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Stenvold, T, Eiken O, Zumberge MA, Sasagawa GS, Nooner SL.  2006.  High-precision relative depth and subsidence mapping from seafloor water-pressure measurements. SPE Journal. 11:380-389. AbstractWebsite

A method to accurately measure seafloor subsidence away from platform locations is presented. The method is based on seafloor water pressure, which is measured on top of predeployed benchmarks visited one after another using a remotely operated vehicle (ROV) and is at the same time measured continuously throughout the survey at one or more reference locations. Because no significant subsidence is expected during a few days of data acquisition, high-precision relative depths representative for the average time of the survey can be obtained. Accurate subsidence estimates between seafloor surveys are found assuming negligible subsidence at benchmarks located outside the field. Results from six seafloor surveys performed at two gas fields in the North Sea are presented. For an area of 1 km(2) at 80 m water depth, single-measurement relative depth precision (standard deviation) of 0.4 cm was obtained. Correspondingly, for an area of 700 km(2) at 295 to 345 m water depth, 0.6 cm was obtained. Single-station subsidence accuracy down to 1 cut is achieved from the two most recent pressure surveys at the large field. A subsidence signal is seen for this difference, and it is compared with modeled subsidence. Error budgets for depth precision and subsidence, incorporating instrumental and environmental errors, are discussed.