Analysis of phase lead "anomalies" in the tidal response of groundwater levels
Abstract. The tidal response to the groundwater level refers to an aquifer under the influence of tidal forces, the pressure head (pore pressure) within the aquifer produces changes that drive the alternating transportation of water between well-aquifers, causing the rise and fall of the water level in the wells. Considering the driving process of force and seepage of water, the groundwater level response should only have a phase lag compared to the Earth's solid tides. However, the actual observation data show that the phase of the groundwater level tidal response exceeded that of the theoretical gravity tides, which is not in accordance with the commonly occurring mechanical process of the phenomenon. Using the theory of trans-current recharge, the seepage of aquifer water was decomposed into lateral and vertical transport, and the two kinds of "lagging" transport processes were superimposed to obtain the final groundwater level tidal response, which may appear as an anomalous phenomenon in which the phase is over the front after superposition. Taking the Lugu Lake well as an example, before the Wenchuan earthquake, the phase of groundwater level was ahead of the theoretical solid tide, indicating the existence of a transgressive aquifer, whereas the groundwater level tidal factor declined from 0.28 mm/uGal before the earthquake to 0.23 mm/uGal after the earthquake. The phase, from 15 min ahead in pre-earthquake to 15 min lagged after the earthquake, combined with the theoretical analysis it can be seen that the Wenchuan earthquake led to develop the new fissure in the Lugu Lake well, thus permanently altering its aquifer response and changing the permeability of the aquifer. However, the subsequent earthquakes did not produce new fissures; only the seismic waves caused by the stress redistribution process were observed. This co-seismic response of the groundwater level shows a step-down phenomenon, phase analysis of the groundwater level has scientific significance for the study of well-aquifer conditions and well-borehole seismic capacity.
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