Influence of Benched Mountain Relief on the Results of Magnetotelluric Soundings

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Дәйексөз келтіру

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Аннотация

The aim of the conducted research was to estimate the influence of the relief, specified as a system of steep benches, on the apparent resistivity curves and the parameters of geoelectric sections determined from these curves during magnetotelluric observations. It was solved using three-dimensional mathematical modeling of magnetotelluric fields by the finite-difference method using the Maxwellf program. The dependences of the curves and the results of their one-dimensional inversion on the block parameters of the two-dimensional and three-dimensional models were analyzed. Distortions that shifted the invariant apparent resistance curves calculated on the steps of the staircase from the MTS curves measured at the flat ground-air boundary were estimated. At the same time, the geoelectric parameters of the blocks located under the flat boundary are equal to those specified under the model with the relief. The problem of constructing geoelectric models under conditions of stepwise changes in terrain can be solved using three-dimensional mathematical modeling of apparent resistivity curves adjusted by normalizing coefficients that take into account the transition to a 3D model with a flat ground-air boundary. However, they depend on the period of variations. Due to this, it is more appropriate to evaluate the use of three-dimensional inversion programs that include terrain topography in the starting three-dimensional models. Before carrying out this procedure, it is necessary to know what displacements of the apparent resistivity curves may occur if the influence of terrain and deviations obtained during their inversion, as well as geoelectric parameters of sections from test models are not taken into account.

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Авторлар туралы

V. Belyavsky

Geoelectromagnetic Research Center of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences

Хат алмасуға жауапты Автор.
Email: victor.belyavsky@list.ru
Ресей, Troitsk

Әдебиет тізімі

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Әрекет
1. JATS XML
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2. Fig. 1. Fragment of the terrain relief map in the vicinity of the Brahmaputra depression. Rectangle indicates the area of three-dimensional modelling.

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3. Fig. 2. Two-dimensional models: 2DT1 with sedimentary cover UES at staircase steps ρsd = ρf = 2000 Ohm ∙ m and crustal layer with ρsr. = ρf; 2DT2, 2DT4b with layers at depths Zcr = 20, 70 km with ρsr = 10 Ohm ∙ m. The 2DT4b model specifies covers with ρsd = 10 Ohm ∙ m on its stages. In the 2D, 3D models, the Y-axis is orientated to the northeast and the X-axis to the southeast.

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4. Fig. 3. (a) - Section of 2DT1 model along Ave. 5zy; (b) - 1D-inversion section - ρin (Hin) of the curves ρxu = ρ||; (c) - pseudo-section of the tipper norm ||W||| at Ave. 5zy. On the right are the resistivity scales ρm and ρin (Hin). Above - points of modelling (t.m.) of MT fields.

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5. Fig. 4. Apparent resistance curves ρyx = ρ⊥, ρxy = ρ|| at Ave. 5zy models 2DT1 (a) and 2DT2 (b), (c).

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6. Fig. 5. (a) - Section of 2DT2 model (pr. 5zy); (b) - results of 1D-inversion of ρxy curves. The resistivity scales ρm and ρin are on the right, and the depths Zm and Hin are on the left. Below the ρin (Hin) section, the area of no ρin (Hin) information is visible.

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7. Fig. 6. (a) - 3DT3 model cross-section at the level of Zm = 6.5 km and numbers of MT field modelling points (t.m.). At the trough cover ρsd = 10 Ohm ∙ m, background ρf = 5000 Ohm ∙ m; (b) - distribution of the tipper norm ||WZ||| at the period T = 4 s.

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8. Fig. 7. Cross-sections along Ave. 2zy (a), Ave. 5zy (c) of 3DT3 model and 3DT3a 3D model (e). Results of 1D-inversion of ρmxH curves in the models: (b), (d) - 3DT3 (ρf = 5000 ohm ∙ m); (e) - 3DT3a (ρf = 2000 ohm ∙ m). On the right are the scales of ρm and ρin; on top are the modelling points. The position of the trough is in Fig. 6а.

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9. Fig. 8. Curves of ρmxH at 5zy pr. and 2zx pr. 2zx models: (a), (b) - ‘ladder’ model 3DT3, ρf = 5000 Ohm ∙ m (Fig. 6, Fig. 7c); (c) - with flat model ground-air interface in 3DT3a model with ρf = 2000 Ohm ∙ m (Fig. 7d).

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10. Fig. 9. Curves of ρmxH (avg. 5zy) models: (a) - 3DT4a; (b) - 2DT4b, with ∆Zsd = 2 km in the trough; (c) - 3DT4c with a flat ground-air interface, with ∆Zsd = 3 km outside the trough and with ∆Zsd = 4 km in the trough.

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11. Fig. 10. (a) - Section along av. 5zy of 2DT4b model; (b), (c) - 1D-inversion sections of ρmxH curves in 2DT4b (∆Zsd = 2 km) and 3DT4a models (in the depression ∆Zsd = 3 km, Fig. 11a); (d) - pseudo-section of ||W||| in 3DT4a model. The dotted line is ||W||| in the 2DT4b model. Right - scales ρm, ρin and ||W|||.

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12. Fig. 11. Cross sections of 3D models and results of 1D-inversion of ρmxH curves at: (a), (b) - 2zy av. (3DT4a) and (c), (d) av. 5zy - (3DT4c). On the right are the scales of specific resistances ρm, ρin.

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