On studying flow through a fracture using self-potential anomaly: application to shallow aquifer recharge at Vilarelho da Raia, northern Portugal

Abstract

We demonstrate a strategy of investigating recharge potential of a shallow aquifer in a hard rock terrain through flow in a fractured zone using self-potential (SP) anomaly and other ancillary (published) data related to the area of investigation. Our study area is Vilarelho da Raia, northern Portugal which possesses a shallow aquifer within a granitic terrain. We explain the phenomenon responsible for generating SP anomaly due to groundwater flow through a fractured conduit associating a fault. We propose delineating parameters associated with such structure from SP profile across it using a thin 2D conducting sheet model buried below subsurface. We propose a data based robust interpretation using derivative analysis of SP anomaly. We demonstrate that for sufficiently smooth SP anomaly across a 2D buried sheet model the first and the second order vertical derivatives of SP anomaly are the only necessary tools in delineating model parameters using characteristic features of derivatives curves and a set of closed form formulas. We propose using Savitzky–Golay derivative filter to get robust estimate of the first and second order horizontal derivatives. The first and second order vertical derivatives are obtained using Hilbert transform and Laplaces harmonic equation respectively. We applied the proposed technique on SP anomaly profile across a fault at Vilarelho da Raia in estimating parameters of a sheet model which represents the fault plane. We used the estimated parameters and the ancillary data taken from published literature to delineate hydrogeological parameters related to recharging of an aquifer within a granitic terrain.