10 January 2005
TSUNAMI
DETECTIONS WITH CODAR SEASONDE® HIGH FREQUENCY (HF) RADAR
An investigation on using our early HF CODAR radars to detect or measure
the strength of tsunami surface signatures was conducted and published in the
late 1970s [Barrick, D. E. (1979), A coastal radar system for tsunami warning,
"Remote Sensing of Environment" , vol. 8, pp. 353-358]. (available for
download as a PDF at the publications section of our website http://www.codaros.com/bib_79-70.htm)
At that time, the result seemed of marginal utility because the radars could see
only 30-40 km at best, providing little tsunami warning time. Nowadays our SeaSonde®
HF radars (successors of the CODAR model) can see out farther, potentially increasing
the time between tsunami detection and it's arrival at the shore.
First,
a little on how tsunami waves propagate into an area and how the HF radar sees
them. Then, a bit on how data from the radar network in parallel “current-mapping
and tsunami modes" might be processed and transferred when needed.
HOW TSUNAMI WAVES PROPAGATE
Typically it is an earthquake, volcanic eruption
or underwater rockslide that generates a tsunami, which is a series of waves that
have time periods between consecutive crest passages typically between 20 and
30 minutes. In any ocean wave, the time period (once generated) never changes;
the spatial period, height of the wave, and velocity of propagation all change
depending on the water column depth, but the time period is invariant.
To use an example, suppose the tsunami wave propagating across an ocean basin
has a height of 20 cm (crest height) in water depth of 4000 m with a period of
20 minutes. In water this depth, it will have a propagation speed near 700 km/hr
(independent of its time period), but a spatial period of 300 km. There are very
few technologies that could possibly detect this potentially damaging wave in
deep water, in real time; with its small crest height, it will simply raise any
floating buoy or ship by 20 cm over a 20 minute period as the crest passes by,
an undetectable motion. Bottom pressure sensors (like those on the NOAA-engineered
DART buoys in the Pacific Ocean) can feel the weight of this water rise as it
passes overhead.
As this wave moves into shallower water (e.g., 180 m
depth at a distance of 100 km from shore, typical of parts of United States bathymetry),
its crest height will increase to 50 cm, and its speed will slow to about 140
km/hr. At even shallower depths closer to shore, crest height increases further,
while propagation speed decreases as does the spatial period.
HOW AN
HF RADAR "SEES" A TSUNAMI WAVE
The most sensitive measurement the
HF radar makes is velocity, from the Doppler shift of the echo. However, what
the HF radar sees is NOT the very fast propagation speed of the tsunami wave (e.g.,
140 km/hr at 180 m depth), but the "current speed" of the orbital velocity
at the crests and troughs. The HF signal is scattering from Bragg waves that are
6 to 30 meters long, depending on the radar transmitted frequency. These, in turn
are shifted by any underlying currents. Near the crest and trough of ANY wave
(including a tsunami wave), the water undergoes an orbital motion: forward at
the crest and backward at the trough. For the case considered here, this velocity
is +10 cm/s, which is enough to be detected by the HF radar from the background
noise. With a spatial period of 45 km at 100 km distance, this pattern will be
apparent in the data, because closer to shore at the next trough 22 km away, the
current will reverse, and be -10 cm/s.
As the tsunami moves into shallower
water 90 m deep, at say, 50 km from shore, this orbital velocity will increase
to +/-17 cm/s, while the spatial period decreases (between crest and trough) to
perhaps 16 km. So, you begin to see even stronger currents with shorter spatial
periods as the tsunami wave gets closer, into shallower water-- an increasingly
robust signal.
The variation of the tsunami wave with distance from shore
is known because the depth is known. What isn't known is the strength of the tsunami
wave. This strength is obtained from the orbital velocity measured by the HF radar
at crests and troughs: the higher the orbital velocity, the more energy in the
tsunami wave, and the higher the crest heights will be when the tsunami steepens
and crashes on shore in very shallow water. Also, by looking at the strength of
the currents as two crests pass the same point, one could make a prediction of
how many tsunami waves will be devastating, till it dies out. In other words,
is it still increasing, or has begun to decrease?
HOW COULD SOFTWARE
PICK OUT A TSUNAMI FROM A BACKGROUND OF SIMILAR PERIOD OR WAVELENGTH SIGNALS?
There really are no other significant waves on the ocean of 10-30 minute periods
other than tsunamis. That is a much longer period than the very longest swells,
which might have 15-20 second periods. And it is much shorter than the shortest
tidal periods of 6 hours (tides are just another type of wave, like tsunamis and
swells.) To put this into perspective, there are typically 20 tsunami-genic earthquakes
that occur anywhere on earth per year, with 4-6 of these having sufficient magnitude
to cause varying amounts of destruction and loss of life.
HOW MUCH TIME
IS THERE BETWEEN TSUNAMI DETECTION AND ARRIVAL AT THE SHORE?
