKONOS imagery and high-resolution DEM: a case study on volcanic hazard monitoring of Nisyros Island (Greece)

Georeferenced high-resolution satellite images can be used for acquisition of topographic information, navigation and visualisation for various environmental studies. The present study is part of the multidisciplinary EU project GEOWARN related to monitoring, warning and emergency planning for volcanic hazards in the island of Nisyros, Greece. Here, the main aim was the orthorectification of a 1-m resolution pan-sharpened IKONOS Geo image of Nisyros island. For the orthorectification, a digital elevation model (DEM) with a cell size of 2 m and an RMS accuracy of ca. 3.5 m was used, as well as 38 selected ground control points (GCPs) measured with differential GPS. An object-to-pixel space transformation using the ground control points was computed using two different models, a relief-corrected affine transformation and the polynomial mapping functions of Kratky. These transformations were used for orthorectification and the orthophoto accuracy was evaluated using GCPs as check points. Postprocessing for radiometric improvement of the orthophotos was applied. The orthophoto and the DEM served as basic tools for subsequent base mapping and visualisation. D 2002 Elsevier Science B.V. All rights reserved.

Subsidence monitoring within the Athens Basin (Greece) using space radar interferometric techniques

The application of conventional SAR Interferometry (InSAR) together with the two techniques of subcentimeter accuracy, the Stacking and the Permanent Scatterers (PS) Interferometry, were used to study the ground deformation in the broader area of Athens for the period 1992 to 2002. Using the Stacking interferometric method, 55 ERS-1&2 SAR scenes, between 1992 and 2002, were acquired producing 264 differential interferograms. Among these only 60 were finally selected as fulfilling certain criteria. The co-seismic deformation associated with the Athens Earthquake (Mw = 5.9, September 7, 1999) was excluded from the analytical procedure in an attempt to present results of only aseismic character. In total ground subsidence results of about 12 mm in the southern suburbs of Athens, but higher value of about 40 mm in the northern ones for the period 1992–2002. Based on the PS technique, a precise average annual deformation rate-map was generated for the period 1992–1999, ending just before the Athens earthquake event. Both circular and elongated-shape areas of subsidence are recognizable especially in the northern part of the Athens Basin (3–4 mm/yr), as well as at its southern part (1–3 mm/yr). In addition, a rate of 2–3 mm/yr is also yielded for some part of the Athens city center. Subsidence rates of 1–2 mm/yr are measured at the western part of the basin over an area of old mining activities, and around the newly built Syntagma Metro Station. The correlation of the observed deformation patterns with respect to the spatial distribution of water pumping, older mining activities, metro line tunneling and other local geological parameters is examined and discussed.

A magnetotelluric survey of the nissyros geothermal field (Greece)

Abstract A preliminary magnetotelluric study consisting of twenty measurements, in the frequency range 128–0.016 Hz, was undertaken on the active volcanic island of Nissyros. Two boreholes identify the existence of high enthalpy manifestations. The results correlate well with the borehole logs and delineate, in a 1-D approximation, the existence and symmetry of a possible geothermal reservoir. Some of the main faulting features were detected as well as an inferred highly conductive zone at the centre of the island.

Integrated geophysical study of the geothermal system in the southern part of Nisyros Island, Greece

Abstract The study of the high-enthalpy geothermal field of Nisyros Island is of great importance, because of the planned construction of a geothermal power station. The purpose of the applied geophysical surveys — gravity, SP, VLF and audio-magnetotelluric — in southernmost Nisyros was to investigate the major and minor faulting zones which are geothermally active, i.e. whether geothermal fluid circulation occurs in these zones. The survey lines, four parallel traverses of about 1500 m length, were chosen to be almost transverse to the main faults of the area. The SP method was the main reconnaissance technique, with the VLF and gravity measurements correlating with the “SP model”. Previously proposed SP data acquisition and reduction techniques were used, followed by a 2-D interpretation of the SP map which apparently locates the position of the fracture zones (geothermally active). The SP and VLF anomalies are believed to be generated by the same source (subsurface flow of fluid, heat and ions). Hence, at the place of a vertical geothermal fluid circulation zone, the curve of SP dipole-like anomaly changes its behaviour and the curve of the VLF anomaly takes maximum values for the in-phase component and minimum values for the out-of-phase component. On the VLF map of the survey area, the zones detected with the SP interpretation coincide with the maximum values of the VLF in-phase component. The geothermal fluid circulation zones, detected by the SP method, appear to be well correlated with corresponding features derived from the gravity and the AMT surveys. In particular, the AMT soundings indicate two zones of geothermal fluid circulation instead of the one the SP method detected in the central part of the investigated area.

Two-dimensional magnetotelluric modelling of the Kos Island geothermal region (Greece)

Abstract The magnetotelluric (MT) method in the period range 0.075–42 s was applied to determine the electrical structure (

The self‐potential method in the geothermal exploration of Greece

Self‐potential (SP) anomalies are generated by flows of fluid, heat, and ions in the earth. SP investigations have been used to locate and delineate sources associated with such flows in three areas of geothermal interest in Greece—Lesvos Island, Loutra Hypatis (central Greece), and Nisyros Island. A combination of geophysical methods, with SP being the primary method, has been applied in these areas. The SP method is adversely influenced by various sources of noise. Field procedures have been suggested to minimize their effects by monitoring electrode polarization and telluric activity. The interpretation of SP contour maps is preferred to using profile data. A procedure was adopted for SP interpretation, and the results were satisfactory. However, this model is based on thermoelectric sources only and is not related directly to hot fluid movement. In all three survey areas, the geothermal zones delimited by the SP interpretation in combination with data acquired by other geophysical methods result in an...

