ASEG Extended Abstracts - 24th International Geophysical Conference and Exhibition – Geophysics and Geology Together for Discovery, 2015

An airborne electromagnetic (AEM) survey was carried out over the Dampier Peninsula, North of Broome, WA during September-October, 2012. The key objectives of this geophysical survey funded by the Department of Water was (i) to obtain a better understanding of the nature of the contact between the base of the Broome Sandstone and the underlying siltstone; (ii) to identify areas of water retentive clay layers in the near surface, (iii) to create a map of the water table; (iv) to study the detailed geometry of the near shore saline intrusion; and thus (v) assist the conceptualisation of the hydrogeology and determine the quantity and quality of available groundwater resources for the benefit of local communities, government and industry. The survey was conducted using SkyTEM, a helicopter-borne time domain AEM system.

The processed AEM data for each of the survey lines were examined and inverted using the industry standard inversion techniques. The results were then compared with available bore-hole geophysical logging as well as the regional geophysical, geological and hydrogeological data. Apart from successfully mapping the depth to water table for the whole project area, this survey has clearly delineated the thickness of Broome Sandstone, shallow impermeable layers within the Broome Sandstone and areas of possible saline sea water intrusions. The survey has also successfully identified a WNW-ESE trending lineament (a basement high) and couple of NW-SE trending structural features (such as fault structures) from the central part of the survey region. The regional geophysical data images obtained from Department of Mines & Petroleum supports this finding.

We describe interpretation of an AEM survey around the La Grange allocation area, WA. This survey was designed to map aquifer bounds and the sea water intrusion, and then to assess groundwater in the region, and to facilitate planning water use.

The simple, stratified nature of sediments of the western onshore Canning Basin allowed us to use blocky layered earth models and we found that five-layer models were the most parsimonious. After deriving surfaces representing the top of the Jarlemai siltstone and the top of the sea water ingress, we were able to effectively characterise the spatial characteristics of the sea water intrusion.

We found that in places, sea water intruded 40 km inland, and could be found at a depth of over 250 m.

A SkyTEM airborne electromagnetic survey was flown in the Cane River area near Onslow, Western Australia in 2011, in order to assist groundwater investigation and borefield development. The survey yielded a range of information relevant to future groundwater investigations.

The detailed geometry of the nearshore saline intrusion has been successfully defined in three dimensions. The intrusion occurs in the unconfined aquifer above the impermeable Muderong Shale.

A broad zone of low conductivity has been mapped within the alluvium and Trealla Limestone, which has been interpreted to indicate the extent of relatively fresh groundwater. The low conductivity zone has greatest extent in the 10 - 20 m depth slice. Between 20 - 50 m depth, the low conductivities are confined to the downstream part of the Cane River. At these depths, the low conductivities extend further to the eastern side of the river than to the west. This result suggest that it may be possible to expand the existing borefield to the east in areas without clay cover, while retaining a reasonable buffer from the nearshore saline intrusion.

A number of shallow granite bedrock highs have been identified in the northern and central parts of the survey area, many of which have not been intersected by existing drilling. The upper weathered and/or fractured parts of the granite may have potential as aquifers where they are not overlain by impermeable clays. However the margins of the granite should be avoided where they are in contact with the onlapping Muderong Shale, which is associated with poor water quality.

In former glaciated areas buried tunnel valleys can often be found. These buried erosional structures can be highly decisive for groundwater recharge and groundwater flow. Delineation of the architecture and infill of the structures are therefore very important in relation to groundwater mapping. The dense data coverage offered by airborne electromagnetic methods makes it possible to map and model the buried valley structures with a high degree of detail. The delineation of the individual valleys and the mapping of the internal cross-cutting relationships are dependent on the geological interpretation, which is founded on the knowledge about geological processes and the regional geology.

In this study, we have investigated a relatively small study area in Denmark with SkyTEM and lithological log data as the main data sources. The area is characterised by a network of cross-cutting buried tunnel valleys, which have been incised into impermeable Paleogene clay deposits. The geological interpretation of the SkyTEM data resulted in modelling of 21 buried valleys belonging to at least 7 different generations. Manual voxel modelling of the infill of these valleys, as well as the surroundings, resulted in a geological 3D model consisting of 43 different units. Most of the valleys show heterogeneous infill, characterized by a predominant lithology (for instance meltwater sand) with local occurrences of secondary lithologies (for instance clay till). In the majority of the valleys, meltwater sand is the main lithology, but clay till and meltwater clay deposits are also commonly found. Due to the heterogeneity of the infill, proper modelling of this type of geology requires voxel modelling instead of layer modelling.

We present a derivative analysis method that automatically detects and selects layers in any geophysical borehole trace. Using a wavelet analysis, we delineate relevant boundaries from inflection points. This allows for the automatic, objective detection of layers.

