Inverting raypath-dependent delay times to compute S-wavevelocities in the near surface

Raul Cova and Kris Innanen

ABSTRACT

Characterizing the near-surface is an important part of solving seismic static problems. It is also a critical step as input for more general iterative inversion methods applied to land seismic data. In the case of converted waves this becomes even more true due to the large magnitudes of the shear-wave statics. Here we propose two solutions based on the inversion of traveltimes in the near-surface. First we address the problem of the parameterization of the inversion. Results show that performing the inversion in the rayparameter domain removes the ambiguities that exist in the raypath-angle domain. The first model we invert is based on transmission traveltimes solely in the low velocity zone. This applies to the inversion of uphole data, but introduces the requirement that multiple offsets be recorded. Secondly, a solution based on the difference in reflection traveltimes between receivers is proposed. This solution may be useful for inverting traveltimes retrieved by interferometric techniques. Due to the complexity of the partial derivatives of the forward modelling operator for this case we decided against local descent-based methods, adopting instead a simulated annealing inversion method. This is also justified by the complex topography of the objective function. Performing a representative number of iterations of the proposed algorithm successfully retrieves the true parameters of the model. Since traveltime differences were used, the inverted parameters only allowed us to compute changes in the depth of the base of the near-surface rather than its absolute value. This can be fixed through a calibration process given the depth of the low velocity zone for at least at one receiver location. Near term plans include extending the results to the full 2D problem and testing on the CREWES 2014 Priddis data set.

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