Ambient noise beneath the seafloor

Simultaneous measurements of ambient noise were first made at several depths in the seafloor in August/September 1989, in a borehole off the Florida coast. Ambient noise levels decreased by a factor of three in amplitude (10 dB) between 10–100 m downhole (see figure) in the frequency range 2–50 Hz. As distance increased between the detector and the seafloor, the ambient noise levels decreased. Why do we care about ambient noise in the seafloor? One reason is that ambient noise levels limit the magnitude of earthquakes that can be detected. On land, earthquake monitoring stations are frequently placed in boreholes where better observations can be obtained. The Incorporated Research Institutions for Seismology consortium has identified the need for borehole seismic installations on the seafloor to improve the resolution of earthquake locations and of the source mechanisms of oceanic earthquakes. However, the study of ambient noise can be a fascinating subject in its own right. We do not yet completely understand how weather generates noise in the ocean, nor how the noise propagates and is distributed within the ocean and seafloor.