GNGTS 2013 - Atti del 32° Convegno Nazionale

The formula used to extract modulus and phase of the MT complex response at the Earth’s surface, i.e., the apparent impedivity function ρ a d (ω) , for the three-layer Earth with a dispersive intermediate layer is given as (Patella, 1993) (4) where t 1 and t 2 are the thickness of the 1st and 2nd layer, respectively, ρ 1 is the DC resistivity of the 1st layer and ρ 3 that of the substratum, and th stands for hyperbolic tangent. Moreover k 1 and k 2 are the wavenumbers in the 1st and the 2nd layer. New examples of 1D and 2D synthetic dispersive MT responses. We show the results from a simulation of the MT responses, when dispersion is assumed to characterize the electrical properties of a region of the explored half-space. A 1D three-layered earth, with its four A, Q, H, K type sections (Kaufman and Keller, 1981), is considered, with only the intermediate layer assumed to be dispersive. A fixed sequence of DC resistivities and thickness is attributed as in Fig. 1. Fig. 2 shows how large is the influence of the dispersion amplitude m on the shape of the MT responses. A large spread appears from the red curves, correspondingwith the lowest value of m, which nearly coincide with the dispersion-free curves, to the blue curves, corresponding with the highest value of m. These simulations show that the dispersion alters the shape of the curves in the same way as a lowering of the DC resistivity of the second layer does in a not dispersive situation. Such equivalence, without any external constraints, may make the interpretation of the curves quite ambiguous, as far as the maximum permitted slopes for 1D dispersion- free curves are not surpassed. It isworth stressing that theH-type layer sequence has been shown to be the most recurrent model, fitting the MT spectrum in oil and geothermal exploration (Pellerin, 1996; Zhdanov, 1994). In fact, strong dispersion phenomena may occur in a permeable rock, underlying a cover layer, because of the diffuse presence of mineral and clay particles formed by the aggressive action of uprising fluids from a subjacent reservoir. For the 2D case a model of magma chamber (10 Ω/m) at a depth of 1 km was considered, buried into a soil of 200 Ω/m. The synthetic responses, as the 1D model, were performed considering both the non- dispersive and the dispersive case. In Fig. 3 we show the differences of the 2D modelled MT responses compared with the true position and the dimensions of the body. As for the 1D case, the dispersion alters the resistivity values, particularly at the boundary of the buried body, leading to an ambiguous interpretation. The magnetotelluric method, as previously mentioned, may recognize induced polarization phenomena when the soil is polarizable; unless we have the certainty to perform surveys of polarizable media, magnetotelluric responses can be interpreted as corresponding to non- Fig. 1 – The three-layer A, Q, H and K type sections used for the 1D dispersive MT simulations. 120 GNGTS 2013 S essione 3.2