GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.2 GNGTS 2024 For the Loupe system, a quick transmiter waveform turn-of (10 μs) allows to get a good resoluton in the near surface while the of 20 A of peak current and 13 turns in the coil allow to reach a good depth of investgaton, exceeding 50 m. With this work we aim to assess if and how the system is sensitve to Induced Polarizaton (IP) efects. IP efects on EM data have been studied since the early ‘80s (e.g. Spies, 1980; Lee, 1981), and have re-gained atenton in recent years thanks to the ability to invert for IP efects on EM data in terms of Cole-Cole models (e.g. Viezzoli et al., 2017; Lin et al, 2019; Couto et al., 2020; Grombacher et al., 2021). However, the importance of IP modelling in EM data is stll ofen overlooked, and false structures (incorrect conductvity-thickness parameters) may be recovered when IP is not properly modelled (Viezzoli et al. 2017). In order to study the IP efect on the Loupe system, we focused on a simple but representatve case: a two-layer model with a chargeable cover over a resistve bedrock. This case may represent targets suitable for the Loupe system: mine wastes; permafrost; weathered covers over bedrock on mountainous areas. The study is performed varying the electrical propertes of the cover layer, and its thickness, in order to highlight quanttatvely the signifcance of the IP efect. Method In order to defne how the Induced Polarizaton afects the Loupe electromagnetc response, we compute the forward response (following Auken et al., 2015) from a large amount of two-layer models, consistng of a chargeable cover over a resistve bedrock. The model space is defned in terms of the Cole-Cole resistvity model, as defned by Pelton et al. (1978): (eq. 1) Table 1 contains the Cole-Cole parameters used for simulatng the frst chargeable layer, in all combinatons. The second layer instead has been always considered a purely resistve layer at 1000 Ω*m. For each Cole-Cole two-layer model, we calculate a corresponding purely resistve two-layer model (using the DC resistvity value of the Cole-Cole model), and we compare the responses. m IP = { ρ j , m 0 j , c j , thk j }, j = 1,2 Chargeable frst layer Rho [Ω*m] m0 [mV/V] Tau [s] C Thk [m] 1 5 1e-04 0.5 5 2 8 3e-04 20 3 13 1e-03 4 20 3e-03 6 30 1e-02 10 50 3e-01