Electrical conductivity imaging of aquifers connected to watercourses : a thesis focused on the Murray Darling Basin, Australia
- Publication Type:
- Thesis
- Issue Date:
- 2007
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Electrical imaging of groundwater that interacts with surface watercourses provides detail
on the extent of intervention needed to accurately manage both resources. It is particularly
important where one resource is saline or otherwise polluted, where spatial quantification
of the interacting resources is critical to water use planning and where losses from surface
waterways need to be minimized in order to transport water long distances. Geo-electric
arrays or transient electromagnetic devices can be towed along watercourses to image
electrical conductivity (EC) at multiple depths within and beneath those watercourses. It
has been found that in such environments, EC is typically related primarily to
groundwater salinity and secondarily to clay content. Submerged geo-electric arrays can
detect detailed canal-bottom variations if correctly designed. Floating arrays pass
obstacles easily and are good for surveying constricted rivers and canals. Transient
electromagnetic devices detect saline features clearly but have inferior ability to detect
fine changes just below beds of watercourses. All require that water depth be measured by
sonar or pressure sensors for successful elimination of effects of the water layer on the
data. The meandering paths of rivers and canals, combined with the sheer volume of data
typically acquired in waterborne surveys, results in a geo-referencing dilemma that cannot
be accommodated using either 2D imaging or 3D voxel imaging. Because of this,
software was developed by the author which allows users to view vertical section images
wrapped along meandering paths in 3D space so that they resemble ribbons.
Geo-electric arrays suitable for simultaneous imaging of both shallow and deep strata
need exponentially spread receiver electrodes and elongated transmitter electrodes. In
order to design and facilitate such arrays, signed monopole notation for arrays with
iv
segmented elongated electrodes was developed. The new notation greatly simplified
generalized geo-electric array equations and led to processing efficiency. It was used in
the development of new array design software and automated inversion software
including a new technique for stable inversion of datasets including data with values
below noise level. The Allen Exponential Bipole (AXB) array configuration was defined
as a collinear arrangement of 2 elongated transmitter electrodes followed by receiver
electrodes spaced exponentially from the end of the second transmitter electrode. A
method for constructing such geo-electric arrays for use in rivers and canals was
developed and the resulting equipment was refined during the creation of an extensive set
of EC imaging case studies distributed across canals and rivers of the Australian Murray-
Darling Basin. Man made and natural variations in aquifers connected to those canals and
rivers have been clearly and precisely identified in more than 1000 kilometres of EC
imagery.
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