Wiso Basin (West) Collaborative Tempest Survey

Abstract

A Tempest airborne electromagnetic survey of 1288 line km covering 1,761 sq km was completed as per the 50 percent NT Government funded Wiso Basin West Collaborative Tempest Survey. The survey area covers a large proportion of Toro Energy's ELs 27123 and 29395, which are situated on the southern edge of the Wiso Basin and northern edge of the Arunta Block. The objective was to refine the geological understanding in the area to assist in exploration drillhole targeting for uranium mineralisation. The Tempest system was utilised because it captures calibrated shallow high-resolution EM data that is highly suited to quantitative geological interpretation of palaeochannel settings, which is more cost effective than helicopter-borne systems. Tempest has proven effective in mapping sandstone packages elsewhere and has been used effectively to target 'roll-front style' uranium mineralisation in Australia and overseas. Main results and conclusions: The southern edge of the Wiso Basin has been accurately mapped by the AEM survey in combination with the available regional magnetic data; The most obvious features of the grids are two large palaeodrainage networks that are evident as broad sinuous conductive 'plumes', which emanate south of the survey area in the Reynolds Ranges, and propagate northward across the flat landscape overlying the Wiso Basin. These palaeochannels were probably deposited in the Tertiary and are now providing a conduit for northward dispersion of saline groundwater. They occur within 200 m of the current land surface. The surrounding sand plain is much less conductive and is interpreted to be comprised of less permeable sands with fresher groundwater; Several generations of palaeochannel are evident in the CDIs; Some of the deeper conductive features are likely to be gently dipping stratigraphy within the Wiso Basin, but some are ambiguous at present and could be either deep palaeochannels or Wiso geology; Within the Wiso Basin and Tertiary cover, there appears to be a series of east-west oriented discontinuities of the conductivity trends that may be indicating the position of active basement faults; The modern drainage systems are not reflected in the subsurface electromagnetic data, indicating that the underlying palaeochannels are of an older generation when the depositional environment and setting were different to today; and Care should be taken when interpreting conductivity grids in isolation, as there are clearly several generations of palaeochannel and confusing relationships with Wiso stratigraphy. This can be partly resolved by undertaking inversions of the data to generate a realistic conductivity-depth model. However, this model then requires 'ground truthing' via framework drilling, to identify subsurface geology and groundwater conditions. This is Toro?s next step.