H. Hamdan

Joint inversion of 2D resistivity and seismic travel time data to image saltwater intrusion over karstic areas

The simultaneous inversion of multiple geophysical data types has been proven to be a powerful tool to both improve subsurface imaging and help in the interpretation process. The main goal of this paper was to develop joint inversion strategies to provide improved resistivity and seismic velocity images for delineating saline water zones in karstic geological formations. The cross-gradient constraint approach was adopted to jointly invert resistivity and seismic first arrival data. The basic idea of this approach is to quantitatively estimate the structural similarity between resistivity and seismic velocity models, using the cross product of their gradients and to achieve a unified geological model which satisfies both data sets. Initially, synthetic data were employed to help develop a joint inversion strategy to be used over such complex geological structures. The proposed strategy uses a weighting factor for the cross-gradient constraints and separate damping factors for the resistivity and seismic data. This strategy was applied successfully on field data from the karstic region of Stilos, Crete, Greece.

Time-varying deconvolution of GPR data in civil engineering

Ground Penetrating Radar (GPR) profiles are often used in civil engineering problems. Overlapping reflections from thin subgrade layers are observed when a relatively low frequency antenna is used. An efficient GPR data processing method, which increases the dominant frequency of GPR data and the temporal resolution, is proposed. It is implemented in the t–f domain. The proposed time-varying deconvolution technique avoids the need for both the calculation of an inverse zero-phase whitening operator and subsequently the application of a band-pass filtering. The user must select the dominant frequency of the Ricker wavelet and use the phase of a reference electromagnetic wavelet, which is acquired experimentally, for stationary dephasing. Apart from delineating thin layers, this method reduces the number of antennas for imaging both shallow and deeper layers in civil engineering. The effectiveness of the proposed method is demonstrated through four civil engineering applications.

Contribution of electrical tomography methods in geotechnical investigations at Mavropigi lignite open pit mine, Northern Greece

In this paper, the application of 2D and 3D electrical resistivity methods in geotechnical investigations is explored through a case study in Northern Greece. These two methods were employed at a lignite surface mining operation where fracture zones and discontinuities have been recently observed close to the pit boundaries. The main aim of the geophysical survey was to estimate the inclination of the contact between the Neogene and Schist/Carbonate formations near the southern limits of the pit, as well as to estimate the thickness of the carbonate rocks on top of the Schist formations to evaluate the stability of the southern slopes. Synthetic data were initially generated to help plan an efficient electrical tomography survey, in a region with complex geology and irregular terrain. Three configurations (Wenner–Schlumberger and dipole–dipole or pole–dipole) proved essential in such conditions and helped improving the resolution of the resistivity section. The sections were then calibrated by boreholes. Finally, the geophysical survey provided invaluable data regarding the geometry of the bedrock and possible faults, which was essential for the slope stability calculations.

Application of Classification Methods on Geophysical Data from the Archaeological Site of Aptera, Chania, Greece

Classification techniques such as the self-organizing-maps (SOM) network are proposed as a tool for efficient interpretation of geophysical data obtained from archaeological investigation. This methodology was useful for the interpretation of georadar data from the archaeological site of Aptera, Chania, Greece where an integrated geophysical survey covered 9730 m2. The survey was realized under the auspices of the 25th Ephorate of Prehistoric and Classical Antiquities, Ministry of Culture. Soil resistance and magnetic gradient maps, resistivity and georadar sections and 3D images, aerial and ground photos, archaeological and geologic maps were registered in a GIS project. The geophysical maps display the ancient city plan. Resistivity depth slices exhibit a stone corridor at a depth of 0.34 m. Three-dimensional inversion was applied on electrical tomography data collected along parallel survey lines. The resulted three-dimensional image reveals a water pipeline. Instantaneous and geometric attributes calculated from the migrated georadar sections over a roman cistern were subsequently classified using SOM. The resulted classified 3D georadar image displays the arc-shaped roof of the cistern as well as stone pipelines which supplied the cistern with rain water.

Inversion Techniques to Improve the Resistivity Images over Karstic Structures

The complexity of karstic structures causes major problems in the processing and interpretation of 2D resistivity data. This paper investigates the optimum inversion method to be applied in such cases. Three inversion methods (combined, smoothness constrained and robust inversion) are applied on real data set and their results are compared. In addition, joint inversion of two electrode arrays, namely Wenner-Schlumberger and dipole-dipole, has been employed to achieve the maximum reliability of the 2D resistivity model. The resistivity data have been collected from a strongly karstic site where saline water intrusion is present.

