Abstract
Abstract In order to decipher complex subsurface structures, various
seismic imaging techniques have been developed. Full Waveform Inversion
(FWI) is one of the most sophisticated approaches in the new generation
of seismic imaging algorithms for 2D/3D Earth modeling. An effective
implementation of FWI requires a good initial model and intensive
computations, among other challenges. To address these challenges, we
developed a new FWI scheme, where we perform just a few local inversions
under the assumption that the layers present only small lateral
variations. We then upscale the local inversions to reconstruct 3D
media. This approach also includes Source Signature inversion and does
not require an initial model. The algorithm is based on the exact
analytical solution of the Navier-Lame equation for a visco-elastic
multi-layer model. This solution is represented in the form of
Fourier-Bessel integrals, and performs forward modeling calculations
about 106 – 107 times faster than conventional finite differences (FD)
algorithms. It is free from various numerical artifacts inherent to FD
methods, and allows modeling of all known types of seismic waves. The
solution also indicates numerous unknown waves in various layered
systems (some of the waves are practically impossible to reproduce
numerically for the thin layers, using the grids, typical for 3D
modeling). The comparison of the exact solution with 3D FDTD results
shows excellent agreement. Here we applied this approach to stratified
media with small lateral variations (the Earth, consisting of the
inclined multi-layer structures). The feasibility of the method was
analyzed using traces from synthetic seismograms for the Marmousi model,
which were simulated by the 3D elastic SW4 code. A few sources with
surrounding geophones were chosen. At the first stage we invert the
traces from each geophone to the local stratum under the source,
deciphering thicknesses, densities and P,S-wave velocities for each
layer. At the second stage we identify the layer boundaries’ slopes and
reconstruct the full 3D model. For the Marmousi model we obtained an
accurate image down to 2.3 km deep, after which lateral variations were
excessive. This result demonstrates that a new FWI scheme exploiting
separate inversions, can be applied successfully to the reconstruction
of 3D media.