Generalized Linear Inversion of Refracted Arrivals
Speaker: Shih Hua-Pei
Department of Earth Sciences, National Central University, Taiwan
Abstract
For an arbitrary source-receiver geometry, given a set of observed traveltimes associated with the refracted arrivals, we can estimate the parameters associated with a single-layer near-surface model using the generalized linear inversion (GLI). The parameters consist of the refractor velocity, and the velocity and thickness of the near-surface layer at all shot/receiver locations. The GLI solution for the parameters satisfies the requirement that the difference between the observed(picked) refracted arrival times and the estimated (modeled) times is minimum in the least-squares sense. The modeled times are computed using the traveltime equation for refracted arrivals for a flat refractor considered as the base of a weathering layer.
The GLI schemes that allow velocity and thickness of a near-surface layer to vary spatially require iterative strategies (Hampson andRusell, 1984; Schneider and Kuo, 1985; De Amorim et al., 1987). Starting with initial estimates for the near-surface layer velocity and thickness, and an initial estimate for the bedrock velocity, these parameters are changed such that the difference between the observed (picked) refracted arrival times and the estimated (modeled) times is minimum in the least-squares sense. The GLI method is not only applicable to 2-D line recording but also to 3-D swath recording geometries (Baixas and Du Pont,1988; Kircheimer, 1988). It is important to parameterize the near-surface layer parsimoniously while conforming with the basic assumptions required for the use of refracted arrivals in estimating the near-surface model.
References
Sawasdee, Y., Christopher, J., Ruediger, G., Calin, C., 2007. Shallow velocity-depth model using first arrival traveltime inversion at the CO2SINK site, Ketzin, Germany. Journal of Applied Geophysics,63,68-79.
Hampson. D. and Russell, B., 1984. First-break interpretation using generalized inversion: J. Can. Soc. Explor. Geophys., 20, 40-54.