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Geophysical surveys: applicability,
practical implementation, experience from the trials, indicative
costs
Pages in this section include:
This page provides a detailed description of the applicability,
practical implementation, experience from the trials, and indicative
costs for the geophysical surveys channel seepage identification
and measurement technique.
Applicability
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 should come
down as new procedures emerge
- Technical expertise is required
to conduct and analyse survey results
Using resistivity techniques it is possible to obtain a multiple-level
distribution of seepage. This is in contrast to the EM31
and EM34 techniques, which have a single measurement representing
a thickness
of the subsurface profile.
In order to map and quantify seepage, using local relationships
from back-to-back ponds in a single location is satisfactory
if estimates close to this location are required, and subsurface
conditions
do not vary considerably between the ponds. However, if the
purpose of the investigation is to predict seepage over a
long section
of channel, pondage tests covering a range of subsurface
conditions should be conducted. If from this data two or
more different
trends can be observed due to identifiable differences in
subsurface conditions,
then separate regression equations could be used for each
trend. If such trends cannot be observed then all ponds should
be
used to generate the regression equation.
Pondage tests selected for calibration should ideally be
over areas of like geophysical response (conductivity/resistivity).
Therefore
the geophysical survey should be conducted before the pondage
tests so that their location can be based on the survey results.
| 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.
| Experience from
the trials |
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A large number of geophysical investigation techniques
were undertaken in the IAL trials. These are documented
in the Project Trials Report (IAL, 2003).
Comparison of techniques at the same site
Comparison of the results of different EM techniques
are shown using the results of trials at the Toolondo
channel, which used both EM31 and EM34. The surveys
were conducted within five days of each other, so
it can be assumed that groundwater conditions were
essentially identical and that the differences reflect
the responses of the different techniques.
Within this narrow range of measurements the trends
are generally similar when plotted against the pondage
test results, with the higher conductivity areas
coinciding with lower seepage zones (refer Figure
1). Differences appear to be slight. The EM31 conductivities,
representing shallower depth profiles, were slightly
higher than the EM34 results for the low-seepage
ponds, whereas the EM34 conductivities were marginally
higher than the EM31 in the three higher seepage
rate ponds.
Even though the target depths of the EM31and EM34
are different, EM responses were similar for the
bulked soil and water profiles. The limited information
suggests that even in this case, where the watertable
is around 7m deep, the EM31 responses adequately
detect the lower salinity and lower clay impacts
in the higher-seepage zones (P1, P2, P4 below) and
the inverse impact (higher salinity, greater clay
content) on the profile in the low-seepage zones.
Given the penetration depth of the EM31, the response
will be dominated by unsaturated zone properties.
Therefore the primary method of detection is via
detection of variation in soil properties, as opposed
to direct impacts on the groundwater.
Figure 1 EM34
and EM31 survey results (August 2001) - Toolondo
channel
EM
mapping and quantification
The main approach in the trials was to compare the distribution of EM conductivities
from survey lines alongside and within channels. There is a general trend at
all sites of lower conductivity relating to higher seepage, although the strength
of the correlation varies between locations (Figure •-2).
The distribution of the EM results for the Toolondo channel in Western Victoria
is shown in Figure •-3, along with the measured base rate seepage estimates
obtained from pondage tests. Sections where low conductivities are identified
tend to have higher seepage rates. Therefore in its simplest form, an EM survey
provides an indication of the areas on either side of a channel where there may
be seepage relative to other parts of the channel.
Figure 2 Toolondo EM31 land survey
Figure 3 Correlation of
seepage measurements and EM31 results for Toolondo channel
Repeat testing
Repeat testing of the Donald main channel in western Victoria found that average
conductivities in each pondage section varied between two surveys (refer table
below). The results from the September 2001 survey returned consistently higher
conductivities than the October 1999 survey.
Table 1 Comparison of Donald main channel EM34 results: October 1999 and September
2001
| Pond |
Seepage (mm/d) |
Conductivity
Av. Sept 2001
(mS/m |
Conductivity
Av. Oct 1999
(mS/m) |
Variation (mS/m) |
| P1 |
32 |
51 |
28 |
23 |
| P2 |
29 |
44 |
25 |
19 |
| P3 |
28 |
39 |
22 |
17 |
| P4 |
35 |
49 |
42 |
7 |
| P5 |
48 |
49 |
36 |
13 |
| P6 |
9 |
66 |
54 |
13 |
The differences were primarily due to the elevated and fresh groundwater
mound present in the October 1999 survey due to six months of prior channel operation.
At the time of the September 2001 survey the channel had only been running four
weeks (at reduced capacity), possibly only enough to flush some of the accumulated
superficial salts down into the profile. The October 1999 survey was therefore
conducted under a subsurface environment dominated by seeped water and a flushed
and therefore relatively salt free profile, while the September 2001 survey was
conducted in a environment probably only just beginning to flush salts through
the relatively salt-rich profile. Therefore in this instance there was a clear
and identifiable reason for the difference in repeat survey results.
| Indicative Costs |
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Approximate costs for
the three types of geophysical
surveys undertaken in
the IAL trials are
provided below:
EM31 surveys:
- Wimmera Mallee
Water: For 6|km,
on-land,
including four traverses on each
side of channel (over
three sites): $400/km
(includes mobilisation,
data processing and
mapping).
- Murray Irrigation:
For 8km, on-land,
including four
traverses on each
side of channel
(over
four sites): $340/km
(includes mobilisation,
data processing
and mapping).
- Murrumbidgee
Irrigation: On-land,
including
four traverses
on each side
of channel: the
unit cost ranged
from $650/km
(3km section)
to $800/km (1km
section). (Includes
mobilisation,
data processing and
mapping).
On-channel
survey cost was
$330/km for a
3km
section.
EM34 surveys:
- Wimmera Mallee
Water: For 4km over
two
sites: $250/km (one traverse
only on one side
of the channel), i.e.
$500/km
for both sides
of channel (excludes mobilisation).
- Murray Irrigation:
For 6km (on
each side of channel)
over three
sites: $435/km
(includes mobilisation).
- Multi-electrode
resistivity
surveying - The following
costs were
for resistivity surveying
across
11 sites
(approximately 2km
each
in length)
in the Wimmera,
Murray and
Murrumbidgee irrigation areas.
- Resistivity
towed array
surveys:
$900/km
(includes mobilisation
from Adelaide,
travel
between
sites,
production
and all
equipment
costs).
- Data processing
costs: $220/km.
Resistivity surveying
costs are difficult
to quantify because
the
technique is
relatively new. Costs are
likely to come
down as the
technique is
refined, equipment
becomes commercially
available, and
competition is
introduced.
| Related
pages |
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For a more detailed description of the geophysical survey technique see:
Geophysical surveys: summary
Geophysical surveys: principle, method |
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