We have developed a case study of crosshole seismic
tomography with a cross-firing geometry in which seismic
sources were placed in two vertical boreholes alternatingly
and receiver arrays were placed in another vertical borehole.
There are two crosshole seismic data sets in a conventional
sense. These two data sets are used jointly in seismic tomography.
Because the local sediment is dominated by periodic,
flat, thin layers, there is seismic anisotropy with different
velocities in the vertical and horizontal directions. The vertical
transverse isotropy anisotropic effect is taken into account
in inversion processing, which consists of three stages
in sequence. First, isotropic traveltime tomography is used
for estimating the maximum horizontal velocity. Then,
anisotropic traveltime tomography is used to invert for
the anisotropic parameter, which is the normalized difference
between the maximum horizontal velocity and the
maximum vertical velocity. Finally, anisotropic waveform
tomography is implemented to refine the maximum horizontal
velocity. The cross-firing acquisition geometry significantly
improves the ray coverage and results in a
relatively even distribution of the ray density in the study
area between two boreholes. Consequently, joint inversion
of two crosshole seismic data sets improves the resolution
and increases the reliability of the velocity model reconstructed
by tomography.