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An OBS records earthquakes and other seismic and acoustic signals in the deep ocean. The instrument package includes two sensors – a seismometer to record vertical and two components of horizontal ground motion and a hydrophone to measure sound. In addition, batteries, a very accurate clock and a data logger are included in a tube that can withstand very high pressures. The instrument is dropped overboard and falls freely to the seafloor. When it is time to recover it, an acoustic signal is sent to the release system to decouple the anchor from the rest of the instrument. Without the anchor, the OBS is buoyant and floats to the sea surface, where it is fished out of the ocean and the data are downloaded to the computer. The steel anchor rusts on the seafloor and dissolves after several years. Some OBSs have a back-up timed release in case the acoustic release malfunctions.

To make it easier to find the OBS when it surfaces, it is normally equipped with a radio sensor and a blinking light source. These devices have a pressure sensor inside which ensures that they switch off below a specified water depth to preserve battery power while on the ocean bottom.

Here are pictures of the two types of OBSs used during PICTURES:

SIO OBS prepared for deployment
GEOMAR OBS prepared for deployment

The main difference between the two OBSs shown are the way the components are assembled and the type of flotation. The SIO-OBSIP OBS uses pressure-resistant glass balls in a yellow plastic shell, whereas the GEOMAR OBS uses syntactic foam, which allows it to go as deep as 8000 m. The deepest OBS during PICTURES is at 7070 m, and the shallowest is at 674 m water depth. Both types of instruments can be left on the seafloor for up to 1 year to record earthquakes.

– Jan Handel, November 2016