Time series analysis of subsidence at Tauhara and Ohaaki geothermal fields, New Zealand, observed by ALOS PALSAR interferometry during 2007–2009

Abstract

We present a modification of the Small Baseline Subset (SBAS) technique and apply it to L-band ALOS PALSAR data for time series analysis of subsidence at Tauhara and Ohaaki geothermal fields in the Taupo Volcanic Zone, North Island, New Zealand. We noticed that residual topographic noise presented in most ALOS PALSAR interferograms that have been acquired with large perpendicular baselines is a significant limiting factor that complicates their interpretation. We modified the original SBAS technique in such a way that the residual topographic contribution is accurately estimated and removed. For testing purposes we acquired 12 ALOS PALSAR images between 13 January 2007 and 18 January 2009 and created 33 differential interferograms for use in SBAS processing. The calculated time series of deformation at both geothermal fields show steady subsidence with rates of 5–6 cm/year that is clearly mapped with the proposed technique. A comparison of results calculated with the proposed technique and with the original SBAS algorithm shows that the proposed approach is more accurate and produces results of higher quality. The technique presented in this paper can be used for time series analysis of any SAR data; however, it is most appropriate for L-band ALOS PALSAR that acquires coherent images with large spatial baselines that also correlate with the time of acquisition.