A wide-angle reflection experiment with Vibroseis sources 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 the 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 obtain velocity information for the upper crust from high-density refraction information for offsets of up to 60 km, and to carry out a comparative study of near-vertical and wide-angle seismic images of the crust in the study area. The survey deployed 120 short period recorders at a spacing of 500 m. Acquisition parameters used for the wide-angle reflection experiment were selected so that it would fit into the schedule and technology of the reflection survey. The signals were recorded by refraction and reflection equipment simultaneously. The major challenge in processing the wide-angle data was to manage the huge set of data collected during the survey. 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 signals from Vibroseis sources were recorded to at least 50 km offsets in the first arrivals, and later arrivals were observed down to 12 s near the sources. A preliminary upper crustal model developed from the wide-angle data shows that the thickness of a high-velocity layer, corresponding to 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 is associated with the deepest reflections modelled, total crustal thickness in the region can be speculatively estimated to be in the range of 32?37 km. This model will be refined when more processed data become available for modelling.