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Seismic signatures of time-dependent reservoir processes, necessary for the interpretation of 4-D seismic data, are still insufficiently described. This experiment was designed to monitor fluid-front movements and saturation changes and to identify the related seismic signatures.Ultrasonic P-and S-wave transmission and reflection measurements were used to monitor the waterflooding of a porous sandstone. The sandstone was flooded in steps by filling a tank in which the room-dry cubic (50-cm side) block of rock was placed.Waterflooding caused the velocity, amplitude, and frequency of the transmitted waves to diminish significantly; however, the changes were reversible by drying. The maximum reduction of the velocities was 7% and 12% for P-and S-waves, respectively. The velocity and amplitude behavior can be explained by the Biot-Gassmann's theory, local fluid flow, and grain-surface effects. The correct interpretation of seismic signatures of fluid processes in reservoirs thus involves a knowledge of rock physical relations and attenuation mechanisms.Even at small saturations, reflections from the block bottom were strongly attenuated, but those from the upgoing water front could be monitored. The latter reflections were best observed in differential seismic traces, confirming that seismic monitoring can observe moving fronts directly.

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Recent geophysical investigations revealed the existence of zones in the Earth's crust with anomalous low seismic velocities. The nature of these zones is still under debate, although there are some indications that they might be correlated with changes of porosity and hydraulic permeability as well as with the presence of fluids. To test this hypothesis an experimental study of the influence of fluids on the elastic properties of sandstone as a model rock system for crustal conditions was performed. The investigations were conducted at a hydrostatic pressure of 300 MPa and temperature up to 850°C. They revealed the strong influence of different fluids (neutral, acid, alkaline) on the elastic properties of sandstone as a result of mineral reactions, phase transition and of changes of the rock microstructure. To evaluate the phase changes the computer modeling was performed. Together with investigations of the role of volatiles in geochemical processes including the influence of mineralized fluids on mineral reactions these studies will contribute to the petrophysical and geochemical interpretation of geophysical measurements.