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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 monitoring 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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Remote sensing: summary
Pages in this section include:
This page provides a summary of the remote sensing
channel seepage identification and measurement technique.
Principle
Remote sensing refers to data recording by a sensor which measures
energy emitted or reflected by objects located at some distance
from the sensor Remote sensing techniques infer channel seepage
based on soil moisture, vegetation vigour and soil profile properties
adjacent to the channel.
In particular, airborne night thermal infrared imagery can provide
an indication of shallow soil saturation resulting from lateral
channel seepage, which may be a precursor to water logging and
soil degradation.
There are no documented studies of remote sensing being used to
quantify channel seepage, although it is now being used for detecting
seepage locations.
More information
| Method |
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Remote sensing assumes that channel seepage has a surface expression
adjacent to the channel.
This may be detected as increased soil
moisture, vegetation vigour and water status. Techniques are limited
to detecting seepage that migrates laterally through channel banks
and surfaces near the channel toe.
Data must be of sufficient resolution to allow definition of seepage
zones. Ground resolutions of less than 10m are required. The
regions most useful for channel seepage detection include visible,
reflected
(near) infrared and thermal infrared. Source data should be multispectral
(i.e. have data collected from more than one distinct region
of the electromagnetic spectrum). Distinct data from the infrared
region is expected to be the most beneficial as this area of
the
spectrum is strongly absorbed by water and is able to most distinctly
separate areas of varying soil moisture and plant water and growth
status.
Remote sensing data can be evaluated in conjunction with other
spatial data such as that from EM surveys and soil survey assessments.
Increased surface moisture and vegetation growth due to channel
seepage are likely to be evident during late summer and early autumn
when surrounding areas (apart from irrigation) would be distinctly
drier. This is likely to be the optimum data collection time. Imagery
from more than one date would be useful to remove the seasonal
variations such as irrigation or rainfall.
Remotely sensed image data sources may include:
- Digital infrared aerial photography
- Airborne high-resolution
sensor data
- Satellite imagery
More information
| Applicability |
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- Remote sensing offers considerable potential for effective
identification of seepage zones of large lengths of a channel
system without interfering with channel operations. Quantification needs
to be based on further analysis.
- Remote sensing is appropriate
for investigations where the primary aim is identification
of land degradation associated with
channel seepage. It should not be used if it is known that the seepage
mechanism is predominantly vertical, which is likely to occur
at sites with a deep watertable.
- Remote sensing is most useful in environments
where lateral seepage is predominant. Sites with a high watertable,
shallow impermeable
layer or bank seepage have conditions that can cause seepage
to have a surface expression.
Remote sensing is a tool for targeting potential seepage sites,
although a drawback is that it assumes seepage will have a
surface expression as moist soil or associated vegetation
adjacent to the
channel.
More information
| Practical
implementation |
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Remote sensing is relatively expensive and requires specialist
technical input at the planning, data gathering, processing and
interpretation stages. However, it can rapidly acquire data over
long distances of channel, and along with geophysical surveys it
is likely to be a key part of future large-scale channel seepage
investigations.
More information
| Indicative costs |
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Costs vary widely depending on the source data used. As an indication
of the likely data collection costs, suitable quality airborne infrared
data (3-5m resolution) for three lengths of channel (10-20km each)
were quoted at around $11,000. Data processing costs, including integration
with a GIS system are in addition to these costs and likely to be
in the order of $5,000-10,000.
More information
| Related
pages |
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For a more detailed description of the remote sensing technique see:
Remote sensing: principle, method
Remote sensing: applicability, practical implementation,
experience from the trials, indicative costs |
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