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Application of electrical resistivity tomography for imaging seawater intrusion in a coastal aquifer

Niculescu Bogdan Mihai, Andrei Gina

Acta Geophysica

2021

Vol. 69, no. 2

613--630

Following previous geoelectrical researches initiated in 2009 for the delineation and characterization of seawater intrusion in a freshwater aquifer of Sarmatian (late Middle Miocene) age, a subsurface imaging survey via 2-D electrical resistivity tomography (ERT) was conducted in 2019 on the outskirts of Vama Veche resort—Romanian Black Sea southern coast. The survey was organized in the framework of a feld camp sponsored by the Foundation of the Society of Exploration Geophysicists (SEG)–Tulsa, OK, USA, with participation of teams from the University of Bucharest—Department of Geophysics and the Geological Institute of Romania. A number of eight ERT profles with N–S, W–E, NNE–SSW, and WNW–ESE orientation and 155–315 m length were imaged with a SuperSting R8/IP instrument (Advanced Geosciences Inc.), using deployments of 32–64 electrodes at 5 m spacing, in Wenner, Schlumberger, and dipole–dipole array confgurations. The processing and interpretation of high-resolution ERT data indicated that the seawater intrusion, evidenced as very low resistivity (5–10 Ω m) anomalous zones starting at approximately 45–49 m depth, has advanced at least 500 m inland. The survey results also allowed the identifcation of a system of fractures or faults with an approximate NW–SE/WNW–ESE and, possibly, N–S orientation, that might have provided potential pathways for saline water intrusion.

Using Vertical Electrical Soundings to characterize seawater intrusions in the southern area of Romanian Black Sea coastline

Mihai Niculescu Bogdan, Andrei Gina

Acta Geophysica

2019

Vol. 67, no. 6

1845--1863

Seawater intrusions are a major environmental hazard for coastal freshwater aquifers. They are generated mainly by the uncontrolled exploitation of freshwater in pumping stations, if the aquifers are in hydraulic connection with the sea. In Romania, such marine intrusions have occurred in the southern part of Black Sea’s coastline, in Costinești and Vama Veche resorts, contaminating the main aquifers hosted in Sarmatian (late Middle Miocene) limestones, at distances ranging from hundreds of meters to over 2 km inland. For the study of these salinization phenomena, Vertical Electrical Sounding (VES) surveys were performed in the affected areas. These surveys allowed the delineation and spatial–temporal monitoring of the intrusions and offered information related to faults that may have provided pathways for seawater migration toward the exploitation wells. The 1D interpretation of VES apparent resistivity data was performed via a set of novel software applications. The forward modeling component of the applications uses digital linear filtering and allows the simulation of theoretical VES responses for horizontally-layered geological media with virtually unlimited number of layers. The pseudo-inversion component of the applications is based on a random sampling of the parameters space of the geoelectrical models. The interpretation of VES surveys recorded in Vama Veche area by using the elaborated software indicates that the seawater intrusion occurs at more than 40 m depth. This agrees with a well flow test which produced saltwater at 40–60 m depth in that area.