Gregory N. Tsokas

Tomography of the crust and upper mantle in southeast Europe

Compressional velocity structure of the crust and the upper mantle in southeastern Europe (broader Aegean area) is studied by inverting residuals of the first P arrivals from earthquakes in this region (16°E-31°E, 34°N-43°N). The data used are from regional events recorded by the permanent network of stations during the period 1971-1987, enriched with data from experiments with portable seismographs in four regions of this broad area. This study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern Mediterranean lithosphere under the southern Aegean and gives further detailed information on the crustal and upper mantle structure of the area. Important new information is the existence of a low-velocity crustal layer in western Greece and Albania and that the velocity anomaly in the mantle under the southern Aegean extends much farther and deeper to the northeast than the Benioff zone of the intermediate depth earthquakes indicates. Furthermore, evidence is presented about the possible existence of older subduction in the northern Aegean and about the influence of the tectonic regime on the velocity field.

The effect of terrain topography on commonly used resistivity arrays

In this work a study of the effect of surface topographical variations on several dc resistivity arrays is presented. A 2.5-D finite‐element modeling scheme is used to examine the performance of several arrays over buried features that exist below a range of different topographical terrain contexts, such as valleys, hills, and steep slopes. A mesh‐generating algorithm allows a realistic representation of terrain topography. The results confirm that topographical variations can have a significant impact on the field resistivity data values for all resistivity arrays. Further, topographical variations can be treated flexibly using a realistic resistivity forward modeling process. Thus, topographic effects, to the degree they can be modeled, are predictable and should be taken into account when designing surveys and interpreting data.

Study of the crustal thickness and the subducting lithosphere in Greece from gravity data

The area of Greece has experienced a complex tectonic history dominated by the subduction of the African plate beneath Eurasia. In this study the variations of the crustal thickness in the area of Greece were obtained by means of the multiple-source Werner deconvolution (MSWD) method applied to gravity data. Thicknesses of 40-49 km are estimated beneath the Hellenides mountain belt to the west. Eastward thinning of the crust, to thicknesses ranging from 25 km in the north to 30 km in the south is seen in the Aegean region. These results are in good agreement with recent seismological results, demonstrating that the MSWD method successfully treated the problem. Using the crustal model we derived, we computed the gravity effect of the crust and extracted it from the Bouguer anomaly. We also extracted the gravity effect of the subducting lithosphere from the Bouguer anomaly, producing a residual map where most of the original gravity variation has been successfully removed. The remaining anomalies appear related to near-surface features and an area of low-velocity mantle in the central Aegean Sea.

Efficient ERT measuring and inversion strategies for 3D imaging of buried antiquities

ABSTRACT The optimum processing technique (2D vs. 3D inversion) to interpret and visualize parallel and/or orthogonal two‐dimensional surface Electrical Resistivity Tomography data collected from archaeological sites is examined in this work. A simple modification of a standard resistance‐meter geophysical instrument was implemented in order to collect parallel two‐dimensional sections along the X‐, Y‐ or XY‐direction in a relatively short time, employing a pole–pole array. The sensitivity analysis showed that the distance between the parallel 2D lines must be smaller or, at the most, equal to the basic inter‐electrode spacing in order to produce reliable 3D resistivity images of the subsurface. This was confirmed by modelling and inversion of both synthetic and real data. Direct comparisons of the quasi‐3D images, resulting from a posteriori combination of the inverted 2D sections, with the full 3D inverted resistivity models indicated the superiority of the 3D inversion algorithm in the reconstruction of buried archaeological structures, even in complex archaeological sites. Due to the inherent three‐dimensionality of many archaeological targets, quasi‐3D images suffer from artefacts. The combination of a single survey‐direction with a full 3D processing and interpretation scheme is adequate to image the 3D subsurface resistivity variation in detail. Furthermore, the implementation of a quasi‐Newton Jacobian matrix update technique reduced the processing time by one‐half without any significant loss of accuracy and resolution.

A large scale geophysical survey in the archaeological site of Europos (northern Greece)

Abstract The results of a large scale exploration of an archaeological site by geophysical means are presented and discussed. The operation took place in the site where the ruins of the ancient city of Europos are buried. This site is in northern Greece. Resistivity prospecting was employed to detect the remnants of wall foundations in the place where the main urban complex of the ancient city once stood. The data were transformed in an image form depicting, thus, the spatial variation of resistivity in a manner that resembles the plane view of the ruins that could have been drawn if an excavation had taken place. This image revealed the urban plan of the latest times of the life of the city. Trial excavations verified the geophysical result. Magnetic prospecting in the same area complemented the resistivity data. The exact location of the fire hearths, kilns and remnants of collapsed roofs were spotted. Magnetic gradient measurements were taken in an area out of the main complex of the ancient city and revealed the location of several kilns. One of these locations was excavated and a pottery kiln was discovered. The resistivity prospecting in one of the graveyards of the ancient city showed anomalies which were expected and corresponded to monumental tombs. The locations of a few of them were excavated and large burial structures were revealed. Ground probing radar profiles were measured over the tombs which showed pronounced resistivity anomalies, so far unearthed. The relatively high resolving ability of the method assisted the interpretation in the sense that a few attributes were added. In the presented case, it was concluded that a particular tomb consists of two rooms and that it is roofless.

