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Previous studies examining the relationship between the groundwater table and seismic velocities have been guided by empirical relationships only. Here, we develop a physics-based model relating fluctuations in groundwater table and pore pressure with seismic velocity variations through changes in effective stress. This model justifies the use of seismic velocity variations for monitoring of the pore pressure. Using a subset of the Groningen seismic network, near-surface velocity changes are estimated over a four-year period, using passive image interferometry. The same velocity changes are predicted by applying the newly derived theory to pressure-head recordings. It is demonstrated that the theory provides a close match of the observed seismic velocity changes.

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<p>Previous studies examining the relationship between the groundwater table and seismic velocities have provided contradictory results, sometimes reporting positive and sometimes negative correlations between seismic velocity and groundwater table changes. Here we introduce a physics-based model relating fluctuation in the groundwater table and the pore pressure to seismic velocity variation through change in effective stress. This model can be used to explain the contradictory results of previous studies and justifies the use of seismic velocity variation for monitoring of the pore pressure and the groundwater table. It further results in a new field method to measure the pressure dependency of the shear modulus. Using data acquired in Groningen, the Netherlands, we demonstrate that measurements of seismic velocity variation can be used to monitor the pore pressure.</p>

<p>We previously developed a physics-based model relating changes in pore pressure and vertical stress to seismic velocity variations and validated the model in a small area of Groningen gas field. Using the entire Groningen seismic network, near-surface velocity changes are estimated over a three-year period, using passive image interferometry. Using our developed model, we invert these observations of velocity change for pore pressure variations as a function of space and time, and thus we construct a 4D pore pressure model for the shallow subsurface of Groningen. Pressure-head recordings in the southeastern region of Groningen allow us to calibrate our inference tool.</p>