Exploiting the Full Wavefield to Overcome Limitations of Ray Based Tomography in the Central North Sea

Full waveform inversion (FWI) using diving waves has in recent years become a standard model-building tool in the Central North Sea (CNS). Below the maximum depth of diving wave penetration however, we have remained reliant on ray based tomographic methods, which although powerful, have many limitations.In this paper, we will describe the use of time-lag FWI (TLFWI). TLFWI uses a modified cost function which aims to minimize travel time differences between recorded and modelled shots. This mitigates many of the issues encountered in other FWI solutions when reflection information is included. The ability to reliably use reflection information make it possible to reduce our dependence on ray-based approaches at depth. This is of particular benefit in areas where these ray-based tomography methods are inherently limited such as fast, layered chalk layers. The application of this technology to a large survey in the Central North Sea will be described.