J. Alexopoulos

Adumbration of Amvrakia’s spring water pathways, based on detailed geophysical data (Kastraki - Meteora)

The Amvrakia spring is located at the bottom of Meteora pillars and more specifically near the village of Kastraki (Kalambaka municipality). It is a seasonal spring since it functions only during the wet period. The Meteora conglomerates which dominate the area are characterized by large discontinuities creating a network of groundwater pathways above the impermeable strata of the underlying marls. The research targets was to define these water pathways in order to understand the mechanism of Amvrakia spring, by mapping the exposed discontinuity network and define their underground extension with the contribution of geophysical techniques. Electrical Resistivity Tomography (ERT) and Very Low Frequency (VLF) methodologies were applied. The VLF method is indicated for the detection of water-bearing fracture zones, but before the application of their filters they had to be processed for topographic corrections, as the area had not smooth relief. Five (5) VLF profiles were conducted with different directions around the spring’s area, in order to detect possible conductive zones in the conglomerates surrounding the study area. Moreover, two (2) ERT sections of a total length of 140m were carried out, parallel to existent VLF sections, for cross-checking the geophysical information. Both techniques revealed important conductive zones (<200 Ohm.m) from the south-eastern Meteora conglomerate pillars, possibly interpreted as discontinuities filled with water feeding the spring.

Application of Geoelectrical Techniques in the Investigation of a Coastal Sand Dune Field

A geophysical research was carried out to investigate the lithostarigraphic substratum characteristics of the dune field of the central Kyparissiakos Gulf (W. Peloponnese, Greece), which is characterised by the presence of four dune lines. For this purpose, the geophysical techniques of Electrical Resistivity Tomography (ERT) and Vertical Electrical Soundings (VES) were applied, along with detailed morphological mapping and the collection of geological and geomorphological information. The processing of the geophysical data that were collected from two ERT soundings, perpendicular to the general direction of the dune lines, and seven geoelectrical soundings revealed four geoelectrical layers with the two surficial resistive layers adumbrating the extent and thickness of the four sand dune ridges and the associated slags. The third layer, corresponding to the underlying geoelectrical formation of 50-75 Ohm.m, is interpreted as a layer of transgressive Holocene deposits saturated with fresh water, while the deepest geoelectrically identified layer seems to correspond to the “Neda” formation, which consists of marls, sandy marls and conglomerates. It is concluded that the combined application of the aforementioned techniques has the potential to provide valuable data for the investigation of complex coastal depositional environments.

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.

A Geophysical Insight for the Occurrence of Mediterranean Temporary Ponds on Mts. Oiti and Kallidromo (Greece)

A geophysical survey was performed at selected locations of Mt. Oiti and Mt. Kallidromon characterized by the hosting of priority habits of Mediterranean temporary ponds and the threatened plant species of Veronica oetaea, in order to understand the geoenvironment and contribute to the conservation of biodiversity. Primarily, the formation of these seasonal ponds, where Veronica oetaea occurs, seems to depend exclusively on the local hydrogeological regime. Thus, we investigated the subsurface structure of “Livadies” and “Nevropolis” ponds with the application of Electrical Resistivity Tomography for high accuracy information and Vertical Electrical Sounding for deeper data acquisition. Four ERT sections and 15 geoelectrical soundings in total were carried out. The combined results of their processing revealed differences at the geological structure beneath the ponds locations. At “Livadies” pond (Mt. Oiti), two geoelectrical layers were distinguished both corresponding to a folded flysch succession, contributing to the formation of this pond. On the contrary, at “Nevropolis” pond two geoelectrical layers were identified and interpreted as a surficial soil deposit stratum covering the geomorphological karst structure of a polje, created on the underlying limestones. The combined geophysical research offered significant data for the formation and the hydrogeological status of the priority habitats.

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.