Kirchhoff synthetics for seismic reflections and diving waves in two dimensions and application to the D" layer

The Kirchhoff-Helmholtz (KH) integration has been used to model the reflected and the diving waves from an interface with a positive velocity gradient. The modelling is carried out for a spherical boundary and for a sinusoidal topography with a long-scale wavelength.
An artefact, which is a major problem in modelling the seismic response using the KH integration, has been reduced by introducing a Hilbert transform sign manipulation. Cleaner synthetic seismograms with correct amplitudes have been produced by this method. A discretization in larger surface elements has been made possible by introducing a smoothing factor that suppresses the noise that normally follows the constructed signal if a large element size is taken.     

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 information from a conventional deep multichannel reflection profile and a coincident wide-angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new traveltime inversion scheme was developed (reported elsewhere) that was used to invert traveltime information from both the wide-angle OBH records and the reflection profile simultaneously. Results from the inversion were evaluated by producing synthetic seismograms from the final inversion model and comparing them with the observed wide-angle data, and an excellent match was obtained. It was possible to fine-tune velocities in less well-resolved parts of the model by considering the critical distance for the Moho reflection. The seismic velocity model was checked for compatibility with the gravity field, and used to migrate and depth-convert the reflection profile. The unreflective upper crust is characterized by a high velocity gradient, whilst the highly reflective lower crust is associated with a low velocity gradient. At the base of the crust there are several subhorizontal reflectors, a few kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike section through northward-dipping reflectors at the base of the crust, identified on orthogonal profiles by Freeman et al. (1988) as being slivers of subducted and imbricated oceanic crust, relics of the mid-Palaeozoic Iapetus Ocean.     

Ivrea seismic array: a study of continental crust and upper mantle

A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P - and S -wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation.
Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times.
We have obtained a three-layer crustal seismic model. The P -wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s⁻¹, 6.5±0.3 km s⁻¹ and 7.8±0.3 km s⁻¹ respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a V _P/ V _S ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station.