SUBSIDENCE RESEARCH IN SOUTHEAST TEXAS BASED ON RESURVEYING OF PREEXISTING BENCHMARKS

The purpose of this study is to determine the rates of subsidence or uplift in SE Texas, particularly in flood-prone areas. This is done through comparing recent survey-grade GPS measurements of horizontal and vertical positions to the original positions of previously installed National Geodetic Survey (NGS) benchmarks. This study will also update the positions of these benchmarks for future use by surveyors, and indicate which ones are stable or should be periodically re-leveled. This research adds to previous studies of elevation changes which suggest subsidence rates of 10 to 15 mm/yr in Louisiana near the SE Texas border and 3 to 12 mm/yr in parts of eastern Harris and Galveston counties. The counties covered in this project include Chambers, Galveston, Hardin, Jefferson, Liberty, Orange, and southern Jasper and Newton counties.

Surveying was conducted using a Trimble R8 survey-grade GNSS GPS system on any NGS benchmarks that could be found in the study area. Differential corrections were applied in real time using a VRS network of base stations. This system yields a nominal vertical accuracy of 1.5 to 2.0 cm for each 2 to 5 minute reading. Usually three of these readings were measured on each benchmark and averaged for the final result. A total of 367 benchmarks were resurveyed, most of which are suitable for subsidence/uplift rate calculations. Original NGS elevations were subtracted from the new elevations and divided by the number of years between the two elevation measurements to determine the average subsidence or uplift rate of the benchmark. Benchmarks used for determining the vertical change rates were monumented between1931 and 2006, thus yielding rates averaged over 5 to 80 year periods.

Initial results show predominantly 1 to 6 mm/yr of subsidence in northeastern and southern Chambers County with one anomalous rate calculation of 16 mm/yr. Uplift rates of 1 to 3 mm/yr were also calculated for parts of western and central Chambers County. Compilation of the measurements for the other counties continues with final results expected soon. Potential factors affecting uplift and subsidence include depletion of groundwater, oil or natural gas, pumping of these products into the subsurface, dewatering of deeper sediments, growth faulting, and expansion or contraction of near-surface soils.