GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.3 149 3D body has horizontal length of 6 km along the y-axis and square edges of 2×2 km 2 . The top face is located at 2 km depth. The simulations have been done along a 8 km long profile parallel to the x-axis which crosses the prism’s mid-length. 17 MT station points, spaced 500 m from each other, have been assumed along the profile. The IPFD spectrum in rocks is mostly modeled using the Cole-Cole type impedivity function ρ CC (ω) (Cole and Cole, 1941), given as ρ CC (ω )= ρ 0 {1− m [ iωτ c /(1+ iωτ c )]}. (1) In Eq. 1, i =√− 1, ω is the angular frequency, ρ 0 is the DC resistivity, and m ∈ [0,1], known in mining geophysics as chargeability (Seigel, 1959), is the IP amplitude, defined as m =( ρ 0 − ρ ∞ )/ ρ 0 , where ρ ∞ ∈ [0, ρ 0 ] is the resistivity at infinite frequency. Moreover, c ∈ [0,1] is the decay spectrum flatting factor and τ ≥ 0 is the main time constant. For the sake of conciseness, only one example of simulation is presented, which refers to the following Cole-Cole parameter set for the dispersive prism: m =0.9, τ =10 s and c =0.75, the blue line refers to the reference non-dispersive case and the red lines to the dispersive assumption. Along the profile, centered over the prism (Fig. 1), the departure of the red lines from the reference blue lines, for both modulus and phase of the TE and TM modes, is very evident. The IP effect manifests with a more pronounced minimum of both the TE and TM apparent impedivity modulus, centered above the median axis of the 3D dispersive body. The amount of distortion of the MT response over a polarizable body depends on the values assigned to the Cole-Cole parameters, m , τ and c . Mauriello et al . (1996) gave a detailed overview on this topic, and inferred from 2D simulations that high values of m (not less than 0.75) and τ (not less than 100 s) are ideal for dispersion effects to be recognizable in MT measurements. They also showed that the TE is always the most distorted MT mode. Esposito and Patella (2009) showed that c has practically no remarkable distortion effect in MT on 1D structures. The low influence of c was also inferred by Mauriello et al . (1996) in 2D cases. The results from a great number of simulations above the model in Fig. 1, carried out following the same approach as in Mauriello et al. (1996) by changing m and τ with fixed c , fully confirm all of the above conclusions. A further consideration is that visible distortion effects can be obtained even when the m and τ vary in opposite directions. The sense is that the choice of an exceedingly low value of one of them must be compensated by a quite high value of the other one. Snake river plain example. We now show a field example already studied by Mauriello and Patella (1999) in the frame of the probability tomography imaging. The area is the eastern Snake River Plain (SRP), Idaho, where a MT profile was performed by Stanley (1982), near the Idaho National Engineering and Environmental Laboratory. The SRP is an arcuate depression bounded on both sides by the Basin and Range structures, and for much of its extent it is underlain by basalt and interbedded continental Quaternary and Tertiary sediments (Mabey, 1982). Mauriello and Patella (1999) applied the 2D probability tomography imaging to the SRP TM and TE pseudosections in Fig. 2 and evinced within the SRP depression a laterally bounded conductive slab with lateral extension of about 17 km and a mean depth to its top of about 2 km. Dispersion effects were also admitted, in order to explain the occurrence of a charge polarity inversion at the top edges of the slab and a tightening of the same edges as frequency decreases. Using the 2D dispersive MT forward modeling by Mauriello et al. (1996), the 2D structure depicted in Fig. 2 was then proposed, as best conforming to the large-scale geometry and dispersion inferences from the probability tomography sections. Finally, by comparing the synthetic TM and TE pseudosections from the 2D model in Fig. 2 with the original ones, Mauriello and Patella (1999) found a good matching of the two TM pseudosections, while the TE ones were judged not properly conforming to each other. They attributed this discrepancy to the circumstance that the TE mode is much more affected than the TM mode whenever