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Remote non-invasive techniques
Direct and point seepage measurements techniques and subsurface
characterisation identify seepage distribution and measure
rates by directly measuring a physical property at a single
location. For example, groundwater
assessment of water levels
in a bore allows a direct measure of the watertable, and
infiltration tests are direct measures of the soil properties
at a point.
Geophysical surveys, in contrast, use remote
sensing, high-density
sampling of subsurface and near-surface properties to provide
continuous data along the channel.
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Geophysical surveys: summary
Pages in this section include:
This page provides a summary of the geophysical surveys
technique for channel seepage identification and measurement.
Principle
Geophysical techniques applied to seepage measurement involve measuring
a contrast in terrain conductivity (or its inverse, resistivity)
in the subsurface profile around the channel. They can be used
in two ways:
- Direct measurement of conductivity of groundwater
and identifying the conductivity contrast of fresher channel
water as it seeps
into and dilutes saltier native groundwater. Decreasing
groundwater salinity causes a decrease in electrical conductivity
(or an
increase in resistivity).
- Identification of contrasts in soil properties
and inference of the likelihood of greater seepage through
more permeable
materials in the zone above the watertable.
More information
| Method |
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The geophysical methods most likely to be applied
to channel seepage detection and which have most
relevance to Australian water industry operations
and conditions, are electromagnetics (specifically
EM31 and EM34) and resistivity.
The preferred technique for geophysical channel
seepage assessment is directly detecting the impact
of seepage on the groundwater. This means that
the instrument must focus on the zone immediately
above and several metres below the watertable:
- For shallow watertables (surface to approximately
5m) EM31 is suitable for direct seepage
detection.
- For watertables deeper than 5m, EM34
(in vertical dipole mode) or resistivity can
be used.
EM31 (vertical dipole) adjacent to the channel
can be used effectively in areas with deeper
watertables, although it does not directly measure
the seepage
impact on the watertable. If this method is used,
however, it must be made certain that seepage
is controlled by the unsaturated zone and not
surface-clogging
processes. Otherwise errors can be introduced
to the assessment process.
Geophysical techniques can be used for seepage
assessment in two ways:
- Mapping the distribution zones. High-
and low-seepage zones (or inferred zones of likely
seepage) can
be effectively mapped using geophysical
techniques alone. Greater confidence can be obtained
by confirmation
with geological investigations.
- Quantification
of seepage rates. Quantification requires integration
of geophysical methods
with other techniques in order to calibrate results.
Geophysics can be used to provide an
estimate of seepage rate, provided a sufficiently
strong relationship
can be developed between geophysical
response and pondage tests. The relative seepage
rate can be
identified by the correlation of geophysics
with other data, particularly pondage tests.
Important variables that need to be considered
when conducting a geophysical channel seepage
investigation include survey timing, on-channel
versus on-land,
off-set distance and location for on-land
surveys, and other potential influences such as trees
and rainfall.
More information
| Applicability |
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Use of geophysical surveys for channel seepage assessment is
an emerging area. Its attraction is the potential for rapid
assessment
of long channel sections. However, care needs to be taken in
the interpretation of results. Seepage can be detected using
geophysical
techniques alone, but quantification requires integration of
geophysical methods with other techniques in order to calibrate
results.
Benefits of using geophysical techniques:
- Potentially the fastest means of seepage assessment
- Essentially
continuous spatial assessment
- No interruption to channel operations
- With adequate local
calibration, can provide reasonable estimates for seepage
quantification
Factors to consider:
- Interpretation can be difficult and will vary from
area to area
- Interpretation may require subsurface investigation
- Can
be relatively expensive, but costs should come down as new
procedures emerge
- Technical expertise is required to
conduct and analyse survey results
More information
| Practical
implementation |
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Geophysical surveys should be conducted while the channel is in
operation, or immediately after the end of the channel operating
season. No interruption to channel operations is required.
A disadvantage of resistivity surveys is that they require substantially
more data processing than EM surveys. This is costly, requires
higher levels of specialist technical input and possibly more time
to deliver final information and reports.
More
information
| Indicative costs |
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Approximate costs for the three types of geophysical surveys
undertaken in the IAL trials are provided below. It is important
to note that the unit costs per kilometre were for very short
sections of channel (1-3|km) and costs would be significantly
lower for longer channel sections.
- EM31 Surveys: For on-land surveys, including 4 traverses
on each side of channel (over 3 sites): $400/km to $800/km.
On-channel survey costs: around $330/km.
- EM34 Surveys: For 4km over 2 sites:
$250/km, (1 traverse only), $500/km for both sides of channel.
For 6km (on each
side of channel) over 3 sites: $435/km.
- Multi-electrode resistivity surveying:
$900/km (includes mobilisation, travel between sites, production
and equipment costs)
- Data processing costs: $220/km.
More information
| Related
pages |
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For a more detailed description of the geophysical survey technique
see:
Geophysical surveys: principle, method
Geophysical surveys: applicability, practical implementation,
experience from the trials, indicative costs |
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