Low-frequency seismic and hydroacustic noise measurements in a fjord

Abstract

Low frequency ambient noise signals from an ocean bottom seismometer (oBS) put on the sea floor of a Norwegian fjord have been studied. The instrument contains 3 seismometers and one hydrophone, and signals from all 4 sensors are analyzed. Measurements have been done over several local areas which have been found to be reasonably well stratified. Depth and bottom parameters like sediment type and -thickness and subbottom type vary from place to place. Noise characteristics i n terms of power spectral densities , amplitude statistics and directionality are treated with emphasis on variations with time and changes in environmental parameters. The statistical processes involved are highly variable and may not in general be considered stationary. Noise spectrum levels have been found to be higher than reported from open waters. Human activityin and around the closed fjord area must be the main cause of this increased noise.