Oil and gas reservoirs can cover hundreds of square kilometers. Observation and production wells sample the environment within the reservoir at only a limited number of locations. We do not have x-ray glasses to see into the reservoir to know what is happening in the rock pore spaces as oil and/or gas is extracted, so we must infer how the fluids move, how pressure changes, and how deformation develops from other measurements.
Illuminating the reservoir with seismic rays can reveal a great deal, but not everything. The change in positions of seismic reflectors can be ambiguous, being affected both by true changes in fluid horizons and apparent changes resulting from seismic velocity variation. Other methods must be used to help develop a complete model of the reservoir. Time-lapse gravity measurements and time-lapse seafloor height determinations are two such powerful methods to build more robust reservoir models.