Tiny ground movements that occur too gradually to be seen by the human eye can
nevertheless be detected by ESA satellites looking down to Earth from 800 km away.
At a workshop in Italy last week, researchers explained how they are using this
ability to monitor volcanoes and earthquake zones, aid oil and gas prospecting,
observe urban subsidence and measure the slow flow of glaciers.
Data from Synthetic Aperture Radar (SAR) instruments like those flown aboard the
ERS spacecraft and Envisat are the basis for a technique called SAR
interferometry, or InSAR for short. InSAR involves combining two or more radar
images of the same ground location in such a way that very precise measurements
— down to a scale of a few millimetres — can be made of any ground motion
taking place between image acquisitions.
Very small movements can potentially be detected across wide areas: tectonic
plates grinding past one another, the slow ‘breathing’ of active volcanoes, the
slight sagging of a city street due to groundwater extraction, even the thermal
expansion of a building on a sunny day.
More than 230 researchers from all across Europe, plus the United States,
Argentina, Korea, Indonesia and China met in Frascati near Rome from Monday 1
December. They were attending the third ESA International FRINGE Workshop, a
five-day gathering devoted to InSAR advances from the ERS and Envisat missions.
More than 110 papers were presented by Principal Investigators during the
Workshop, with dedicated sessions on specialist subjects including tectonics,
land motion, volcanoes and ice movement.
Precise views of changing landscapes
“Collecting multiple images of the same landscape might at first sound boring,
until you realise the extraordinary level of precision with which InSAR shows us
how that landscape changes,” explained Prof. Fabio Rocca of the Milan
Politecnico, who has worked in this field for the last two decades.
“The technique really came into its own since ESA launched its first ERS
satellite in 1991. The decision was made to archive all ERS data, which was
courageous as data storage was so much more expensive then. Now that decision is
paying off because all the archive is available for InSAR use.”
Radar images record the backscatter of microwave pulses reflected off the
Earth’s surface, and so measure relative surface roughness — the brighter a
given point shows up, the higher its roughness, and backscatter. Smoother
surfaces tend to bounce radar pulses away from the spacecraft’s field of view.
“It is very different to looking at optical wavelengths, and one of the subjects
we are discussing at the Workshop is how to work out more accurately what we are
seeing,” said Rocca. “We are looking with different eyes — think of it as like
the eyes of the Terminator! At optical wavelengths the surface of a building
reflects light, but at radar wavelengths we pierce through the walls of the
building to the steel skeleton beneath — its sharp corners give it high radar
reflectivity.
“Features like vegetation or loose soil can be difficult to image clearly, and
can move between acquisitions causing an InSAR image to lose coherence. So for
reference we use fixed and high reflecting points in a landscape like buildings,
large rocks or even the poles holding up a tennis net. We call them permanent
scatterers and they function in the same way as trig points for ground-level
mappers.”
Watching over a ‘breathing’ volcano
Researcher Paul Lundgren of the California-based Jet Propulsion Laboratory —
working with Italy’s National Research Centre Institute for Remote Sensing of
the Environment (IREA-CNR) in Naples — has mined the ERS data archive to
produce more than a hundred interferograms of Mount Etna. Acquired between 1992
and 2001 they reveal terrain shifts as large as 14 cm taking place between
measurements.
The volcano appears to alternately inflate and deflate depending on the pressure
of its underground magma chamber, and a gravity-driven spreading movement has
also been observed. By turning the interferograms into an animation, the volcano
appears to be breathing.
Lundgren’s intention is to better understand the connection of surface
deformation to subsequent volcanic activity, and increase our ability to predict
volcano behaviour. He plans to make use of Envisat data in future to continue
his survey.
“With ERS overflying it every orbit, Etna is a great volcano to study because
there’s lots of data, and there’s lots of relatively new lava flows on its
slopes which make for good InSAR coherence,” said Lundgren. “It’s not as
dangerous as the likes of Vesuvius but while that volcano stays dormant for long
periods Etna does a lot, and by measuring its displacement we’ve learned a lot
about the complexity of its underlying structure and what is happening inside.”
