P. Soupios

Use of engineering geophysics to investigate a site for a building foundation

The combination of geophysical data and geotechnical measurements may greatly improve the quality of buildings under construction in civil engineering. A case study is presented here at a vacant building site. Initially, boreholes indicated a complex geology. A dipole–dipole configuration was selected for electrical resistivity tomography (ERT) implementation and the data were processed and interpreted by applying 2D and 3D inversions. An electromagnetic survey was also carried out at a different time periods and successfully used to verify the results of the resistivity measurements. It is demonstrated that engineering geophysics is able to provide solutions for determining subsurface properties and that different prospection techniques are necessary for developing a reasonable model of the subsurface structure.

Energy efficiency and environmental impact of biogas utilization in landfills

This study investigates the utilization of landfill biogas as a fuel for electrical power generation. Landfills can be regarded as conversion biogas plants to electricity, not only covering internal consumptions of the facility but contributing in the power grid as well. A landfill gas plant consists of a recovery and a production system. The recovery of landfill gas is an area of vital interest since it combines both alternative energy production and reduction of environmental impact through reduction of methane and carbon dioxide, two of the main greenhouse gases emissions. This study follows two main objectives. First, to determine whether active extraction of landfill gas in the examined municipal solid waste sites would produce adequate electric power for utilisation and grid connection and second, to estimate the reduction of sequential greenhouse gases emissions. However, in order to optimize the designing of a plant fed by biogas, it is necessary to quantify biogas production over several years. The investigation results of energy efficiency and environmental impact of biogas utilization in landfills are considering satisfactory enough both in electric energy production and in contribution to greenhouse gases mitigation.

Spatial spectral variations of microtremors and electrical resistivity tomography surveys for fault determination in southwestern Crete, Greece

The horizontal to vertical spectral ratio (HVSR) technique using microtremors and electrical resistivity tomography (ERT) surveys reveal a potentially seismic active source in southwestern Crete located within the outer forearc of the Hellenic subduction zone in one of the most seismically active deformed regions in Europe. The combined approach is applied on the Pahia Ammos coast southwest of the Paleohora peninsula and reveals an almost E–W-striking fault crosscutting the dense populated area. Spatial HVSR variations in the fundamental frequencies and HVSR shapes using microtremors pattern the effects of surface and subsurface structure on seismic ground motion and are capable of delineating fault zones. One clear HVSR peak in the low frequencies is related to the thickness of the alluvial deposits. Two amplified frequencies are attribute to lateral heterogeneities/irregularities induced by the fault zone and thickness variations of the geological column overlying the lateral irregularities of near-subsurface structure. Dipole–dipole and Wenner–Schlumberger configuration arrays are conducted to model the surface and subsurface structure variations. The identified fault zone striking E–W inland is capable of enhancing ground seismic motion and significantly contributes to the seismic hazard assessment of the studied area. Geophysical results are cross-correlated, verifying the validity of the research outcome.

Landslide susceptibility mapping by comparing the WLC and WofE multi-criteria methods in the West Crete Island, Greece

In the current research, the weighted linear combination (WLC) and the weights of evidence (WofE) methods were applied for landslide susceptibility zonation mapping in the Chania Prefecture of Crete Island, Greece. Several thematic maps representing various landslide conditioning factors, such as geology formations, faults proximity, altitude, slope gradient, aspect, curvature, rivers proximity, precipitation, roads proximity and land use types have been generated in a GIS environment. Three landslide susceptibility maps were created, one for each method and a combined one by applying the logistic regression approach to the WLC and WofE susceptibility maps. The maps were evaluated and validated using the efficiency rate curve, the receiver-operator curve and the spatially agreed areas methods. The resulting landslide susceptibility maps have uncertainties introduced due to the subjective knowledge of experts in the case of WLC method and to the quality of the recorded landslides sample in the case of the WofE method. Nevertheless, the performance of the three approaches was found to be almost equal with all methods to produce quite satisfactory results. Besides the comparison of the applied methods, the current study contributes to the risk management of the area, providing the first landslide susceptibility maps for Chania Prefecture.

Saltwater intrusion in an irrigated agricultural area: combining density-dependent modeling and geophysical methods

Saltwater intrusion is one of the most important water quality problems in coastal aquifers, especially in areas with increased water demands. Geophysical techniques can provide a non-invasive and cost-effective approach for determining the geometrical characteristics of an aquifer and for guiding the saltwater intrusion modeling process and in turn reducing the model’s inherent uncertainty. In this work, the above concept was applied in the Tympaki basin in Heraklion, Greece. The transient electromagnetic method was used to obtain an accurate 3-D geomodel (bedrock geometry and fault detection) of the basin. This, in turn, was used to guide the construction of a density-dependent groundwater flow and transport simulation model. The results show significant advancement of the saltwater intrusion front in the northern part of the study area, while the phenomenon is less pronounced in the central and southern parts. This is mainly attributed to the combined effect of the fault in the northern part of the basin, the uplifted Neogene deposits in the central part and the freshwater inflow from the Festos corridor in the southern part.

