Investigations of shallow aquifer groundwater systems of a Gallo-roman anthropized site using earth tide analysis

The prospect for the sustainability of the Gallo-Roman archeological site located in Genainville (France), where relics and artifacts dated to the 2rd century CE have been excavated is threaten by groundwater ingression. The in-situ building heritage materials and structures comprising a two cellea temple and an amphitheater made of limestones, are submitted all days to rising and changing groundwater levels; water being the main agent or vector of damage processes (salt weathering or freeze-thaw cycles). As part of a multi-disciplinary effort to support archeological expeditions and conserve the site structures. We report the results of groundwater monitoring and hydrogeological studies as well as tidal subsurface analysis executed to quantitatively evaluate hydraulic and geo-mechanical characteristics of the subsurface sequences toward a nondestructive approach. Continuous groundwater level data recorded in three wells in the archeological site were decomposed into constituent events that impact the observed fluctuations. The groundwater levels and barometric pressure data were acquired at 60 seconds intervals to study the response of the aquifer to strain and stress prevalence at the site. Using the method of regression deconvolution, the response to barometric pressure was disentangled from the measured water levels. Theoretical Earth tides parameters were computed using the PyGtide code, based on the ETERNA PREDICT program, at intervals of 1 minute. Harmonic analysis of the raw and filtered data using the classical Fast Fourier transform (FFT), and Singular Spectral Analysis (SSA) identify M₂, S₂, K₁ and O₁ tidal constituents as the dominant amplitudes. The SSA technique has the advantage of resolving the events into individual strands compare to the spectra of the composite data produced by the FFT. Hence, an event decomposed in the data is isolated in terms of it frequency and amplitude, and visualized. The K₁ and S₂ harmonic constituents were present in the filtered and raw data sets with different amplitudes. The amplitude response method was used to compute the poroelastic properties of the aquifer and characterize the subsurface heterogeneity. The model identified a semi-confined aquifer as the main groundwater storage system in the site.

Keyword: Heritage site, groundwater ingress, harmonic constituents, hydraulic properties