A seismic survey in Antarctica, parallel schemes for seismic migration and target oriented velocity analysis

This dissertation comprises three different studies. The first part describes the acquisition and data processing techniques utilized during a seismic survey conducted in the austral summer of 1994–95 in the interior of Antarctica. Three multichannel seismic reflection profiles and two wide-angle profiles were collected over the central-west Antarctica ice sheet to investigate methods to obtain a shallow to mid-crustal section of the lithosphere below the Byrd subglacial basin. The multichannel seismic data were analysed to develop images of the shallow crustal structure, the base of ice, and intra-ice reflections that (with minor exceptions) conform to the ice-floor topography. The high energy, low frequency seismic energy generated by the larger charges of the wide angle data was more successful in imaging the deep crustal section. The upper crust in this area was determined to be fairly non-reflective. Along the main traverse, the base of ice has significant topographical undulation in both inline and crossline directions and several half grabens and localized basins can be identified. More efficient surveys can be conducted and better signal quality can be obtained by using longer streamers (∼4.5 km) and larger and buried charges.; The second part describes a parallel implementation of 3D pre-stack Kirchhoff depth migration using the Parallel Virtual Machine (PVM) environment of message passing and clustering. A simple yet robust strategy has been proposed to distribute the computation load among the nodes of a virtual parallel machine and the performance of the parallel method has been compared with conventional sequential schemes. A near linear speedup was achieved in this implementation which implies that the reduction in computation time (compared to the sequential run time) was almost directly proportional to the number of nodes in the virtual machine.; The third part of this dissertation describes an approach for target oriented migration velocity analysis that is customized for imaging tabular salt structures. An inverse problem is formulated where a multidimensional model space is iteratively searched using a modified simulated annealing scheme to determine the background velocities and salt structure. At each iteration, common image gathers are constructed by migrating common offset gathers with the current velocity model and an error measure is computed on the basis of the residual moveout in the common image gathers. A 2D parallel implementation, based on the algorithm discussed in the second part of this dissertation, was executed on an 8-node, 8 Gbytes SGI Origin 2000 and used to illustrate the feasibility of the method for a test case. Keeping the basic approach unchanged, the method can be easily extended to the 3D case.