Observation of groundwater-related subsidence and thermal effects in tilt and strain measurements

Subsurface fluid pressure changes due to groundwater extraction are commonly interacted with Earth tides phenomena and observations. We present the results of tiltmeter and strainmeter recordings, which show this mechanism of non-tidal loading and seem to be important for tide data analyses and prediction. Shallow (up to few meters) instrument installations may be subjected to anthropogenic subsurface fluid and thermal effects exceeding several times the tidal signals. Weekly cycles of the order of 10(-7) have been recorded by the laser strainmeter installed in 2-3 m underground trench. Public water supplies by operating extraction wells increase extremely in the weekend (Saturday-Sunday) that causes the observed strain to be on a rise up to 1-2 10(-7) per day. At the same time, measuring data of the near-surface tiltmeter have been subjected to anthropogenic thermal effects yielding 0.3-0.7 arcsec amplitudes in weekly and 0.1 arcsec amplitudes in daily cycles consequently. It is remarkable that EW-component of weekly tilts takes the lead over NS-component by 10-15 hours. The comparison with the instruments of deep (15 - 90 m) installations has been performed. For example, tiltmeters  at a deep gallery of an open-pit mine show significant tilting in a yearly cycle caused by groundwater level change and mining progress.