Open File Report 175
Constrained inversion of Resolve electromagnetic data - Riverland,
Brodie R, Green A, Munday T
PREFACE AND EXECUTIVE SUMMARY
This report describes a subset of activities undertaken CRCLEME
as part of contribution to the Riverland and Tintinara projects
of the South Australia Salinity Mapping and Monitoring Project conducted
under the auspices of The National Action Plan for Salinity and
Water Quality (NAP). Specifically, it concerns the development of
constrained inversion methodologies to invert RESOLVE helicopter
EM data in order to map the location and thickness of near-surface
clay-rich materials. In the Riverland region, the focus of this
report, these materials are principally associated with the Blanchetown
Clay sedimentary unit. The Blanchetown clay, and units of like texture,
act to delay the rate of groundwater recharge and thus locations
where it is present are more favourable for agricultural development.
After a preliminary analysis to select the optimum system and survey
characteristics, approximately 12,000 line-km were surveyed in the
Riverland region at line spacings of 150 or 300 m. The data were
recalibrated with measurements from down-hole induction logs and
then inverted using a 1-D layered-earth model. In order to improve
the sensitivity to the unknown aspects of the ground section, the
inversion was constrained with as much local geological and hydrological
information as possible. These constraints included information
about the depth of the water table, the conductivity of the groundwater,
the variability of the conductivity and thickness of two sand units.
An understanding of the geological and geomorphic history of the
area was also used to help define the probable geometry, distribution
and disposition of relevant sedimentary units.
A map of the distribution and thickness of the Blanchetown Clay
and like-units was generated. The results of the inversion also
allowed us to reconstruct the beach strandline-dominated palaeo-topography
left when the sea retreated from the Murray Basin in the early Pliocene.
They also can be used to define the landscape left after the demise
of Lake Bungunnia. This latter surface, defined by the top of the
Blanchetown Clay, is a complicated result of fluvial and aeolian
redistribution processes that have, in some locations, reworked
clays to positions well above the maximum level of the lake.
The survey also revealed a hitherto unsuspected, deeper variability
in conductivity following the Pliocene strand line pattern. The
cause of this pattern is not, as yet, clear.
There are a number of potential uses for a detailed map of the
distribution of the Blanchetown Clay. At present it is being used
to model the recharge behaviour of the area as an input into a model
predicting the future course of salinity inflows to the Murray River.
If more areas are to be released for irrigation, the map could also
be used to select areas of thicker clay that may be preferred locations
for irrigation. Finally, areas of thicker clay are might also be
the preferred location for disposal of saline water from salt interception
Dr. Tim Munday