Airborne Electromagnetic Research Articles

Calculation of One-dimensional Forward Modelling of Helicopter- borne Electromagnetic Data and a Sensitivity Matrix Using Fast Hankel Transforms

The helicopter-borne electromagnetic (HEM) frequency-domain exploration method is an airborne electromagnetic (AEM) technique that is widely used for vast and rough areas for resistivity imaging. The vast amount of digitized data flowing from the HEM method requires an efficient and accurate inversion algorithm. Generally, the inverse modelling of HEM data in the first step requires a precise and efficient technique provided by a forward modelling algorithm. The exact calculation of the sensitivity matrix or Jacobian is also of the utmost importance. As such, the main objective of this study is to design an efficient algorithm for the forward modelling of HEM frequency-domain data for the configuration of horizontal coplanar (HCP) coils using fast Hankel transforms (FHTs). An attempt is also made to use an analytical approach to derive the required equations for the Jacobian matrix. To achieve these goals, an elaborated algorithm for the simultaneous calculation of the forward computation and sensitivity matrix is provided. Finally, using two synthetic models, the accuracy of the calculations of the proposed algorithm is verified. A comparison indicates that the obtained results of forward modelling are highly consistent with those reported in Simon et al. (2009) for a four-layer model. Furthermore, the comparison of the results for the sensitivity matrix for a two-layer model with those obtained from software is being used by the BGR Centre in Germany, showing that the proposed algorithm enjoys a high degree of accuracy in calculating this matrix.

Application of high resolution airborne geophysical data in geological modelling of Mohar Cauldron Complex, Bundelkhand Massif, central India: implications for uranium exploration

The Mohar cauldron, located near Mohar village, Shivpuri District, Madhya Pradesh, India, represents an explosive felsic volcanic event. The Mohar Cauldron Complex (MCC) is an important target area for uranium exploration as collapse breccias associated with extensional tectonics are traditionally important for multi-metal deposits, including uranium. Advanced processing and interpretation of the high resolution airborne electromagnetic (AEM), magnetic and radiometric data acquired over the Mohar cauldron and the surrounding environs by Fugro Airborne surveys, successfully mapped the major geological domains in the area based on their distinct geophysical characteristics. Interpretation of the data indicated the presence of three felsic intrusive bodies, only one of which, the MCC, reached the surface and collapsed. Variation in the geophysical characteristics of the three bodies is attributed to variations in hydrothermal alteration. Magnetic signature and radiometric response of the MCC and surrounding area also show signs of intense alteration. AEM data has allowed the boundary of the sediments within the MCC to be mapped accurately, along with the surrounding brecciated zone. Conductivity depth imaging calculated to a depth of 500 m clearly indicated the geometry and disposition of different layers of MCC. 3D voxel modelling of the MCC also allowed for the identification of the different lithologies that constitute the cauldron structure. 3D conductivity isosurfaces provided a thorough understanding of the subsurface distribution of conductivities.

Applying regional airborne electromagnetic (AEM) surveying to understand the architecture of sandstone-hosted uranium mineral systems in the Callabonna Sub-basin, Lake Frome region, South Australia

The Frome airborne electromagnetic (AEM) survey was designed to provide reliable pre-competitive AEM data to aid the search for energy and mineral resources around the Lake Frome region of South Australia. Flown in 2010, a total of 32,317 line kilometres of high-quality airborne geophysical data was collected over an area of 95,450 km2 at a flight line spacing mostly of 2.5 km, opening to 5 km spaced lines in the Marree–Strzelecki Desert area to the north. The Lake Frome region hosts a large number of sandstone-hosted uranium deposits with known resources of ∼60 000 tonnes of U3O8 including the working In Situ Recovery operations at Beverley, Pepegoona, Pannikin and Honeymoon, and deposits at Four Mile East, Four Mile West, Yagdlin, Goulds Dam, Oban, East Kalkaroo, Yarramba and Junction Dam. The aims of the Frome AEM Survey were to map and interpret critical elements of sandstone-hosted uranium mineral systems including basin architecture, paleovalley morphology, sedimentary facies changes, hydrological connections between uranium sources and uranium deposition sites and structures. Interpretations of the data show the utility of regional AEM surveying for mapping crucial elements of sandstone-hosted uranium mineral systems as well as for mapping geological surfaces, structures and depth of cover over a wide area. Data from the Frome AEM Survey allow mineral explorers to put their own high-resolution AEM surveys into a regional context. Survey data were used to map and interpret a range of geological features that are associated with, or control the location of, sandstone-hosted uranium mineral systems and have been used to assess the uranium prospectivity of new areas to the north of the Flinders Ranges.

