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Abstract

Many marine magnetotelluric (MT) datasets contain dominant, quasi-harmonic signals at a period of several seconds in the magnetic and (to a lesser degree) in the electric channels. In some cases these signals are strong enough to drive a standard processing into failure. Coherence with tilt records shows that these signals are generated by motion of the instrument. However, the source of this motion is controversial. It is unclear whether the surrounding water performs such oscillations that might be caused by microseisms, or if streaming water incites oscillations of the instrument. This contribution contains a discussion resulting in a preference of the latter possibility. Furthermore, the mechanism of electromagnetic noise generation due to that motion is explained, and possible noise-correction and processing strategies for disturbed data are presented. Data examples are from GEOMAR (Kiel) campaigns offshore Costa Rica and Namibia.