Use of Earth tide analysis for study of unstable aquifer regime.

In here we estimate hydrogeological parameters of fractured fluid collector using water level variations caused by Earth tides. Main hydrogeological parameters, such as transmissivity, specific storage and porosity, are usually estimated with aquifer tests and core samples investigations in a lab. Nevertheless we usually have to control them in the course of wellbore operating that may strongly affect massif mode of deformation and therefore change aquifer state. Prompt response to such variations might be significant for hydrocarbons mining, improvement of urban environment, construction works in unstable regions, etc.

Hsieh et al (1987) showed that aquifer transmissivity could be found on base of phase shift between the water level variations and the pressure head variations. In [Cooper et al, 1965] the pressure head variations were linked with the amplitude of the vertical component of ground motion. Authors of these papers consider isotropic and homogeneous aquifers and have rather good correlation between theoretical and experimental results.

We applied noted above approach to the water saturated fractured hydrocarbon collector with transmissivity of about 4 m²/day and non-periodical level variations up to 160 mm per day with tidal peak-to-peak amplitude about 30-32 mm. Observation area is located within the territory of the Mikhnevo Geophysical Observatory in the central part of the Russian Platform, 80 km to the South from Moscow. We found that the phase shift between tidal water level oscillations and theoretically calculated ground motion strongly varied especially in the time intervals of high flow rate in the aquifer. Thus critically significant was to choose time intervals when proposed approach is suitable that is aquifer regime might be considered as quasi-stable.

For most of such time intervals phase shift was about (-5.5˚) - (-3˚) that corresponds to transmissibility of about 1.5-3 m²/day according to calculations as offered in [Hsieh et al, 1987]. This value is close to what was estimated with aquifer test and the difference can be associated with complex fractured-porous structure of the aquifer.

For the same time intervals water level variations to ground motion tidal amplitude ratio (amplification factor) was considered to estimate aquifer porosity that was found to be about 20-22% and corresponded with known values for limestones.

Real aquifer is a complex system that can strongly evolve either in the course of wellbore operating or due to technogenic or natural impact. Precise passive monitoring of aquifer parameters can aim at both fundamental purposes, including hydrogeological influence on fractured and unstable areas, and applied research in engineering and mining areas.

Suggested approach is supposed to be more precise in case of consideration of causes of flow rate change and possible dependence on absolute pressure head value.

This work was supported by the Russian Science Foundation (Project No. 14-17-00719).