By bouncing radar off the ground, two European satellites detected the inflation of an Alaskan volcano preparing for its next eruption. In another measurement, they revealed sinkage in a section of Naples, Italy where subway tunnels were being dug.

Scientists accidentally discovered Naples downward movement as the satellites monitored nearby Campi Flegrei caldera, a large volcanic crater, said engineer Gianfranco Fornaro of the Research Institute for Electromagnetism and Electronic Components in the Italian metropolis.

Italian researchers speculated that subway construction "triggered a more complex subsidence mechanism that extends over a larger area," perhaps related to the presence of faults along an ancient volcanic caldera, Fornaro said by e-mail from Italy. Calderas first form during cataclysmic eruptions that begin when the crater floor sinks along a ring-shaped set of faults.

Engineer Riccardo Lanari led the study, which was financed partly by the Italian Space Agency. Lanari, Fornaro and colleagues from their institute and the University of Neapolitan Studies reported the findings last month in the journal Geophysical Research Letters.

In June, the same journal published a report on how the ERS 1 and ERS 2 satellites, which are operated by the European Space Agency, bounced radar off Westdahl volcano on Alaskas Unimak Island.

"Immediately after it erupted, it began filling the magma chamber [with molten rock] again," said study coauthor Charles Wicks, a geophysicist at the U.S. Geological Survey (USGS) in Menlo Park, California. "Maybe most of the inflation occurs right after the eruption, and the rate slows down as it gets closer to the next eruption. Hopefully it will be good for predictive value" by warning that an eruption is likely within years.

The new study concluded: "On the basis of timing and volume of recent eruptions at Westdahl and the fact it has been inflating for more than five years, the next eruption can be expected within the next several years."

The research was led by geophysicist Zhong Lu of Raytheon STX Corp., a USGS contractor at a satellite data center in Sioux Falls, South Dakota. Lu and Wicks studied Westdahl with five other researchers from the USGS, the Alaska Volcano Observatory and the Cascades Volcano Observatory.

Based on the measurements, the scientists estimate that a magma chamber located about 5.6 miles (9 kilometers) beneath the volcano was injected with magma equivalent to a cube a quarter mile (more than one-third of a kilometer) long on each side.

Using satellite radar to measure inflation of volcanoes has been tried only on Westdahl and on Alaskas Okmok volcano, so its value for predicting eruptions remains uncertain.

"It certainly has great potential," said Tom Miller, acting scientist-in-charge of the Alaska Volcano Observatory. "But we need to go through a few cycles [of volcanic eruption and inflation] and see what it means."

Lu said: "If we know how much magma needs to be restored before an eruption, this would be very useful for pinning down the window for the next eruption," particularly when used in concert with other forecasting methods.

Using radar to measure uplift or sinking is difficult. Scientists compare radar images of the same area over periods of months to years. Data from two images at a time are combined to create an "interferogram." The difference in returning radar waves in the two images creates a pattern of colorful "fringes" on the combined picture. Each fringe represents a certain amount of vertical movement.

The ERS satellites, launched in 1991 and 1995, were not designed to measure volcanoes and land sinking. Their main mission is to study oceans, atmospheric conditions, ice caps and coastal regions.

Meanwhile, the National Aeronautics and Space Administration (NASA) announced this week that its DC 8 jetliner Flying Laboratory will spend the next two and a half months using radar to collect data in more than 15 nations in the volcanically active Pacific Rim. Sites include Cambodias Angkor Wat Temple, French Polynesia, Papua New Guinea, the Philippines and Australia's coastal wetlands.

The airborne radar, which is used to demonstrate technology for spaceborne radar missions, will not only collect geological data, but information for coastal analysis, oceanography, forestry, hydrology and archaeology.