Design and testing of a free-fall device for recording seismic activity beneath the ocean floor

For the last thirty years ocean bottom seismometers (OBSs) have been used to survey the oceans. However, the quality of the data they produce is often degraded due to poor coupling or high levels of ambient noise. In contrast, sensors placed beneath the sea-floor avoid many of these problems. For the last two decades burials have relied on Ocean Drilling Program (ODP) bore-holes. This approach is expensive and demands numerous human resources. In addition, the locations of these bore-holes, determined by the ODP, are rarely desirable for monitoring seismic activity. In this paper a sub-bottom seismic acquisition system is presented that uses free-fall devices, or Deep Ocean Seismic Penetrators (DOSP), to place sensors several tens of metres beneath the sea-floor. The DOSPs weigh approximately 1,800 kg, achieve terminal velocities between 30-50 m/s and penetrate to depths of 20-30 metres in soft sea sediments. Once buried, they record seismic activity and transmit data back to the surface using a Frequency Shift Keyed (FSK) modulation technique. The results of an experiment conducted in the Mediterranean using this system are presented. These confirm the predicted dynamic and kinematic behaviour of the DOSP and allow an assessment of the ambient seismic noise level at a depth of ≈30 metres beneath the sea-floor. In conclusion this paper discusses the potential use of free-fall devices to increase our understanding of processes in the deep oceans, with particular emphasis on their applicability to future deep ocean seismology.