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.
Ocean bottom hydrophon
Ocean Bottom Hydrophone System (OBH-System)
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 (iXSea or KUMQUAT) and the 0.8 m long pressure tube with the data logger (supplied by the company SEND: www.send.de) and its batteries (up to 60 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 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.
The Ocean Bottom Seismometer construction is based on the experiences with the GEOMAR OBH. For system compatibility acoustic release, pressure tubes, and the hydrophone are identical to those used for the OBH. Syntactic foam was used as floatation again but of larger diameter due to the increased payload. Other than the OBH the OBS has three legs around its center post to which a special anchor weight is attached. While the OBH is floating about 1 m above the sea bottom, the OBS touches the sea bottom to avoid collision of the seismometer cable with the anchor. The sensible seismometer is deployed about 1 m to the side of the system. A burn wire releases the sensor about two hours after seawater contact and let him drop to the sea floor from about 1 m height. This should ensure the coupling of the seismometer to the sea floor. At this time the only connection from the seismometer to the instrument is a cable and an attached wire which will take the pull load while rising to the sea surface later. A movement or current on the instrument is thus not transmitted mechanically to the seismometer. All three channels are recorded by the standard data logger as used in the OBH units. Parallel to these three channels the standard hydrophone is recorded on the fourth channel.
OBS design 2002
In 2002 a completely revised design was developed for the 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 fixed to the main frame by four screws and can be changed according to the type of seismometer requested. A variety of short period (4.5 Hz, 15 Hz) and broadband sensors are available. The anchor weight underneath the system carrier keeps floatation, pressure tube and seismometer 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. Besides the floatation all components are compatible with the other OBH and OBS type instruments and can be interchanged.