CRC LEME
Open File Report 174
ABSTRACT
The validation of Resolve helicopter EM data: Mineralogical and
petrophysical results from field investigations for the Tintinara
East survey area in the south east of South Australia
Tan KP, Munday T, Leaney F
As part of the South Australian Salinity Mapping and Management
Support Project (SA-SMMSP), helicopter airborne electromagnetic
(HEM) data of the Tintinara East, have been acquired to map subtle
conductivity variations in near surface materials (~1-10 m) at a
high spatial resolution as an aid to managing the existing groundwater
resource. In the absence of much surface water, the groundwater
resources at Tintinara represent an important asset. The lifetime
of this resource is not limited by the volume of abstraction, but
by increasing salinity due to irrigation practices. Salt that has
accumulated in the soil and sediments is being leached by increased
recharge, exacerbated by the recent increase in irrigation. The
presence or absence of a near surface clay unit is important. In
the presence of a thick clay unit, the impacts of irrigation on
the aquifer will be over a period of hundreds of years. A management
strategy being developed for the region involves steering new irrigation
development to those areas of thicker, near surface, clay layers,
targeting water use efficiency. In areas where clay is absent, relocating
irrigation-based enterprises to more appropriate sites may be necessary,
in order to significantly prolong the lifetime of the groundwater
resource.
This study had four objectives:
- To define the principal factors driving variations in observed
electrical conductivity in the sediments of the Tintinara East
study area.
- To provide appropriate geo-electrical constraints for the conduct
of a constrained inversion of the HEM data to better map the distribution
of the shallow clay.
- To ascertain whether the conductivity depth image (CDI’s,
e.g. 6 - 8 m) reflects the distribution of conductive clay materials,
which, in the vicinity of the study area, are associated with
the fine textured back-barrier sedimentary facies of the Pliocene
Loxton-Parilla Sands.
- To verify whether the constrained inversion product, i.e. the
Clay Thickness Image was a good representation of true clay thickness.
Five boreholes were selected to target the various conductivity
responses observed in the conductivity-depth intervals generated
from the inversion of the HEM data. Both field and laboratory analyses
were employed, including down-hole conductivity logs and measurement
of water and chloride content, as well as a determination of grain
size distribution in sampled materials. An examination of the drill
cuttings in light of the regional geology suggests that four lithologic
units are represented in the study area, including the Molineaux-Lowan
Sands (Quaternary), Bridgewater Formation (Quaternary), Loxton-Parilla
Sands (Pliocene) and time equivalent fine-textured lagoonal facies
sediments.
From correlation of the inverted HEM data with borehole information,
and statistical analyses carried out on the petrophysical attributes
of the lithologic units, the following can be concluded.
- In the Tintinara AEM survey area, the primary driving factor
of electrical conductivity is salt load, which is a function of
water content and its salinity. Electrical conductivity is correlated
to clay abundance due to the positive causal relationship of the
latter with water content, which denotes the degree of saturation
of pore spaces in the sediments.
- The 25th percentile of 240 mS/m of the fine-textured lagoonal
facies sediments can be used as a threshold value in differentiating
between conductive and resistive sediments. From line of regression,
240 mS/m will equate to approximately 24 vol. % clay. The sedimentary
texture ternary diagram indicates that samples containing more
than 24 vol. % clay comprise sandy mud and sandy clay. Since the
sands (i.e. muddy sand and sand) are dominantly resistive with
only a handful of wet and conductive saline sand units, it is
possible to utilize electrical conductivity to map the conductive
mud and clay.
- The relationship between the geological units and their respective
electrical conductivity suggest the adoption of a three layer
model for constraining the inversion of HEM data. The layers comprises
a resistive layer 1 (Molineaux-Lowan Sands), a conductive layer
2 (lagoonal facies) and a resistive layer 3 (Loxton-Parilla Sands).
4. Evidence from textural information and borehole conductivity
logs, as obtained from the 5 selected boreholes which targeted
specific conductivity signatures shown on the CDI (6- 8 m depth
slice) and the clay thickness product derived from the constrained
inversion, confirms that the Helicopter EM system used at Tintinara
was successful in mapping near surface clay-rich materials. The
conductivity patterns suggest the presence of paired linear resistors
comprising barrier sand, which is accompanied by conductive back-barrier
fine-textured sediments.
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