RAPID DETERMINATION OF SALT-LOADS ALONG THE RIVER MURRAY USING AN AIRBORNE ELECTROMAGNETIC SYSTEM
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Inverted RESOLVE FDEM data for the Bookpurnong stretch of the River Murray, were compared with the Insteam NanoTEM data and available river-bed core data. The AEM data were not collected along the river itself, but extracted from gridded profile data flown in a WNW-ESE direction. The HEM data show very similar trends in conductivity variation identified in the corresponding NanoTEM data as shown in Figures 1 and 2. There are some minor differences between the two images shown in Figure 1, but this is attributed to the NanoTEM data representing the conductivity of riverbed sediments, whereas the RESOLVE data represents the 1.5-3 metre depth from the waterlevel surface of the river. The inverted conductivity depth sections shown in Figure 2 provide a better means of comparing the two techniques. For this reach of the river the RESOLVE HEM and NanoTEM data effectively map gaining and losing stream conditions and provide significant insight into the interplay between an irrigation induced groundwater mound, the regional groundwater system and river salinity. Regolith 2006 - Consolidation and Dispersion of IdeasKeywords:
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Bathymetry and Seafloor Mapping Via One Dimensional Inversion lAnd Conductivity Depth Imaging of AEM
This study examines the application of airborne electromagnetic (AEM) methodologies to bathymetry in shallow seawater and to map seafloor conductivity. Conductivity versus depth sections have been generated from a recent helicopter-borne DIGHEMV survey (operating vertical coaxial and horizontal coplanar transmitter-receiver coil geometries) of lower Port Jackson, Sydney Harbour. The sea depth ranges from about 1 to 30 m. Acoustic bathymetric soundings and marine seismic survey data provide the true seawater layer thickness and estimates of depth to bedrock respectively over most of the EM survey region. This complementary data can be used to evaluate the accuracy of airborne electromagnetic bathymetry. The efficacy of 1D conductivity inversion and rapid conductivity-depth imaging was investigated for shallow seawater overlaying marine sand sediments and sandstone. The inversion constructs layered conductivities which satisfy the AEM data to an accuracy consistent with the observational uncertainties. Inverted frequencies ranged from 328 to 55300 Hz. Resolution of the sea depth gave good agreement with known bathymetry (within about 10% or better) when inversion was unconstrained. Approximate conductivity-depth images obtained using program “EM Flow” gave similar agreement. Both inversion methods clearly identify the location and burial depth of higher resistivity regions associated with shallow marine sandstone bedrock. In addition to measuring water depths to about 30 m, this study has shown that the AEM DIGHEM technique provides a capability for remote sensing of seabed properties and offers the potential to detect areas of shallow bedrock and differentiate between consolidated and unconsolidated sediment in areas of seawater deeper than 25 m.
Seafloor Spreading
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The flooding extent area in a river valley is related to river gauge observations such as discharge and water elevations. The higher the water elevations, or discharge, the larger the flooding area. Flooding extent maps are often derived from synthetic aperture radar (SAR) images using thresholding methods. The thresholding methods vary in complexity and number of required parameters. We proposed a simple thresholding method that takes advantage of the correlation between the river gauge and the flooding area. To show the applicability of the method, we used a 2014-2018 time series of 161 Sentinel 1 SAR images acquired over a wetland floodplain located in Northeast Poland. We validated the method by extracting local water line elevations from a high-resolution digital elevation model for three river gauges, which resulted in a root-mean-square error of 0.16 m, a bias of 0.07 m, and a correlation of 0.86 for the best scenario. The scenario analysis showed that the most important factor affecting the method's accuracy was a proper delineation of the zone in which the flooding extent area was calculated. This was because other water sources, uncorrelated with river flow, were present in the floodplain as open water. Additionally, higher accuracy was obtained for the VV than VH polarization. The discharge can be used instead of water elevations as a river gauge variable, but this results in more bias in the water extent estimates.
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Small coil separation (hand-held) electromagnetic (EM) mapping systems are frequently used in environmental and hydrological surveys. Multi-frequency capability allows shallow earth resistivity soundings to be conducted. Apparent depth of the response is approximately square root of the skin-depth value. In frequency domain true measurement accuracy and system stability factors are important because earth responses can be small in magnitude. Measurement height needs to be taken into account in absolute values and in data inversion. Case study concerns with mapping of shallow overburden variations for light (diesel) oil spill flow routes in eastern Finland. Oil contamination occurred years ago and current interest was to map potential flow routes along bedrock surface or in bedrock depressions where flow along the groundwater layer surface can happen. Survey was taken with electromagnetic frequency domain instrument. Data was interpreted with converted apparent resistivity maps, 2-layer 1-D model inversion, calculating electrical conductance maps and producing bedrock surface elevation map. Results indicate that EM surveying is useful method in detailed hydrological studies. Results were used with success to place new investigation holes and install groundwater standpipes into potential flow routes.
