The influence of angled survey lines on the data and results of 2D ERT surveys using the Wenner (α) array
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Date
2020-01
Authors
Mukhwathi, Unarine
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
The electrical resistivity method was first developed in the 1920s but has been widely used since the
1970s due to advances in the technology to collect, analyse and process data (Loke, 1999). This
geophysical technique is one of the main methods used to investigate the subsurface resistivity
distribution. Until recently, electrical resistivity surveys were mostly conducted in a one-dimensional
(1D) mode, either by performing vertical electrical sounding (VES) where resistivity variations with
depth are investigated, or horizontal profiling where lateral resistivity variations are studied.
However, 1D surveys have several limitations. VES allows the modelling of only horizontally layered
earths and it does not provide tangible information for the interpretation of structures and the extent
of subsurface features (Loke et al., 2013). Horizontal profiling, in turn, assumes a constant depth of
investigation along the profile, which is not strictly true since the depth of investigation is dependent
on the subsurface resistivities. To overcome the limitations of a 1D surveying, two-dimensional (2D)
and three-dimensional (3D) resistivity methods were developed. These methods are more accurate,
convenient, and field worthy, since they are more technologically advanced and easier to use than the
1D modes of investigation.
The 2D electrical resistivity tomography (ERT) method has become one of the most widely used
geophysical technique in investigating near-surface structures. This method was developed to satisfy
the need for new technologies that can generate high-resolution sections of the sub-surface (Daily et
al., 2005). The ERT method is based on the fact that different geological units or structures in the
earth’s subsurface are more or less sensitive to electrical current flow and that different geological
units have different electrical conductivity (Milsom and Ericksen, 2011).
During 2D ERT surveys, apparent resistivity data are collected by using numerous collinear
electrodes inserted into the earth along a straight line. The measured resistivity data are later edited,
processed, and inverted using 2D inversion software to yield models of the subsurface resistivity
distribution. The 2D ERT technique has been recently applied to address a number of engineering,
geological and geo-hydrological problems such as detection of faults and fractured zones,
investigation of the slope stability, and delineation of cave systems (Yadav, 1988; Kumar, 2012; Obi,
2012; Mohamaden and Ehab, 2017)
In geohydrological studies, the main applications of the ERT technique are to investigate 1) the
presence of geological structures potentially associated with groundwater, 2) the movement of
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groundwater and 3) the presence and migration of contaminants in the subsurface. The technique has
been widely used in this field because quality water resources have become a primary concern in
many societies due to increasing population and industrialisation (Donnenfeld et al., 2018). To
increase water supply and sustain industrial demands, economic growth and growing population, all
potential water resources should be appropriately utilised and managed. Groundwater often forms a
large component of the total water supply to communities. However, unlike surface water that can be
seen or measured, groundwater is generally not visible from surface. In addition, the occurrence of
groundwater is localised and often associated with geological structures. The ERT method may be
used to investigate the subsurface and locate sites where production boreholes have a high probability
of intersecting high-yielding aquifer systems, thereby avoiding the wasteful expenditure of drilling
unsuccessful production boreholes.
Studies conducted by Woodford and Chevallier (2002) in the Karoo Basin, show that dolerite
intrusions (dykes and sills) are considered as the main targets for groundwater exploration. Dolerite
dykes in the Karoo Basin are linear or circular (ring-dykes) geological bodies that intruded the
sedimentary country rock during the Jurassic Age. According to Makhokha and Fourie (2016), the
intrusions were associated with the high temperatures and pressures, causing fractured zones along
the margins of the intrusive bodies. Moreover, the fractured zones are often associated with
significant permeability and high-yielding aquifers (Woodford and Chevallier, 2002).
Since large resistivity contrasts generally exist between the dolerite intrusives and the sedimentary
country rock, the 2D ERT method is well-suited to locating and delineating such intrusives. However,
a major assumption of the method is that the survey lines are straight and electrodes are collinear.
Due to the presence of surface infrastructure or other surface constraints (e.g. rivers, vegetation, large
outcrops, and steep topographic gradients) it is not always possible to conduct 2D ERT surveys along
straight lines. The study investigates the influence of angled survey lines on the data recorded and the
resistivity models obtained when using the Wenner (α) array. This array is one of the most commonly
used arrays in resistivity surveying and has a high signal-to-noise ratio compared to the other
commonly used arrays.
Description
Keywords
Dissertation (M.Sc. (Geohydrology))--University of the Free State, 2020, Electrical resistivity., Vertical electrical sounding., Karoo - Geohydrology.