A portable borehole extensometer and tiltmeter for characterizing aquifers

Abstract

[1] The solid skeleton of an aquifer moves in response to changes in water pressure, barometric pressure, Earth tides and other factors, and measuring these movements can help characterize aquifers. A device for measuring the vertical displacement and horizontal shear, or tilt, at the same time and place has been developed by combining a tiltmeter and extensometer on a frame that can be temporarily anchored to borehole walls. The device can resolve displacements on the order of 10−8 m and tilts on the order of 10−8 rad. During a 4 h long pumping test in fractured gneiss, the rock contracted by 3 µm in response to 2 m of drawdown, a compliance of roughly 1.5 µm of displacement/meter of head change (1.5 × 10−10 m/Pa). Tilt of 10 s of µrad to the southwest occurred during pumping and both the displacement and the tilt returned during recovery. The tilt signal is repeatable during sequential tests and it changes in magnitude with depth. It can be explained by a fracture zone that strikes roughly EW and dips to the south. Semidiurnal tilts of roughly 0.1 µrad occur during ambient conditions at periods of the O1 and M2 Earth tides. Diurnal displacements of several tens of nanometers are associated with ambient fluctuations in water level when barometric pressure changed by several 100 Pa. Increasing barometric pressure causes water level to fall, but elevates the total pressure in the well. This correlates with dilational displacement, while falling total pressure correlates with contraction during ambient conditions.