Our software classifies layers based on importance in the geophysical data, and allows a user to select blocked layers based on total number of layers detected, a portion of the total layers, minimum layer thickness or the number of layers detected using a minimum operator width.

We demonstrate the effectiveness of the layer blocking technique with some field examples in Western Australia and New South Wales for aquifer detection and soil classification.

Electrical conductivity, or its inverse, the resistivity, is an important geophysical property within groundwater mapping. It is known to correlate empirically to lithology, primarily through clay minerals and pore water ions. Although, in Denmark, geoelectric and electromagnetic surveys have been carried out for decades, no systematic, nationwide study on the relationship between resistivity and lithology has been carried out.

We present a procedure for generating a resistivity atlas based on resistivity measurements, which can be related directly to specific and well described soil samples. Data are obtained from archives, literature and the Danish national databases. The procedure implies a restricted use of wireline logging data in combination with direct measurements on samples, resulting in resistivity distributions for specific lithologies or geological formations. The use of documented high-quality data ensures reliable results, reflecting actual resistivity of a specific lithology.

This procedure is illustrated on clay till. The resistivity variations obtained for this lithology seems to be related to real compositional variations, which reflect the process of forming the clay till.

Our procedure is likely to provide equally reliable results for other main lithologies. Future detailed studies, in particular on sediments with low clay content, should consider resistivity differences related to the degree of saturation and variations in the formation water resistivity.

We present an automatic method for parameterization of a 3D model of the subsurface, integrating lithological information from boreholes with resistivity models through an inverse optimization, with the objective of creating a direct input to groundwater models. The parameter of interest is the clay fraction, expressed as the relative length of clay-units in a depth interval. The clay fraction is obtained from lithological logs and the clay fraction from the resistivity is obtained by establishing a simple petrophysical relationship, a translator function, between resistivity and the clay fraction. Through inversion we use the lithological data and the resistivity data to determine the optimum spatially distributed translator function. Applying the translator function we get a 3D clay fraction model, which holds information from the resistivity dataset and the borehole dataset in one variable. Finally, we use k-means clustering to generate a 3D model of the subsurface structures, which we then use as direct input in a groundwater model. We apply the concept to the Norsminde survey in Denmark integrating approximately 700 boreholes and more than 100,000 resistivity models from an airborne survey in the parameterization of the 3D model covering 156 km2. The final five-cluster 3D model is input to a groundwater model and it performs equally well or slightly better than traditional groundwater models from the area.

High-resolution hydrogeophysical data are increasingly acquired as part of investigations to underpin groundwater mapping. However, optimization of AEM data requires careful consideration of AEM system suitability, calibration, validation and inversion methods.

In modern laterally-correlated inversions of AEM data, the usefulness of the resulting inversion models depends critically on an optimal choice of the vertical and horizontal regularization of the inversion. Set the constraints too tight, and the resulting models will become overly smooth and potential resolution is lost. Set the constraints too loose, and spurious model details will appear that have no bearing on the hydrogeology. There are several approaches to an automatic choice of the regularization level in AEM inversion based predominantly on obtaining a certain pre-defined data misfit with the smoothest possible model.

However, we advocate a pragmatic approach to optimizing the constraints by an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding. In this approach, in a process of both confirming and negating established interpretations and underlying assumptions, the inversion results are judged by their ability to support a coherent conceptual model based on all available information. This approach has been essential to the identification and assessment of MAR and groundwater extraction options in the Broken Hill Managed Aquifer Recharge project.

We present a new approach to improve the sensitivity and efficiency of geophysical surface nuclear magnetic resonance (NMR) measurements. An extremely powerful tool in groundwater investigations, surface NMR inherently has a relatively low signal-to-noise ratio (SNR), which sometimes necessitates long survey times for signal averaging. In pursuit of faster survey speeds, we show that replacing the standard on-resonance excitation pulse with an adiabatic, frequency-swept pulse can provide significant increases in the NMR signal amplitude. This increase results from the fact that adiabatic pulses can excite larger volumes of groundwater more efficiently than conventional pulses. Using numerical simulations and full-scale field experiments, we show that adiabatic pulses can provide a factor of ~3 increase in signal, and suggest other advantages for groundwater imaging. The signal increase alone allows for data of equivalent SNR to be acquired in a fraction of the time required for conventional on-resonance pulses. Ultimately these improvements can allow surface NMR to be exploited in an expanding range of applications.

