Dynamics of the Ocean Floor

Ocean Bottom Hydrophones (OBH) and Seismometers (OBS)

Ocean Bottom Hydrophones (OBH) and Seismometers (OBS) have been developed and operated by the MARINE GEODYNAMICS Group since 1991.

Since then three types of instrument carriers have been developed. Modifications of data loggers and modularity of system components allow to operate a variety of sensors used for active and passive seismic experiments as well as for Tsunami detection purposes.

The mechanical construction of the OBH/S is available from the company "Umwelt- und Meerestechnik GmbH, KUM" (www.kum-kiel.de). Today, all metal parts of the system carrier as well as the pressure tubes of the acoustic release and data logger are made from Titanium. All buoyant bodies consist of syntactic foam which has proven to be highly reliable even under harsh conditions. All components of the systems are designed for operation in 6000 m water depth. A few of OBS are rated for 8000 m.

Own recorder development and the new 6D6 logger by KUM are available for active seismic or passive long-term monitoring projects with lowest power consumption.

Ocean bottom hydrophone (OBH)

For the OBH all components are centered around a main tube with a handhold on its top to make it possible to retrieve the instrument from the water. On this handhold a  2 m long flagstick is mounted. The buoyant body is made of two hemispheres, 550 mm in diameter. Between the two hemispheres additional discs can be placed, each of them provides an additional 3.5 kg of flotation. The fixed upper hemisphere is used as the attachment for a flasher and a radio beacon.

Underneath the buoyant body the acoustic release (iXblue or KUMQUAT) and the 0.8 m long pressure tube with the data logger (own development or 6D6 by KUM) and its batteries (up to 72 R20 or lithium cells) are mounted. The hydrophone is mounted between the acoustic release and the pressure cylinder. The acoustic release has its own power supply and an additional time release programmable for safety reasons.

On the bottom of the acoustic release there is a movable hook  to which an anchor (a 40-50 kg piece of railway track) is attached to fix the whole system to the bottom. A 2 m long wire is used between the anchor and the release unit to prevent the system from contacting the seafloor. Via  a transponder which hangs on a cable beneath the hull of the research vessel an acoustic signal is sent from the ship to release the anchor. 

The total weight of the system is about 125 kg without the anchor. The descent and ascent speeds are about 1 to 1.2 m/s.

OBS design 2002

In 2002 a completely revised design was developed for the four component OBS system carrier. Aim of the redesign was to decrease the size of the instrument in the water column, to enable the operation of short period and broadband seismometers without complicated deployment frames and to increase the numbers of instruments stored in one container.

The floatation is now provided by cylinders, which are manufactured from syntactic foam again. The pressure tube with the data logger is located on the frontal part of the frame next to the cylinders. This part of the frame is used to fix a variety of seismometers to the anchor. Alternatively seismometers of small size (4.5 Hz) can be deployed from a lever by burn wire.

Standard seismometer is a 4.5 Hz sensor but Trillium broadband sensors are available as well. Both components are manufactured by "Umwelt- und Meerestechnik GmbH, KUM" (www.kum-kiel.de). A hydrophone (HTI) is always mounted as fourth component.

The anchor weight, fixed by an acoustic release (KumQuat, KUM) underneath the system carrier keeps floatation, pressure tube and sensors in a horizontal plane during descent to the seafloor.

The seismometer is fixed between frame and anchor. This position avoids the risk of the seismometer to touch the ground with a too tilted angle and avoids expensive gimbal mechanisms, but ensures good coupling to the ground. Test measurements confirmed that frequencies of 100 Hz and above could still be recorded.

Upon recovery the instruments rises to the sea surface with the floatation first while the heavy pressure tube and seismometer are oriented vertical below. This ensures the acoustic release, mounted in the centre to be kept under water at any time. Hence acoustic ranging is still possible when the system floats at the surface. In the vertical position recovery aids as flag, flash and radio beacon are lifted out of the water.

Besides the floatation all components are compatible with the other OBH instruments and can be interchanged. All OBS are rated for 6000 m water depth, a few of them can operate at 8000 m.

Contact

Prof. Dr. Heidrun Kopp

Phone: +49 (0)431 600-2324

e-mail: hkopp(at)geomar.de

 

Dr. Joerg Bialas

Phone: +49 (0)431 600-2329

e-mail: jbialas(at)geomar.de