  Shifts in magnetotelluric (MT) responses owing to localized source currents
should be considered when visualizing deep subsurface resistivity structures
such as the earth mantle. The earth is not flat but spherical; nevertheless,
non-uniform geomagnetic temporal variations arising from localized source
currents are evaluated on the basis of components in a Cartesian coordinate
system. To address this issue, this study assesses the difference in source
bias within MT responses of a flat earth and spherical earth. Apparent
resistivity and phases are calculated by setting the time period and
conductivity of the earth interior to 200/2000 s and 0.001 S/m, respectively,
and by changing the vertical and horizontal distances between the source
current and an observation station. A deviation in the biased MT responses of
the flat earth and spherical earth is not observed although both shift strongly
from the true values. We can thus treat the source bias in a Cartesian
coordinate system although it originally arises in a spherical coordinate
system.
