A common problem in deepwater node surveys is the lack of accurate information on the depths of nodes deployed at the seafloor. Tides, rapidly changing water properties, pressure sensor drift and calibration errors limit the accuracy of depth measurements performed during the operations. Depth uncertainties at the level of a few meters are not uncommon.
Previous work on gravity-subsidence surveys that were carried out on the Norwegian continental shelf using permanent concrete monuments have produced depth repeatability at the mm level that enable the measurement of small subsidence rates over years.
In this paper, we introduce the principles of gravity-subsidence surveys into an OBN operation with the goal of accurately determining relative node depths and developing the capability to measure long-term subsidence between repeat OBN surveys. We applied this technique on a node survey acquired in ~2500 m water depth and demonstrated an average relative depth accuracy of 4 cm. We also carried out a set of base-line measurements on fixed seafloor frames that can be used to deliver cm-level accuracy in subsidence measurement during future surveys. This experience demonstrates the concept of introducing the well-proven survey method for depth determinations into OBN campaigns.
Presentation Date: Wednesday, September 18, 2019
Session Start Time: 1:50 PM
Presentation Time: 2:40 PM
Presentation Type: Oral