1 Introduction
Knowledge of groundwater flow and groundwater level variation is crucial
to groundwater management and water quality control (Tesfaldet et al.,
2019). A better understanding of the mechanism governing groundwater
movement in the vadose zone requires investigations on both groundwater
flow characterization and the recharging process (Nielsen et al., 1986).
Traditional hydrographic groundwater surveys include float, pressure,
and automatic tracking observations. However, when measuring the
dynamics of the groundwater level over a long period, the measurement is
usually indirect, costly, and infrequent (Ogilvy et al., 2009). In
contrast, the nonintrusive time-lapse geophysics tools provide an
opportunity to complement traditional measurements (Fetter, 2001),
especially given that geophysical surveys can be implemented over a
large region with dense sampling intervals in both space and time. In
particular, electrical resistivity tomography (ERT) and self-potential
(SP) tomography are appropriate to monitor groundwater dynamics due to
the sensitivity of resistivity and apparent current density to changes
in flow or water chemistry (Carey, 2017; Revil & Linde, 2006). The
time-lapse method carries out periodic measurements at a fixed location
and determines the 2D/3D groundwater flow by analyzing the response
variation in the subsurface over a specific period (Doetsch et al.,
2012).
The ERT is sensitive to changes in pore water electrical conductivity
and temperature. Therefore, it has been used to track the subsurface
migration of conductive tracers and image the hydraulic conductivity of
heterogeneous aquifers (Bowling et al., 2006). The time-lapse ERT method
has been well established for soil moisture estimation and groundwater
flow monitoring (Jongmans & Garambois, 2007; Niesner, 2010). It
measures the resistivity of the subsurface by using an electrode dipole
to inject direct current into the ground and using additional dipoles to
measure the resulting voltage. Many studies have explored the challenges
and uncertainties associated with predicting groundwater flow using ERT.
However, for small-scale groundwater flow, there is not apparent
resistivity change, and the ERT is not sensitive to describe the
groundwater movement (Fagerlund and Heinson, 2003). The SP method is
used to monitor the groundwater based on its flow characteristic. The SP
signal has two main components: (1) the electrokinetic contribution
associated with groundwater flow through the permeable soil and (2)
oxide-reduction phenomena (Naudet et al., 2003). Sill (1983) used
physical approaches to simulate the SP anomalies related to groundwater
flow by solving the constitutive equation corresponding to groundwater
flow. By observing the SP data on the ground surface, both potential and
current density distributions of the underground space can be
effectively calculated to quantify the abnormal distribution
characteristics. In recent years, the application of the geophysics
potential method in the inversion of geophysical parameters, such as
changes in the hydraulic head using SP technology to reconstruct pumping
tests, has attracted increasing attention (Revil & Linde, 2006).
The ERT and SP methods have different physical mechanisms to describe
groundwater flow characterization. ERT utilizes the change of
resistivity caused by the variations of moisture content in the soil,
while the groundwater movement produces the SP signal. Combining the two
methods for cross-validation will provide a more robust result (Fan et
al., 2020; Guo et al., 2020). This study addressed two primary
questions: 1) What is the advantage of the combined time-lapse strategy;
and 2) How does the magnitude and timing of water input change
groundwater flow dynamics? To answer these questions, we propose the
combined ERT and SP method strategy to monitor the groundwater flow
variation in a pumping experiment. The groundwater level is controlled
by pumping water from shallow water well to create various conditions of
groundwater flow. The time-lapse ERT data can be used to derive the
dynamics in resistivity distribution, which reflects the evolution of
soil moisture content over time. Then, the SP data and ERT result can be
combined to invert the resistivity and estimate the permeability
distribution. The proposed method was tested in a pumping experiment,
and the geophysical results agreed well with direct monitoring of the
groundwater level in wells. The results of this study provide a reliable
and low-cost way to identify and monitor the groundwater movement in the
vadose zone.
2 The Self-potential Inversion Theory
Self-potential (SP) refers to passively measure electric potentials that
are generated through coupling with some other forcing mechanism, which
is often hydraulic, chemical, or thermal. This coupled flow mechanism in
this stratigraphic setting was detected on the surface by Minsley,
(2007). Over the years, there has been a growing interest in the
application of the SP method in various fields of earth science,
including hydrology, geothermal and geotechnical and environmental
engineering (Darnet& Marquis, 2004). In many cases, this method is
relatively easy to use and convenient for qualitative interpretation. In
this section, we present the forward and inverse theory of the SP
method. Meanwhile, the permeability tensor can be determined directly
according to the coupling coefficient. The problem can then be solved
independently by resistivity tomography.