3,200 metres under the sea

More than 20 years after the involvement of Guralp Systems Ltd. in the design and manufacturing of the first ever broadband ocean-bottom borehole seismometer, we recently achieved another milestone in this high-end market. Japanese scientists installed a three-component string of Guralp CMG-3T seismometers and a CMG-DM24 down-hole digitiser in a 400 metre deep ocean-bottom borehole located under 2800 metres of water off the coast of central Honshu. The installation is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a complex ocean drilling project conducted over several years off the Pacific coast of Japan (see www.jamstec.go.jp/chikyu/eng/Expedition/NantroSEIZE).

The diagram shows the drill-ship and, to its right, the Statue of Liberty, drawn to the same scale.

This experiment is a major venture within the frame work of the International Ocean Drilling Program (IODP) and is carried out from the Japanese drill ship R/V Chikyu. During the ship’s cruise IODP 332, the string of Guralp CMG-3T seismometers and the CMG-DM24 digitiser were deployed in borehole C0002, together with other geophysical equipment.

Before their delivery to Japan, the sensors were rigorously tested on a shake-table in Southern England. During the extensive tests, the sensors in their titanium OBS housing were exposed to three hours of continuous shaking with accelerations of up to 4 g over a wide frequency range. Every sensor tested passed the test with flying colors.

Our company has a long history in deploying Ocean Bottom Seismometers (OBS) and their technically more complex variant, the Ocean Bottom Borehole Seismometers. In 1989 a Guralp three-component broadband sensor was deployed at 715 metre depth in an IODP-Borehole located in 2800 metre deep water in the Japan Sea. This deployment was a world premier, because never before had a broadband seismometer been installed in a borehole deep beneath the ocean surface.

Photo of R/V Chikyu courtesy of Gleam