There are several
factors affecting the time between tsunami detection by the radar and it's arrival
at the shore. The bathymetry is a very important factor; a wider, shallower shelf
will make it possible to detect the signature wave farther offshore than in an
area where the bathymetry is very deep right up to the coast. A second important
factor is the amplitude of the tsunami wave that gets generated by the earthquake
or rockslide. This varies widely also. CODAR staff have written simple routines
that will estimate the wave detection-versus-onshore arrival time for any place
on the globe.
HOW WOULD "TSUNAMI MODE" DATA PROCESSING WORK?
Only very rarely would there be a tsunami-like wave in an HF radar's field of
view. In the meantime, one could be performing parallel processing of data for
standard surface current mapping and monitoring of general wave parameters (such
as significant wave height), with background parallel processing for the tsunami
mode taking place in real time on the field computers at the radar receive stations.
The tsunami monitoring is like the normal SeaSonde current and wave processing,
but update rates occur at a much shorter interval (2-4 minutes) so as to catch
the rapidly evolving nature of the tsunami field. Because, tsunamis are present
so rarely, one wouldn't necessarily need or want to transfer all of the tsunami
data back at near real-time intervals, if there are limited communication baud
rates available at radar field station. However, one could have the data sent
back regularly at near real-time, or could program the system for rapid transmissions
only when an earthquake and/or tsunami has been detected (by another sensor).
Typically any earthquake that can generate a significant tsunami is detected seconds
after it occurs (and the "tsunami watch" begins). At that point, the
central station (typically located at client's headquarters) linked to the remote
SeaSonde radar computers could be triggered to begin requesting the higher-rate
tsunami-mode data from the remote stations. Then at the central station(s), you
begin to do the calculations to look for the telltale orbital pattern and estimate
the strength of the tsunami as it approaches that stretch of coast. With SeaSondes,
it is usually the central station at the users office that initiates and requests
data transfers from the remote stations, but this could also work in reverse.
Such special software for an operational "tsunami mode" configuration
must still be created. CODAR Ocean Sensors is interested in spearheading or assisting
this effort if #1. It would be used in the field (after plenty of bench testing),
#2. The SeaSonde owner does comparison work with other tsunami detection/monitoring
equipment (or at least share such data and allow us to do comparisons), and, most
important, #3. This work is viewed as a research effort. Results from a research
effort such as this often produce something a bit different that what was initially
sought, but in many occasions that turns out to be even more useful than the original
target.
THE REALITY -- WHAT OTHER TECHNOLOGY IS OUT THERE, AND OF WHAT
BENEFIT COULD USE OF HF RADARS REALLY BE FOR TSUNAMI OBSERVATION AND RESEARCH?
There are clever numerical computer model programs that can predict earthquake-generated
tsunami impacts, along with when, where and how strong these impacts will be.
The longer the wave (and tsunamis are very long waves), the simpler and more robust
the model program becomes. Such programs includes ocean scale bathymetry. Since
earthquakes at or near the sea can only occur at fault lines (most of which are
known), one wouldn't even need to wait for an earthquake to happen to run such
a program. Every possible epicenter point (e.g. 50 km spacing) along these fault
lines could have the model run for it, with different levels of intensity and
duration --all prior to any actual seismic activity. (Today's processing horsepower
and data-base storage are more than adequate for this job. ) The coastal locations
at risk for impact can be predicted, along with time from epicenter to impact
at coast. At that point it is a question of whether the international community
has created protocol for rapid data dissemination of seismic data and tsunami
model predictions between foreign governments; if so, then local authorities would
receive warnings and have capability to spread instructions to civilians in the
at-risk locations. Such models, modes of cooperation, and local emergency warnings
are in effect at very few regions on the globe, and might take years/decades to
develop.
While models taking into account fault lines, and bathymetry
are obviously critical for tsunami prediction and planning, models are not perfect,
and will benefit from real-time improvement. Localized and quantitative data from
instruments such as HF radars and bottom-mounted pressure sensors (also not considered
fully operational for this application) will fine tune the cruder model forecasts.
Any better refinements will aid in preparations, e.g., by focusing on which areas
are going to need helicopter and rescue vessels. Having real data gathered by
a country's own HF radar could also be another source of early detection directly
available to local authorities, in case the string of required international communications
fails or hits a fatal choke point. Global models may not necessarily work perfectly
for a particular area whose bathymetry or physical parameters have not accurately
been quantified in the model database. (This is especially true in the developing
nations who do not conduct regular bathymetric/hydrological surveys.) And, those
instances when a quake epicenter is close to shore may not allow enough time for
the international communication chain to occur, leaving a direct, local detection
with HF radar or a pressure sensor as the only hope for advance detection/warning
and information refinement.
Finally, the present tsunami prediction
models and early warning schemes have been designed for those generated by earthquakes,
so if the source of the tsunami wave train is an underwater rockslide, then the
present scheme of an earthquake detection activating the tsunami warning will
not occur.
A more detailed analysis of tsunami detection using SeaSonde
HF radars is presently underway, including simulations. Results of this study
should be available within the coming weeks, and will be posted on our website
(www.codaros.com).
For further information, contact Laura Pederson at
phone +1 (408) 773-8240 or E-mail  |
