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FORWARD MODELING OF VERTICAL ELECTRICAL SOUNDINGS WITH APPLICATIONS IN THE STUDY OF SEA WATER INTRUSIONS

Bogdan Mihai Niculescu

Abstract

Salt water intrusions of marine origin are a major cause of contamination for coastal fresh water aquifers. They are generated mainly by the uncontrolled exploitation of fresh water in pumping stations from coastal areas. In Romania, such marine intrusions have occurred in the southern part of Black Sea's coastline, in the area of Vama Veche and Costine?ti resorts, contaminating the main aquifers which are hosted within Sarmatian (late Middle Miocene) lumachelic and oolitic limestones. Fresh water exploitation management requires the identification, mapping and monitoring of marine intrusions. These are achievable by means of an adequate application of geoelectrical methods, because sea water intrusions lead to major variations of the aquifers resistivity. Apparent resistivity surveys such as electrical profiling and vertical electrical sounding (VES) are able to delineate the areal extent of these intrusions and to identify the approximate depth of the fresh water / salt water interface. Assuming the subsurface geology consists of horizontal and homogeneous layers, the VES apparent resistivity data can be interpreted in terms of layer thicknesses and true resistivities, i.e. a one-dimensional (1-D) geoelectrical model with a stepwise variation of true resistivity with depth. The optimal model is obtained by repeated forward modeling trials or through the inversion of measured apparent resistivities. This paper presents an algorithm and a set of software applications for the forward modeling of VES apparent resistivity curves, by using digital linear filtering. The codes were elaborated in MATLAB programming environment and allow the calculation of the theoretical VES response for horizontally-layered geological media with virtually unlimited number of layers, including layers with a quasi-continuous resistivity variation. The latter option can be used to simulate particular hydrogeological situations regarding the sea water intrusions in fresh water aquifers, which often show a mixing/transition zone of variable salinity and resistivity. The interpretation of VES curves recorded in Vama Veche area by using the elaborated software indicates that the sea water intrusion occurs at more than 50 m depth. This agrees with water wells tests which confirm the presence of salt water at 40-60 m depth in that area.

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DOI resolver: doi.org/10.5593/sgem2018/1.1/s05.101

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