The detection of monumental tombs buried in tumuli by seismic refraction

A tumulus is a construction erected to cover a tomb. Some tumuli are impressively massive and may conceal architectural masterpieces. Seismic refraction is employed to locate the tomb and to allow selective excavation without destroying the tumulus. The detectors are spread along a circular profile on the periphery of the tumulus, and acoustic waves are generated on its top. Time delays observed in the arrivals of the headwaves reveal the position of the monument. The delays are not caused by the monument itself, but are an effect caused by the presence of a ramp that was dug in the undisturbed soil to help in the construction of the tomb. Three case histories in Northern Greece establish the efficiency of the technique. In the first example, an experiment was conducted at a previously excavated tumulus, and time delays attributed to the revealed ramp are observed. The second case study led to the discovery of an impressive monument; 3-D modeling by finite difference verifies the interpretation. A third s...

Non-destructive DC resistivity surveying using flat-base electrodes

The application of flat-base electrodes to geoelectrical measurements is examined. This study is motivated by the inability to apply geoelectrical techniques in environments where conventional spike electrodes cannot be inserted into the ground. The performance of flat-base electrodes is examined in various environments, using different measuring modes. It is shown that flat-base electrodes can be satisfactorily used in most cases, producing data that are almost identical to the measurements obtained using standard electrodes. Several case studies in various urban locations in Greece, where flat-base electrodes have been successfully employed, are also presented. The results indicate that flat-base electrodes provide the advantage of a fully non-destructive application and, therefore, the extension of geoelectrical methods to environments that, otherwise, would not have been considered suitable.

Nature of the Levantine (eastern Mediterranean) crust from multiple‐source Werner deconvolution of Bouguer gravity anomalies

The nature of the Levantine (eastern Mediterranean) crust has been the subject of controversy for many years, revolving around two hypotheses: the continental crust hypothesis and the oceanic crust hypothesis. The proponents of the first hypothesis suggest that the Levantine (eastern Mediterranean) basin is characterized by a thick sedimentary succession overlying thinned crust of continental origin, through which a number of aborted Mesozoic rifts were etched. However, multiple-source Werner deconvolution (MSWD) estimates and other geophysical data, integrated with earlier geological and geophysical results, provide further support to the second hypothesis (oceanic crust) and lead to the following conclusions: (1) The depth to Moho ranges from about 20 km to about 28 km below sea level, with an average crustal thickness of about 22 km. (2) The large thickness (about 10 km) of Phanerozoic section leaves only about 12 km of thickness for the igneous/metamorphic (basement) complex. (3) The northern boundary of the Levantine (easternmost Mediterranean) lithosphere is delineated by an arcuate belt of seismic activities along the southern margin of Cyprus. (4) The formation of the Phoenician and Latakia basins and the Iskenderun Bay is probably controlled by the counterclockwise rotation of the lithospheric slices southeast of Cyprus. (5) The apparent absence of magnetic anomaly lineations (reversals) is due probably to the thick Phanerozoic cover, and/or the formation of the oceanic crust during a long magnetic chron.

A large-scale magnetic survey in Makrygialos (Pieria) Greece

A large-scale magnetic survey was conducted in the archaeological area of Makrygialos. The site was threatened due to the construction activities carried out in the area, as part of the national highway re-route project. Geophysical prospection contributed to the archaeological evaluation of the site, which was based mainly on the salvage excavations that took place prior to and after the geophysical survey. Magnetic prospecting was applied on a routine base, in order to cover a large area in a short period of time. Also, magnetic susceptibility was used to acquire detailed information of the stratigraphy of the ditches revealed by the excavations. The Le Borgne contrast was calculated and was used as an index of the magnitude of the magnetic anomalies. Geophysical data were processed by a number of filtering techniques, including the removal of regional trends and Hanning smoothing. Fourier transformation was applied and bandpass filtering procedure was based on the examination of the power spectrum of the data. In addition, two-dimensional inversion filtering was performed at specific parts of the data set, in an effort to rectify the significant geophysical anomalies of the site and obtain more information about their width and magnetization. The results of the geophysical survey were able to highlight a system of three curvilinear ditches, which excavation data suggested were probably dug during the Neolithic period. Various linear and geometrical anomalies, related to subsurface structures, are included among the other geophysical features encountered at the site. The geophysical prospecting techniques were able to map more than 60,000 m2 of the site, a large portion of which has now been destroyed by the construction activities for the national road. In this way, geophysical maps can be used as a valuable source of information for the future study of the site. The present case study illustrates the impact of geophysical exploration in the management of archaeological sites threatened by large-scale construction projects.

Two-dimensional inversion filters in magnetic prospecting: Application to the exploration for buried antiquities

The magnetic total field anomaly is considered as the convolution of two analytically determined functions. One of them is chosen such that it modifies the amplitude of the anomaly while the other controls the anomaly pattern. Using the anomaly of vertical‐sided finite prisms, the inverse of the shape function is computed in the Wiener mode. This is an optimum filter operator, in the least‐square sense, to be convolved with the data. The majority of the structures which are the targets of exploration at archaeological sites can be represented as assemblages of vertical‐sided prisms. This fact motivates the selection of the basic model. On that basis the filtering scheme results in anomalies centered at the epicenter of the disturbing bodies, delineates their lateral extent fairly well, and gives a measure of their magnetization. Applications of synthetic and actual data clarify the merits and disadvantages of the technique.