Long term underwater monitoring of seismic areas: Design of an Ocean Bottom Seismometer with Hydrophone and its performance evaluation

0 The Ocean Bottom Seismometer with Hydrophone (OBS/H) is an autonomous data-acquisition system which free falls to the sea floor to record seismic and pressure signals generated by earthquakes and tsunamis. The development and the improvement of OBS/H's is a necessary process for seismological research institutions, because in the world about 90 percent of all natural earthquakes has epicenters in offshore areas. This paper describes the OBS/H developed by the “Istituto Nazionale di Geofisica e Vulcanologia” (INGV), designed to reliably collect broadband data from seafloor sites during long term deployment. The data recorded by seismometers on the sea floor is influenced by background noise. In particular, the free fall of the instrument and the subsequent dropping of the seismic sensor on the superficial sediments, could cause coupling problems with signals distortion and high level noise due to the sensor tilt. Supported by data acquired during several monitoring campaigns in the Mediterranean Sea, we will show that the INGV OBS/H is able to record high quality seismic signals with low noise levels and low distortion, proving a good coupling of the seismometer with the seabed.