GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.3 ______ ___ GNGTS 2023 Method This works follows what has been shown by (Hernandez et al. 2022) and its main goal is to verify if it is possible to find a relationship between the MT data measured in the surface and the resistivity values measured in an exploratory well, and if the relationship exists, we want to assess if it could be used to obtain a geophysical model directly from the data. To determine if such assessment is possible an electrical resistivity model was proposed, and apparent resistivity measurement were simulated. Afterwards, following the approach proposed by (Gomez-Treviño, 1996) the apparent resistivity measurements were transformed into a first depth-apparent resistivity model. However, if we compare the differences between the model proposed and the model derived by the apparent resistivity measurements it becomes apparent that there are many differences that should be addressed. To model the behaviour between model and the data measured and to see if an expression can be used to correct those differences, we used a transformation of the resistivity data using an approach proposed by (Florio 2018) and (Socco et al. 2017) in which the resistivities from the model and from the MT measurements are transformed into their cumulative versions. These cumulative representations of the data are compared, and we use a polynomial relationship to model the difference in depth of equal resistivity values (cumulative and apparent resistivity respectively). This polynomial function can then be used to rescale the data and directly transform it into a cumulative resistivity model. The cumulative resistivity can then be used to calculate a layered resistivity model through a numerical derivative. The retrieved rescaling function is then applied to a different apparent resistivity curve for which the model parameters are different and used to directly transform the apparent resistivity data into a resistivity model. The input of the MT method will be the natural magnetic field ( B ) traveling downward in the z direction inducing a perpendicular electric field ( E ) that does not have a z component ( ). = 0 Considering a uniform Earth, the apparent resistivity measured at the surface for a particular ρ ( ) period T can be obtained by (Vozoff 1991) ρ ( ) = | | 2 2πµ 0 (1) Where is the electrical impedance and is the magnetic permeability of the void. The ( / ) µ 0 relation between to a penetration depth was derived by (Niblett and Sayn-Wittgenstein 1960) σ providing the following relationship ρ ( ) = 2πµ 0 0 ℎ ∫ σ( ) ⎡⎢⎢⎣ ⎤⎥⎥⎦ (2)