Monitoring Land Subsidence Using GPS And INSAR

Abstract

Land subsidence is one of the most varied forms of ground failure with devastating effects, ranging from broad regional lowering of the land surface to local collapse. Geological reasons like tectonic and volcanic activities affect at regional scale while localized phenomena can be mainly attributed to either human activities or natural activities like sink holes, fissures etc. With the increase in population, demand of natural resources like water, oil and gas has also increased, resulting in the extraction of these important natural resources in a haphazard manner. Such unplanned extraction of natural resources is one of the major reasons for land subsidence. Uniform settlement of ground does not create any major problem; however, uneven settlement causes lots of damage to infrastructure built over the affected area. There is a significant financial loss in repair of such the infrastructure. Hence attention should be paid to this man-made hazard. To monitor and measure land subsidence, many techniques are available, like conventional levelling, Global Positioning System (GPS), Synthetic Aperture Radar Interferometer (InSAR), in situ techniques like Bore hole extensometer and Radio Active Marker method, etc. With the advancement of space technology, land subsidence is increasingly being measured and monitored with GPS. GPS has proven to be a reliable technique for monitoring the land subsidence with the precision of few mm levels, using geodetic dual frequency receivers, precise ephemeris and processing the data in post processing mode with scientific software. To measure subsidence near Surat in Gujarat state, India, GPS survey has been carried out by the Indian Institute of Technology Bombay (IITB) GPS team. A total of 10 GPS field campaigns have been carried out during the period: February 2004 to October 2006, at an interval of 3-4 months. A GPS monitoring network was established in February 2004 with four reference stations and 27 deformation stations. Geodetic dual-frequency GPS receivers have been used for these observations. The reference station GPS receivers were continuously observing during the entire field campaign, and at each deformation station, minimum five hours of continuous GPS data was collected. The collected data and base lines were processed with scientific GPS data processing software: Bernese v 4.2. IGS data files as well as precise ephemeris were downloaded from IGS data bank, which were used for post processing the data. The data was processed considering saastamoinen troposphere model and ionosphere free solution combining L1 and L2 frequencies. In this paper, the results of ten campaigns are compared to understand land subsidence phenomena in the area of study. During a period of two years, 67 mm subsidence has been measured within the gas reservoir area. It has been confirmed that the rate of subsidence is more within reservoir boundary compared to that out side the reservoir boundary. Estimated subsidence is also correlated with the parameters responsible for land subsidence like gas extraction rate, pressure depletion, water level. It is established that, gas extraction is one of the main causes of subsidence over the study area. It was planned to compare the GPS results with InSAR method, but the results of INSAR method could not get due to limitations of INSAR method in highly vegetated area with changing in the crop pattern with time. (Key Words: Land Subsidence, GPS, InSAR)