Subsidence Study in Northwest Harris County Using GPS, LiDAR, and InSAR Techniques
Karacay, Ayca 1986-
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Subsidence has been affecting many cities around the world, such as Nagoya (Japan), Venice (Italy), San Joaquin Valley and Long Beach (California), and Houston (Texas). This phenomenon can be caused by natural processes and/or human activities, including but not limited to carbonate dissolution, extraction of material from mines, soil compaction, and fluid withdrawal. Recent studies on Harris County, Texas suggested that surface deformation is driven by four major mechanisms: faulting, soil compaction, salt tectonics, and fluid withdrawal (groundwater withdrawal and hydrocarbon extraction). The objective of this study was to assess the land deformation rate in the northwest Harris and detect the effect of fluid withdrawal on subsidence. To achieve this goal, data from three complimentary remote sensing techniques Global Positioning System (GPS), Light Detection and Ranging (LiDAR), and Interferometric Synthetic Aperture Radar (InSAR) were used. The data of twenty (20) GPS stations acquired from Harris-Galveston Subsidence District (HGCSD) were processed using Online Positioning User Service (OPUS) of National Geodetic Survey (NGS). Two (2) of these GPS stations are Continuously Operating Reference Stations (CORS), and eighteen (18) are Port-A-Measure (PAM) sites. The zonal statistic method was applied on 2001 and 2008 Digital Elevation Models (DEMs) generated using LiDAR data. The Persistent Scatterer Interferometry (PSI) was performed using twenty five (25) ERS1/2 data. The rates of change in groundwater level and hydrocarbon production were calculated using data from 261 water wells and 658 hydrocarbon wells. Furthermore, the rates of change in groundwater level and hydrocarbon production were compared to the results of remote sensing techniques. The results of this study revealed the rates of subsidence ranging from 0.3 to 4.5 cm/y for GPS, LiDAR, and InSAR. The level of groundwater drops with a rate of 4 m/y close to the area where subsidence is the highest. Also, the hydrocarbon withdrawals are highest (~70 million m3/y) in areas sinking more rapidly. This study found strong correlation between fluid withdrawals and subsidence. Therefore, both groundwater and hydrocarbon withdrawal in northwest Harris are considered to be the major drivers of the subsidence deformation.