Supplementary material to “Satellites, Tsunamis, and Early Warnings”
Tom D. Allan, Satellite Observing Systems, Godalming, United Kingdom, tom@satobsys.co.uk
Citation:
Allan, T. D. (2007),
Satellites, Tsunamis, and Early Warnings,
Eos Trans. AGU, 88(33), 327.
[Full Article (pdf)]
Background
In the early morning of December 26th 2004 the ocean research satellite Jason flew over the Indian Ocean and recorded a surface tsunami wave less than 1 metre high. If it was no more than chance that a satellite equipped with a sensitive radar passed over at that precise moment, the potential contribution of orbiting spacecraft to future early warning systems had been demonstrated. A synoptic view of the tsunami’s devastating impact on coastal resorts was subsequently revealed in the optical imagery also provided by satellites.
Curiously, satellites have not featured so far in the detection systems now being implemented to provide a more reliable warning of future tsunamis. UNESCO’s Intergovernmental Oceanographic Commission is co-ordinating the building and installation of surface buoys with acoustic links to sensitive bottom-mounted pressure sensors. One is already launched in the area of the Indian Ocean disaster and in the Pacific Ocean NOAA is increasing its own network of buoys.
The IOC has expressed the view that only ‘proven technology’ should be used to detect future tsunamis and that satellites are at least 10 years away from forming part of an integrated system. This assertion derives mostly from the initial response of the ocean research community that it might take several hours for an altimeter’s tsunami signal to be extracted and identified—clearly too late in most cases to provide adequate warning.
This is a view now challenged by a number of marine scientists working in the field of satellite altimetry. They argue that a direct transmission to the relevant spacecraft the instant the global seismological network located an underwater earthquake would allow the onset of a possible tsunami wave to be confirmed or not in near real-time by subtracting the historical record of observations along that altimeter’s track from the ongoing set of measurements.
Doubts have also been raised about the ultimate effectiveness of buoys. Their coverage over all global oceans is unlikely to match that of a constellation of low, polar-orbiting platforms. The cost of regularly servicing a network of buoys, some far from shore, may be high; for, to be effective, they must perform reliably year in year out in all weather conditions. By contrast, satellites fly above the weather, and some, equipped with altimeters, have operated continuously for over 12 years.
PORSEC
The Pan Ocean Remote Sensing Conference was initiated in 1990. At the time the ‘P’ stood for Pacific and its membership was more or less confined to Pacific rim countries before it widened its remit to include all oceans. Its biennial meetings are aimed at attracting students and trainees in the business of marine remote sensing, mostly, but not exclusively, from satellites. PORSEC aims to assist in the transfer of technology by reporting the state-of-the-art and latest developments in this field. To help fulfil its aims it has received in the past some funding from the national space agencies of USA (NASA), Europe (ESA), and Japan (JAXA). It usually also receives support from the local remote sensing and photogrammetry organisations that have hosted the conferences—most recently from the Korean Aerospace and Research Institute (KARI). Previous PORSEC training programmes enjoyed the support of SCOR and UNESCO.
Its most recent biennial conferences have been held in Concepcion, Chile (2004); Bali, Indonesia (2002); and Goa, India (2000). The decision to hold this meeting in South Korea had been taken prior to the disaster of December 26 2004. In the circumstances it seemed appropriate to convene a Special Workshop on Tsunamis—the more so since, as we have seen, satellites appeared to have been given little serious consideration by the competent international agencies.
The Workshop was held on the afternoon of November 1 2006 at the University of Pukyong in Busan, South Korea’s second largest city. The majority of the 100 participants were drawn form universities and public authorities in SE Asia. The broad aim was to review the performance of satellites and consider the extent to which future space systems could contribute to reducing the impact of a range of marine hazards including tsunamis.