The Impact on Geological and Hydrogeological Mapping Results of Moving from Ground to Airborne TEM

In the past three decades, airborne electromagnetic (AEM) systems have been used for many groundwater exploration purposes. This contribution of airborne geophysics for both groundwater resource mapping and water quality evaluations and management has increased dramatically over the past ten years, proving how these systems are appropriate for large-scale and efficient groundwater surveying. One of the major reasons for its popularity is the time and cost efficiency in producing spatially extensive datasets that can be applied to multiple purposes. In this paper, we carry out a simple, yet rigorous, simulation showing the impact of an AEM dataset towards hydrogeological mapping, comparing it to having only a ground-based transient electromagnetic (TEM) dataset (even if large and dense), and to having only boreholes. We start from an AEM survey and then simulate two different ground TEM datasets: a high resolution survey and a reconnaissance survey. The electrical resistivity model, which is the final geophysical product after data processing and inversion, changes with different levels of data density. We then extend the study to describe the impact on the geological and hydrogeological output models, which can be derived from these different geophysical results, and the potential consequences for groundwater management. Different data density results in significant differences not only in the spatial resolution of the output resistivity model, but also in the model uncertainty, the accuracy of geological interpretations and, in turn, the appropriateness of groundwater management decisions. The AEM dataset provides high resolution results and well-connected geological interpretations, which result in a more detailed and confident description of all of the existing geological structures. In contrast, a low density dataset from a ground-based TEM survey yields low resolution resistivity models, and an uncertain description of the geological setting.

Three-dimensional resistivity modelling of grounded electrical-source airborne transient electromagnetic (GREATEM) survey data from the Nojima Fault, Awaji Island, south-east Japan

An airborne electromagnetic (AEM) survey using the grounded electrical-source airborne transient electromagnetic (GREATEM) system was conducted over the Nojima Fault on Awaji Island, south-east Japan, to assess GREATEM survey applicability for studying coastal areas with complex topographic features. To obtain high-quality data with an optimised signal-to-noise ratio, a series of data processing techniques was used to acquire the final transient response curves from the field survey data.The 1D inversion results were feasible in that the horizontal resistivity contrast was not much higher than the true contrast, but they were not reasonable in that the horizontal resistivity values were greatly changed. To circumvent this problem, we performed numerical forward modelling using a finite-difference staggered-grid method (Fomenko and Mogi, 2002) adding a finite-length electrical dipole source routine to generate a three-dimensional (3D) resistivity structure model from GREATEM survey data of the Nojima Fault area. The 3D model was based on an initial model consisting of two adjacent onshore and offshore layers of different conductivity such that, a highly conductive sea of depth (10–40 m) is placed on top of a uniform half-space, assuming the presence of topographic features on the inland side. We examined the fit of the magnetic transient responses between field data and 3D forward-model computed data, the latter were convolved with the measured system response of the corresponding dataset. The inverted 3D resistivity structures showed that the GREATEM system has the capability to map underground resistivity structures as deep as 500 m onshore and offshore. The GREATEM survey delineated how seawater intrudes on the landside of the fault and indicated that the fault is a barrier to seawater invasion.

