Fracture characterisation using shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea

Abstract:

The measurement of seismic anisotropy using shear-wave splitting (SWS) allows the presence, orientation and density of subsurface fracture networks to be determined. We apply this technique to the investigation of a seismic chimney structure located beneath Scanner Pockmark, a ~900 m by 450 m, ~22 m-deep seafloor depression, located in the North Sea. At this location, methane venting is observed, indicating that the chimney acts as a fluid escape pathway. Based on models for the fracture geometries associated with chimney structures, we would expect to observe differences between the anisotropy measured inside and outside the chimney. A multi-component seismic experiment was conducted at Scanner Pockmark, during which an array of ocean bottom seismographs (OBS) recorded signals produced by airgun and sparker seismic sources, on hydrophones and three-component geophones.

Here we analyze the evidence for SWS in the signals generated using two different airgun seismic sources, within a frequency range of ~10-150 Hz. Initial observations of SWS from the GI-gun source identify shallow anisotropy, at 70 ms twt beneath the seafloor, with orientations 70^oN and 150^oN, consistent with the orientations of ice ploughmarks, and roughly perpendicular to the minimum horizontal stress in the region respectively. We construct a shallow P-wave velocity structure by forward modelling of OBS hydrophone recorded arrivals from a sparker source, and convert to S-wave velocity using an empirical relationship. We then correlate P-to-S converted arrivals observed on the OBS to their most likely stratigraphic origin. This allows us to relate the observed anisotropy to shallow subsurface structures mapped using other geophysical data, and permits application of a layer-stripping approach to progressively determine and correct for anisotropy in individual layers.

By comparing observations at Scanner Pockmark with a nearby reference site, we aim to contribute to the understanding of chimney structures and their role in fluid migration. Our interpretation is informed by combining the field observations with laboratory measurements and rock physics models. Further work will use higher frequency (250-2000 Hz) sources, to study the potential frequency-dependence of anisotropy over a much wider frequency range than previous studies.