Global sensitivity analysis of water level response to harmonic aquifer disturbances considering wellbore effects

Abstract: Oscillatory water level is often observed in groundwater monitoring wells when the aquifer is disturbed by some periodic pressure sources, such as oscillatory hydraulic test, ocean tide, and far-field seismic wave. The amplitude attenuation (A) and phase shift (P) between the source and water level response are utilized to estimate aquifer properties in many related studies. Water level response, in essence, is not only affected by aquifer parameters but also the characteristics of disturbance source and wellbore effects, which are determined by multiple parameters and the highly nonlinear well-aquifer system. To clarify the impacts of A and P on all relevant parameters, a global sensitivity analysis of water level response to harmonic aquifer disturbance is conducted in this study. A general numerical model of water level response that integrates different types of sources and considers wellbore effects is first introduced. Based on the quasi-Monte Carlo method, nine relevant parameters regarding wellbore geometry, aquifer property, and characteristics of the source, are sampled within their normal ranges. A data-driven model of the water level response to the selected parameters is trained and cross-validated using the random forest regression method. The global sensitivity analysis of the A and P is then implemented by using the variance-based method.  The first-order Sobol index shows that for both A and P, the oscillatory period of the disturbance source is the most sensitive factor among others. The second-order Sobol index indicates that the interaction between the period of disturbance source and water column height in the wellbore is the most important to A and P. preliminary results show that wellbore effects have significant impacts on water level responses, especially under a high-frequency disturbance.