Key to the effective management of natural ecosystems that characterise the floodplains of the Murray Basin in south eastern Australia, and maintenance of river health (a critical water resource) in a setting where severe salinisation is an ever-present threat, requires a sound understanding of surface water-groundwater processes. This paper presents results from an examination of hydrogeophysics, specifically airborne electromagnetics (AEM) data acquired by the SkyTEM time domain helicopter EM system, as a means for improving our knowledge of spatial patterns associated with inter-aquifer mixing where groundwater flow is complex. In the south-eastern part of the Murray Basin, AEM data shows considerable promise as a means for understanding of groundwater quality and its lateral variability. In the Bookpurnong and Loxton irrigation areas the high moment capability of SkyTEM permits us to investigate variations in the quality of groundwater at depth (>100m), which in turn allows us to visualise how groundwater may be moving across aquitards and within particular aquifer systems.
Although the notion of spatio-temporal monitoring of natural landscapes and phenomena using multi-date airborne electromagnetic (AEM) surveys has been around for some time, examples are very limited in scope, particularly when defining vertical and lateral changes with time. We demonstrate an effective procedure for defining spatio-temporal variations in ground conductivity across a salinised floodplain in South Australia, using multi-date FDHEM data. Lateral and vertical changes in the conductivity of the floodplain have been resolved. We believe the advent of improved calibration procedures, geometry correction, calibrated broad band AEM systems and advanced inversion procedures that obviate the necessity of system calibration – recalibration, such as the holistic inversion, provide for the realistic proposition of using AEM data for the semi-quantitative and quantitative monitoring of landscape change in the subsurface. However, we emphasize the need for caution when considering observed spatial variations, stressing the importance of accounting for system investigation depth and the potential for artifacts that might be introduced from noise, system geometry and/or data interpretation procedures, when comparing data and derived conductivity models from different dates.
The recent advent of calibrated airborne EM systems, coupled with effective ground-based calibration procedures and more robust inversion tools that can account for system geometry, have given added impetus to their deployment as tools to aid the quantitative monitoring of variations associated with floodplain ecosystems, and in particular processes connected with surface water and groundwater interactions. This also has implications for understanding the consequences of floodplain management.
Salinity in the River Murray and in adjacent floodplains of south‐central Australia, has important environmental, economic and social consequences. Methods to monitor the temporal state of river and particularly river‐groundwater interactions, have been in place for many years now. However, few have the capacity to define variability at a resolution appropriate for developing effective salinity management strategies, such as salt interception schemes. The use of geophysical methods for rapid high resolution mapping of river sediments has been successfully trialed in Australia, particularly using the "in stream" NanoTEM, a time domain ground EM system, deployed in a boat with the transmitter and receiver towed behind on a rigid floating boom. More recently, tests have been conducted using two different helicopter EM systems; a frequency domain EM system (FDHEM) and a time‐domain EM system (TDHEM). Comparisons between conductivity‐depth sections derived from the "in stream" NanoTEM and the airborne datasets suggest that the different approaches are comparable. This paper examines the potential of using the FDHEM RESOLVE system as basis for mapping reaches of the river that contribute to elevated salt loads in the Murray River to the south east of Mildura in Victoria. The advantages of the airborne systems become more apparent when data coverage and acquisition costs are considered, particularly in a situation where a parallel swath approached is employed. This entails the acquisition of adjacent lines of EM data along the centre and along the margins of the river. We suggest this approach provides for a better understanding of recharge and discharge processes and links between the floodplain and the main‐river channel. Compared with data acquired along the river alone, this study demonstrated our ability to use Helicopter EM data to map losing and gaining (from a salt load perspective) stretches of the river and to provide insight into which parts of the groundwater‐floodplain system were significant contributors to river salt loads. The rapid acquisition of airborne EM data makes these systems more suited to providing temporal snapshots of a river‐floodplain environment during dramatic climatic events, such as flooding. In the Murray basin this may assist our understanding of how salt stores are mobilised during such occasions.
