Document Type : Research Article
Authors
1
Assistant Professor, Institute of Geophysics, University of Tehran, Tehran, Iran
2
M.Sc student, Institute of Geophysics, University of Tehran, Tehran, Iran
3
Ph.D., Iranian Gas and Engineering developmentcompany, Iran
Abstract
We investigated an MT dataset composed of 284 broadband (10-4-3414 sec) MT stations along seven profiles to unravel the electrical properties of sub-surface structures in the Nar-Abad region, west Central Iran. The region is composed of five tertiary salt diapirs developed along the Ab-Shirin-Shurab strike-slip fault zone. The MT profiles are extended perpendicular to the general trend of the Zagros Orogenic belt (in an SW-NE direction) which is one of the main structural elements controlling regional deformation in the Iran plateau.
A crude image of electrical conductivity distribution is provided by analysis of impedance data:
(1) A shallow conductive layer appears throughout the study region; while apparent resistivity-sounding curves show small values throughout the region, high values of impedance phases occur at short periods (< 1 sec).
(2) Moving along the profile: (i) apparent resistivity generally decreases from 423 Ωm in the SW to 0.2 Ωm in the NE. (ii) the splitting between sounding curves of ρTE and ρTM decreases, and it occurs at longer periods. Accordingly, one can conclude that the shallow conductive layer extends to the deeper part beneath the NE of the profile. Additionally, a more complex conductivity structure is expected beneath the SW of the profile.
(3) The Abshirin-Shurab fault significantly influences the apparent resistivities at the SW end of most profiles. While the TM mode apparent resistivity soundings predominate those of the TE mode values to the west of the fault trace, this setting is flipped over to the east of the fault.
In the next step, we characterize the structural dimensionality of MT data by commonly used Bahr rotational invariants (κ, μ, η, ∑) and the phase tensor skew angle (β). The phase-sensitive skew (η), the regional 1-D indicator (μ), and β skew angle depend on the phase information inherent in the impedance tensor.Therefore, they are affected primarily by large-scale induction anomalies and are immune to low-frequency galvanic distortions. The thresholds assigned for μ, η, and β are 0.1, 0.3, and 3˚ respectively. The skew values calculated from the Nasr-Abad MT data set suggest that the regional conductivity structure is 2D rather than 3D or 1D as the calculated η remains below 0.3 and μ above 0.1. The appropriate category of data is therefore, responses from a regional 2D structure contaminated by galvanic distortion effects.
The strike determination method applied is the phase tensor analysis. It does not require any assumption about regional conductivity structure, and its results are not susceptible to galvanic distortion. The method determines the electrical strike from the axis direction of phase tensor ellipses. The analysis reveals a scatter pattern of strikes at short periods (<1 sec) due to the small sampling area of EM fields at these periods. As the period increases, two dominant azimuths are obtained for regional geoelectric structures. Between 1-300 sec, the strike direction is constantly between 15-30˚ west of geographic north, and at the longest periods (300-3000 sec) it conforms to approximately 30˚E of the north.
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