| Summary: | We identify a few unique issues that are important for performing a nonlinear refraction
traveltime tomography effectively. These include accuracy of traveltime and raypath
calculation for a turning ray and physical information in a refraction traveltime
curve. Consequently, we develop a shortest path raytracing method with an optimized
node distribution that can accurately calculate refraction traveltimes and raypaths in
any velocity model. We find that minimizing misfit of refraction traveltimes with the
least-squares criterion does not account for the whole physical meaning of a refraction
traveltime curve. We therefore pose a different nonlinear inverse problem that explicitly
minimizes misfits of both traveltimes (integrated slownesses) and traveltime gradients
(apparent slownesses). As a result, we enhance the resolution of the tomographic inversion as well as the convergence speed. We regularize our inverse problem with the
Tikhonov method as opposed to applying ad hoc smoothing to keep the inversion stable.
The use of the Tikhonov regularization avoids solving an ill-posed problem and allows
us to invert an infinite number of unknowns. We apply this tomographic technique to
image the shallow velocity structure at a coastal site near Boston, Massachusetts. The
results are consistent with a local boring survey.
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