  Monitoring the rock-physics properties of the subsurface is of great
importance for reservoir management. For either oil and gas applications or CO2
storage, seismic data are a valuable source of information for tracking changes
in elastic properties which can be related to fluids saturation and pressure
changes within the reservoir. Changes in elastic properties can be estimated
with time-lapse full-waveform inversion. Monitoring rock-physics properties,
such as saturation, with time-lapse full-waveform inversion is usually a
two-step process: first, elastic properties are estimated with full-waveform
inversion, then, the rock-physics properties are estimated with rock-physics
inversion. However, multiparameter time-lapse full-waveform inversion is prone
to crosstalk between parameter classes across different vintages. This leads to
leakage from one parameter class to another, which, in turn, can introduce
large errors in the estimated rock-physics parameters. To avoid inaccuracies
caused by crosstalk and the two-step inversion strategy, we reformulate
time-lapse full-waveform inversion to estimate directly the changes in the
rock-physics properties. Using Gassmann's model, we adopt a new
parameterization containing porosity, clay content, and water saturation. In
the context of reservoir monitoring, changes are assumed to be induced by fluid
substitution only. The porosity and clay content can thus be kept constant
during time-lapse inversion. We compare this parameterization with the usual
density-velocity parameterization for different benchmark models. Results
indicate that the proposed parameterization eliminates crosstalk between
parameters of different vintages, leading to more accurate estimation of
saturation changes. We also show that using the parameterization based on
porosity, clay content, and water saturation, the elastic changes can be
monitored more accurately.
