Assessment of crustal velocity models using seismic refraction and reflection tomography

Abstract

SUMMARY Two tomographic methods for assessing velocity models obtained from wide‐angle seismic traveltime data are presented through four case studies. The modelling/inversion of wide‐angle traveltimes usually involves some aspects that are quite subjective. For example: (1) identifying and including later phases that are often difficult to pick within the seismic coda, (2) assigning specific layers to arrivals, (3) incorporating pre‐conceived structure not specifically required by the data and (4) selecting a model parametrization. These steps are applied to maximize model constraint and minimize model non‐uniqueness. However, these steps may cause the overall approach to appear ad hoc, and thereby diminish the credibility of the final model. The effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide‐angle data set: the first‐arrival times. For data sets with Moho reflections, the tomographic velocity model can be used to invert the PmP times for a minimum‐structure Moho. In this way, crustal velocity and Moho models can be obtained that require the least amount of subjective input, and the model structure that is required by the wide‐angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data. The tomographic models are not intended to supersede the preferred models, since the latter model is typically better resolved and more interpretable. This form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models. Four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above. This was followed by conventional first‐arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth, minimum‐structure models. The case studies are from the SE Canadian Cordillera across the Rocky Mountain Trench, central India across the Narmada‐Son lineament, the Iberia margin across the Galicia Bank, and the central Chilean margin across the Valparaiso Basin and a subducting seamount. These case studies span the range of modern wide‐angle experiments and data sets in terms of shot–receiver spacing, marine and land acquisition, lateral heterogeneity of the study area, and availability of wide‐angle reflections and coincident near‐vertical reflection data. The results are surprising given the amount of structure in the smooth, tomographically derived models that is consistent with the more subjectively derived models. The results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide‐angle traveltime data.

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

“…1A) has resulted in the extreme thinning of the continental crust over distances of 100 -200km. Unaltered crust landward of the proximal rift margin is ~30 km thick and has been thinned through a complex pattern of faulting to only a few km at the distal limits of the margin (Zelt et al 2003;Reston 2009). Initial extensional deformation is inferred to have occurred as high-angle normal faulting, which Downloaded from https://academic.oup.com/gji/article-abstract/doi/10.1093/gji/ggx415/4331637/Resolving-the-fine-scale-velocity-structure-of by GEOMAR Bibliothek Helmholtz-Zentrum fuer Ozeanforschung user on 09 October 2017 formed large fault-bound blocks between 10 and 20 km wide, thinning the crust to between 20 and 30 km thick (Ranero and Pérez-Gussinyé 2010).…”

“…This model was developed using OBS data collected from the Galicia-3D seismic experiment, supplemented with data from the ISE-1 seismic profile (Sawyer et al 1997;Zelt et al 2003), and inverted using the "TOMO2D" travel-time inversion code of Korenaga et al (2000). The final TOMO2D model has an overall travel-time misfit of 53 ms, and a chi-squared value of 0.97.…”

Vp/Vs-ratios and anisotropy on the northern Jan Mayen Ridge, North Atlantic, determined from ocean bottom seismic data

“…The modeling thus consists of estimating the conversion boundary for each interpreted arrival, and the Vp/Vs-ratio within each layer. We modeled the data from each instrument separately and derived the Vp/Vs ratio in each layer by a layer stripping approach (Zelt, 1999;Zelt et al, 2003). A variation in Vp/Vs-ratio within each layer was achieved by merging the models from all instruments (Figs.…”

“…A c 2 value around one means an optimal fit between the observed and calculated travel-times for the given uncertainty. However, 3-D wave propagation effects (out-of plane ray paths), unresolvable small scale structures and structural inhomogeneities (Zelt et al, 2003) will prohibit an ideal fit. Therefore, additional indicators must be considered when estimating the reliability of the final model.…”

Crustal structure of the South American–Caribbean plate boundary at 67°W from controlled source seismic data

“…The nonlinear iterative approach used here is essentially the same as that used in 3-D by Zelt and Barton [1998] and in 2-D by Zelt et al [2003]. Therefore, only a brief overview of the method is provided here.…”