Increasing resolution in the North Sea

Abstract

Recent step changes in seismic processing and imaging technology have delivered dramatic improvements in resolution, velocity model building and multiple attenuation. This article will explore improvements in resolution that have been demonstrated on two multi-client surveys in the North Sea: Cornerstone and Northern Viking Graben (NVG). Extending full-waveform inversion (FWI) to include absorption effects as well as velocity has delivered improved imaging, higher resolution and more reliable AVO products for the NVG survey, and this will now be applied to the reprocessed Cornerstone data. Both surveys cover more than 35,000 km2 each, so application of high-resolution processing sequences to these two data sets will deliver advanced high-resolution data over two large areas of the North Sea. The NVG survey was acquired using BroadSeis variable-depth streamers in conjunction with a broadband source to maximize the bandwidth. However, new developments in broadband processing, including ghost wavefield elimination (GWE) and bandwidth extension, can be extended to the reprocessing of conventionally-acquired data sets, which can now be uplifted to deliver data of almost similar quality to modern broadband data by mitigating many of the limitations of traditional acquisition. This is demonstrated by the reprocessing of the Cornerstone project in the Central North Sea. Although this survey was recently reprocessed in depth using multi-layer tomography, the rapid improvements in GWE, multiple and noise attenuation, velocity modelling and imaging in the last few years mean that it can still benefit from being reprocessed again. Modern processing techniques ensure compliance with amplitude versus offset (AVO) analysis, and AVO attributes are used for QC purposes during the processing flow to ensure that the final data sets can be used directly for reservoir characterization. The AVO attribute maps for Cornerstone shown later in this article have already delivered impressive high-resolution images of the Forties field