Wide-angle reflection experiment with vibroseis source as part of a multidisciplinary seismic study of the Leonora-Laverton tectonic zone, Northeastern Yilgarn Craton

Abstract

A wide-angle reflection seismic survey coincident with a regional transect through Northeastern Yilgarn Craton focused on the Leonora-Laverton Tectonic Zone, Western Australia, was carried out to supplement deep seismic reflection studies. The major objectives were: to collect high-density refraction information for offsets of up to 60 km; to carry out a comparative study of near-vertical and wide-angle seismic images of the crust in the study area; to obtain velocity information for the upper crust. The survey deployed 120 short period recorders with a 500 m spacing. Acquisition parameters used for the wide-angle reflection experiment were selected so that it would to fit into the schedule and technology of the conventional reflection survey. The same vibrations were recorded in both surveys simultaneously. The major challenge in processing the wide-angle data was to manage the huge volume of information. The processing sequence included sorting into receiver and source gathers, cross-correlation with reference sweeps and stacking original seismic traces to form single source point traces, producing seismograms from individual traces and finally creating seismic record sections from separate seismograms. High amplitude seismic signal from vibroseis sources was recorded at least up to 45 km offsets in the first arrivals, and later arrivals were observed down to 12 s next to sources. A preliminary upper crustal model developed from the wide-angle data shows that the thickness of a high velocity layer, corresponding to the greenstone rocks, is 4.0-4.5 km. The boundary separating this layer from a low velocity layer below it is possibly a compositional boundary between greenstones and underlying felsic gneisses. There is no evidence for high velocity material below this boundary. Assuming the Moho belongs to the deepest reflections modelled, total crustal thickness in the region can be speculatively estimated in the range 32-37 km. This model will be refined when more processed data become available for modelling.