Three-Dimensional Velocity Structure of the Crust and Upper Mantle beneath the Nevada Test Site

We have constructed a model of the three-dimensional velocity structure in the crust and upper mantle beneath NTS. This model was obtained by inversion of teleseismic travel times from NTS explosions reported for a set of globally distributed seismograph stations. Although the geometry is conceptually the reciprocal of the classical geometry involving a localized network and distributed teleseismic sources, several important differences emerge. In particular, the data are contaminated by different error processes, and different correction terms need be applied to the raw travel time data. We present algorithms which take advantage of the large number of raw data initially available to improve to some extent the signal-to-noise ratio, and lead to a reduction of the inverse problem to computationally manageable size. In addition, we show how projection operators can be used to formulate an equivalent inverse problem with smaller dimensions which is insensitive to unknown baseline corrections and similar nuisance parameters. Finally, we develop an algorithm which permits further reduction in problem size by grouping redundant data with minimal loss of information and with further noise reduction.