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Case study

Underpinning the Indian Tsunami early warning system

Client: National Institute of Ocean Technology

November 17, 2021
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The challenge 

Sonardyne bottom pressure gauges (BPR) have been at the heart of the Indian Tsunami Early Warning System (ITEWS) since its establishment in 2007. Based on Sonardyne’s workhorse Compatt transponder, our BPR instrument was developed in direct response to the devastating 2004 Indian Ocean Tsunami.

30% of India’s population live along the country’s coast and are highly vulnerable to devastating Tsunamis such the one that occurred on 26th December 2004. This particular event killed over 230,000 people in the Indian Ocean region, with another 5,640 confirmed missing. Although seismometers are an important component of Tsunami warning, they are unable to detect the path and height of waves created by a seismic event. Only wireless Bottom Pressure Gauge Recorders (BPR) can detect the passage of an actual Tsunami. Tsunami warnings based only on seismic data have the potential to produce false alarms, which are costly and undermine public confidence in government warnings.

To improve Tsunami Detection Systems (TDS), it is imperative to not only note the point of seismic activity, but also track the elevation in sea levels along with the speed and the direction of the waves produced. This provides the essential data needed for early warning systems to be effective – the anticipated height of a tsunami and the expected arrival time.

While a Tsunami may arrive at the coast many metres high, in the open ocean they pass almost imperceptibly, sometimes mere centimetres in height. However, this elevation in sea-level can be maintained for as long as 20 minutes.

Functionality that provides detention ashore with sufficient warning needs to track the mass of water along with its acceleration. A tsunami travels faster in deeper water. For example, in 1,000m depth of water it will travel at over 1,100 kmph. In India’s case, the Andaman-Sumatra and Makran subduction zones are located within just a few hours of Tsunami travel time from the Indian coastline, making robust early detection more difficult.

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The solution

Sonardyne’s founder, John Partridge, initiated the development of a variant of the Compatt 5 seabed transponder in response to the catastrophic 2004 Indian Ocean Tsunami. With some custom engineering, this new variant would be able to detect a Tsunami passing overhead.

The Compatt 5 variant was integrated with a Digiquartz pressure sensor and developed to enable the Compatt 5 to function as a Bottom Pressure Recorder. The development involved reducing the power required for continuous operation on battery. A new transceiver with low quiescent power, capable of long endurance deployment on a surface telemetry buoy, was also developed. These innovations ensured the variant of the Compatt 5 seabed transponder, was ready for competitive field trials when India’s National Institute of Ocean Technology (NIOT) in Chennai, started looking for a TDS in 2005. Sonardyne’s solution was subsequently selected in 2006, leading to deployment of operational systems in the Bay of Bengal and the Arabian Sea in 2007.

In normal operation the Digiquartz pressure sensor in the BPR continuously measures water pressure storing data every 15 seconds. The pressure data is then acoustically transmitted hourly to the surface, where an acoustically baffled transceiver, mounted beneath a buoy, receives the data. The buoy is linked to NIOT’s Mission Control Centre by satellite communications. This ensures data arrives ashore quickly and the health of the BPR can be remotely monitored and, if necessary, reconfigured.

Embedded in the BPR is the National Oceanic and Atmospheric Administration’s (NOAA) Tsunami detection algorithm. This compares each measurement to the predicted pressure [Figure]. Predicted pressure uses the previous 3-hour history to take account of tide, weather and temperature variation. Should the difference between the two exceed a programmable default threshold of 3 cm for two consecutive samples, the BPR switches into Tsunami Alert Mode. This also initiates a sequence of data transmissions for the following few hours.

The results 

Soon after the first batch of BPRs were deployed, the system detected its first Tsunami. The seismic event of September 12th, 2007 off the coast of Sumatra at 04° 30’ S 101° 18’ E was of 8.2 magnitude. The Tsunami generated by this tracked across three of the Indian Tsunami Buoys that had been equipped with the upgraded Compatt 5 sensors and transceivers.

Located between 1,760 – 2,300 km away, all three stations recorded the seismic ground wave arrival between 7 – 12 mins after the event started.  Tsunami warnings were successfully issued and the states of Tamil Nadu, Kerala, Andhra Pradesh and the union territories of Andaman and Nicobar Islands along with Puducherry were put on high alert.

Between 2 – 3 hrs later, the wave itself, which was less than 10 cm in height, passed over the three BPRs, indicating that it had travelled at between 740 – 800 km/h over this period. This subsequent information about the speed of travel and anticipated height of the tsunami on reaching India’s coast enabled the Government of India to confidently recall the warning.

No deaths were attributed to the tsunami that reached India’s coast, although power outages from the earthquake disabled communications in Bengkulu and 21 deaths and 88 injuries were recorded as a consequence of the tremors.

Fourteen years later Sonardyne’s TDS, continues to deliver bottom pressure and event data to NIOT. The sea-level data is used by the Indian Tsunami Early Warning Center (ITEWC) at the Indian National Centre for Ocean Information Sciences (INCOIS) in Hyderabad.

In 2014, NIOT reported that system data availability had been 98.83%, with a Mean Time Between Failure (MTBF) of 1.62 years. The majority of these failures were due to external impacts, including weather and human interference with the surface buoys, resulting in damage to surface communication and data systems. Seven years after implementation, Sonardyne’s TDS is still keeping India’s coastal populations safe from devastating tsunamis.

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