1 Introduction
The groundwater flow or level variation is crucial to groundwater
management and site soil pollution control (Tesfaldet et al., 2019). To
understand the location and timing of groundwater traveling through the
vadose zone, we must investigate groundwater flow characterization and
recharge mechanism. The traditional hydrographic groundwater surveys
usually include float observation, pressure observation, and automatic
tracking observation. However, when measuring the dynamic change of
groundwater level or content over a long period, the measurement is
usually indirect, difficult, costly, and infrequent, except for the
traditional methods such as volume weighing and borehole sampling to
detect the properties of groundwater (Ogilvy et al., 2009). In contrast,
the nonintrusive time-lapse geophysics tools provide an opportunity to
complement hydraulic campaigns (Fetter, 2001) since they can be
implemented over a large region with dense sampling in both space and
time. In particular, electrical resistivity tomography (ERT) and
self-potential (SP) tomography are particularly appropriate to
monitoring groundwater dynamics in resistivity and apparent current
density because it is sensitive to changes in flow or chemistry (Carey,
2017; Revil & Linde, 2006). The time-lapse method carries out periodic
measurements at a fixed location and provides the perception of the
2D/3D groundwater flow by analyzing the response variation in the
subsurface. Compared with the hydrographic survey, and it is a
non-invasive, practical, and cost-effective method for characterizing
and delineating special recharge zones.
The time-lapse ERT is a mature technology for the hydrogeological study.
There have been many recent examples of applying this technology for
vadose zone soil moisture estimation, groundwater flow monitoring based
on the resistivity characteristic (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, the ERT method is an indirect
approach that depends on the change of soil moisture content. There are
inherent uncertainties, and the sensitivity of ERT is also not enough
for small-scale groundwater flow.
The SP method corresponds to the generation of an underground natural
current source (Ahmed & Jardani, 2013). This method is used to monitor
the groundwater based on the flow characteristic. Sill (1983) uses
physical approaches to simulate the SP anomalies related to groundwater
flow by solving major mathematical problems corresponding to groundwater
flow problems. By observing the SP data on the ground surface, the
potential distribution and current density distribution of the
underground space can be effectively calculated to quantify the abnormal
distribution characteristics. In recent years, the application of the
natural potential method in the inversion of geophysical parameters,
such as changes in the hydraulic head using SP technology to reconstruct
pumping tests, has gradually attracted attention.
The ERT and SP method monitors the groundwater flow from different
aspects. ERT utilizes the change of resistivity caused by the variations
of moisture content in the soil. The groundwater movement produces the
SP signal. These two methods have different physical mechanisms to
describe groundwater flow characterization. Therefore, it is necessary
to use a variety of geophysics methods for cross-validation and
interpretation using ground truth constraints. This study addressed two
primary questions: 1) What is the advantage of the joint 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
joint ERT and SP method strategy to monitor the groundwater flow
variation in the pumping experiment. The groundwater level is controlled
by pumping water from well to create various conditions of groundwater
flow. The ERT data invert the resistivity distribution, which relates to
the soil moisture content. Then, we combine the SP data and resistivity
result to invert the apparent current density and estimate the
permeability.
The paper is organized as follows. After the introduction, we introduce
the basic forward and inversion theory of SP data. By solving the
hydraulic problem and the Poisson equation, the underground current
density distribution is restored. Then, the pumping experiment is used
to test the ability of ERT and SP data in groundwater flow monitoring.
The final sections are the conclusion and discussions.
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, the forward and inverse problems of the SP method will be
introduced. Meanwhile, the permeability tensor can be determined
directly according to the coupling coefficient. The problem can then be
determined independently by resistivity tomography.