Tim Munday

Spatially constrained inversion for quasi 3D modelling of airborne electromagnetic data – an application for environmental assessment in the Lower Murray Region of South Australia

We present an application of spatially constrained inversion (SCI) of SkyTEM (airborne electromagnetic) data for defining spatial patterns of salinisation in the Bookpurnong irrigation area located in the lower Murray Basin of South Australia. SCI uses Delaunay triangulation to set 3D constraints between neighbouring soundings, taking advantage of the spatial coherency that may be present in the dataset. Conductivity information for individual soundings is linked through the spatial constraints, from well determined parameters to locally poorly determined parameters. For the survey presented here, SCI generated maps detail the spatial variability of floodplain salinisation, the extent of floodplain sediments influenced by lateral recharge and flushing along stretches of the Murray River, and the variable quality of groundwater in deeper semi-confined aquifers of the Murray Group. Available borehole and other ancillary information, such as vegetation density and health patterns, match the observed conductivity variations seen in the SCI results, even at the very near surface (≈2m depth). The SCI provides more accurate and spatially consistent results compared with those from single site inversions. They are also more uniform and detailed than maps obtained with single point Layered Earth Inversions or a laterally constrained inversion. In this example, the SCI provided reliable quasi 3D modelling, that confirmed and improved the hydrogeological knowledge of the area, indicating that the technique would have application with helicopter electromagnetic data in similar settings throughout the lower Murray Basin of Australia.

A Small-Diameter NMR Logging Tool for Groundwater Investigations

A small-diameter nuclear magnetic resonance (NMR) logging tool has been developed and field tested at various sites in the United States and Australia. A novel design approach has produced relatively inexpensive, small-diameter probes that can be run in open or PVC-cased boreholes as small as 2 inches in diameter. The complete system, including surface electronics and various downhole probes, has been successfully tested in small-diameter monitoring wells in a range of hydrogeological settings. A variant of the probe that can be deployed by a direct-push machine has also been developed and tested in the field. The new NMR logging tool provides reliable, direct, and high-resolution information that is of importance for groundwater studies. Specifically, the technology provides direct measurement of total water content (total porosity in the saturated zone or moisture content in the unsaturated zone), and estimates of relative pore-size distribution (bound vs. mobile water content) and hydraulic conductivity. The NMR measurements show good agreement with ancillary data from lithologic logs, geophysical logs, and hydrogeologic measurements, and provide valuable information for groundwater investigations.

A comparative study of in-river geophysical techniques to define variations in riverbed salt load and aid managing river salinization

Increased interest in the character of sediments at the base of waterways, for the purpose of managing river salinization, has led to the application of several geophysical techniques for collecting information from this zone. These instream methods are based on established ground and airborne electrical and electromagnetic technologies, including towed transient electromagnetic systems, towed direct current resistivity array systems, and frequency-domain helicopter electromagnetic systems. Although these systems are individually successful, a systematic examination of their relative effectiveness for identifying variations in substrate conductivity for a common stretch of a river remains lacking. We have compared results obtained from data collected using three instream geophysical techniques for a common stretch of the Murray River in southeastern Australia. The Murray River is an important water resource for drinking and agricultural purposes. Data from these surveys were acquired tolocate areas of sig...

Airborne electromagnetic modelling options and their consequences in target definition

Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that the 2D and 3D methods used to extract models that are geologically meaningful would be favoured over 1D inversion and transforms. We do after all deal with an Earth that constantly undergoes, faulting, intrusions, and erosive processes that yield a subsurface morphology, which is, for most parts, dissimilar to a horizontal layered earth. We analyse data from a survey collected in the Musgrave province, South Australia. It is of particular interest since it has been used for mineral prospecting and for a regional hydro-geological assessment. The survey comprises abrupt lateral variations, more-subtle lateral continuous sedimentary sequences and filled palaeovalleys. As consequence, we deal with several geophysical targets of contrasting conductivities, varying geometries and at different depths. We invert the observations by using several algorithms characterised by the different dimensionality of the forward operator. Inversion of airborne EM data is known to be an ill-posed problem. We can generate a variety of models that numerically adequately fit the measured data, which makes the solution non-unique. The application of different deterministic inversion codes or transforms to the same dataset can give dissimilar results, as shown in this paper. This ambiguity suggests the choice of processes and algorithms used to interpret AEM data cannot be resolved as a matter of personal choice and preference. The degree to which models generated by a 1D algorithm replicate/or not measured data, can be an indicator of the data’s dimensionality, which perse does not imply that data that can be fitted with a 1D model cannot be multidimensional. On the other hand, it is crucial that codes that can generate 2D and 3D models do reproduce the measured data in order for them to be considered as a plausible solution. In the absence of ancillary information, it could be argued that the simplest model with the simplest physics might be preferred. Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that 2D and 3D methods used to extract models of geological significance would be favoured over 1D inversion and transforms. We analyse data from the Musgrave province, South Australia, used for mineral and for hydro-geological prospecting.

Airborne electromagnetics — Providing new perspectives on geomorphic process and landscape development in regolith-dominated terrains

Abstract In regolith-dominated terrains, the nature of contemporary processes and the surface distribution of regolith materials may be a poor guide to the character and history of regolith materials at depth. The nature of regolith materials at depth is often critical to unravelling the development of a landscape. Conventional mapping aids such as air photos, multispectral remote sensing and airborne radiometrics are not wholly adequate in this context, as they penetrate limited depths ( 100m). The application of AEM to regolith mapping and its potential as a tool in geomorphology are illustrated by reference to an AEM survey flown at Lawlers in the Yilgarn Craton of Western Australia. At Lawlers, AEM identifies a palaeochannel that has no surface expression. It cannot be seen in images of the Landsat, airborne radiometric or airborne magnetic data. The disposition of this channel in the landscape, and in particular its association with ferruginous materials forming breakaways, suggest that inversion of relief has been a significant factor in the evolution of the Lawlers landscape. The AEM data at Lawlers have also been used to map the weathering front. The topography of the weathering front not only reflects the movement of water through the landscape in a general sense, but also reflects the influence of lithology and structure. Different lithologies are clearly weathering to different depths. Information on the nature of the weathering front is potentially an important constraint on models of groundwater flow, and by association, models of solute dispersion.

Characterisation of a Coastal Aquifer System in the Eyre Peninsula, South Australia, Using Nuclear Magnetic Resonance Methods

The coastal aquifers of the Uley Basin, which are the most important source of potable groundwater for the Eyre Peninsula, consist of unconfined Quaternary limestone overlying Tertiary clays and sandstones. Despite its importance, elements of the connectivity and total water resource basin remain relatively poorly understood. To address this, hydrogeophysical methods have been employed to better characterise the aquifer systems present. Interpretation of airborne electromagnetic data provided evidence for the delineation of the base of the Quaternary (limestone) aquifer and a basement low in the southwest corner of the South Uley Groundwater Lens, where there is a limited number of lithological bores or groundwater wells. The basement low, adjacent to the coast, suggests a preferential groundwater flow path and a possible connection between the Basin aquifers and the Southern Ocean.

Near-surface geophysics for informed water-management decisions in the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands of South Australia

The Aboriginal population of the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands in South Australia is dependent on groundwater for nearly all water needs. In that region, placement of wells in productive aquifers of appropriate water quality is challenging because of lack of hydrologic data and variable aquifer properties. It is desirable to have an improved ability to identify and evaluate groundwater resources in this remote region with cost-effective methods that make minimal impact on the environment. A project supported by the Society of Exploration Geophysicists program Geoscientists Without Borders tested a combined geophysical approach with airborne and ground-based data sets to locate a potential aquifer, confirm water content, and estimate the subsurface extent of the water-bearing zone. This hydrogeophysical approach was an effective means for exploration and evaluation of groundwater resources in APY lands generally, and it characterized a specific aquifer as a case study.

Exploration for kimberlites through a complex regolith cover — A case study in the application of AEM in the deeply weathered Archaean Yilgarn Craton, Western Australia.

