Hydrophone as an accelerometer

Abstract

We present the equivalence of the ground acceleration and water pressure variation induced by earthquakes, using data from co-located seafloor instruments in shallow water. The equivalence is evident from a strong similarity of the waveforms, which completely and persistently overlap along the entire duration of the recordings, and it is supported by a high cross-correlation value. Moreover, the comparison of the amplitude spectra of the acceleration and pressure signals confirms the existence of a frequency range of ‘forced oscillations’, where the pressure variations are proportional to the ground vertical component of the acceleration. We demonstrate the equivalence of the signals for a set of local and regional earthquakes on a wide range of magnitude (2.7<M<6.8), recorded by seismometers and hydrophones operating in a coastal monitoring infrastructure, at less than 80 m of water depth, located in the Gulf of Pozzuoli (Campi Flegrei caldera, Southern Italy). The infrastructure consists of four marine platforms which integrate a wide set of sensors, in particular a seafloor multi-sensor module, hosting a broad-band seismometer (120s – 100 Hz), a low frequency hydrophone (down to 0.01 Hz) and a bottom pressure recorder. The simple linear relation between sea bottom pressure variation and vertical seafloor acceleration measurement is widely reported in the scientific literature and well verified in previous works. All these works use data from low frequency waveforms of large magnitude tsunamigenic earthquakes, recorded by pairs of bottom pressure recorders and accelerometers. Here, for the first time, short period data recorded by pairs of hydrophones and seismometers are used expanding the theory validity to a larger frequency range. The high correlation value between the ground acceleration, derived from the ground velocity, and hydrophone pressure signals allows to calibrate hydrophones by comparison with co-located accelerometers, or seismometers, used as a reference in a range of frequencies that is very difficult to reproduce in lab. The proved similarity of the recordings underpins the possibility that hydrophones can be more extensively used in place of accelerometers in those marine environment, where seismic sensor accurate installation is usually not easy and not always affordable.