Microgravimetry in Greece: applications to earthquake and volcano-eruption prediction

Abstract The Atalanti Fault Zone in Central Greece is an area of high seismic risk, where destructive large magnitude earthquakes have taken place in the past. The microgravimetric network set up in the region to detect gravity changes associated with big tremors, has been monitored annually and the observations of the last five years, from 1981 to 1986, are presented here. All data were analyzed using a newly developed algorithm of network adjustment. Gravity changes of more than 40 μGal were detected after 1982 in the region between Atalanti and Arkitsa, and across the downthrow side of the fault. These gravity changes, implying uplift and observed only in this very closely monitored part of the network, increased gradually in 1983 and 1984 and declined in 1985. The inferred uplift may exceed 200 mm at the center of the bulge, applying the Bouguer gravity gradient of −0.2 μGal/mm. Examination of the water table data from wells in the region suggests that the amplitude of the observed gravity changes is not likely to have been caused by changes in the water level; only one third of the observed changes, at the most, could be thus accounted for. Since dilatancy cannot be established on the basis of the existing geophysical information there is no evidence for any association between the inferred uplift and the occurrence of a recent tremor in the region ( M l = 4.3 , June 6, 1986). However, it is possible that the aseismic movements monitored in the area after 1982, over a relatively small region in a small time-period, may offer some additional information about the deformation of the upper crustal layers. Another application of microgravimetry, as part of a multidisciplinary project, is being tried out on Santonni Island, which has one of the most active volcanoes in the Eastern Mediterranean. A network of 15 stations has been established there to try to detect upward movements of magma. It was set up in 1984 using three LaCoste and Romberg gravity meters and has been monitored annually since then. The rms error of adjustment was 7.0, 3.2 and 2.6 μGal, in 1984, 1985 and 1986, respectively. Small amplitude gravity changes were observed at spme stations in the network. Attempts to interpret these changes will be made if a clear pattern is confirmed by future observations and in the light of other available geophysical information.

Thermal imaging of Nisyros volcano Aegean Sea using ASTER data: estimation of radiative heat flux

A time series of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images (AST08) have been processed to allow radiative heat flux estimations of the Nisyros volcano. ASTER night-time images were acquired on four different dates: 7 April 2001, 13 June 2002, 26 October 2002 and 23 July 2005. The results show a steady state energy release with heat fluxes ranging from 15 to 30 W m−2 over the craters of Kaminakia, Polyvotis and Stefanos, respectively. It is suggested that this range of values indicates a background heat flux of this volcano following the unrest of 1995–1997, and that the volcano has entered again into the quiet phase. Also, on the basis of the average spatial extent of the thermal anomaly a total radiative heat flux of 36 MW was estimated at the moment of ASTER overpass. Heat flux values for Nisyros are in good agreement with other published estimates derived from low-temperature fumarolic volcanoes (Stromboli and Vulcano, Italy) using Landsat TM data. It is also evidenced that the ASTER radiometer can be used as an important imaging tool for the monitoring of geophysical properties associated to volcanic activity, as is the volcanic heat flux.

Seismotectonic investigation on the Bam earthquake prone area (Iran) based on ASAR interferometry

In this study the Satellite Radar Interferometry (InSAR) was applied to investigate the crustal deformation caused by the Bam Earthquake (M/sub w/=6.5) that occurred in Iran on December 26, 2003. It was a multiple seismic event that destroyed the historical City of Bam and provoked great damages in the urban centers of the region. The Bam area in the south-eastern part of Iran is an active seismic zone and the Bam Fault System is comprised by three specific segments (north, east and SE of Bam). The three-pass interferometric technique was applied using ENVISAT ASAR scenes. According to the interferometric processing results subsidence has occurred in the area NE of Bam City, while dextral strike-slip displacement has taken place in the southern area of Bam. The last deformation could be attribute to a parallel hidden segment of the Bam Fault that is reaching the surface according to the shear displacement during the earthquake.

GROUND DEFORMATION OF ZAKYNTHOS ISLAND (WESTERN GREECE) OBSERVED BY PSI AND DGPS

Ground deformation studies based on Differential GPS (DGPS) measurements and Permanent Scatterers InSAR (PSI)* analysis using ERS (1992-2000) and ENVISAT (2003-2010) radar data have been conducted on Zakynthos Island (Western Greece) covering the period 1992 to 2012. These results were compared, validated and integrated with geological, geotectonic and seismological data to evaluate possible pre-earthquake deformation process, and the present tectonic regime. The PSI results indicate that a slight subsidence had occurred during the period 1992-2000, while uplift has mainly observed in 2003-2010. DGPS results for 2005-2006 indicated strong opening of the southern part of the island, while in the period 2006-2012 the amplitude of deformation is relatively smaller. The occurrence of the seismic outbreak that took place offshore to the south of Zakynthos during 2005-2006 may have contributed to the different deformational pattern as revealed by the ERS and ENVISAT PSI products, and also elucidate the DGPS results.