Ocean tides induce a tidal signal in coastal aquifers. The same applies to the Earth’s solid tides, which induce a piezometric tidal signal far inside continents. This signal has been observed and studied for several decades in oil reservoirs, some mines and some confined aquifers.
Since the influence of the Moon and the Sun is everywhere, it can be assumed that the piezometric tidal signal is universal and we have tested this hypothesis by studying piezometric records on unconfined, shallow aquifers. Our observations confirm the existence of a piezometric tide in all aquifers, including the shallowest.
The signal amplitude is small (2 to 10mm). Oscillations can thus go unnoticed or be masked by other phenomena (barometric variations, rain, pump start-ups, etc.). To study the piezometric tide, rigorous signal processing is therefore required (barometric compensation, high-pass and low-pass filters).
Harmonic analysis of the signal highlights the main components of the Earth tides, where solar and lunar semi-diurnal components (M2 and S2) predominate.
The piezometric tidal signal reflects the aquifer's response to a well-known periodic disturbance (the Earth tide). This makes it all the more interesting: it serves as a universal instrument for studying the properties of shallow aquifers (the most widely used by humankind).
To test this instrument, we have multiplied high-precision piezometric records in 50 wells in 6 countries across Europe and Africa. This paper sets out the first lessons learned from these measurements:
o The piezometric tide has a specific signature for each aquifer (and it is identical within very large portions of each aquifer);
o The signal is not just a simple sinusoid (which can be defined by its amplitude and phase); its complexity reflects that of gravimetric forcing, as well as the structure of the aquifer;
o The amplitude of the signal is related to the storage (S) of the aquifer; piezometric tides could thus be extremely useful for determining S;
o The phase shift of the signal (relative to the Earth tides phase) is not constant; it varies during the lunar month and differs from one aquifer to another;
o The shape of the signal is much more complex than that of the Earth tide; it sometimes includes multiple peaks and hysteresis effects, which in all likelihood reflect the complex structure of the aquifers themselves.
In conclusion, the piezometric tidal signal constitutes a new instrument for investigating the properties of aquifers. This method is particularly useful for studying aquifers where conducting very long-term pump tests is unfeasible, and aquifers where only one observation well is available.