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Abstract
Differential Synthetic Aperture Radar Interferometry (DInSAR) is a remote sensing technique that is capable of detecting land surface deformation with centimeter accuracy. In this research, this technique was applied to two pairs of Advanced Land Observing Satellite (ALOS) Phased Array L-band SAR (PALSAR) data to detect land subsidence in the Kathmandu valley from 2007 to 2010. The result revealed several subsidence areas towards the center of the valley ranging from a maximum of 9.9 km2 to a minimum of 1 km2 coverage with a maximum velocity of 4.8 cm/year, and a minimum velocity of 1.1 cm/year, respectively. The majority of the subsidence was observed in old settlement areas with mixed use development. The subsidence depth was found to gradually increase from the periphery towards the center in almost all detected subsidence areas. The subsidence depth was found to be in a range of 1 cm to 17 cm. It was found that the concentration of deep water extraction wells was higher in areas with higher subsidence rates. It was also found that the detected subsidence area was situated over geological formations mainly consisting of unconsolidated fine-grained sediments (silica, sand, silt, clay and silty sandy gravel), which is the major factor affecting the occurrence of land subsidence due to groundwater extraction.
Highlights
A study conducted by Calo et al, 2015 in Istanbul Megacity, Turkey, where the major geological constituents are mainly composed of clay, sand, gravel and silt, revealed subsidence occurrence over urbanized areas during the observation period of 2010–2012
Differential Synthetic Aperture Radar Interferometry (DInSAR) technique was applied to 43 TerraSAR-X data, which revealed an average subsidence velocity of 3 cm/year, with most of the detected subsidence found to be occurring in the Quaternary layers [50]
The phase difference between an interferometry data pair can be expressed as follows: Int = φdisp + φatm + φnoise + φtopo + φflat where φdisp refers to the phase difference from ground displacement along the slant range; φatm refers to the atmospheric effect; φnoise refers to the noise from the radar instrument and temporal deceleration; φtopo refers to the topographic height information; and φflat refers to the assumption of an ideally flat earth terrain [42]
Summary
Land subsidence is defined as an environmental geological phenomenon that causes the slow lowering of ground surface elevation [1] It is often a result of the natural compaction of sediments and extraction of ground water, geothermal fluids, oil, gas, coal and other solids through mining [2]. Large amounts of groundwater extraction from certain types of underlying sediments, such as fine-grained sediments, result in compaction of these sediments, because the groundwater is partly responsible for the subsurface support Considering the relation of groundwater exploitation and land subsidence from the case studies of various countries, it can be assumed that the same may occur in the Kathmandu valley, as well. Mapping, continuous monitoring and risk assessment of land subsidence is critical in a place like the Kathmandu valley. In Antelope Valley, California [38]; Coachella Valley, California [39]; Kolkata, India [40]; Iran [41]; Jakarta [42]; and Alto Guadalentin Basin, Spain [43]
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