K15-L13-K18-L16 3D: The Netherlands’ First OBN Survey Addresses Pre-Salt Imaging Challenges

We describe the design, processing, velocity model building, and imaging of the first Ocean Bottom Node survey in the Netherlands, acquired in blocks K15, L13, K18, and L16 in the Broad Fourteens Basin. The basin’s complex structural history has caused significant deformation in the Zechstein salt, forming salt walls and creating strong lateral lithology variations. These complexities pose substantial challenges for velocity model building (VMB) and imaging, while the shallow water adds to the processing difficulties.

The new acquisition aimed to enhance subsurface illumination and enable VMB to the Rotliegend target level. Key improvements included acquiring inline offsets of up to at least 9 km and full azimuth coverage up to 6 km for imaging. The long offsets allowed diving wave Full Waveform Inversion (FWI) to reach target depth, achieving one of the main objectives. This was supplemented with reflection FWI, tomographic updates, and salt scenario-testing. Full azimuth illumination, combined with the improved velocity model and iterative least-squares reverse-time migration, produced more continuous horizons, even in complex areas under the salt wall. Compared to the legacy NAZ data the current imaging results represents a step change in pre-salt imaging, paving the way for quantitative interpretation of pre-salt reservoir properties.