Recent global tomography models, constructed through linearized inversion,
by van der Hilst et al. (Nature, 1997)
and Zhou (JGR, 1996) show smaller scale mantle structures than had
been imaged on a global scale before.
In a similar, but still linear approach we
have been able to take their improvement a step further, mapping upper
mantle structure with detail (60-100 km) directly comparable with that
obtained in high resolution regional studies. This was achieved through
the implementation of an accurate global data set and
a model parameterization with cells of variable sizes.
To further improve global travel-time tomography
we extend the method to nonlinear inversion.
Up to now the nonlinear global tomography problem has always
been linearized about the ray paths in the 1-D reference
Earth model used.
The bending of rays due to lateral heterogeneity
has not been taken into account,
but may be very important
when imaging small-scale structure.
We will present results from the linear inversion (focussing on the
behaviour of slabs in the transition zone, deep subduction, remnants of
past subduction in the lower mantle and the deep roots of hotspots) and
first results from nonlinear inversion.