The use of velocity analysis of long-offset seismic data to interpret the seaward-dipping reflectors imaged along the Argentinian and Uruguayan rifted margins

Seaward-dipping reflectors are packages of syn-rift volcanic effusive material and are a key characteristic of volcanic margins. These packages are composed of sub-aerial tholeiitic lavas, thin tuffs and are commonly interbedded with thin layers of terrestrial sediment. This complex structure presents difficulties with conventional seismic imaging of the volcanic packages themselves and underlying structures. Here, we present a detailed view of the internal geometry and surrounding structure of the seaward-dipping reflectors offshore the southern segment of South America based on high-quality multichannel industry seismic data. We demonstrate that the interpretation ofthe internal and sub-basalt seismic reflections can be enhanced by detailed velocity analysis of pre-stack seismic gathers.

The seismic data were acquired by ION-GXT between 2009 and 2012 offshore Argentina, Uruguay and southern Brazil using 10200 rn long-offset streamers, with a 12.5 rn CDP interval and towed at a water depth of 10m. As a result of recording long-offsets and low-frequencies, down to 3 Hz, this data is capable of imaging the internal and sub-basalt features along the continental margin. In the analysis we prepared the data by picking mutes, band-pass filtering (3-40 Hz), attenuating random noise, and making supergathers fi·orn 10 CDPs. The velocity analysis was completed every 250 m to ensure the resulting model was not too fine. The analysis shows that the seaward-dipping reflectors are conunonly defined by a steep velocity increase at their top, a variable internal velocity structure and a prominent velocity inversion at their base. Internally, the velocity structure of these packages varies by up to 1 km/s, where the highest velocities are observed at the centre of the thickest packages. In some regions, the basal velocity inversion is as much as 0.5-1 km/s. The data were then re-migrated using the refined velocity model. The resulting images showed a more geologically reasonable geometric relationship with potential underlying tilted fault blocks.

The seismic velocity and imaging results are interpreted in terms of a change between heterogeneous basaltic flows (hyaloclastites) and more homogeneous continuous flows, a change in the ratio of sediment-basalt, or perhaps, a transition from tholeiitic basalt to crystalline crust. This study has increased our understanding of the internal structure of the volcanic sequences, provided a distinction between the volcanic material and potential sub-basalt structures and produced a better image ofthe sub-basalt structure.