Geophysical methods are used in Australia to provide detailed spatial information to help predict the impact of current and future irrigation developments, the design of salt interception schemes and protection of floodplain values. RESOLVE frequency domain helicopter electromagnetic data were acquired over the Chowilla Floodplains, in the Lower Murray region of southern Australia, to provide detailed baseline data on the spatial distribution of near‐surface salt stores and materials in the floodplain and their relationship with in‐river salinity. Degradation across the floodplain and wetlands has resulted primarily from a significant reduction in flood events, and overgrazing. Restoration of the floodplain will involve the reduction of salinity flow from groundwater into the river and increasing environmental flows across the floodplain. Conductivity models predicted from HEM data help identify local recharge and discharge areas, and links with river salinity. The baseline data provided by the airborne data are used with high resolution ground EM surveys including EM31 and time‐domain EM, over targeted areas. Ground methods can be repeated, to monitor affects of artificial flooding designed to restore vegetation health. Similarly the combination of airborne and ground data, allows piezometers to be effectively targeted with the resulting information interpreted within the context of the baseline conductivity structure defined form airborne data.
Increasing salinity in the River Murray is well documented and is of concern environmentally, economically and socially. The Murray Darling Basin Commission and the Mallee Catchment Management Authority engaged the authors to collect base-line in-stream NanoTEM data within the River Murray from Lock 1 to Mallee Cliffs (675 km). This is a new application of a high resolution fast sampling Transient Electro-Magnetic (TEM) system, towed behind a boat, taking soundings every seven to ten metres along the river. The observed NanoTEM response was interpreted against the current understanding of the regional hydrogeology and groundwater processes in and around the river. This paper summarises some of the results from this investigation. The observed response correlates strongly with previously mapped major changes in underlying lithostratigraphy along the Murray River, and with gaining and losing reaches of the river. The extensive length of the survey provides an insight into potential interactions between the river, floodplain and groundwater, but does not replace the need for focussed ground-truthing programs to examine specific correlations. This rapid, portable technique should be applicable outside the Murray-Darling Basin as well as at additional locations within the Basin.
Groundwater in the Eyre Peninsula of South Australia is scarce with potable resources limited to the western coastal margin and the southern tip of the peninsula. Consequently an understanding of their extent has become increasingly important particularly with demand being close to current extraction limits. In September 2006, about 1000 line km of TEMPEST AEM data were acquired over the Southern Eyre Peninsula, in order to assist in the definition of freshwater lens systems and in particular aquifer bounds associated with them as part of a resource definition project. Following their acquisition, the TEMPEST data set was analysed for data quality and then transformed into conductivity depth images (CDI) using EMFLOW and subsequently using a smooth model inversion (see Fitzpatrick and Munday, 2007). In an effort to better define to better define the geometry of specific bounding surfaces of hydrogeological relevance the TEMPEST data were inverted through the application of the laterally constrained inversion (LCI) technique. This paper describes the initial results from the first application of the LCI to data from a fixed wing AEM system.
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.
Geophysical methods are used in Australia to provide detailed spatial information to help predict the impact of current and future irrigation developments, the design of salt interception schemes and protection of floodplain values. RESOLVE frequency domain helicopter electromagnetic data were acquired over the Chowilla Floodplains, in the Lower Murray region of southern Australia, to provide detailed baseline data on the spatial distribution of near-surface salt stores and materials in the floodplain and their relationship with in-river salinity. Degradation across the floodplain and wetlands has resulted primarily from a significant reduction in flood events, and overgrazing. Restoration of the floodplain will involve the reduction of salinity flow from groundwater into the river and increasing H87environmental flows across the floodplain. Conductivity models predicted from HEM data help identify local recharge and discharge areas,<br>and links with river salinity. The baseline data provided by the airborne data are used with high resolution ground EM surveys including EM31 and time-domain EM, over targeted areas. Ground methods can be repeated, to monitor affects of artificial flooding designed to restore vegetation health. Similarly the combination of airborne and ground data, allows piezometers to be effectively targeted with the resulting information interpreted within the context of the baseline conductivity structure defined form airborne data.
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