Indirect velocity monitoring fromapassive electrometer through non-coherent noise statistics of induced voltages

Measurements of the near-bottom electrical field made by a short-baseline carbon-fiber electrometer are presented. Observations are made for a range of different tidal phases. It is shown that the signals over the whole measured frequency range (0.01-20 Hz) were mainly of a high-coherency plane-wave type, as emitted from a distant source suggesting an atmospheric or ionospheric origin. When extracting the non-coherent part of the signals (the locally induced part) it is shown that the power spectral estimates of the residual vary with current speed. The affected frequency region encompasses the range from 0.01 Hz to around 1 Hz, whereas higher frequencies conform to average system noise levels. This indicates that the non-coherent part of the signal is connected to the level of the turbulence generated in close proximity to the instruments, which hence provides a means for indirect current monitoring by shifting up the slowly varying mean-flow into a frequency regime where most electromagnetic sensor systems are at their peak performance.