Wide-angle seismic profiling

Wide-angle seismic profiling uses refractions and reflections to determine seismic velocity structure of a region. This can yield key constraints on parameters such as crustal thickness variations which record the long-term tectonic history, or the past addition of melt to the crust which affects the thermal history. It can also give more accurate depths in deep basins, or be used to infer the presence of overpressured sediments.
Ocean-bottom seismometer being deployed offshore Sumatra, 2008.
More recent developments have enabled us to use wide-angle methods in high-resolution studies of shallow targets. NOCS is engaged in all aspects of wide-angle profiling from data acquisition through interpretation for seismic structure to geodynamic modelling of the seismic results. Our experience includes experiments in all environments, from fully marine through onshore-offshore to land-based studies; we have research interests in a wide range of geological settings, from active tectonics to passive continental margins. Projects in the last few years have included major 3D experiments offshore Sumatra, high-resolution studies of gas hydrates in the Arctic, and detailed 2D profiles in the Black Sea.

Survey design and data acquisition

We have extensive experience in designing surveys for different types of objective, and the compromises involved in studies using denselysampled 2D profiles compared with more sparsely-sampled 3D volumes. We have a strong data acquisition capability through National Marine Facilities large airgun and GI-gun arrays and the Ocean Bottom Instrument Consortium 2-component and 4-component seismometers.
[image] Seismic velocity structure along three profiles in the Black Sea; heavy black lines show depths to the Moho constrained by wide-angle reflections.

Data analysis and modelling

Depending on the objectives we can interpret traveltimes in 2D or 3D either using refracted first arrivals, or by explicitly distinguishing different refracted and reflected phases. We test and refine final velocity models using synthetic seismograms and by comparing with other constraints such as seismic reflection results or gravity measurements. The seismic velocities determined from wideangle data can also be used to improve the processing of normal incidence reflection data. For appropriate targets we can use converted shear waves to model coincident S-wave velocity structure and Poisson’s ratios. We have additional experience in seismic imaging using pre-stack migration of wide-angle reflections, and full waveform inversion.

Geological context

Our research extends to the geological understanding of the seismic results by interpreting the seismic velocity results within the context of other datasets and theoretical models. Crustal thinning allows us to determine how much a region has extended and therefore what its thermal history will be. In addition the seismic velocity model can often distinguish whether melt has been added to the crust; this can help to constrain temperatures in the mantle and the tectonic history of the region.
Deep crustal structure of the San Andreas Fault in Northern California. The background colours show seismic velocities determined from inversion of travel time picks; the shading shows pre-stack depth migration of reflected phases. Geodynamic model for the rifting of the North Atlantic showing temperature distribution and melting at several times since the onset of extension.