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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Traditionally, seismic imaging of the shallow subsurface is done with active sources. Seismic or acoustic sources from explosives or airguns excite downwards propagating waves, of which the reflections can be used to map geologic interfaces. Over the last decades, however, we have seen a shift toward passive imaging and monitoring. Seismic signals that were initially considered noise (e.g., microseisms) are now used to acquire subsurface data (e.g., Curtis et al., 2006).Passive image interferometry (Sens-Schönfelder & Wegler, 2006) allows us to estimate seismic velocity changes using measurements of seismic ambient noise. This method consists of two steps. First, approximate Green's functions are estimated using cross-correlations of seismic noise measured at two receivers. This is referred to as seismic interferometry (Wapenaar, Draganov, et al., 2010). Second, velocity changes as a function of time are retrieved by comparing the coda of time-lapse cross-correlations to a reference. This step is referred to as coda wave interferometry (Lobkis & Weaver, 2003;Snieder, 2006). With passive image interferometry, a single lapse cross-correlation is generally constructed from noise measurements with a duration of a few hours to a few weeks, while the reference cross-correlation is often an average over one to a few years. The relative difference in arrival times dt/t then represents the relative velocity change dv/v = −dt/t with respect to the average reference velocity.Seismic velocity variations have been empirically linked to many physical processes or observations, including temperature variations (e.g.,