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Mapping of Deltaic Aquifers with the Combined Application of DC and TEM Soundings

Authors:

Abstract

The current study aims to clarify the hydrogeological regime of the deltaic valley of Pinios river (Thessaly, Greece). Its purpose is to map the existing aquifers below the deltaic plain, but also to assess the quality of their water (detect possible seawater intrusion), through the combined application of VES and TEM soundings. The results of the geophysical data processing revealed that the shallow (phreatic) aquifer is not detected throughout the entire deltaic plain, but only in the central and northern parts of the region, with thickness of 5-10 meters. Additionally, a deeper aquifer has been detected, with a maximum thickness equal to 100 meters. The interpretation of the geophysical soundings indicated that great part of the deeper aquifer has been affected by a saline intrusion that has also been noticed by hydrochemical data. Based on the fact that Pinios deltaic plain is a highly productive agricultural area, the irrigation system has to be reevaluated in order to constrain the extension of the seawater intrusion.
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... On the basis of the results of some preliminary geophysical research in the area, the results of applying the Vertical Electrical Soundings (VES) and Transient Electromagnetic Soundings (TEM) in the deltaic area of Pineios have somehow been discussed in the past by the authors (Alexopoulos, 2014;Alexopoulos et al., 2014aAlexopoulos et al., , 2014bAlexopoulos et al., , 2019aMatiatos et al., 2018), but mainly focused on hydro-geophysical characterization and the adumbration of the deltaic aquifers. The present study aims to investigate the deeper geological structure of the deltaic valley of Pineios river, through the combined application of gravity measurements, Vertical Electrical Soundings (VES) and Transient Electromagnetic Soundings (TEM). ...
... The geoelectrical and geoelectromagnetic methods seem to reach relatively smaller depths of investigation (up to 250-300 m). The geological interpretation of the VES and TEM data is practically based on the layered model of each station (Alexopoulos, 2014;Alexopoulos et al., 2014aAlexopoulos et al., , 2014bAlexopoulos et al., , 2019a along with some re-evaluations based on the gravity results. These data were taken into account in order to calibrate or/and control the gravity interpretation (García-Pérez et al., 2018). ...
... The resistivity values along the sections of Fig. 20 along with previous publications of the authors (Alexopoulos, 2014;Alexopoulos et al., 2014aAlexopoulos et al., , 2014bAlexopoulos et al., , 2019aMatiatos et al., 2018), dealing with the preliminary results of the geoelectrical and geoelectromagnetic methods, indicate that the Neogene formations (Ng) are saturated with water. It is also important to notice that moving towards the northern and eastern part of the deltaic area the resistivity values decrease at such a level that indicate seawater intrusion (also mentioned in Alexopoulos et al., 2019a). ...
Article
The current study aims to clarify the structural regime of the deltaic valley of Pineios river (Thessaly, Greece). The structural control of a deltaic area is usually a crucial parameter for its Palaio-geographical evolution and the latter needs to be clear for the contemporary conservation of such a sensitive ecotope environmentally. The investigation of the concealed subsurface tectonic structures was accomplished through the combined interpretation of gravity measurements, VES and TEM soundings. The standard gravity data reduction has been carried out and the residual anomaly was isolated with the contribution of the Fourier filters. The Euler deconvolution has been applied, providing the corresponding depth solutions between 159.8 and 1716.6 m. In the context of the qualitative interpretation, we produced several structural maps (THDR, VDR, Tilt and Theta) in order to enhance the edges of density sources that may reflect fault zones. Severe indications for the delineation of fault zones of the area were provided by these maps. Moreover, 3D density models of the area have been constructed illustrating the subsurface density distribution, up to depths of 3370 m. A main zone of lower densities in the central part of the delta has been revealed, surrounded by three zones of higher densities. Afterwards, the densities of the majority of the existing geological formations were determined with laboratory measurements from geological specimens. Therefore, three geophysical-geological profiles have been constructed, based not only on the gravity modelling but also on the geological interpretation of the geoelectrical layered models from the VES and TEM measurements. In conclusion, concealed and unknown tectonic structures of the Pineios deltaic valley have been identified beneath the surface.
... Regarding the presented case study of Pineios river, the authors have already discussed the results and the interpretation from the application of several geophysical studies, including the Vertical Electrical Soundings (VES), the Transient Electromagnetic Soundings (TEM) and gravity measurements [23][24][25][26][27] . ...
... The geological interpretation of the geophysical data was achieved calibrated with available drilling data ( Fig. 1 ) from [27] . Beyond that, the interpretation of the TEM soundings, was also cross-checked based on the published results of the geoelectrical method carried out by the authors [23][24][25][26] but also with the recently published results by [28] regarding the investigation of the deeper geological and tectonic structure of the deltaic plain by gravity measurements. ...
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This paper presents the methodology of an applied geophysical and remote sensing research at river deltas for their subsurface and surface structure and its application in the deltaic plain of Pineios River (Thessaly, Greece). The scope is to primarily calculate the thickness of deltaic post alpine deposits, with the contribution of Transient Electromagnetic Method (TEM), but also identify the surficial characteristics (biotic/ abiotic) of the area with the appropriate combination of spectral bands and spectral indices. Regarding the remote sensing approach, some geomorphological features were outlined. This was managed from six false color composites of the area, produced by combining spectral bands and additional five false color composites by combining several spectral indices. • The results of the TEM method along with their statistical analysis provided important information regarding the spatial distribution and thickness of the lithological units along the deltaic plain. • The combination of spectral bands 8, 7, 6 seems to adumbrate the hydrographic network of the area and even detects possible changes in the inflow. • The combination of spectral indices MSAVI2, WV-BI, WV-WI was helpful enough to geomorphological mapping of the deltaic plain.
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Relevance. In this paper the feeding mechanism of springs distributed around the broader area of Delfini at NE side of Chios Island is explored, in an effort to locate possible fresh water paths inland before they are discharged to the sea. The drilling of hydro wells for more production is of vital importance to cover the drinking and irrigation needs of the broader area. Aim. Although Chios has been extensively drilled in the past years the demand for further investigations comes from an increased water consumption during summer time in one hand and the inefficiency of alternative water resources (e.g. construction of dams, desalination units, etc) on the other hand. Methods. Geophysical methods were used (VLF and resistivity) to explore in detail water potential areas, like the broader area of Delfini, where during winter time huge quantities of fresh water discharge through springs to sea level. Results and its discussion. The presence of a confined aquifer composed mainly of limestones, which are overlain by clastic impermeable deposits, together with the existence of water potential bearing fracture zones determine the hydrogeological regime of the nearby area. It is also explained the presence of brackish waters at positive elevations found during summer time at nearby springs. It seems that the lateral extent of impermeable clastic deposits plays a crucial role in allowing the inland flow of seawater during summer time and combined with the presence of the confined aquifer of fresh water produce an upward mixing flow of fresh and seawater at springs of positive elevations. During winter time the huge amount of fresh water discharges do not allow the inland flow of seawater.
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