Journal Article

Lateral velocity resolution from three-dimensional seismic refraction data

Colin A. Zelt

in Geophysical Journal International

Volume 135, issue 3, pages 1101-1112
Published in print December 1998 | ISSN: 0956-540X
Published online December 1998 | e-ISSN: 1365-246X | DOI:
Lateral velocity resolution from three-dimensional seismic refraction data

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A method for estimating the lateral velocity resolution from 3-D seismic refraction traveltimes is presented. The method is non-linear in that synthetic data calculated from ‘checkerboard’ models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration. Two applications are presented: (1) real data from the Faeroe Basin experiment using the ray coverage and noise level of approximately 50000 picks, and (2) ideal data from the same experiment corresponding to 100 per cent data recovery, equivalent to about three times as many ray paths. The refraction data constrain the sedimentary and basement structure to approximately 12 km depth. A comparison of the results from the two data sets provides a qualitative check on the resolution method, illustrates the pitfalls of using ray coverage as an indicator of resolution, and demonstrates the potential resolution of currently feasible 3-D refraction experiments. 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested, each with a different cell size, position or orientation. The problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small. Using models with four cell sizes allowed for resolution estimates from 3 to 15 km, this range being imposed by the frequency content of the data and the areal dimensions of the study area (44×26 km). An operator centred on each model node measures the local semblance between the known and recovered checkerboard models. A determination of the smallest well-resolved cell size at each model node is then made using a 0.7 semblance threshold to provide a spatially dependent lateral resolution estimate. Resolution for the real data set varies from an average of just over 3 km at 2 km depth, to 10 km at 10 km depth, where there is a local maximum in ray coverage due to a concentration of ray-bottoming points. For the ideal data set, the average lateral resolution drops off roughly linearly from better than 3 km at 2 km depth, to 6.3 km at 10 km depth. The largest difference between the resolution associated with the ideal and real data occurs at 7.2 km depth, where it averages 4.6 km for the former and 10.8 km for the latter due to the near absence of ray-bottoming points between 7 and 9 km. The results show that the final model for the Faeroe Basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point. The results for the ideal data show that good-quality 3-D refraction experiments can provide a lateral resolution of no worse than the receiver spacing (shot spacing for land data) throughout most of the sampled volume.

Keywords: seismic refraction; seismic resolution; seismic velocity; tomography; traveltime

Journal Article.  6363 words.  Illustrated.

Subjects: Geophysics

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