Seismic velocity variations in the northern Amadeus Basin, Central Australia, from an expanding spread reflection profile

Abstract

An expanding reflection spread with a maximum shot-receiver offset of 25 km was recorded in the northern part of the Amadeus Basin, Central Australia, to obtain seismic velocity estimates throughout the crust. Arrival times of reflections indicate significant lateral variations in P-wave velocities superimposed on the small northerly dip of the sedimentary sequence. Nevertheless, a useful velocity profile that contains two pronounced low velocity zones has been derived for the basin sedimentary rocks to depths of 8.6 km. Below depths of 3 km, these interval velocities are a considerable improvement on those obtainable by standard velocity analysis techniques on the coincident near-vertical incidence reflection data; the results thus enable the determination of more reliable estimates of the thicknesses of the deeper sedimentary formations. The refracted arrivals from the expanding spread yield well-constrained P-wave velocities for shallow depths, but below about 1.5 km depth, the refraction interpretation is problematic owing to the absence of a refracted arrival through the high velocity lid in the interval velocities at depths between 1.8 and 2.7 km. The determination of useful velocity estimates for basement rocks is hampered by the presence of peg-leg multiples produced within the overlying sedimentary formations.