Introduction to the Time Domain Electromagnetic Special Issue of JEEG

The idea to propose a special issue on time domain electromagnetic (TEM) methods was based on my perception that recent innovations and developments, both in instrumentation and processing, deemed it time to perform a checkup on the state-of-the-art of the method. Many of these advancements address offshore and airborne applications rather than ground-based ones. For example, the popular use of the marine controlled source electromagnetic (MCSEM) method in the oil industry, undoubtedly a real “renaissance” for EM, marked a surge in the development of new devices and approaches, together with the continuous improvement in technology and processing of airborne EM data. As shown in this issue, the application of ground-based TEM has also grown, with advances in instrumentation, data processing, and its application. The papers by Xue et al. and Li …

Relevance of AEM and TEM to Detect the Groundwater Aquifer at Faiyum Oasis Area, Faiyum, Egypt

This study investigates the groundwater aquifer located in Fayuim oasis. In this study, two of the electromagnetic measurement methods have been used in determining the hydrological situation in the Fayoum oasis. The first is airborne electromagnetic (AEM) which, sometimes is referred to as Helicopter electromagnetic (HEM) and the second is ground Time-domain Electromagnetic method (TEM). The subsurface consists of four geoelectrical layers with a rough slope towards the center. The third and the fourth layers in the succession are suggested to be the two-groundwater aquifers. The third layer saturates with fresh water overlying saline water which exists in the bottom of the second one. It is worth mentioning that the depth of the fresh water surface undulates between the surface level in two lakes in the study area and 57 meters below the ground, whereas the thickness of the fresh water aquifer varies from 13 to 36 meters. The depth of the saline water surface undulates between 59 and 81 meters below the ground. In general, airborne electromagnetic surveying has the advantage of fast resistivity mapping with high lateral resolution. Groundbased geophysical surveys are often more accurate, but they are definitely slower than airborne surveys. It depends on targets of interest, time, budget, and manpower available by the method or the combination of methods that will be chosen. A combination of different methods is useful to obtain a detailed understanding of the subsurface resistivity distribution.

Magnetotellurics and Airborne Electromagnetics - a combined method for assessing basin structure and exploring for unconformity-related uranium

Unconformity-type uranium deposits are characterised by high-grade and constitute over a third of the world's uranium resources. The Cariewerloo Basin, South Australia, is a region of high prospectivity for unconformity-related uranium as it contains many similarities to an Athabasca-style unconformity deposit. These include features such as Mesoproterozoic red-bed sediments, Paleoproterozoic reduced crystalline basement enriched in uranium (~15-20 ppm) and reactivated basement faults. An airborne electromagnetic (AEM) survey was flown in 2010 using the Fugro TEMPEST system to delineate the unconformity surface at the base of the Pandurra Formation. However highly conductive regolith attenuated the signal in the northern and eastern regions at a depth of 300 m or less, requiring the application of a deeper geophysical imaging methods. In 2012 a magnetotelluric (MT) survey was conducted along a 110 km transect of the north-south trending AEM line. Braodband (300 Hz - 300 s) MT data were collected at 29 stations and successfully imaged the depth to basement, furthermore providing evidence for deeper fluid pathways. The AEM data were integrated into the regularisation mesh as a-priori information generating an AEM constrained resistivty model, and also correcting for static shift. It was found that the combined techniques provided better structural information over depths of 0 - 5 km than could be obtained by inversion of each method alone.

