Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

We report results from the Seismic Wide-Angle and Broadband Survey carried out over the Mid North Sea High. This paper focuses on integrating the info rmation from a conventional deep multichannel reflection profile and a coin cident wide-angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new travelti me inversion scheme was developed (reported elsewhere) that was used to inv ert traveltime information from both the wide-angle OBH records and the ref lection profile simultaneously. Results from the inversion were evaluated b y producing synthetic seismograms from the final inversion model and compar ing them with the observed wide-angle data, and an excellent match was obta ined. It was possible to fine-tune velocities in less well-resolved parts o f the model by considering the critical distance for the Moho reflection. T he seismic velocity model was checked for compatibility with the gravity fi eld, and used to migrate and depth-convert the reflection profile. The unre flective upper crust is characterized by a high velocity gradient, whilst t he highly reflective lower crust is associated with a low velocity gradient . At the base of the crust there are several subhorizontal reflectors, a fe w kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike sect ion through northward-dipping reflectors at the base of the crust, identifi ed on orthogonal profiles by Freeman et al. (1988) as being slivers of subd ucted and imbricated oceanic crust, relics of the mid-Palaeozoic Iapetus Oc ean.