There is increasing use, throughout the world, of groundwater as the primary source of freshwater. The evaluation and management of this resource requires information about the extent and connectivity of groundwater aquifers, the contained volume of producible water, the changes in stored water, and the processes that can impact the quantity and/or quality of the water. Such information is required at a density of spatial and temporal sampling best provided by various forms of geophysical data. For the past decade, we have been working in partnerships with groundwater districts and managers to advance the use of geophysical methods as a central component of groundwater evaluation and management. Examples include the use of surface and logging nuclear magnetic resonance to estimate water content and hydraulic conductivity, electrical resistivity tomography for imaging saltwater intrusion along the California coast, and satellite InSAR data for estimating changing hydraulic head levels in confined aquifers in the San Luis Valley, Colorado. Such examples illustrate the tremendous potential for - and need for - geophysical methods to ensure the long-term health of our groundwater resources.

The Broken Hill Managed Aquifer Recharge (BHMAR) project aimed to define key groundwater resources and aquifer storage options in the lower Darling River floodplain of western NSW. The project was multi-disciplinary and utilised airborne electromagnetics (AEM), borehole nuclear magnetic resonance (NMR) and LiDAR DEM data and lithological, hydrostratigraphic and hydrochemical information to develop a suite of hydrogeological and groundwater property maps and products.

This abstract discusses the methods and results of estimating the transmissivity of the semi-confined target aquifer. Hydrostratigraphy and hydraulic texture classes were mapped by interpreting the AEM data in conjunction with borehole geophysics and lithological information. Aquifer transmissivity was statistically derived by combining borehole NMR hydraulic conductivity estimates with the mapped 3D distribution of texture classes and hydrostratigraphic units. Using a statistical and GIS approach, the derived aquifer thicknesses in the key areas ranged from 20 - 40 m and the lower and upper transmissivity bounds ranged from 1 to 10 m²/d, and 10 m²/d to 1000 m²/d, respectively.

Regional scale fault structures are considered a first order control on hydrothermal ore systems. Recognition and delineation of such features is essential for search space reduction and project selection in exploration. The New England Orogen in northeastern New South Wales has significant potential for the discovery of new hydrothermal ore systems. However, limits to interpretation of broad scale geophysics in the region and limited exposure for ground-based mapping have hampered the recognition of the first order fault architecture in many areas.

As part of the Geological Survey of New South Wales 3D mapping program, we aim to further the understanding of strike extensive and depth penetrative regional scale fault architecture in the southern New England Orogen. The workflow for constraining the regional 3D fault architecture involves integrating a limited number of deep seismic lines with broader gravity and magnetic wavelet-based multiscale edges. All the geophysical data sets are further constrained by the Geological Survey of New South Wales’ seamless geology mapping and surface structural orientation data. The work to date demonstrates correlation between the lateral position of multiscale edges and their dip inferred from upward continuation, with steeper dipping structures interpreted in seismic lines. Strike orientations of edges, or systematic breaks in edges, are broadly consistent with structural orientations previously recognised in mapping, but often not at the true regional scale as suggested by edge continuity. Known hydrothermal ore systems in the southern New England Orogen display a strong correlation with the deeply penetrating edges.

The Far South project is located five kilometres along strike from the Deep South mine, where gold mineralisation is commonly associated with semi-massive pyrrhotite and pyrite. Data from a Sub-Audio Magnetic (SAM) survey set up in galvanic configuration were acquired over the project principally to map stratigraphy and structure using the on-time Magnetometric Conductivity (MMC) and Total Magnetic Intensity (TMI) responses. The off-time Galvanic Source EM (GSEM) data were subsequently extracted from the raw data and examined. Four late time anomalous responses were identified. Two of these responses are strong late-time (>45ms) anomalies up to 350m in strike length, and the remaining two are weaker mid-time, more subtle and less diagnostic responses. Follow-up Moving Loop Transient Electromagnetics (MLEM) and Fixed Loop Transient Electromagnetics (FLEM) surveys confirmed well defined conductive responses over all four follow-up areas. Modelling of the GSEM data over the two strongest anomalies is in good agreement with modelling of the MLEM/ FLEM data, confirming the ability to identify and model conductive targets from SAM GSEM data. The two weaker GSEM responses could not be reliably modelled and use of the MLEM/ FLEM data was necessary to produce robust models. The identified conductors were all interpreted as having good exploration potential, and a subsequent drill program intersected the source of all four as sulphide zones of varying widths and types.

The Eureka massive sulphide lens is the first new discovery of VMS mineralisation at the Stockman Project since 1979. The discovery was made in early 2013 through the integration of geophysical techniques, particularly down-hole electromagnetics, with a robust geological interpretation.

The lens is located approximately 350m northeast of the Currawong deposit, immediately along strike and beneath the Bigfoot lens at a depth of 360m. Though surface EM methods played key roles in the discovery of the main deposits at Currawong and Wilga, airborne and fixed-loop EM surveys failed to detect the Eureka lens due to its moderate conductance and increased depth. Interpretation of subtle DHTEM responses in two exploration drill holes was a key component of the discovery. Additional geological input, including short wavelength infrared modelling and structural reinterpretation, presented a compelling drill target, which led to the discovery of the Eureka massive sulphide lens.

