On thin‐layer telluric modeling of magnetotelluric responses

Modeling of three‐dimensional (3-D) thin‐sheet telluric anomalies has been a popular means of estimating the distortions of magnetotelluric (MT) response functions in the vicinity of an upper crustal resistivity inhomogeneity (Kaikkonen, 1986) because structures with an arbitrary 3-D shape in plan view can be simulated with modest computational effort. In the class of thin‐layer problems of this note, the sheet structure overlies an infinitely resistive basement; and anomalous fields are computed in the long‐period or zero‐frequency (dc) limit (e.g., Hermance, 1982). Under the assumption of no vertical current flow into the basement, this 3-D problem reduces to a two‐dimensional (2-D) dc solution across the sheet. Since only the long‐period limit is considered, however, the conclusions that can be drawn about the effects of general 3-D structures are restricted. The purpose of this note is to clarify restrictions on the use of 2-D MT interpretations in 3-D areas based on just thin‐sheet telluric modeling.