European Space Agency Press Information Note No. 07-95 Paris, France 27 March 1995
Although the eruption of Vesuvius over 1900 years ago is probably the best known example of such a phenomenon, mainly because of the description of it by Pliny the Younger, it was by no means the most violent or caused the most damage. When the Tambora volcano erupted on the Indonesian island of Sumbawa east of Java in April 1815, falling stones and ash killed about 12 000 people and on 8 May 1902, the hot cloud of gas and rivers of lava that raced down the slopes following the "explosion" of Mount Pele, in the north-west of the Carribean island of Martinique, wiped out almost the entire population -- about 38 000 -- of the town of St. Pierre. The only survivor was a prisoner who had been held in an underground cell. But the most violent volcanic eruption in recent times took place on 27 August 1883 on the island of Krakatoa, between Sumatra and Java. The equivalent of about 21 cubic kilometres of stone exploded into the air and for two and a half days the skies were darkened by volcanic ash, which, when it fell, covered an area 800 kilometers square (as large as France and Great Britain put together). The force of the eruption was twenty-five times that of the most powerful H-bomb ever exploded and destroyed all forms of life on the 50 kilometer square island. The wave that followed in the wake of the eruption destroyed over 150 villages along the coast of the surrounding islands, taking some 40 000 lives.
As far as we know, there are at present over 500 active volcanoes, whose successive eruptions gradually alter the Earth's surface. Almost half of them are to be found along the western rim of the Pacific and on offshore islands. A sixth of them comprise the "belt of fire" around the Pacific. But Europe too has active volcano chimneys and one in fourteen of the world's volcanoes lies along a line stretching from east to west across the Mediterranean Sea on the boundary between the European and African continental plates. The inhabitants of Iceland have to live with 27 active volcanoes in their country, which has a surface area of about 100 000 square kilometers. So it is not surprising that even in Europe, as much scientific data as possible are gathered and analyzed with a view to acquiring the knowledge needed to forecast volcanic eruptions reliably.
For some years now, volcanologists have had at their disposal a completely new and extraordinary reliable method based on observations and measurements from space of those regions of the world that are prone to volcanic eruptions. It is called synthetic aperture radar (SAR) interferometry and has been tested for the first time through radar images produced by ESA's European Remote-Sensing Satellite, ERS-1.
In this method, two or more high-resolution radar images of the same area are taken from slightly different positions -- for instance on two consecutive satellite passes, and are then superimposed ("interfered") using a computer to show up changes in the surface configuration (relief). For this purpose, data are gathered using a SAR, part of an active microwave instrument whose geometric resolution is considerably improved by skillful exploitation of the satellite's movement. Generally speaking, the resolution or acuity of vision of a detector depends on the size of the antenna used. In the case of ERS-1, the antenna is about ten meters long. However, because the satellite moves a little further along its roughly 800-kilometer high orbit around the Earth between the time of emission of the radar beam and reception of the return signal, the resulting succession of backscatter signals show the same swath of the Earth's surface from slightly staggered angles of vision. Once this series of signals has been processed by computer, recption with a considerably larger antenna can be simulated, thereby producing an artificial or synthetic increase in aperture size. Using this method gives ERS-1 a synthetic antenna diameter of about 800 meters and in "image mode" it can thus produce radar pictures of the surface of the Earth or its oceans with a pixel size corresponding to an area as small as 30 m x 30 m and in special cases even 4 m x 20 m.
SAR interferometry -- or INSAR -- is, as it were, the next logical step leading from flat images of the Earth's surface to three-dimensional relief map. Professor Philipp Hartl of the Institute for Navigation at Stuttgart University, who played a crucial role in the development and testing of this new method explains its principle: "Just as a relief map of a scene can be established from an optical stereo image pair by triangulation, using analog or digital techniques, the superposition of complex digital SAR image pairs produces an interferogram providing information about the topography of the zone observed."
