Article,
Assessment of crustal velocity models using seismic refraction and reflection tomography
Geophysical Journal International, 153 (3): 609--626 (March 2003)
DOI: 10.1046/j.1365-246X.2003.01919.x
Abstract
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.
