Lessons from a joint interpretation of vibroseis wide-angle and near-vertical reflection data in the northeastern Yilgarn, Western Australia

A wide-angle reflection seismic experiment was carried out in the Eastern Goldfields granite-greenstone terrane of the Archaean Yilgarn Craton during 2001. This was the first time in Australia that wide-angle data were collected using a vibrator source and with a high density of observations. Unlike other wide-angle surveys carried out in other parts of the world, our survey used both a smaller number of sweeps, and shorter sweeps. We recorded three sweeps (each with its own frequency range) at each vibration point. The experiment demonstrated that the sum of three 12 s sweeps using 3 large vibrators provides enough energy to record signal at offsets up to up to 60-70 km. A comparison of individual shot gathers from near-vertical data and receiver gathers from wide-angle data demonstrated higher reflectivity in near-vertical data. This may be due to differences in the frequency bands of the recording equipment. The after stack section obtained from dense wide-angle data is different from that obtained from conventional near-vertical reflection data. The conventional reflection section provides higher quality image of the crust compared to the wide-angle section. This could be explained by the low-fold in wide-angle data and differences in the acquisition and processing methodology. The wide-angle survey, which was coincident with a regional vibroseis seismic reflection transect, was focused on the Leonora-Laverton region. The survey was designed to supplement the deep seismic reflection studies with velocity information. This also created an opportunity to compare velocity model derived from wide-angle reflection seismic data with a structural image obtained from the deep common mid-point seismic reflection data, and thus refine our geological understanding of the area. A high velocity body reaching a maximum thickness of 2 km was identified exclusively from the seismic velocity model derived from wide-angle study. This body is interpreted as mafic rocks within the Archaean Granite-Greenstone Belt. The joint interpretation also shows that structural boundaries do not always follow lithological boundaries in our study area. The combination of wide-angle reflection and near-vertical reflection data has facilitated a more complete geological interpretation of the seismic data.