The effect of screening of the magnetotelluric field by high resistivity layers: Three-dimensional numerical modeling constraints

Abstract

A 3-D numerical modeling approach is applied to the effect of screening of the magnetotelluric (MT) field by high resistance (≥10⁶ Ω m²) layers, which up to now has mainly been studied in terms of 2-D heterogeneous models of geological sections [Berdichevsky and Yakovlev, 1990]. Three-layer models above a poorly conducting basement (ρ=10⁴ Ω m) are used. The resistance of the screening layer was generally taken equal to 2×10⁶ Ω m² and its thickness was varied from 200 to 2000 m. The resistivity of the host medium was set at 10 Ω m. Heterogeneities of a low and a high resistivity (ρ=10⁻³ and 10⁴ Ω m, respectively) ranging in horizontal size from 4 to 40 km and having a height of 1–2.8 km (a protrusion on the basement) were examined. Based on calculations of these models, 2-D and 3-D screening effects were compared. The 3-D modeling determined values of model parameters at which the screening properties are preserved both in the case of a screen of limited horizontal dimensions and in the presence of a rupture in the screen comparable in horizontal size with the 3-D heterogeneity. As follows from the modeling results, the screening effect of the high resistivity model layers seriously complicates the use of MT soundings for the identification of a local heterogeneity of both a low and a high resistivity (ρ=10⁻³ and 10⁴ Ω m, respectively) if its lateral size is smaller than the 4-km thickness of the studied three-layer section in question and its height is 1–1.5 km. The regular patterns of the screening effect revealed in this work are of interest in electromagnetic sounding applications.