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Magnetotelluric method (CSAMT) in the exploration of deep hydrogeological targets

A groundwater research cannot be imaged today without application of the electrical resistivity tomography (ERT), especially in the areas that are characterised by very complex geological models. The method can be even considered as a fundamental geophysical method, especially in karst areas. However, there is a serious limitation of the method in the groundwater exploration. Namely, standard equipment for ERT is declared to reach target depths up to 130 m. The problem is even greater because the effective (real) depth penetration is in the range 70-100 m, depending on the resistivity relationships at the interface. Experience has shown that in most terrains the real depth penetration is 70-80 m. On the other hand, deeper hydrogeological investigations seek depth penetration of several hundred meters. In such cases the use of Controlled Source Audio Magnetotelluric (CSAMT) can solve the problem (ŠUMANOVAC & OREŠKOVIĆ, 2018). The capabilities of CSAMT method are analysed on the exploration case study in the Apatovec area. The study area is characterised by the complex geological relationships. According to the previous studies and surface geological data, the study area is covered by Quaternary sediments, Miocene rocks (Pontian, Pannonian and Badenian), Oligo-Miocene rocks, Cretaceous sediments and diabase. There is a wide range of permeable and impermeable clastic and carbonate rocks, which can be determined on the basis of different resistivities. Field measurements were performed by Stratagem EH4 system that allows frequencies up to 10 kHz to be recorded. The investigation depth depends on frequency and subsurface resistivity, so the depths between 500 m and 1000 m can be reached. Electrical field components (Ex, Ey) were measured on two perpendicular dipoles and two horizontal magnetic field components (Hx, Hy) were measured using induction coils. The surface impedance (Z) at a measurement site is calculated in two orthogonal directions and the apparent resistivity and phase are calculated from impedance components (Zxy and Zyx) The CSAMT data were recorded along profiles on MT-sounding stations with distances between stations from 15 to 50 m. The CSAMT data recorded along profiles were interpreted to obtain resistivity models that reflect subsurface geology in two ways, using inverse 2D modelling and forward 1D modelling (Fig. 1). We have employed an Occam’s inversion (DE GROOT-HEDLIN & CONSTABLE, 1990), with use of smoothing operator and additional contrast minimization to obtain simple and rather smooth resistivity model. The inverse resistivity model shows small resistivities, pointing to the impermeable rocks, at the surface and large resistivities, indicating permeable rocks, at greater depth (Fig. 1). But, the thickness of the low resistivity body is small (up to 40 m) at the beginning of the profile, and large (around 200 m) on the other part of the profile. The high resistivity body at the beginning of the profile probably consists of several high resistivity bodies and shallowest one is caused by the permeable Badenian rocks. The shape of high resistivity body points to reverse fault at the position of MT-4 station, which is also determined by the geological mapping. The 1D resistivity models can assist to define the interface of the high resistivity body. This study, but also many other exploration cases show that the CSAMT method provides the most complete data on lithological and structural relationships if deep hydrogeological targets should be reached (deeper than 100 m). It can reach targets at larger depths with dense space sampling at significantly lower costs. The CSAMT method provides information on the existence of aquifers and their general burial depths. The interpretation of CSAMT data is much more complex in relation to resistivity methods. More reliable models are obtained by the application of both, forward and inverse modelling since the interpretation is under the control of interpreter.

CSAMT, electrical resistivity tomography, groundwater, Apatovec

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