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Abstract Airborne electromagnetic (AEM) data can be inverted to recover models of the electrical resistivity of the subsurface; these, in turn, can be transformed to obtain models of sediment type. AEM data were acquired in Butte and Glenn Counties, California, USA to improve the understanding of the aquifer system. Around 800 line‐kilometers of high‐quality data were acquired, imaging to a depth of ∼300 m. We developed a workflow designed to obtain, from the AEM data, information about the large‐scale structure and heterogeneity of the aquifer system to better understand the vertical connectivity. Using six different inversions incorporating various forms of available information and posterior sampling of the recovered resistivity models, we produced 6,006 resistivity models. These models were transformed to models of sediment type and estimates of percentage of sand/gravel. Exploring the model space, containing the resistivity models and the derived models, allowed us to delineate the large‐scale structure of the aquifer system in a way that captures and communicates the uncertainty in the identified sediment type. The uncertainty increased, as expected, with depth, but also served to indicate, as areas of high uncertainty in sediment type, the location of both large‐scale and small‐scale interfaces between sediment types. A plan view map of the integrated percentage of sand/gravel, when compared to existing hydrographs, revealed the extent of lateral changes in vertical connectivity within the aquifer system throughout the study area.
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Airborne EM methods have considerable potential for providing detailed spatial information on the distribution of salinity in soils and groundwater, across the floodplains of the Lower Murray River in southern Australia. This potential is examined along with the relative merits of high resolution airborne electromagnetic technologies, specifically the RESOLVE frequency domain helicopter EM (FDHEM) and the SkyTEM time domain helicopter EM (TDHEM) systems. For a coincident area, these two data sets were inverted for conductivity and depth using a smooth model Occams inversion, a conductivity depth transform (CDI) obtained using EMFlow and a Laterally Constrained Inversion (LCI) technique for defining variations in near surface conductivity and sediment salt load . Results from the two systems are comparable, both indicating the presence of an extensive flushed zone adjacent to the Murray River, and identifying finer scale variations between losing and gaining groundwater on the floodplains adjacent to the Murray River. Both systems effectively map the near surface conductive Blanchetown Clay in the adjacent highlands and the high saline groundwater system at depth
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We report on results from a coordinated geophysical and drilling program on highly conductive floodplains near Mildura, Victoria. High resolution ground TEM (the Zonge NanoTEM system, configured as a towed rig) and low frequency GPR (the Mala Pro Ex system, combined with a towed 25 MHz antenna) surveys were run over three lines to evaluate the effectiveness of near surface geophysical methods for resolving important floodplain characteristics, including the depth to water table, location of perched water lenses, and extent and location of flushed zones near waterways. We were specifically concerned with whether the results from the two techniques could be used together to better inform the hydrogeology of these environments. The TEM data were processed using standard techniques, i.e. depth sections were prepared based on smooth-model inversion. The GPR data were initially processed using standard techniques to produce wiggle traces. Due to the conductive nature of this environment and the use of a relatively low frequency GPR system, the results from the standard processing were not satisfactory. These data were then reprocessed using conductivity information from the TEM section to improve velocity estimates at each GPR sounding. Results show improved resolution of water table elevation and delineation of river flush zones, while also providing information about lithological variations in the unsaturated zone. In summary, improving velocity estimation by incorporating information about the conductivity structure of the survey area has improved the interpretation of GPR data collected in this study, allowing us to interpret the TEM and GPR data sets together.
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SummaryThe distribution of groundwater salinity is a key input for management of water resources. Estimates of the three dimensional distribution of groundwater salinity below areas spanning thousands of square kilometres may be required. Airborne transient electromagnetic methods provide the possibility of recovering first pass large scale solute concentration distributions provided lithological influences on electrical conductivity distribution are not dominant. The Allanooka airborne TEM survey is located in the northern most portion of the Perth Basin in Western Australia. We use data from this Airborne TEM survey combined with data recovered from monitoring wells to highlight the steps used to construct a first pass large scale solute distribution model. We provide a method for converting airborne TEM datasets to estimates of solute concentration distribution for sandstone dominated sediments at a basin scale. For the Allanooka monitoring well network, base line empirical relationships are developed between laboratory derived total dissolved solids and formation conductivity derived from wire line logs. This relationship is then extended to include airborne TEM derived formation conductivities. Appropriate layer discretisation of input seed models for inversion of the airborne TEM data set are based on analysis of resistivities derived from wire-line logs. The interpretation of the inverted airborne TEM was assisted by geological constraints and high resolution seismic reflection transects. Selected inversion statistics were also mapped throughout the 3D volume to provide a quick method for assessing the “importance” of particular layers to the outcome of the inversion.An approximate volume of low solute concentration sandstone dominated formation below the regional water table was extracted from the airborne TEM data. Our first pass basin-scale Airborne TEM derived 3D solute concentration provides a starting point for more detailed interpretation to commence.
