Far-offset detection of normal modes and diving waves: A case study from the Valhall Field, southern North Sea

Abstract

The use of permanent arrays for continuous reservoir monitoring has become a reality in the past decades, with Ekofisk and Valhall being its flagships. One of the possibilities when such a solution is available is to passively record data while acquisitions with an active source are ongoing in nearby areas. These recordings might contain ultrafar-offset data (more than 30 km), which are hardly used in standard reservoir exploration and monitoring because they are mostly a combination of normal modes, deep reflections, and diving waves. We present data from the Valhall life-of-field seismic array, recorded while an active seismic survey was acquired at Ekofisk, in April 2014. Despite the lack of control on source firing time and position, analysis of the data indicates that the normal modes are remarkably clear, overcoming the ambient noise in the field. The normal modes can be well explained by a two-layer acoustic model, whereas a combination of diving waves and refracted waves can be fairly well reproduced with a regional 1D velocity model. We suggest a method to use far-offset recordings to monitor changes in shallow sediments between source and receivers, with and without a coherent seismic source in the area.