A review of AEM in South Australia

Subsurface resistivity is a key component of many mineralization models including unconformity-related uranium, palaeochannel hosted uranium and nickel sulphide exploration. It is also a key component in understanding environmental aspects of the subsurface including groundwater detection, and civil engineering applications including detection of buried pipes and cables. Measuring subsurface resistivity is the aim of electromagnetic (EM) geophysical techniques. It involves transmitting an electromagnetic field into the Earth, and then recording this field - and the Earth response - on a receiver. The transmitted field signal can be removed from the received field to determine the Earth response. Airborne EM (AEM) is a geophysical technique that allows this process to be undertaken from an airborne (aeroplane or helicopter) platform. AEM exploration first commenced in South Australia with AFMAG (Audio-Frequency Magnetic technique) surveys in the 1960s, and VLF (Very Low Frequency) surveys from 1971. Numerous platforms including RepTEM, GeoTEM, Input, QUESTEM, HoistEM, TEMPEST and VTEM are now routinely used within South Australia. Each technique provides a different view of the subsurface dependant on the system parameters and the processing undertaken on the data. Given the increase in AEM surveying within South Australia and the wide applications available for this data a review of this technique within the State has been undertaken. This poster presents a summary of AEM within South Australia, focussing on a number of significant surveys and their outcomes. Surveys in the Cariewerloo Basin and Fowler Domain in particular have been used to model uranium prospectivity and help define nickel deposits. All data reviewed is now downloadable online via SARIG.

Inversion of SPECTREM AEM data for conductivity and system geometry

We evaluate the use of data from the fixed-wing Spectrem 2000 airborne electromagnetic (AEM) survey flown for ore body detection, regolith mapping and assessment of aquifers. Since the position and orientation of the receiver bird are not measured in the Spectrem system, the primary field at the bird cannot be known and removed precisely. In typical Spectrem data processing it is estimated and removed by subtracting out the last time window, relying on the assumption that the entire secondary field has decayed to zero by late-time. This is not generally a good assumption under the conductive regolith conditions and prevents the simultaneous fitting of both X- and Z-component data to within the noise envelope. In order to successfully invert the AEM data, and produce conductivity's depth maps and sections, we first reinstate the removed primary field and convert the data from ppm units to Teslas. We then inverted the total (primary plus secondary) field data, to solve for a 1D conductivity model as well as the horizontal and vertical offsets and pitch of the receiver bird using Geoscience Australia's sample-by-sample inversion algorithm. High-altitude data and synthetic forward models were also analysed to better understand the system's noise. Spectrem has flown many kilometres in other parts of the world but not that much in Australia. Our results highlight many similarities between Spectrem 2000 data and other systems previously flown over the same areas. Through our further processing and inversions we have resolved conductivity's depth structures very similar to those previously obtained from other well-established AEM systems flown under Australian conditions. We also present sections of AEM with project logged drilling core from both reverse circulation and diamond drill as means of cross validation of our derived inverted models with other data sets; which is always desirable.

Eureka! AEM uncovers more than expected

Eureka! AEM uncovers more than expected Precompetitive AEM data and associated scientific analysis assists exploration under cover by reducing risk; stimulating investment and promoting exploration for commodities. In recent years, Geoscience Australia has flown three regional Airborne Electromagnetic (AEM) surveys covering three percent of Australia. Data and associated interpretations from regional surveys in the Paterson, Pine Creek and Lake Frome regions have led to tenement take up, stimulated exploration for a number of commodities and have given rise to many Eureka moments. This presentation highlights new results from the use of the regional AEM data and interpretations for commodities other than uranium; results that have been announced by industry via the Australian Stock exchange and other publications. A copper exploration company reported high-grade copper intercepts in target areas defined by data from the regional Paterson survey. A gold exploration company identified horizons associated with the gold-rich Cosmo-Howley corridor using the Pine Creek AEM data. The AEM data have also helped to identify Triassic coal measures at Leigh Creek.

Correlating ERT with AEM in a rock slide mapping project, same shape but different quantities

SUMMARY We investigate an active rock slide in Western Norway with ground- and airborne resistivity mapping to ultimately find weakness zones & sliding planes embedded in crystalline bedrock. The study area comprises phyllite, a low grade metamorphic rock type that tends to be reworked to clay in disturbed zones. Mapping these electrically conductive clay zones was the aim of the survey. GPS measurements over the last 5 years indicate that precipitation drives rock slide movements. The role of ground water is thus a crucial factor to investigate for risk assessment in the area. Based on a successful airborne electromagnetic (AEM) demonstration survey, we conducted a total of 1.600 profile meters of ground resistivity (ERT) measurements to confirm AEM anomalies, to gain precise 2D geometries and to link conductivity anomalies with geology. All resistivity results confirm AEM anomalies and refine their lateral extent. In the East we find consistency between a strong conductor, dipping sub horizontal SW with an outcropping thrust fault, separating phyllite and gneiss. In the West a conductor dipping steeply NNW seems to be fed by surface water and may represent a formerly unknown sliding plane. While ERT and AEM anomaly shapes generally agree within their mutual resolution limitations, the resistivity values significantly deviate. It remains unclear whether anisotropy or strong 3D artefacts cause this disagreement.