Geoelectrical Mapping of the Omalos Plateau/Polje, Chania, Western Crete, Greece

This study case presents the results of a geoelectrical survey that was carried out at Omalos plateau in Chania, Western Crete, Greece. The survey included measurements of resistivity mapping and sounding as well as electrical tomography. The aim of this survey is to image the complex structure of the active karstic system of the Omalos polje. The processing of the measurements indicates three geoelectrical layers. The shallow layer exhibits extremely high electrical resistivity and is attributed to alluvial – dilluvial deposits. The intermediate layer exhibits very low electrical resistivity and reflects mainly the presence of marls (Neogene?). Finally, the lower geoelectrical layer outlines the metamorphic mainly carbonatic bedrock (Tripali unit and Plattenkalk group), whose relief appears highly irregular. At Omalos polje the thickness of post-Mesozoic deposits (terra rossa, clays, sands and gravels) ranges from 40-130 m.

Imaging the Saline/Fresh Water Interface at Bafra, Turkey Using Joint Inversion of Seismic Refraction and ERT Data

A geophysical survey was conducted at Bafra, Samsun, Turkey, Greece in order to support the geological and hydrogeological studies of the existing phenomenon of seawater intrusion. This survey employed seismic refraction tomography (ST), time domain electromagnetic (TDEM) and geoelectrical methods. The later, consisted of Electrical Resistivity Tomography (ERT). The objective of this survey was to image the saltwater intrusion at this area and map possible favorable sea water pathways. We present the interpretation of geophysical data obtained under the research project ClearWater project. We applied both individual and joint inversion of electric and seismic data from Bafra. At Bafra, the geophysical survey provided images of the saltwater intrusion which is present at the north eastern of the study area. These geophysical methods proved useful as prediction and prevention tools in qualitative characterization of water bearing formations for an optimum groundwater management plan for the Bafra basin.

Imaging of Alpine and Post-Alpine Formations Using ERT at Loutsa- Remeri Open Pit Mine, Scourta, Viotia, Greece

A geophysical survey carried out at Scourta plateau in Central Greece employed the electrical tomography method in order to image the interface between Alpine and Post-Alpine formations and estimate the thickness of Terra Rossa deposits. The later are crucial for TITAN-SA cement company open pit mine. This mine provides row materials for cement production. This survey is related with future extension plan of the existing Loutsa-Remeri open pit mine. Scourta is an active polje and is filled with a thick Terra Rossa deposit which is a row material for cement industry. The geophysical survey revealed two distinct layers, the upper is characterized by lower resistivity values (less than 20 Ωm), minimum thickness to the east (25 m) and maximum thickness at least 50 m and is attributed to Terra Rossa deposits. The underlying layer exhibits higher resistivity values (more than 70 Ωm) and is attributed to karstified carbonate relief. The variation of the polje depth is accosiated with tectonokarstic processes. The E-W direction fault system delineates the karstic structure and the N-S ridge in the east part of the survey area is delineated by N-S direction faults.

Salt Water Intrusion Imaging at Tybaki (Greece) Using Geophysical Methods

A geophysical survey was conducted at Tybaki, Crete, Greece in order to support the geological and hydrogeological studies of the existing phenomenon of seawater intrusion. This survey employed seismic refraction, transient electromagnetic (TEM) and geoelectrical methods. The later, consisted of Vertical Electrical Sounding (VES) and Electrical Resistivity Tomography (ERT). The objective of this survey was to image the saltwater intrusion at this area and map possible favorable sea water pathways. We present the combined interpretation of vintage geophysical data obtained under the research project MEDIS and the ClearWater project data. At Tybaki, the geophysical survey provided images of the saltwater intrusion which is present at the north west of the basin.

Imaging of Karstic Carbonates Using Electrical Resistivity Data at Malia, Crete, Greece

This work involves the use of resistivity and borehole data for the delineation of weak zones in karstic carbonates at Malia, Crete, Greece, where two water reservoirs will be constructed. Site M mostly consists of cretaceous carbonates belonging to the Tripolis zone while Site K consists of upper triassic carbonates of the Tripolis zone. Electrical tomography data were combined with seismic refraction and borehole data. Electrical tomography imaged karstic structures of the Tripolis carbonates at sites M and K. A 3D resistivity image was generated by inverting resistivity data collected along parallel lines on a grid at site M. At this site, electrical resistivity imaging delineated fractured and weathered limestone zones whose thickness ranges from 20 to 40 m. 2D ERT images of the northern part of site M indicate a major low resistivity anomaly. The formations at site K are mostly characterized as slightly weathered limestone (H) with higher RQD values (more than 60). The existing voids are small and the fractured zones exhibit limited extent at site K.