Experimental seismic crosshole setup to investigate the application of rock physical models at the field scale

Seismic crosshole techniques are powerful tools to characterize the properties of near-surface aquifers. Knowledge of rock-physical relations at the field scale is essential for interpreting geophysical measurements. However, it remains difficult to extend the results of existing laboratory studies to the field scale due to the usage of different frequency ranges. To address this, we develop an experimental layout that successfully determines the dependency of gas saturation on seismic properties. Integrating geophysical measurements into a hydrogeological research question allows us to prove the applicability of theoretical rock physical concepts at the field scale, filling a gap in the discipline of hydrogeophysics. We use crosshole seismics to perform a time lapse study on a gas injection experiment at the “TestUM” test site. With a controlled two-day gaseous CH4 injection at 17.5 m depth, we monitor the alteration of water saturation in the sediments over a period of twelve months, encompassing an observational depth of 8–13 m. The investigation contains an initial P-wave simulation followed by a data-based P-wave velocity analysis. Subsequently, we discuss different approaches on quantifying gas content changes by comparing Gassmann’s equation and the time-average relation. With the idea of “patchy saturation”, we discover that analyzing P-wave velocities in the subsurface is a suitable method for our experiment, resulting in a measurement accuracy of 0.2 vol.%. We demonstrate that our seismic crosshole setup is able to describe the relation of the rock’s elastic parameter on modified fluid properties at the field scale. With this method, we are able to quantify relative water content changes in the subsurface.