GSL install OBS network under the Sea of Marmara

February 11

The North Anatolian Fault is a major, active strike-slip fault running east/west across Turkey. Over the last 100 years, eight earthquakes of magnitude 7 or greater have occurred along the fault, each to the west of the previous one. There is a consensus that a major earthquake will soon strike further west, threatening the city of Istanbul, with its 12.8 million inhabitants and priceless cultural and architectural heritage.

Observation of the fault is complicated by that fact that, for over 250km of its length, it runs beneath the Sea of Marmara at depths of over 1km, making it impossible, until recently, to place instruments close to the fault. Guralp Systems Ltd., working with national distributor Sentez Earth and Structure Engineering Ltd., have spent two years designing, developing and building the specialist instrumentation, communication and installation systems necessary to overcome the significant challenges associated with placing seismometers and other sensors on the sea-bed, directly adjacent to the fault line, and providing real-time data streams and control facilities. The project came to fruition in the winter of 2010, when a five-station network was deployed and commissioned. The stations are up to 20km from the coastline at depths of up to 1,300m.

The CMG-3T/5T “Tripod” OBS instrument is a combined seismometer and accelerometer in a custom-designed titanium case, suitable for deployment at depths of up to 3,000 metres. The seismometer component is based on Guralp Systems Ltd’s industry-leading CMG-3T weak-motion very-broadband force-feedback instruments. Three uniaxial sensor components are each mounted in separate gimbals, with individual levelling control systems. The accelerometer component is based on the CMG-5T compact, high dynamic range, strong motion instrument and also uses a true-orthogonal force-feedback system. In addition to these sensors, each instrument is equipped with a hydrophone, a differential pressure gauge, a precision thermometer, a flux-gate magnetometer, a camera and an inclinometer, effectively providing a complete, sub-sea, multi-disciplinary observatory.

The 3T/5T Tripod includes a high-quality six-channel CMG-DM24 digitiser module with full 24-bit resolution, capable of multiple concurrent data rates up to 1000 samples per second. Also in the case is a CMG-EAM embedded acquisition module, which deploys a powerful Linux processor to handle all communications and data buffering requirements. The CMG-EAM’s convenient web-based and command-line interfaces provide full control and monitoring of the system over a permanent fibre-optic link.

The whole system is packaged in a strong, welded titanium case designed for optimal coupling to the sea-bed. The three conically-tipped feet apply the entire weight of the instrument to a very small area, digging deeply into any gravel or silt in order to provide the best isolation from water-borne signals as well as superb conduction of seismic signals from the ocean bottom to the sensor components.

Each instrument is protected by a cast concrete surround, which serves as both an anchor and an anti-trawler device. The entire weight of the instrument and its concrete mounting (some 1.2 tonnes) is supported, during deployment, by a custom-designed, load-bearing power/data/suspension cable. The instrument is powered up and running while still on board the deployment ship and produces data constantly while being lowered to the sea-bed and while the shore cable is being paid out, allowing any problems to be spotted instantly and reducing the expense of any corrective action. In the event, each instrument behaved faultlessly, both during the deployment and subsequently. This technique eliminates the need for both submarine remote-operated vehicles and wet-mating connectors. The same cable can be used for instrument recovery, if required.

The combined weight of the sensor, its concrete surround and up to a kilometer of cable presents a particular challenge during deployment and a custom cable management system was designed and employed specifically for this project. The “caterpuller” consists of a length of continuous, rubber-studded steel belt, as seen on track-laying vehicles such as tanks, on to which the cable is pressed by hydraulically driven feet, each equipped with a rubber belt. The system is welded to the ship’s deck and provides the grip and control necessary to pay out accurately measured lengths of cable while still supporting the entire suspended mass. As the installation ship proceeds shorewards from the deployment site, GPS signals and a sea-bed contour map are used to calculate the correct rate of cable pay-out in order to ensure that it lies, without tension, firmly on the sea bed for its whole extent.

Each sensor has its own dedicated cable and shore station, from where the data and control signals are routed via both terrestrial and satellite Internet links to Bogazici University’s Kandilli Observatory and the Earthquake Research Institute UDIM (National Earthquake Monitoring Center) data center.

The project has highlighted Guralp Systems Ltd’s resourcefulness as a solution provider alongside our more well-known capabilities as designers and manufacturers of high-precision instrumentation. As 71% of the Earth’s surface is covered by water, the importance of ocean-bottom seismometry and the novel engineering solutions required for deployment in this still largely unexplored territory will only increase.