Abstract

Seafloor gravity and pressure measurements for 4D reservoir monitoring require precise models of the time-varying tidal signals. Current seafloor instrumentation can resolve 0.003mGal in time-lapse gravity differences and 0.05kPa(5mm) in pressure. To verify model accuracy, a seafloor gravimeter and pressure gauge were operated continuously for 446days next to the Troll A gas platform in the North Sea (60.64227° north, 3.72417° east) at a depth of 303m. The seafloor gravity and pressure time series were filtered and corrected with estimates from the tidal model, which predicts the solid earth tide, ocean loading, and direct gravitational attraction of the varying water level. The rms difference between the observed tidal gravity signal and the prediction is about 0.0013mGal during periods when there are no surface storms. A slight difference is observed for the direct attraction of the water overhead as computed from the tidal prediction versus that computed from direct seafloor pressure measurements when the entire 446-day record is analyzed; it shows an rms difference of 0.708kPa, equivalent to 7cm of water-height variation, yielding a gravity effect of 0.003mGal. We conclude that existing theoretical tide models in combination with in situ pressure records are sufficiently precise for correcting time-lapse gravity observations.

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