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Open File Report 174

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:

  1. To define the principal factors driving variations in observed electrical conductivity in the sediments of the Tintinara East study area.
  2. 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.
  3. 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.
  4. 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.

  1. 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.
  2. 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.
  3. 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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