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Open File Report 188

The validation of Resolve helicopter EM data: Mineralogical and petrophysical results from field investigations in the Riverland area, South Australia

Tan KP, Munday T, Leaney F

As part of the South Australia Salinity Mapping and Management Support Project (SA-SMMSP), the Riverland project was set up to use airborne electromagnetic (AEM) data to map the near surface distribution of fine textured materials at an appropriate resolution to provide information for determining recharge rates to the saline groundwater which forms the basis for managing salt being discharged into the River Murray A frequency domain helicopter electromagnetic system (RESOLVE) was selected to delineate the near surface conductors, which are mainly associated with fine-textured materials such as the Pleistocene Blanchetown Clay unit.

The study had two specific objectives:

  1. To define the principal factors driving variations in electrical conductivity in the sediments of the Riverland region.
  2. To ascertain whether the conductive patterns shown in the high frequency apparent conductivity data (eg. 100,000 Hz and 25,000 Hz) and EM inversion products such as conductivity depth imaging (CDI’s) and the “Clay Thickness Map” accurately reflect the distribution of clay materials.

These objectives were achieved through targeted drilling, geological logging, the geochemical, mineralogical and petrophysical analysis of bore cuttings and cores, and acquisition of borehole geophysical logs, including conductivity and natural gamma. From an analysis of the laboratory results, geophysical logs and cross referencing with the inverted EM data the following can be concluded.

  1. In the Riverland survey area, variations in the observed apparent electrical conductivity (ECa) of near surface materials are primarily determined by the texture of sediments. A detailed analysis of texture and water content established that finer textured materials are generally characterized by higher water contents. Although chloride concentrations may vary, and remain high in sand-rich materials, the low water content associated with sand and the higher water content associated with clay-rich sediments resulted in a low ECa for the former and an elevated ECa for the latter.
  2. Textural information and borehole conductivity logs obtained from the 14 selected boreholes, which target specific conductivity patterns shown on the high frequency (25,000 Hz) apparent conductivity image, suggest that the observed conductivity response is largely determined by the distribution of near surface clayey materials. A discrepancy arises in the vicinity of low lying areas where shallow and saline groundwater occurs, in which case, the conductivity signature is attributed to conductive saturated sand. A comparison between inversion procedures (apparent conductivity, CDI and a constrained 1D Layered Earth Inversion) in the vicinity of borehole RIV9HC and RIV9LC shows that the RESOLVE HEM system is capable of differentiating between conductive shallow clay and a saline, conductive groundwater (i.e. saturated sand). The best results from a clay mapping perspective, were obtained from the constrained inversion confirming the value of a full inversion in reducing ambiguities..

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