Geophysical technologies, and in particular electrical and electromagnetic methods, have the potential to provide a rapid and relatively inexpensive approach to determining the location and extent of seepage along irrigation canals or channels. Although showing potential, the application of airborne electromagnetic (AEM) systems for these purposes has been very limited, in part because of the fine scale information required and also the costs associated with acquisition. However, recent developments in AEM system technologies have contributed to substantial improvements in the definition of conductivity at shallow depths and we believe these trends have made these systems a more relevant technology for the systematic mapping and detection of variations associated with irrigation infrastructure. In this paper we examine that potential through the analysis of high resolution HEM data for an irrigation system located in Victoria, Australia. Inverted data from a RESOLVE FDHEM survey along an irrigation channel were compared with an inverted ground resistivity array data set. Results demonstrated that the spatial patterns and magnitude of conductivity variations are generally comparable. The ground geophysical technique benefitted from being able to map variations at finer scales. However, there may be merit in considering the deployment of airborne mapping methods if large surveys are considered and a rapid turn‐around of information is required.
A methane (CH4) and carbon dioxide (CO2) release experiment was held from April to June 2015 at the Ginninderra Controlled Release Facility in Canberra, Australia. The experiment provided an opportunity to compare different emission quantification techniques against a simulated CH4 and CO2 point source release, where the actual release rates were unknown to the participants. Eight quantification techniques were assessed: three tracer ratio techniques (two mobile); backwards Lagrangian stochastic modelling; forwards Lagrangian stochastic modelling; Lagrangian stochastic (LS) footprint modelling; atmospheric tomography using point and using integrated line sensors. The majority of CH4 estimates were within 20% of the actual CH4 release rate (5.8 g/min), with the tracer ratio technique providing the closest estimate to both the CH4 and CO2 release rates (100 g/min). Once the release rate was known, the majority of revised estimates were within 10% of the actual release rate. The study illustrates the power of measuring the emission rate using multiple simultaneous methods and obtaining an ensemble median or mean. An ensemble approach to estimating the CH4 emission rate proved successful with the ensemble median estimate within 16% for the actual release rate for the blind release experiment and within 2% once the release rate was known. The release also provided an opportunity to assess the effectiveness of stationary and mobile ground and aerial CH4 detection technologies. Sensor detection limits and sampling rates were found to be significant limitations for CH4 and CO2 detection. A hyperspectral imager's capacity to image the CH4 release from 100 m, and a Boreal CH4 laser sensor's ability to track moving targets suggest the future possibility to map gas plumes using a single laser and mobile aerial reflector.
Salinisation of the River Murray in South Australia has been a constant threat to this vital water resource. River salinity, a consequence of discharging saline groundwater, is being limited through the implementation of salt interception schemes, where numerous water bores were installed along the river bank to lower groundwater levels and reduce discharge. The efficient removal of this water relies on our ability to locate areas of salt accession to the river. This study shows the use of in-river NanoTEM to map the resistivity of the river sediments. Following a ground validation exercise, which included the collection of saturated sediment cores and subsequent laboratory analyses, it was concluded that variations in the observed resistivity response were associated with changes in salinity. Together with hydrological information, conductive areas surveyed along the river were interpreted as zones of salt accession whereas the resistive areas were zones where fresh river water recharged the underlying sediments.
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