RAPID SUBSIDENCE OVER OIL-FIELDS MEASURED BY SAR INTERFEROMETRY

The Lost Hills and Belridge oilfields are in the San Joaquin Valley, C alifornia. The major oil reservoir is high porosity and low permeabili ty diatomite. Extraction of large volumes from shallow depths causes r eduction in pore pressure and subsequent compaction, forming a surface subsidence bowl. We measure this subsidence from space using interfer ometric analysis of SAR (Synthetic Aperture Radar) data collected by t he European Space Agency Remote Sensing Satellites (ERS-1 and ERS-2). Maximum subsidence rates are as high as 40 mm in 35 days or > 400 mm/y r, measured from interferograms with time separations ranging from one day to 26 months. The 8- and 26-month interferograms contain areas wh ere the subsidence gradient exceeds the measurement possible with ERS SAR, but shows increased detail in areas of less rapid subsidence. Syn optic mapping of subsidence distribution from satellite data powerfull y complements ground-based techniques, permits measurements where acce ss is difficult, and aids identification of underlying causes.