T. Papadopoulos

Microseismic activity and seismotectonics of Heraklion Area (central Crete Island, Greece)

Abstract A seismological network of 10 portable analogue stations was installed in the area of Heraklion (central Crete) from September to December 1995. During this period, more than 1000 events were recorded by at least 4 stations with magnitudes ranging from 0.5 to 4.6 and depths up to 70 km. Analysis of 336 well located events revealed high seismic activity. In the onshore area seismicity is shallow (

1-D and 2-D spatial frequency analysis of self-potential field anomalies, produced by a polarized sphere

This paper gives analytical expressions for the 1-D and 2-D frequency spectra of the self-potential field produced by a polarized sphere. In 1-D, the amplitude spectrum of the potential field leads to a criterion for determination of the depthh to the centre of the sphere. The polarization angle of the buried sphere can be calculated from the maximum point of the amplitude spectrum of the electric field. In 2-D, the depth to the centre of the polarized sphere can be calculated if the polarization is vertical.

Combined geophysical methods for detailed microzoning studies

The peak ground motion parameters, related to earthquake hazard at a given site, are strongly dependent on the source process, the ray path and the local geological conditions. The area of Thiva city, situated in Central Greece, is surrounded by seismogenetic zones characterized by high seismicity level (figure 1). The broad region of Thiva has sustained severe damage from earthquakes occurred in near and moderate distances.

Combined geophysical methods for cavity detection in highly inhomogeneous material over karstic basement. A case history

Sometimes the need for an immediate action for applying a geophysical investigation is of vital importance particularly in cases where remedial measures are necessary to be conducted soon after the termination of field surveys. An example is given in this paper for an off-river reservoir at an elevation of approximately 80m above sea level, few hundred meters from the coastline, in the southwestern part of the Chania prefecture, in Crete island, Greece. The reservoir was under construction, partly by excavation and partly by filling the depressions, when the authors were called to investigate the reservoir area. They were faced with a sizeable subsidence in the form of an almost perfect cylindrical cavity having a diameter of 10 m and a height of 9 m, downstream of the embankment. The surface material is highly inhomogeneous and basically is composed of brown silty sand, gravel of very variable composition and huge blocks of rock. This overburden layer lies above a karstic breccia formation which is easily eroded due to the solution of gypsum present in the initial composition of breccias and extends up to sea level. In such a geologic environment subsidence phenomena are prone to be developed due to weathering factors that continuously enlarge the cavities by the downward flow of meteoric water and widening the existing karstic openings in the basement, as it has been shown in the past (Sowers 1984 and Newton 1984).

Outlining complicated subsurface geological conditions by elaborating 1-D electrical soundings for hydrogeological purposes

Subsurface investigations for hydrogeological purposes using geophysical (mainly geoelectrical) methods are widely used today. For geoelectrical methods, the array type and the way the electrodes are expanded, are highly depended on the nature of the hydrogeological target, the depth of investigation and the strike of the geological setting. For deep investigations (ABI2=1000 m), the Schlumberger array is more suitable and is mostly used, although the dipole-dipole array gives similar results. In addition, the inherent difficulty of interpreting 1D Schlumberger soundings, arising from non horizontal layering and lateral variations, has recently been faced by proposing several methods, for recognizing lateral resistivity inhomogeneities (Renning and Tennensen 1990) and detecting them by combining the Schlumberger sounding array with two dipole-dipole soundings (Morris et.al. 1997). During the last two decades emphasis has also been given among the geophysicists, to develop 2D interpretation algorithms for a reliable earth structure simulation (Dewand Morrison 1979, Barker 1981, Dahlin 1993, Loke and Barker 1994). Furthermore, it has been pointed out that in some cases the 1D approximation of Schlumberger curve interpretation is adequate (Beard and Morgan 1991, Morris et. al. 1997). In contrast, less attention has been paid among the geophysicists on the role of the in situ electrical measurements, mainly because of the diversity of the measured values, and dealing with detailed surface geology observations. Bath investigations can contribute to the construction of a reliable 2D structure.

A New method for quantitative interpretation of SP anomalies produced by a polarized inclined sheet

In this paper is presented a new method for quantitative interpretation of the SP field produced by an inclined sheet, based on the study of its amplitude spectrum Part of the theoretical background, although it concerns the polarized sphere, has been already published (Skianis et.al., 1991). Here the authors give more emphasis in the practical aspects of the new method and propose the procedure to be followed for quantitative interpretation of SP anomalies due (or simulated) to an inclined sheet.

A Study for the role of the surface layer on the conformation of a SP anomaly produced by a point-pole

In the present paper, a mathematical treatment is attempted to describe the influence of a surface layer of resistivity p1, on the formation of self-potential (SP) curve over a point-pole (vertical dipole with infinite depth extent) seated in a medium of resistivity p2 (see fig . 1). The whole elaboration is based on the fact that ground resistivity variations distort the circulation of the electric current which is produced by a polarized body. Therefore, the measured potential at ground surface is influenced by the presence of different geological layers and its magnitude depends on the underground resistivity distribution.

Investigation for geothermal sources on Lesvos Island (Greece) by means of resistivity and P-wave seismic techniques

Combined geophysical investigations including deep Schlumberger resistivity soundings, microearthquake monitoring and velocity inversion of local earthquake arrival time data have been performed on Lesvos island . Lesvos island, situated in the northeastern part of Aegean Sea, belongs to a Tertiary island arc system and is certainly affected by a regional anomaly of geothermal gradient, whose extent and importance are as yet unknown . The tectonics of the island, together with the occurence of some thermal springs, indicate that geothermal conditions could exist on it . The areas between villages Petra Argenos, Kalloni - Stipsi and Polychnitos area can be considered of potential interest . The joint geophysical survey aimed to delineate areas through the tectonic structure of which there are indications of thermal fluid circulation .