A lost world sealed off under tonnes of ice
InSAR has also been used to look beneath a polar ice sheet four kilometres deep
and learn more about conditions prevailing in one of the strangest environments
on Earth. Lake Vostok in the East Antarctic is a 280-km-long freshwater lake
that has been buried beneath the ice sheet for at least half a million years.
A combination of crushing pressure, geothermal heat and the insulation of the
thick ice above it is thought to keep the waters of Lake Vostok liquid. What
remains unknown is whether any life exists in this dark, cold, low-energy
environment, entirely cut off from the rest of the world. Researchers have
decided not to drill into the Lake until they can be certain they will not
contaminate its pristine waters with topside bacteria.
Although Lake Vostok is off-limits for now there are indirect ways of seeing
beneath the ice. Back in 1993 ERS data was employed to help map the Lake’s full
extent, establishing the ice directly over it was much flatter than that around
it. More recently, German researchers have used ERS interferograms to establish
that — despite their distance from the surface — the waters of Lake Vostok are
stirred by daily tides.
During the FRINGE ice session, Anja Poetzch of the Dresden Technical University
presented details on how pair of interferograms acquired during ERS-1 and 2
tandem operations in 1996 demonstrated a maximum vertical displacement of 15 mm
above Lake Vostok, corresponding to tidal motion. Results from in-situ GPS
observations carried out during the last two Antarctic summers confirm the
conclusion.
Missions to come
Future planned radar satellites were discussed on the final day of FRINGE, and
high on the list of recommendations from participants was the need for to ensure
continuity of coverage so ESA’s SAR data archive will extend well into the 21st
century.
Both ERS and Envisat have identical orbits and their radar instruments are based
around C-band wavelengths (code letters are an inheritance from radar’s early
use in World War Two). For a future spacecraft’s radar imagery to be back
compatible for InSAR, it would have to follow the same orbit and radar wavelength.
The importance of having a follow-on C-band mission to ensure continuity of both
wide area SAR and InSAR capability was also a key finding of October’s meeting
of ten consortia developing services within Global Monitoring for Environment
Security (GMES) Service Element programme. GMES is a joint venture between ESA
and the European Union to use satellite data to gather global environmental and
security intelligence.
Radar missions that go beyond C-band were also discussed during FRINGE. A
proposed ESA Earth Watch mission called TerraSAR-L is currently in the study
phase. It is so named because it would employ longer wavelength L-band radar, of
greater use over vegetated surfaces.
Dr. Richard Bamler of the German Aerospace Centre DLR explained how TerraSAR-L
could work in concert with TerraSAR-X, a German mission using the short
wavelength X-band wavelength to resolve objects as small as one metre and
intended for launch in spring 2006.
“Potentially our spacecraft might map the corners of a field while TerraSAR-L
identifies its contents,” said Bamler. “X-band can map the layout of suburban
roads, or see details of forests that appear greyed out on longer wavelengths.
There is also experimental instrumentation to measure fast velocities on the
ground — we will track the speed that cars on a motorway move.”
The mission is an indication of the growing market for InSAR data, Bamler added:
“It is a public-private partnership between Astrium, whose subsidiary Infoterra
gets commercial rights to the data, and DLR, taking charge of scientific
exploitation. We have a detailed business plan.”
The increasing size of the market was a point also emphasised by Rocca: “A
distinctive community of InSAR users has developed, and now we are coming back
to ESA with our user requirements for further missions. The growing number of
end users — including the insurance industry, railways and oil and gas
companies — recognise the value of the technique and require its continuity
into the future.”
Related articles
* Expert’s Roundtable: ASAR interferometry promises hyper-accurate measurements
from orbit
http://www.esa.int/esaSA/ESAGSWTHN6D_earth_0.html
* ERS data ‘breathes’ life into Etna
http://www.esa.int/esaCP/ESALE48708D_index_0.html
* Mapping Britain’s crumbling coastline
http://www.esa.int/esaCP/ESAZDD2VMOC_index_0.html
Related links
* FRINGE Workshop
http://earth.esa.int/fringe03/
* Eduspace — Radar technology
http://www.eduspace.esa.int/eduspace/subdocument/default.asp?document=323
[NOTE: Images supporting this release are available at
http://www.esa.int/export/esaCP/SEM913VZJND_index_1.html ]
