A modified reconstruction algorithm for Fresnel volume tomography
Grandjean and Sage (2004) have presented a 2D seismic tomography software for estimating the velocity field using the first arrival traveltimes in paper ‘‘JaTS: A fully portable seismic tomography algorithm based on Fresnel wavepaths and a probabilistic reconstruction approach’’, Computers & Geosciences 30 (2004) 925–935. Unfortunately, the reconstruction algorithm includes errors in the paper. I correct the errors and present a modified reconstruction algorithm for Fresnel volume tomography.
A Method of Shortest Path Raytracing with Dynamic Networks
The shortest path raytracing algorithm in networks has been widely applied to calculate seismic ray paths and traveltimes, in which the beelines between network nodes are used to approximate the ray paths. When using a few nodes the ray usually follows a zigzag path that is longer than the real raypath, and the calculated traveltime at a receiver is also larger than the real one. To solve this problem, we develop a shortest path raytracing method with dynamic networks based on the shortest path algorithm. In the process of expanding wavefront away from a source throughout the entire model, the wavefront times are sampled at all nodes on the cell edges. In each cell the traveltime at any point on an edge is expressed with linear interpolation between calculated traveltimes at adjacent nodes on the same cell edge, and then Fermat's Principle is used to determine the first arrival times at the nodes on the other edges of this cell. After the first arrival times at all nodes are calculated, trace raypath from a receiver to a source in the same way as the forward sampling for all pairs of receivers and sources is performed. So the drawbacks of conventional shortest path raytracing methods are overcome, and the precision of calculated minimal traveltimes and the correspondent raypaths are improved greatly.
Regional strategies for the minerals industries of the states of the SADCC
The SADCC has been in existence for eleven years but little progress has been made in regional coordination of the minerals sector even though this sector is the most important exporter of the region. There is however scope for minerals development at the regional level.