A fast and reliable method for surface wave tomography

We describe a method to invert regional or global scale surface-wave group or phase-velocity measurements to estimate 2-D models of the distribution a nd strength of isotropic and azimuthally anisotropic velocity variations. S uch maps have at least two purposes in monitoring the nuclear Comprehensive Test-Ban Treaty (CTBT): (1) They can be used as data to estimate the shear velocity of the crust and uppermost mantle and topography on internal inte rfaces which are important in event location, and (2) they can be used to e stimate surface-wave travel-time correction surfaces to be used in phase-ma tched filters designed to extract low signal-to-noise surface-wave packets. The purpose of this paper is to describe one useful path through the large number of options available in an inversion of surface-wave data. Our metho d appears to provide robust and reliable dispersion maps on both global and regional scales. The technique we describe has a number of features that h ave motivated its development and commend its use: (1) It is developed in a spherical geometry; (2) the region of inference is defined by an arbitrary simple closed curve so that the method works equally well on local, region al, or global scales; (3) spatial smoothness and model amplitude constraint s can be applied simultaneously; (4) the selection of model regularization and the smoothing parameters is highly flexible which allows for the assess ment of the effect of variations in these parameters; (5) the method allows for the simultaneous estimation of spatial resolution and amplitude bias o f the images; and (6) the method optionally allows for the estimation of az imuthal anisotropy. We present examples of the application of this technique to observed surfac e-wave group and phase velocities globally and regionally across Eurasia an d Antarctica.