Internal deformation and compaction of the Makran accretionary wedge

The Makran accretionary wedge is one of the largest on Earth. A 7-km-thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets. We present the results of prestack depth migration and focusing-error analysis (migration velocity analysis) performed on a profile across the Makran wedge. The depth section shows the deformation style of the accreted sediments, and the migration velocities allow us to estimate porosity variations in the sediments. The thrust sheets show evidence of fault-propagation folding, with a long wavelength of deformation (≈ 12 km) and secondary thrusting in the kink bands of the folds, such that the central part of each thrust sheet is elevated to form an additional ridge. This deformation style and the 15° steep surface slope of the first ridge suggest a high degree of consolidation. Porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure λ was estimated for 5 locations along the profile. Rather than being undercompacted and overpressured as in most accretionary wedges, the sedimentary input is normally compacted (exponential porosity decay) throughout almost the whole wedge. However, a slight increase in porosity and λ at depth, with respect to the normal compaction curve indicates, that the turbiditic sequence might be overpressured landward of the deformation front.     

Accurate Modelling of Sonobuoy Refraction Data to Determine Velocity Variations in Oceanic Crust

Modern disposable sonobuoys can provide a simple and cost-effective alternative to ocean bottom seismometers for marine refraction experiments over oceanic crust. Unfortunately, the fact that they are free to drift with the prevailing ocean currents can introduce significant travel-time errors into the modelling process if the seafloor topography is large. For sonobuoys recorded during and after turns the drift rate and direction can be uniquely determined by inversion of the shot-receiver ranges derived from the water-wave arrival. The same method can be used to determine a best fitting average drift vector for the whole dataset. A modification to conventional two-dimensional travel-time modelling techniques has been developed to account for this drift. Each sonobuoy profile is divided into several subsets, typically of 100 shots each, and each subset is then modelled as a separate common receiver gather, significantly reducing the errors in the calculated travel-times. For re alistic bathymetry, the magnitude of these travel-time errors is up to 200 ms, significantly larger than the estimated picking uncertainty. Real data from a typical sonobuoy refraction experiment on the Mid-Atlantic Ridge were modelled with and without the drift correction applied. Much of the lateral variation in the velocity structure was removed when the drift correction was applied, indicating that this structure was due to variations in the travel-times caused by sonobuoy drift.