Geophysical data acquisition - OGS

The joint inversion of
direct, reflected and
refracted P waves
allows reconstructing
the velocity and depth
in a model (above)
without any
redundancy, unlike in
the elastic case (below).
For a proper comparison of the differences in the seismic response, one has to
compensate all changes that do not depend on the reservoir itself, as the recording
geometry and equipment. A factor usually neglected that we studied is the seasonal
variation of the seawater velocity, due to changing currents and the temperature
of the mixed layer. These variations can be of the same order of magnitude (or even
larger) than those expected at the reservoir; furthermore, since they are spatially
organized, they can be attributed to the reservoir, so totally distorting the timelapse
analysis. In the Figure, we see that two 3D surveys acquired at the North sea
in 1989 and 1992 provide very different estimates for the sound speed in the sea
water, obtained by the joint inversion of reflected and head waves. (We remark that
these plots can be quite interesting for oceanographic studies too).
Outside the reservoir, we do not expect variations of the seismic velocities and the
reflector structure. Thus, we can impose that the Earth model in depth, obtained
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