In the case of the radar image pairs, the information necessary for determining the satellite's distance in relation to individual pixels is contained in the data gathered. The SAR antenna not only records the brightness of the radar signals backscattered from the Earth's surface but also their "phase". In a comparison of the peaks and troughs of a wave with he movements of a clock hand over a period of one hour, this phase gives, as it were, the position of the clock hand.
For each pixel, the phase values recorded from measurement positions are subtracted in order to produce an interferogram. This provides a "picture" of the scene and represents the differences in the distance of individual pixels from both measurement positions. In the first instance, these are not absolute as the phase value of the backscattered radar signal is repeated in cycles without it being known how many such cycles (or wavelengths) there are between the Earth's surface and the satellite antenna. However, with the aid of complex algorithms it is possible to reconstruct these distances in absolute terms using precise data on the satellite's movements and phase values recorded from adjacent measurement positions.
Volcanologists are particularly interested in minor ground movements for the purpose of forecasting imminent volcanic activity. Previous studies have shown that the upward migration of magma from the Earth's crust in the run-up to an eruption inflates the volcanic cone, rather like a human chest inflating before an intake of breath. Such premonitory signs can easily and quickly be detected with the aid of differential SAR interferometry. This requires radar data from only three satellite passes, of which two must follow each other after a slightly longer period. When image pairs 1+2 and 2+3 are processed respectively to produce two SAR interferograms, the difference between the pairs clearly show the (average) relief changes for each pixel. Extensive calibrations in a variety of test areas have shown that by using this technique, rising and falling of the Earth's surface can be detected to centimeter accuracy.
In the case of Vesuvius, such measurements and long-term observations have now become a routine part of ERS-1's job. In addition, researchers are also able to study developments in the Phlegraean Fields to the west of Naples, a region about 220 kilometers square, containing a great many volcanoes with numerous tuff craters, solfataras, fumaroles and mofettes. This region can look back on a past that has been -- in the true sense of the word -- turbulent as it has risen and sunk on many occasions due to "bradyseismic" activity, with coastal areas being submerged by the sea only to re-emerge later. These sinking and rising movements are explained by volcanic gases and magma that rise up and inflate underground chambers, which subsequently contract. In the normal course of events, such movements occur very slowly and imperceptibly. However, for some time in the sixteenth century they picked up momentum rather quickly, culminating in the eruption of Monte Nuovo in 1538. For some time now, the upward movement has again been gathering pace, this time in the area around Pozzuoli. In the last twenty-five years this area has twice been devastated by spectacular surface activity: between 1970 and 1972 the ground rose up 1.7 meters and ten years later was again pushed up by about 2 meters. Large parts of the town's old port area were destroyed, forcing thousands of people to leave.
The most striking consequences are in fact to be found in the port area where fishermen today have to descend steps to the bottom of the old harbour wall in order to reach their boats. In contrasts, some sites that sank into the water long ago have re-emerged, such as the ruins of the Roman marketplace of Serapis. A small archaeological site has since grown around it but the extensive mussel deposits on the pillar are proof of length of time spent under water.
Nobody yet knows whether recent undergoround activity near Pozzuoli presages a bigger eruption and there is no telling how Vesuvius might react in future. The last occurrence of any major eruption was over fifty years ago. But keeping an eye on possible developments is now much easier thanks to SAR interferometry using ERS-1. Until now land surveyors had to rely on a water-level gauge in Pozzuoli harbour to record their measurements. This gauge was the reference for a system of measuring points throughout the town and each time measurements were taken, it took several days. With SAR interferograms produced by ERS-1 (and in due course by its successor ERS-2) the surveyors can now gather the same data much more quickly and conveniently.
The advantage of interferometry are obvious in that it can be used to monitor inaccessible volcanic regions in order to forecast any impending danger at an early stage.
-- Andrew Yee Staff Scientist, Science North 100 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 5S9
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