Regolith electrical structures associated with a small kimberlite dyke located in the Yilgarn Craton, Western Australia are examined. This study was concerned with determining The kimberlite is hosted by a deeply weathered granitic host. The saprolite associated with the granite and the kimberlites are conductive as defined from downhole geophysical logging. The kimberlite shows as a conductor in ground EM (SIROTEM and GEM-2 systems) surrounded by a resistive host. It also shows a small conductive response in 56Khz DIGHEM data but is not apparent in lower frequency data or in SALTMAP.a fixed wing time domain airborne EM system. The geometry of the latter suggests that any response of the kimberlite is likely to be lost in a coupling of transmitter with conductive wedges of granitic saprolite to the north and south of the blow. Stitched sections of 1D LEI’s for SIROTEM, DIGHEM and SALTMAP show similar trends and suggest that the regolith electrical structure is effectively modelled as a layered earth, with some exceptions. An AEM “response map” in this area is essentially a map of the variation of regolith thickness and conductivity.

Determining cover variability in the Capricorn Orogen with airborne EM

This paper focuses on elucidating cover variability throughout the Capricorn Orogen in Western Australia. We use, as a baseline, data from a widely spaced airborne electromagnetic (AEM) fixed-wing survey acquired for the Geological Survey of Western Australia in 2014. The Capricorn 2013 AEM survey is the largest AEM survey by area flown in Australia to date, covering over 146 300 km2. The Capricorn Orogen is a highly mineral prospective under explored orogeny located between the Pilbara and the Yilgarn Craton. Whilst the western part of the Orogen is particularly well exposed, and as a result the surface geology, geological history tectonic setting is well understood, the north west and eastern regions are characterised by a variably thick and complex regolith. The region is relatively under-explored, although host to significant mineralisation, including mesothermal orogenic gold, copper-gold volcanogenic massive sulphides, and channel iron ore deposits. In a region of variable cover, geophysical (aeromagnetic, electromagnetic and gravity) and geochemical techniques are critical aids to the mapping of lithostratigraphic units that are covered by regolith materials, but also in providing an understanding of the regional geological factors that control the mineralisation. Here we discuss some initial results from the smooth model layered earth inversion of 30,119 line km of AEM data. We consider sections from geologically contrasting parts of the Orogen. The results show the complexity and variability of conductive cover in the region and suggest some areas in the orogen could be beneath 200m of transported and in-situ regolith cover. The regional regolith framework that is being developed from the AEM data will provide a basis for better understanding and interpreting regolith geochemistry that has been acquired across a region, particularly where outcrop is limited.

Integrated interpretation of overlapping AEM datasets achieved through standardisation

Numerous airborne electromagnetic surveys have been acquired in Australia using a variety of systems. It is not uncommon to find two or more surveys covering the same ground, but acquired using different systems and at different times. Being able to combine overlapping datasets and get a spatially coherent resistivity-depth image of the ground can assist geological interpretation, particularly when more subtle geophysical responses are important. Combining resistivity-depth models obtained from the inversion of airborne electromagnetic (AEM) data can be challenging, given differences in system configuration, geometry, flying height and preservation or monitoring of system acquisition parameters such as waveform. In this study, we define and apply an approach to overlapping AEM surveys, acquired by fixed wing and helicopter time domain electromagnetic (EM) systems flown in the vicinity of the Goulds Dam uranium deposit in the Frome Embayment, South Australia, with the aim of mapping the basement geometry and the extent of the Billeroo palaeovalley. Ground EM soundings were used to standardise the AEM data, although results indicated that only data from the REPTEM system needed to be corrected to bring the two surveys into agreement and to achieve coherent spatial resistivity-depth intervals. Combining resistivity-depth models obtained from the inversion of airborne electromagnetic data can be challenging, but spatially coherent resistivity-depth images of the ground can assist geological interpretations. We use ground EM data to standardise AEM data so as to be able to combine overlapping AEM datasets acquired with different systems.