High Precision Seabed Deformation Monitoring

Abstract

Abstract Deformation monitoring has been conducted on land for many decades to observe fluid movement in reservoirs, determine hydraulic fracture orientation, and confirm cap rock integrity. The three technologies currently in common use for land-based deformation monitoring are: electrolytic tiltmeters, GNSS and InSAR (Interferometric Synthetic Aperture Radar). Each technology has a different set of strengths and weaknesses, which are in many ways complementary. Tiltmeters have a high temporal rate and provide far higher precision than GNSS or InSAR but require careful instrument layout planning to ensure that precision translates from gradient changes to elevation changes. The uncertainty from tiltmeter arrays also increases with time if there is no absolute reference in the array. GNSS is much lower precision and relatively high cost per installation, but its stability over time can mitigate long term drawbacks of a tiltmeter-only installation. InSAR has a limited temporal rate and can suffer from uncertainties due to a variety of causes, but the enormous coverage area that is often available with minimal or no ground instruments makes it a strong asset in many monitoring programs. GNSS and InSAR both rely on microwave frequency signals, and so are not directly applicable in the subsea environment. Acoustic analogs exist, but only with lower resolution that renders them of very limited use for monitoring small deformations, including those induced by most processes that are well below the seabed. Tiltmeters can be used in the subsea environment, but deployment methods to date, mostly by the scientific community for seamount and tectonic fault monitoring, have placed instruments directly on the seabed and suffered the consequences of low precision. Instruments on land are typically buried 6 to 12m to isolate them from noise sources at the near surface, which are primarily diurnal thermal disturbances. Best precision typically requires maximum thermal excursions on the order of 1e-3 C near the instrument. Since even the deep ocean generally experiences temperature fluctuations of several tenths to several whole degrees C, placing the instruments on the seabed severely limits the usefulness and application range. A new deployment mechanism enables mechanically and thermally stable placement of tiltmeters in seabed sediment at depth, with acoustic transmission for data retrieval. High resolution tilt measurement of the seabed allows the same diagnostics previously only available on land to be obtained in the more challenging and higher consequence offshore environment. An array of instruments deployed off the coast of Brazil in March 2021 is furnishing key information on reservoir fluid balance, providing insight into the impacts of changes in injection and production and enabling better decisions on reservoir management.