Latin America Volcano Monitoring With ALOS
By Matt Pritchard and Tom Fournier, Cornell University
Deformation of the Earth’s surface at volcanoes provides clues to the myriad processes occurring below and above the surface, and might provide warning of an imminent eruption. Unfortunately, experience has shown that different volcanoes have different behaviors before eruptions, such that a history of precursory activity and eruption should be established for each individual volcano. As of 1997, surface deformation had been observed at only 44 different volcanoes using ground-based methods (e.g., traditional surveying, tiltmeters, or Global Positioning System) out of the more than 1,500 potentially active volcanoes around the world [Dvorak and Dzurisin, 1997]. In the last decade or so, observations of deformation at volcanoes have more than doubled to about 110, due largely to the use of satellite-based Interferometric Synthetic Aperture Radar (InSAR).
One limitation of these InSAR studies has been the difficulty in applying the available C-band radar data (wavelength of 5.6 cm) to vegetated volcanoes like the 300 Holocene volcanoes in Latin America [including Mexico, Central America, the Caribbean, and the Northern, Central, Southern, and Austral Andes (see Figure 1)]. L-band radar data (wavelength of 23.6 cm) from the Japanese Advanced Land Observing Satellite (ALOS), archived at the Alaska Satellite Facility (ASF), are more successful at imaging the deformation of Latin American volcanoes.
The data from ALOS is used to make preliminary surveys of the volcanoes of Central America, the Caribbean, and the Northern and Southern Andes with data that spans 2006-2008. Not included, are the more arid Central Andes as previous C-band studies have already revealed eight areas of volcanic deformation. While the survey is spatially comprehensive, it is quite possible that some deformation was missed that is small in magnitude or spatial scale or temporally aliased. Because data acquisitions are infrequent, data quantity and quality are not uniform across all volcanoes. Nonetheless, the new observations reveal volcanic deformation in 11 different areas. Several of these areas were thought to be dormant by the science community, demonstrating the current incompleteness of global-volcano monitoring and the potential for ALOS data to reveal unsuspected activity.
In the Northern Andes, data quality, or for these purposes coherence of the InSAR signal, seems to be highest at high elevations where there is no snow and less vegetation (Figure 1b). A 46-day repeat provides coherent interferograms in the vegetated lowlands when the spatial baseline is small and/or when spatial averaging is applied. For longer time spans, the lower elevation regions become decorrelated. In the Southern Andes, it is necessary to avoid austral winter. Even considering summer only, coherence is the lowest in the foothills on the western side of Cordillera (Figure 1c) which receives the most precipitation and has the most vegetation. In the Caribbean (Figure 1d), the coherence is the lowest of any region studied and the short time-period interferograms with small baselines are essential. Vegetation and terrain cause the most problems, with the inland areas of most of the islands becoming decorrelated the fastest. In the Northern Andes, Central America, and the Caribbean, the L-band coherence results are clearly superior to previous C-band results from these areas (e.g., Zebker, et al., 2000). On the other hand, L-band is more sensitive to ionospheric effects than C-band and such effects are observed in a few percent of scenes from the Northern, Central, and Southern Andes.
With good control of the orbital baseline (<250 m), the conclusion is that at L-band, sufficient coherence is maintained for volcanic applications in vegetated areas at 46 days, most of the time. Sufficient coherence for interferograms will not be possible all of the time, as shown by incoherent L-band nterferograms made by InSAR systems on aircraft and on the space shuttle with baselines spanning one to 7 days. The incoherence in the interferograms has been associated with weather systems. Furthermore, 46-day interferograms do not work in most snow or ice areas (e.g., the volcanoes of the Southern and Austral Andes during austral winter). A significant fraction of volcanoes around the world have snow cover for at least part of the year. A shorter repeat time for future InSAR systems [like the National Aeronautics and Space Administration’s (NASA) proposed Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI) InSAR mission] will maximize the chance that there will not be a precipitation, wind, or melting event that might diminish coherence
Dvorak, J. J., and D. Dzurisin (1997), Volcano geodesy: The Search for Magma Reservoirs and the Formation of Eruptive Vents, Rev. Geophys., 35, 343-384.
Zebker, H. A., F. Amelung, and S. Jonsson (2000), Remote Sensing of Volcano Surface and Internal Processes Using Radar Interferometry, in Remote Sensing of Active Volcanism, Geophys. Monogr. Ser., Vol. 116, edited by P. J. Mouginis-Mark,
J. A. Crisp, and J. H. Fink, pp. 179-205, AGU, Washington, D.C.