The Workshop
Speakers from Indonesia (Bambang Trisakti) and Thailand (Absornsuda Siripong) presented graphic images of coastal resorts and demonstrated how this imagery was subsequently used by Indonesian and Thai authorities to define areas of risk, and draw up evacuation plans. Satellite imagery was also used by DanLing Tang of the South China Sea Institute of Oceanology to illustrate the tsunami’s effect on regional marine ecosystems.
Mal Heron from James Cook University, Australia then demonstrated how shore-based HF-radar could play a key role in following the approach of a tsunami wave as it moved towards the shore.
The remaining speakers addressed the issue of how satellites could feasibly be used to improve the early detection and warning of tsunamis and other marine threats.
Tony Song from JPL, Pasadena, USA presented evidence from satellite observations that sea-floor uplift from the seismic inversion is not enough to have generated the powerful 2004 Indian Ocean tsunami. Using a three-dimensional coupled earthquake-tsunami model, he demonstrated that the momentum force transferred by the horizontal motions of faulting slopes is the major cause.
Yuliya Troitskaya from Russia’s Institute of Applied Physics presented experimental evidence for space-observed manifestations of the open ocean tsunami in the microwave radar backscatter found in the Jason altimeter record as it crossed the head wave of the tsunami.
Jim Gower of IOS, Sidney, BC Canada, processed the altimeter data to show how short-wave energy generated by the earthquake was also detected, and could serve as a useful signal for a detection system based on satellite altimetry. He showed signals from the DART buoys now being laid down in selected ocean areas, and demonstrated that these are capable of considerably higher sensitivity than can be achieved by altimetry, partly because the tsunami signal for a Bottom Pressure Recorder may be less affected by ocean mesoscale eddies. In his opinion the relative advantages of DART buoys and satellite altimetry need to be carefully balanced.
In a written contribution, Yutaka Hayashi from Japan’s Meteorological Research Institute, reported his investigations relating the lowest height of the tsunami signal which satellite altimeters can detect. Based on this, and the general relation between the magnitude of a tsunami-inducing earthquake and the height of the wave generated in the open ocean, a satellite mission would have the potential to detect one tsunami in its lifetime, which corresponds to the average interval between major tsunami-inducing earthquakes.
Tom Allan (Satellite Observing Systems, UK) argued for a constellation of altimeter-carrying microsatellites as the only feasible method of providing uniform global coverage at an affordable cost. He proposed that nations facing the greatest threat from future tsunamis might form a ‘club’ whereby, for a relatively modest outlay (in terms of space budgets), they would jointly own and acquire access to the observations of the whole constellation of microplatforms, triggered to seek out and track a tsunami wave the instant an underwater ‘quake was detected.
A similar concept was conceived in the UK and is now in operation over land where 5 multi-national satellites make up the Disaster Monitoring Constellation, with several more countries willing and able to contribute another platform. Could the same concept apply to the oceans where it might be argued that the need for surveillance is even greater?
Under normal daily operations the marine constellation would transmit regular bulletins of observed surface wind and wave heights to ships and forecasting centres around the globe allowing storms at sea to be located and tracked with much greater precision. In a longer timeframe there would also be a substantial dividend to climate research programmes since heat-carrying mesoscale ocean eddies would be resolved to greater accuracy by a constellation of altimeters.
Conclusions
Although not discussed in any detail at the Workshop it was recognised that the 2004 disaster had generated renewed research effort into improved understanding of geophysical prediction models. It was also noted that there appeared to be some evidence of a periodic cycle to tsunami activity of around 40–50 years. Around the Pacific rim major tsunamis were recorded in 1946, 1952, 1957 and 1960 followed by a period of relative quiescence until the Indian Ocean event in 2004.
The consensus that emerged from the Workshop was the need to capitalise on the coverage afforded by satellites as part of a more closely integrated, global detection system. Each element of this system—buoy, model and satellite—requires to be evaluated to determine how best they might be complemented.
The fact that a constellation of several satellites would be required to provide a reliable detection and tracking capability would be balanced against the cost savings introduced by using microplatforms. In observing the ocean from space it may now be time to break the mould.