Efficiently extracting the maximum amount of information from gravity gradient data is challenging. Interpretation often takes place in either the data domain or model domain. Here, we present a workflow that utilizes two interpretation techniques that can result in better characterization of the subsurface. Using a method that estimates depth to source, we obtain a depth volume of estimated source locations. The depth volume is then used to constrain inversion of gravity gradient data in the form of a reference model and 3D model weighting. We demonstrate that this combined approach improves the ability to recover sources at depth.

The Romero gold copper zinc silver deposits are located in the Province of San Juan, Dominican Republic, approximately 165 km west-northwest of Santo Domingo. Romero and Romero South orebodies contain stratabound gold mineralization with copper, silver and zinc of intermediate sulphidation epithermal style. The gold mineralization is associated with disseminated to semi-massive sulphides, sulphide veinlets and quartz-sulphides within quartz-pyrite, quartz-illite-pyrite and illite-chlorite-pyrite alteration.

Ground DC resistivity and induced polarization (DCIP) supported by ground magnetics remain the main targeting tools for drill follow-up along with geologic mapping and geochemistry. However ZTEM passive airborne electromagnetics have recently also been applied with success for reconnaissance mapping of deep alteration and increased porosity regionally.

Our case-study compares ground DCIP and airborne EM-magnetic geophysical responses, supported by 3D inversions, over the known Romero and Romero South Au-Cu-Zn-Ag intermediate sulphidation deposit area.

The Cannington deposit is a high grade Ag-Zn-Pb deposit found in 1990 by BHP Minerals drilling an isolated 1,000 nT aeromagnetic feature. Following the discovery of Cannington, numerous airborne, ground and borehole surveys have been carried out which overall, provided some assistance at better defining the ore system but did not lead to the discovery of new major deposit in the area.

While Cannington possessed a clear magnetic response, the presence of a thick conductive cover made the use of EM and electrical techniques challenging. BHP used Cannington as a test ground for a variety of new techniques including a ground SQUID EM sensor, modified airborne EM technology (higher power and lower base frequency) and over 10 years after discovery, the first ever Falcon airborne gravity gradiometer survey in Australia.

The Bunbury Basalt is a series of lava flows in the Perth Basin, Western Australia, and is interpreted to be related to the breakup of eastern Gondwana during the Early Cretaceous. We integrate new aeromagnetic images with available well intersections and outcrop constraints to establish the first 3D model of the Bunbury Basalt. The model reveals that the flows can be up to 100 m thick and are predominantly confined to two paleochannels and their tributaries situated in the Bunbury Trough in the southern Perth Basin. The Donnybrook paleochannel flows proximal to the Darling Fault and is cut by the Bunbury paleochannel, which is positioned centrally in the Bunbury Trough and contains two flow episodes. The model illustrates a dominantly north-south axial paleo-drainage pattern that existed in the southern Perth Basin prior to Gondwana breakup. The 3D model provides a present-day volume of 90 km³ with an aerial extent of 3300 km². Offsets of the Bunbury Basalt have been used to identify new northeast and northwest trending faults in the southern Perth Basin, and broad folding is interpreted to a consequence of drag into the Darling and Busselton Faults. The source vents for the Bunbury Basalt were probably located at extensional jogs at intersections between the Darling and Busselton Faults with subordinate oblique faults.

It is now possible to automate the extraction of magnetic depths over large areas as depth profiles. A depth profile is a graph of the probability of a layer at each depth. Presented in the form of a transect, depth profiles allow layers to be traced across significant distances. The appearance of discontinuous layers, and multiple layers, raises questions for interpretation, here addressed with modelling.

Modelling of layers requires simulating the heterogeneity of the material. Accordingly, a method of modelling is demonstrated where flat prisms are populated with very large numbers of dipoles and their fields accumulated for spectral analysis.

Thick layers give a depth signal in the transects about 20 m below their top surface. The distinction is minor given that the layer is assumed to extend across a 20 km square.

In general, only one depth signal credibly represents the depth of its source. Multiple layers can be picked out on traverses when the deeper layer is sufficiently more magnetised than the layer above it. A weaker depth signal appears closer to a stronger signal. Signals within 100 m of each other tend to merge.

The sensitivity of the method is significantly better when the survey has been flown north-south rather than east-west.

Near-surface layers are not picked up by the method. The sampling frequency of the original survey dictates how close to the surface estimates can be provided. A rough rule of thumb is that no reliable depth estimates can be expected for sources shallower than half the flight line spacing.