Considerations for the Design of Sinusoidal Slug Testing Methods

Modern methods of aquifer hydraulic testing estimate subsurface properties by creating frequency-dependent variations in groundwater well levels. These methods are variously known as periodic, harmonic, or oscillatory hydraulic testing, of which sinusoidal testing is a specific case. Periodic testing provides several potential advantages over traditional methods, including larger signal-to-noise ratios, larger distances over which hydraulic disturbances propagate, and the ability to undertake zero net water extraction. One of three approaches are used to induce groundwater pressure fluctuations: (1) extraction/reinjection of water using motorized pumps, (2) pressurization/depressurization using compressed air, and (3) transient displacement of the water column by slug testing. The latter was the focus of the present study; specifically, how to improve sinusoidal slug testing methods by ensuring accurate generation of sinusoidal variations in well water levels. Two previously published sinusoidal testing designs were evaluated in terms of the ratio of effective transfer link length, $L$ , to effective flywheel radius, $r_e$ . The first published design featured $L/r_e$ ratio values ranging from 13 to 9, which corresponded to maximum discrepancies between intended and actual slug movement of 7% to 10%, respectively. The second design featured $L/r_e$ ratio values ranging from 12 to 2, which corresponded to maximum discrepancies of 8% to 29%, respectively. These analyses suggest that methods featuring a rotating drive coupled to an effective transfer link are suboptimal. Instead, designs featuring either modified flywheel apparatus or winches driven by digitally controlled stepper motors can minimize the potential for discrepancies between intended and actual slug movement. The accurate generation of sinusoidal slug movement will minimize uncertainties associated with hydraulic properties inversely estimated from observations acquired during sinusoidal slug testing.