Using finite dipole lengths in complete earth 3D MT modelling

Summary We have quantified the use of finite electric dipole lengths from the point measurement assumptions typical in 3D MT inversion modeling. Electric fields are measured across dipoles of typically 50 m to 200 m at MT soundings. Modeling algorithms, however, normally use point electric field values at the surface of single cells to calculate MT transfer functions. This is perfectly reasonable for the majority of cases, but there are situations with strong shallow variability of resistivity, where measurements may not be simulated well by point electric fields, and detailed information might not be used optimally. We explore the consequences of this omission by quantifying the difference between point solutions and electric field integrations across dipoles in 3D forward calculations for selected cases. The topic ties closely with galvanic distortion and inversion for related parameters, lateral magnetic field variations, and the benefit of providing shallower constraints for the imaging of deeper targets. As a side product, the analysis led us to focus on the fields output from the 3D modeling, and we illustrate electric current systems through the cases analyzed. We observe that in the presence of strong topography and outcropping inhomogeneities, finite dipole solutions can differ considerably from point solutions, while over a variable regolith case the effect appears more contained