Mitigating the ill-posedness of first-arrival traveltime tomography
using slopes: application to crustal imaging from OBS data
Abstract
First-arrival traveltime tomography (FATT) has been one of the
commanding tomographic approaches for long wavelength velocity model
building. Nowadays, FATT models are often used as initial models for
more resolving imaging methods such as full-waveform inversion (FWI). In
this context, improving the accuracy of FATT model is crucial to
mitigate the nonlinearity of FWI. In spite of the widespread use of FATT
at all scales (from near surface to global scale), it suffers from
ill-posedness in terms of non-uniqueness of the solution due to the
deficient information carried out by the sole traveltime attribute. We
promote the use of the traveltime perturbation with respect to the
source/receiver positions (horizontal component of the slowness vector
or slope) as a supplement to the first-arrival traveltime (Tavakoli et
al., 2018). For multi-component controlled-source experiments, we can
generally take advantage of the fine source sampling to measure slopes
by tracking local coherency over few neighboring traces in addition to
traveltimes in common-receiver gathers, while slopes at the receiver
position may be estimated by polarization analysis. The information of
differential nature carried out by slopes yields a higher order
sensitivity with respect to the subsurface parameters. We implement our
first-arrival slope+traveltime (FASTT) tomography with eikonal solver
and the matrix-free adjoint-state method, as previously proposed by
Taillandier et al.(2009) for FATT. We illustrate the resolution power of
FASTT relatively to FATT for coarse ocean bottom seismometer (OBS)
acquisitions using two benchmarks based upon the EAGE/SEG Overthrust
model at the exploration scale and a synthetic model of the
eastern-Nankai subduction zone at the deep crustal scale (Górszczyk et
al., 2018), then assess the quality of the tomographic models as initial
models for FWI. We finally present an application to real OBS data
collected during the SFJ experiment in the eastern Nankai Trough (Tokai
area).