Influence of Earth Tides on Inland Aquifers

Tidal influence is a well-known phenomenon in the hydrogeology of coastal areas, as it is transmitted through aquifers connected to the sea, generating periodic fluctuations in borehole piezometric levels. The analysis of these fluctuations makes it possible to assess subsurface properties and derive characteristic parameters from the transmission of the pressure wave. Tidal influence results from a complex combination of the gravitational attraction exerted by the Sun and the Moon on the oceanic water mass, the tidal distribution conditioned by ocean morphology, and variable meteorological effects that may be highly localised. The combination of these factors generates oscillations ranging in magnitude from metres to centimetres. Under certain circumstances, such as spring tides coinciding with low atmospheric pressure, these oscillations may cause flooding and other undesirable effects in sensitive areas. The effect of earth tides in inland areas disconnected from the sea, sometimes at high elevations, is also well known and has received increasing scientific attention. Tidal oscillations, which are readily apparent in coastal areas, can also be recorded in certain geological materials, producing fluctuations that can be detected using high-precision sensors. These oscillations are caused by the same gravitational attraction responsible for ocean tides; however, in this case, the gravitational forcing acts on the geological formation itself, giving rise to what is known as an Earth tide or astronomical tide. Recent technological advances have facilitated the detection of these phenomena, which were previously difficult to identify. This study analyses the astronomical influence detected in areas clearly isolated from the sea, focusing on small, low-permeability aquifers where direct gravitational forcing of the groundwater mass cannot account for the observed response. Instead, astronomical forcing acts on the rock mass and indirectly affects piezometric levels. The selected experimental site comprises variable lithologies and different degrees of aquifer confinement and includes numerous research boreholes. It therefore provides an optimal setting for analysing the relationship between lithology and astronomical influence and may serve as a basis for future research in different hydrogeological settings.