SURFACE DEFORMATION AND COHERENCE MEASUREMENTS OF KILAUEA VOLCANO, HAWAII, FROM SIR-C RADAR INTERFEROMETRY

The shuttle imaging radar C/X synthetic aperture radar (SIR-C/X-SAR) r adar on board the space shuttle Endeavor imaged Kilauea Volcano, Hawai i, in April and October 1994 for the purpose of measuring active surfa ce deformation by the methods of repeat-pass differential radar interf erometry. Observations at 24 cm (L band) and 5.6 cm (C band) wavelengt hs were reduced to interferograms showing apparent surface deformation over the 6-month interval and over a succession of 1-day intervals in October. A statistically significant local phase signature in the 6-m onth interferogram is coincident with the Pu'u O'o lava vent. Interpre ted as deformation, the signal implies centimeter-scale deflation in a n area several kilometers wide surrounding the vent. Peak deflation is roughly 14 cm if the deformation is purely vertical, centered southwa rd of the Pu'u O'o caldera. Delays in the radar signal phase induced b y atmospheric refractivity anomalies introduce spurious apparent defor mation signatures, at the level of 12 cm peak-to-peak in the radar lin e-of-sight direction. Though the phase observations are suggestive of the wide-area deformation measured by Global Positioning System (GPS) methods, the atmospheric effects are large enough to limit the interpr etation of the result. It is difficult to characterize centimeter-scal e deformations spatially distribute over tens of kilometers using diff erential interferometry without supporting simultaneous, spatially dis tributed measurements of refractivity along the radar line of sight. S tudies of the interferometric correlation of images acquired at differ ent times show that L band is far superior to C band in the vegetated areas, even when the observations are separated by only 1 day. These r esults imply longer wavelength instruments are more appropriate for st udying surfaces by repeat-pass observations.