This paper deals with the monitoring of land subsidence using Global Positioning System (GPS) in the South Gujarat, India. In this area, hydrocarbon is being extracted since many years. It has been established that hydrocarbon extraction is one of the main causes of subsidence over the study area. Land subsidence studies have been carried out by the GPS team of Indian Institute of Technology Bombay, with dual frequency GPS receivers. A total of nine field campaigns have been completed. The collected GPS data have been processed in post-processing mode, using scientific GPS data processing software with precise ephemeris. To achieve millimeter (mm) level accuracy, ionospheric correction and tropospheric corrections were estimated and applied during data processing. Statistical testing was done to check the significance of the GPS data. The results of statistical testing confirm that there is significant subsidence within the reservoir boundary, rather than outside the boundary.

The term `subsidence' is used to indicate a slow downward change in ground elevation with little or no horizontal motion. Land subsidence arises from either regional or local phenomena. Geological phenomena including tectonic or volcanic activities are the main causes of land subsidence on the regional scale, while localized subsidence is caused by either natural or anthropogenic activities. Land subsidence caused by natural reasons are sink holes in limestone areas, while man-made subsidence is caused due the removal of subsurface material during underground mining operations, underground construction activities and withdrawal of natural resources like water, oil and gas. Over the last several decades, subsidence has caused problems in urban, rural and unpopulated areas of the world. Major events of subsidence observed in the last 50 years are man-made and due to haphazard extraction of natural resources like water, oil and gas. The effects of subsidence are different from other hazards like floods and earthquakes, because the rate of subsidence is very slow. Hence, the effects of subsidence are not sudden and disastrous.

Application of GPS for Monitoring Land Subsidence, natural, subsidence, effects, hydrocarbon, phenomena, resources, correction, disastrous, earthquakes, ephemeris, extracted, haphazard, Geological, ionospheric, Institute, limestone, localized, monitoring, millimeter, operations, postprocessing, Positioning, anthropogenic, scientific