Comparing Higher-dimensional Velocity Models for Seismic Location
Accuracy using a Consistent Travel Time Framework
Abstract
Historically, location algorithms have relied on simple, one-dimensional
(1D, with depth) velocity models for fast, seismic event locations. The
speed of these 1D models made them the preferred type of velocity model
for operational needs, mainly due to computational requirements.
Higher-dimensional (2D-3D) seismic velocity models are becoming more
readily available from the scientific community and can provide
significantly more accurate event locations over 1D models. The
computational requirements of these higher-dimensional models tend to
make their operational use prohibitive. The benefit of a 1D model is
that it is generally used as travel-time lookup tables, one for each
seismic phase, with travel-time predictions pre-calculated for event
distance and depth. This simple, lookup structure makes the travel-time
computation extremely fast. Comparing location accuracy for 2D and 3D
seismic velocity models tends to be problematic because each model is
usually determined using different inversion parameters and ray-tracing
algorithms. Attempting to use a different ray-tracing algorithm than
used to develop a model almost always results in poor travel-time
prediction compared to the algorithm used when developing the model. We
will demonstrate that using an open-source framework (GeoTess,
www.sandia.gov/geotess) that can easily store 3D travel-time data can
overcome the ray-tracing algorithm hurdle because the lookup tables (one
for each station and phase) can be generated using the exact ray-tracing
algorithm that is preferred for a specified model. The lookup surfaces
are generally applied as corrections to a simple 1D model and also
include variations in event depth, as opposed to legacy source-specific
station corrections (SSSCs), as well as estimates of path-specific
travel-time uncertainty. Having a common travel-time framework used for
a location algorithm allows individual 2D and 3D velocity models to be
compared in a fair, consistent manner.