CRC LEME
Open File Report 188
ABSTRACT
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:
- To define the principal factors driving variations in electrical
conductivity in the sediments of the Riverland region.
- 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.
- 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.
- 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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