The robust nonlinear approach by Tarantola & Valette, consisting in direct evaluation of the "probability" density function, is supplemented with the paraxial ray approximation of the travel time. A sufficiently dense 2-parametric system of rays from each receiver is evaluated only once for all hypocentre determinations. The interpolation formulae for the travel times apply to all travel-time branches. Their derivation is based on the summation of Gaussian packets. The proposed algorithm for determining the hypocentre is able to find all of its possible locations.
Nonlinear inverse problems, hypocentre determination, paraxial ray approximation, summation of Gaussian packets.
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