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The focus of this thesis lies on the joint application of ground-based and airborne electromagnetic methods for the investigation of a glacial valley. For the first time two different airborne electromagnetic (AEM) surveying methods were employed to determine the resistivity structure of a single geological target: the frequency-domain helicopter-borne electromagnetic (HEM) system operated by the Federal Institute for Geosciences and Natural Resources (BGR), Germany, and the time-domain SkyTEM system developed at the University of Aarhus, Denmark. For verification of the airborne results, ground-based transient electromagnetics (TEM) and 2D resistivity surveying were also performed. The target survey area was the Cuxhaven valley in northern Germany, a significant local groundwater reservoir. The course of this buried valley was revealed by drillings, and the shape determined by reflection seismics along several transects across the valley. Electrical and electromagnetic methods were applied to investigate the structure of the valley fill, consisting of gravel, sand, silt and clay. Here, the extension and the thickness of clay layers are of particular interest. They have a low hydraulic permeability and often serve as protection for underlying aquifers against pollution from the surface. The standard tools for presenting AEM data are apparent-resistivity maps and resistivity-depth sections. Although the large and dense data sets are favorable for 3D interpretation, it is still not common to perform 3D inversion in AEM, as the effort in terms of computing time is too high. Therefore, 1D inversion models are still used to display 3D resistivity distributions by stitching together the 1D layered inversion models. Besides the 3D inversion the combination of different data sets in one inversion scheme is an ongoing research issue. One approach is the classical joint inversion, which results in one resistivity model at the shared survey sites, whereas each site is regarded as individual. In this thesis I follow a different approach: spatially constrained inversion (SCI). SCI is a technique where different data sets are combined in one inversion scheme and spatial constraints are applied to the resistivity structure revealed at adjacent survey sites. Thus, the method is particularly useful for large data sets as obtained in AEM. Using spatial constraints, information can be propagated horizontally to adjacent models. With this technique it is then possible to resolve layers which are locally poorly resolved. SCI was originally developed at University of Aarhus for SkyTEM data. In this thesis I adapt the technique for the use on HEM data and apply it to both, SkyTEM and HEM data of the Cuxhaven valley using a priori information from geology, drilling, and seismics. Systematic studies of the SCI parameters show that a) the inversion results applying SCI are less dependent on the starting model in comparison to single-site inversion, b) HEM data resolve the base of a conductive layer which can be identified as the Lauenburg clay, and c) SkyTEM data reveal the base of the Cuxhaven valley which is also confirmed by a seismic section and high-moment TEM measurements. The influence of the valley geometry on the 1D inversion results were systematically studied by 3D forward modeling of different slope geometries. 1D inversions of the synthetic data across the slope were performed with and without constraints between neighboring sites. The resulting 1D models are affected by the slope and simulate a valley base at shallower depth than in the 3D model. Using constraints again decreases the dependency on the starting model. This thesis demonstrates that 1.) by the joint application of ground-based and airborne electromagnetic methods, 2.) by the application of the SCI including a priori information and 3.) by explaining 2D effects of valley slopes using a 3D forward code, a better understanding of the structure of the Cuxhaven valley is obtained.
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Silt
Ground-Penetrating Radar
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A RESOLVE frequency domain helicopter electromagnetic (HEM) survey has been flown in and around the Riverland irrigation districts of South Australia. The purpose was to map the distribution and thickness of near-surface clay-rich sediments which impact on irrigation, groundwater and salinity management strategies.The survey data were re-calibrated after their conventional processing by utilising independent ground geoelectric data.Data were inverted using a five-layer 1-D parameterisation of the Earth. Reduction of the ambiguity in the unknown aspects of the geological section was sought by constraining the inversion with as much local geological and hydrological information as was available. Groundwater depth information was incorporated as an extra datum to constrain the upper layer thicknesses. A combination of drillhole lithologic, groundwater and downhole conductivity data were used to construct a spatially variable reference model and impose constraints on the estimated parameters in the form of prior probability information.The resulting detailed map of the thickness of the Blanchetown Clay is more detailed than previous compilations based on drilling. The results also provide insight into other important hydrogeological features of the Riverland area.
Lithology
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