Seafloor permanent platforms, which secure the same measurement location for time-lapse repeats.
Quad Geometrics

Permanently deployed monuments on the seafloor serve as foundations to place the instrument for measurements. These shall be stable over the lifetime of a project (typically 10-30 years for a gas field). For the soft clay seafloor conditions often encountered in the North Sea, Quad Geometrics has good experience with conical concrete benchmarks. About 500 such stations have been deployed on the seafloor above 10 fields since 1997. The time-lapse accuracy proves that these platforms are stable within a few mm, with only a handful of exceptions where external forces like possibly trawl doors have hit a benchmark.

The benchmarks in the 80 m deep Sleipner area has experienced scouring and vertical movements of a few cm. Concrete mats with a podium in the center can provide erosion protection.