3D conductivity models of Lalor Lake VMS deposit from ground loop and airborne EM data sets

Lalor Lake is a VMS deposit in central Manitoba, Canada. The deep ore body is buried under the cover rocks up to 1000 m. Multiple EM data sets were collected to delineate the compact and conductive alteration zones and two data sets are available to us. The first is HELITEM, an airborne time-domain EM survey that covers the entire exploration area. The second is a ground loop EM data measured by SQUID magnetometers that have high precision at late times. The two data sets map the conductivity structures at Lalor Lake in different ways: the airborne survey covers a broad area but has limited resolving power at depth; the ground survey provides information about the deep targets through very late times but the measurements were made in a smaller area. Individual 3D inversions were carried out for both data sets assuming little a prior information. Both are able to recover the trace of the expected ore body, but the airborne model is smooth and the ground model contains highly conductive anomalies. Then we invert the ground data again with the airborne model as the reference model. The new inversion again confirms the existence of the VMS ore body but also rearranges the conductive material according to the constraints from the reference model. The new model differs significantly from the blind inversion model at the deposit scale. Based on the information from the inversion so far, we conclude both surveys have picked up signals from the ore body in different levels of detail. More analysis and further data are still required to better delineate the target’s geometry.

3D Inversion of Large-scale Time Domain Electromagnetic Data

Airborne time-domain electromagnetic (EM) surveys are effective tools for mineral exploration, geologic mapping and environmental applications. These surveys can be an economical way to explore large prospective regions. Traditionally, the data from such surveys have been interpreted using time constant analysis, conductivity to depth imaging (CDI) or possibly 1D inversions. These methods assist in a simple interpretation of the data, however because they do not fully model the physics in 3D, they can fail to accurately represent environments such as real world structures and geological targets. Airborne EM datasets are characterized by large volumes of data, as each EM sounding implies a new transmitter location. As a result, inverting this data in 3D is a computationally difficult problem that until recently has not been possible for the exploration community. In this abstract we demonstrate a methodology for inverting large airborne EM surveys in 3D using multiple meshes, each spanning the full model domain. By using an adaptive meshing procedure during the forward modeling, each mesh is optimally designed for computational efficiency on the local domains defined by a subset of transmitters. This procedure allows the computational cost of each mesh to be dependent on the number of subset transmitters considered, rather than the total survey size. Since the forward modeling operation is the bottleneck for any inversion, this results in a highly parallel algorithm that can handle arbitrarily large datasets. In this abstract we outline our airborne electromagnetic inversion methodology and demonstrate it using synthetic and field data examples.

Correcting for SPM effects in airborne EM

Recent noise reductions in airborne electromagnetic (AEM) systems have allowed detection of conductors at great depths, but systems now have also become sensitive to superparamagnetic (SPM) effects. We distinguish SPM effects in airborne electromagnetic survey data from the response of good conductors. In electromagnetic data processing, off-time data can be accurately represented as amplitudes of a set of basis functions that are comprised of decays that decrease exponentially as a function of time. The SPM impulse response can be approximated by a decay that is proportional to time to the inverse power, a time dependence associated with magnetic viscosity. We identify the presence of SPM effects, as distinct from the decay of good conductors, by using inverse powerlaw decays as additional basis functions in constrained least-squares fitting. Application of the method to airborne time-domain electromagnetic (TEM) surveys shows that the method allows correction of SPM and hence aids significantly in conductive target identification.

