Electric and Magnetic Field Galvanic Distortion Decomposition of BC87 Data.

The BC87 magnetotelluric data exhibit distortions due to three-dimensional structures at all scale sizes, from very local to regional. Previous work has shown that these distortions can sometimes, but not always, be described by an electric field galvanic distortion model, and hence can be removed using a tensor decomposition approach. This work extends the analysis to include galvanic distortion of the magnetic field, and shows its importance for many of the BC87 sites. However, even a combined electric and magnetic field galvanic distortion approach fails for quite a few of the sites studied. This is pervasive for periods shorter than 10 s, suggesting local 3D inductive effects, but model inadequacy at longer periods may indicate a breakdown of the model assumptions, particularly the requirement that the regional electric field be both uniform across the distorting body and comparable to that at the observation point.