GPS Monitoring and Land Subsidence in the Houston Metropolitan Area

This study aims to (1) establish a stable local reference frame (Houston16) for integrating all available GPS observations within the Houston metropolitan area and conducting precise subsidence and faulting monitoring over time and space, (2) evaluate the effects of groundwater withdrawal on land subsidence, and (3) assess the possible effect of petroleum production on ongoing minor subsidence within Galveston County.
In order to realize Houston16, 15 long-term (> 5 years) stable GPS stations outside the greater Houston area were selected as reference stations. The precision (stability) of the local reference frame is less than 1 mm/year. Long-term observations from approximately 200 GPS and 13 extensometer stations indicate that the southeast Houston area has ceased subsiding at historic rates (< 3 mm/year) as a result of groundwater withdrawal regulations, but to the north and in western portions of the Houston metropolitan area subsidence bowls have developed and are expanding. Moderate subsidence (2 to 3 cm/year) is currently occurring in The Woodlands, Jersey Village, and Katy areas. Slight land uplift at the level of 2 to 3 mm/year has also been observed along the Houston Ship Channel and within the Houston downtown area. Groundwater level measurements from 170 wells screened in the Chicot aquifer and 320 wells screened in the Evangeline aquifer were investigated in order to evaluate the interaction between land subsidence and groundwater level change. The results further verify that groundwater withdrawal is the primary driver of land subsidence within the greater Houston area. Subsidence of 5 to 9 mm/year was observed within the southeastern region of Galveston County near the city of La Marque from 2005 to 2012, despite that groundwater levels within the Chicot and Evangeline aquifers have been restored to the preconsolidation head and have remained stable for over two decades. In order to evaluate the contribution of petroleum withdrawal to the local subsidence, 3,570 petroleum wells across 457 leases and 217 fields within Galveston County were investigated. Production from these petroleum wells includes the gas, condensate and oil fractions. Within the localized subsiding area, 2.7 times more petroleum was produced per square mile compared to the surrounding area that had not experienced subsidence during the period from 2005 to 2012. After peak production in 2004, petroleum withdrawal was gradually reduced, but subsidence continued until 2013, and then localized subsidence decreased to its current rate of 3 mm/year. The results suggest that petroleum withdrawal could be the primary cause of the localized subsidence observed from 2005 to 2012.