Observations of Motional Electromagnetic Fields during EMSLAB.

The long period (>1 day) behavior of the seafloor electromagnetic fields during EMSLAB is considered in detail with an emphasis on interpretation in terms of oceanic motions. This begins with a summary of the physics of motional electromagnetic induction, in which the depth-averaged velocity measured by the horizontal electric field is emphasized. Using frequency-domain methods, it is shown that seafloor and terrestrial magnetic variations have similar spectral shapes, indicating a common origin, but the seafloor electric field is not consistent with either at periods longer than four days, suggesting an oceanic source. The magnetic field variations are highly coherent across the EMSLAB array at periods shorter than nine days, but the similarity decreases at longer periods due either to long-term instrumental drift or motional magnetic fields.