2-D GEOELECTRICAL SURVEY FOR THE PRELIMINARY ROUTE OF THE NATURAL GAS PIPELINE INSTALLATION AT ALISTRATI CAVES, N. GREECE

An extensive 2-D geoelectrical survey was carried out at the Alistrati caves, prefecture of Serres, Ν. Greece, as part of a preliminary route survey for the planned natural gas pipeline that was about to cross these regions of Northern Greece. Geophysics can play a significant role in this procedure. Synthetic modeling and inversion examples presented in this work support the above claim. The dipole-dipole array was used with inter-electrode spacing of 5m and maximum dipole separation of n equal to 8 dipole spacing. Two groups of personnel, including geophysicists and technicians, worked for one month to realize this survey. The number of measurements is really enormous for such type of research. A total of 7616 measurements (952 electrode points) were obtained. The area of investigation was chosen on the basis of the local geological conditions, which appeared to favor the existence of void structures. The survey line had a length of approximately 5 km. The 2-D geoelectrical survey proved to be an effective tool for delineating the subsurface lithology and structure of the surveyed area. Most importantly, the technique proved successful in locating highly karstified areas and can be used to aid void mapping as well as to provide useful information about their general setting

A geophysical survey in the archaeological site of Archontiko, Yannitsa

The ancient settlement of Archontiko is 4.5 km NW of ancient Pella in North Greece (Figure 1). The findings showed that the area was first occupied by the end of the Iron Age, i.e. 650-550 B.C. Also, showed that the topographic table of Archontiko was a major settlement of the Yannitsa province due to its concessive position by the main roads of Macedonia (Chrysostomou A. and Chrysostomou P., 1993). At the upper layers of the ruins, findings of the Roman and Byzantine times were also unearthed. The geophysical methods have been used in order to detect and map antiquities in various sites in Greece (e.g., Tsokas et al., 1994; 1995; Sawaidis et al., 1999). The resistivity mapping employing the twin probe array, the total magnetic field variations, the airborne photos and the Ground Probing Radar are the most popular methods in this respect. However, almost all geophysical methods can be used to tackle specific problems. From 1992 till 1994, many geophysical surveys were carried out in the area of Archontiko to collect mainly magnetic data. Resistivity measurements were also conducted in a small part of the area. The data presented in this study cover the northern side of the topographic table of Archontiko and they were collected during two campaigns during the summer of the years 1992 and 1993 (Figure 2)

The structure of the ophiolitic beltin Albania inferred from geomagnetic anomalies

The ground magnetic measurements in Albania were used for the compilation of the Total Magnetic Field (TMF) anomaly map of Albania. The magnetic data were processed and interpreted in order to study the structure of the ophiolitic belts of Albania. The ophiolites of Albania are placed at the Mirdita zone and are divided into two parallel alignments which are called the eastern and western ophiolitic belts. They are associated with strong potential field anomalies and their characteristics are considered crucial for a better understanding of the tectonic settings of Albania. The ground TMF data used in this study were acquired over various campaigns (1990-1994) and cover most of Albania's territory. The data were compiled to a map after reduction to the epoch 1990.4. The strongest magnetic anomalies in Albania appear along the known ophiolitic belts which trend NE-SW to the north and NW-SE to the south. Several processing steps were applied to the unified and gridded data in order to obtain information on the distribution of the magnetic sources. The magnetic sources were subsequently modeled using a 2.5D inversion technique. The magnetic properties of the ophiolites determined from laboratory measurements on rock samples, while their lateral extent was calculated from the processing of the magnetic data and used as constraints to the inversion procedure. The bottom of the ophiolitic belts is considered to be predominated by harzburgites. They exhibit lower magnetization than other rocks of the complex, i.e. gabbros, basalt. In fact gabbros are associated with the observed high frequency magnetic anomalies. The modeling results indicate that the thickness of the ophiolites is reduced from east to west. The eastern ophiolitic belt has a maximum thickness of approximately 12 km at its northern section (Kukesi and Lura massifs). The magnetic sources appear with relatively small thickness at the western ophiolitic belt. Boundaries of the eastern ophiolites are characterized by vertical contacts

Application of Electrical Resistivity Tomography to the detection of the Ermakia (Northern Greece) cavity system

Η μέθοδος της Ηλεκτρικής Τομογραφίας εφαρμόστηκε για την εξερεύνηση ενός καρστικού σπηλαίου στην περιοχή της Ερμακίας (Μ. Πτολεμαίδας). Οι γεωλογικές έρευνες έδειξαν ότι η γνωστή κεντρική αίθουσα του σπηλαίου της Ερμακιάς "επικοινωνεί" με τουλάχιστον έναν υπόγειο καρστικό έγκοιλο. Αυτά τα στοιχεία μας οδήγησαν στη διερεύνηση της περιβάλλουσας περιοχής με την εφαρμογή δισδιάστατης ηλεκτρικών τομογραφιών σε ένα πυκνό πλέγμα μετρήσεων σε δυο κάθετες διευθύνσεις. Κάποιες από τις γραμμές έρευνας τοποθετήθηκαν πάνω από το χαρτογραφημένο τμήμα του σπηλαίου ώστε να βεβαιωθεί η καταλληλότητα της μεθόδου και να επιτευχθεί βαθμονόμηση των τιμών της ειδικής ηλεκτρικής αντίστασης. Τα αποτελέσματα της αντιστροφής των γεωηλεκτρικών δεδομένων συνδυάστηκαν ώστε να παραχθούν ψευδοτρισδιάστατες εικόνες της αντίστασης. Η ερμηνεία των αποτελεσμάτων είχε ως αποτέλεσμα να εντοπιστούν νέοι θάλαμοι του σπηλαίου όπως επιβεβαιώνεται και από γεωλογικές παρατηρήσεις στο σπήλαιο.Electrical resistivity tomography has been applied for the exploration of a karstic cave system at the region of Ermakia, next to the city of Ptolemais (NWt Greece). Geological investigations indicated that the main known Ermakia cave chamber "communicates " with at least one underground karstic ovoid. This evidence lead us to prospect the surrounding area by applying 2-D Electrical tomography survey on a regular grid in both x,y directions. Survey lines crossed the known cave in order to verify k the suitability of the method at the specific area and calibrate the resistivity values. Resistivity data were inverted and results were compiled in qusi3-D resistivity images. The interpreted results indicated the existence of two more chambers next to the known one which is in agreement with in-situ observations