Geochemistry and quality of the groundwater from the karstic and coastal aquifer of Geropotamos River Basin at north-central Crete, Greece

In Geropotamos River Basin, located on the north-central part of Crete, Greece, two main factors were believed to be affecting the geochemistry of the groundwater with high salt contents: seawater intrusion and/or Miocene evaporates. To identify the origin of the high salinity in groundwater, a hydrogeochemical and isotopic study was performed. Water samples from 22 wells and 2 springs were analyzed for physico-chemical parameters, major ions analysis, as well as stable isotopes (δ18O, δD). From the present survey, in which detailed hydrogeochemical investigation was conducted, the uncertainty of the contamination sources was decreased in the northern part of Geropotamos Basin. The results complement the scenario in which seawater and the widespread human activities are the principal sources of groundwater contamination. Moreover, the results of the stable isotopes analyses (δ18O and δD) support the same hypothesis and make seawater intrusion the most probable cause for the highest salinity waters. It is indicated that saline intrusion is likely to occur along fractures in a fault zone through otherwise low-permeability phyllite–quartzite bedrock, which demonstrates the critical role of fracture pathways in salination problems of coastal aquifers.

Aquifer characterization using shallow geophysics in the Keritis Basin of Western Crete, Greece

Effective management of groundwater resources is a well-known problem in several areas around the world. It is especially important to areas suffering from an intrinsic lack of fresh water, such as islands. Detailed study of available aquifers is of particular interest in the highly touristic Greek island of Crete. The increasing water demand makes water resources management extremely important for sustainable development. This is the case in the prefecture of Chania in Western Crete, where there has been a lack of success of many different groundwater management plans that have been produced over the years and submitted for application to water management authority. The inefficiency of the management plans is verified by the continuous water shortage reported every year which forces the public authorities to transfer water from far away sources and wells. Until today there are only geological data as well as information from the borehole logs; therefore, the current work is focused to study in detail the tectonic and hydrogeological characteristics of the Keritis watershed to make a correlation of the available geoenvironmental information with the geophysical data of the study aquifer. Transient electromagnetic soundings were conducted to obtain detailed information about: the tectonic, hydrogeological, hydrolithological, and geometrical characteristics of the aquifer under investigation. In addition, optimum areas of water well drilling were identified to minimize the uncertainty and the total cost (economical, managerial, etc.) of future groundwater surveys.

Modelling tectonic features of the Kissamos and Paleohora areas, Western Crete (Greece): combining geological and geophysical surveys

The purpose of this survey is the fault zone determination in Kissamos (NW Crete) and Paleohora (SW Crete) basins. The study area is located within the central forearc of the Hellenic subduction zone. 2D electrical resistivity tomography (ERT) has been applied to reveal fault zones. Ground-truthed fault evidence in the coast northwest of Kastelli–Kissamou and northeast of Paleohora is incorporated into the ERT data. Thirteen ERT profiles were obtained at several sites. Seven of the eight ERT profiles intersect fault zones in Kissamos. Five ERT profiles were conducted in Paleohora and three significant faults were identified. The results indicated the continuation of previously mapped faults as well as revealing unreported faults. The coastline of Paleohora and Grammeno can be associated with a system of fault zone striking almost E–W. We propose that the ERT method is a reliable and economic method at identifying buried fault zones in populated areas. Considering the proximity of the revealed fault zones in densely populated areas and the high seismic activity of the region the determination of the identified fault zones could contribute in earthquake hazard assessment studies for future seismic mitigation and urban planning strategies in the two areas for western Crete.

Geophysical investigations at the Istron archaeological site, eastern Crete, Greece using seismic refraction and electrical resistivity tomography

The subsurface characteristics of the archaeological site of Priniatikos Pyrgos at Istron, Eastern Crete were studied through the combined use of seismic and electrical tomography techniques. Twenty-nine seismic refraction profiles were applied to determine soil thickness and bedrock geometrical characteristics. First arrivals were picked and a 3D algorithm of seismic tomography was used to invert the travel times. To confirm the reliability of the final results a checkboard test was used. Part of the seismic area was overlapped with six electrical resistivity tomography sections. The ERT data were used to validate the resulted tomographic images. The integrated application of these tomographic methods enhanced the knowledge regarding the geological conditions and contributed in the archaeo-environmental reconstruction of Istron area by providing indications regarding the ancient harbour of the nearby settlement.

Reconstructing former urban environments by combining geophysical electrical methods and geotechnical investigations—an example from Chania, Greece

Nowadays, geophysical prospecting is implemented in order to resolve a diversity of geological, hydrogeological, environmental and geotechnical problems. Although plenty of applications and a lot of research have been conducted in the countryside, only a few cases have been reported in the literature concerning urban areas, mainly due to high levels of noise present that aggravate most of the geophysical methods or due to spatial limitations that hinder normal method implementation. Among all geophysical methods, electrical resistivity tomography has proven to be a rapid technique and the most robust with regard to urban noise. This work presents a case study in the urban area of Chania (Crete Island, Greece), where electrical resistivity tomography (ERT) has been applied for the detection and identification of possible buried ancient ruins or other man-made structures, prior to the construction of a building. The results of the detailed geophysical survey indicated eight areas of interest providing resistivity anomalies. Those anomalies were analysed and interpreted combining the resistivity readings with the geotechnical borehole data and the historical bibliographic reports—referring to the 1940s (Xalkiadakis 1997 Industrial Archaeology in Chania Territory pp 51–62). The collected ERT-data were processed by applying advanced algorithms in order to obtain a 3D-model of the study area that depicts the interesting subsurface structures more clearly and accurately.