Monte Carlo Inversion of SkyTEMTM AEM data from Lake Thetis, Western Australia

During 2011 Groundprobe Geophysics flew a SkyTEM airborne electromagnetic (AEM) survey in the vicinity of Lake Thetis near Cervantes in Western Australia. The survey was commissioned by the Western Australian Department of Water as part of the Mid West Groundwater Dependent Ecosystem Vulnerability Project. Groundprobe Geophysics processed and then initially inverted the dataset using the iTEM fast approximate inversion. Subsequently the data were inverted using a reversible jump Markov chain Monte Carlo (rj-McMC) 1D inversion algorithm recently developed at Geoscience Australia. Both the high- and super-low-moment data were inverted simultaneously taking into account the tilts of the transmitter-receiver frame. In the inversion of each dual-moment AEM sounding an ensemble of 300,000 models were generated in a Markov Chain, about 290,000 of which fit the data within the estimated noise envelope. The reversible jump aspect of the algorithm means that the number of layers in the 1D models varies, and the algorithm tends to favour models with the fewest number of layers that allow the data to be fitted, in essence providing a data-driven Occam's Razor. The algorithm is considerably more computationally expensive than deterministic inversions. However its advantage is that a great deal of information can be extracted from the ensemble. For example, the most likely, mean, mode and median models were all extracted and made into conductivity depth slice maps and sections. We extract the 10th and 90th percentile models and use the spread between them as a measure of model uncertainty, which we convey on maps by making uncertain areas more transparent. Another output of the algorithm is a change-point histogram which provides information on the most probable depths of the layer interfaces. By extracting the peaks from this histogram we begin to be able to automatically interpret layer boundaries to be plotted onto conductivity sections.

The Hood 10 VHMS deposit of Nunavut, Canada: a case history

Hood 10 is a small polymetallic (Cu - Zn) VHMS mineral deposit located in the Archean aged Slave Craton in arctic Canada, and is 100% owned by MMG Ltd. The deposit is hosted in the Napaktulik volcanic belt, a bimodal greenstone belt of approximately 2.67 Ga age. The deposit was initially discovered in the early 1970’s by Texas Gulf and covered with airborne and ground frequency domain EM (FDEM) and magnetics. The property has changed hands several times over the intervening years, with drill programs in the mid 1970’s, early 1980’s and early 1990’s resulting in a non-JORC compliant resource of 1.2 million tonnes of 4.4% zinc and 4.1% copper, and defining the ore body along a 250 m strike length and to a depth of 220 m. The property was acquired by MMG Ltd in 2009. In 2012, MMG Ltd conducted significant geophysical exploration on the property for the first time, acquiring surface and airborne time domain EM (TDEM) coverage of the deposit, as well as TDEM on 11 of 12 holes drilled into Hood 10 in 2012. New geophysical modelling of the TDEM results, drilling results, recent surface geological mapping and structural studies have significantly increased the understanding of the deposit. The current non-JORC compliant resource stands at 2.4 million tonnes of 3.5% zinc and 4.5% copper.

Passive and active helicopter EM survey comparisons over 501 Project Cu- Zn volcanogenic massive sulphide at McFauld’s Lake, northern Ontario

Helicopter AeroTEM, VTEM and ZTEM surveys were flown over the 501 zone in the McFauld’s Lake area, northern Ontario. The 501 zone is a relatively small VMS deposit that appears to respond well to all three active and passive airborne EM systems that have surveyed the property. Comparisons between these data sets and the geology are showcased using 1D-2D-3D EM inversion modeling.

An assessment of 3D ZTEM results over three deposits

The ZTEM airborne EM system was introduced into commercial service by Geotech Ltd. in 2006. ZTEM is unlike other EM systems in that it relies on the measurement of natural occurring EM fields in the AFMAG frequency range of 25-720 Hz. As a primary survey outcome, a ZTEM survey produces in-line and cross-line tipper data; Tzx and Tzy, at a series of frequencies. The contractor provides a suite of filtered X and Y spatial grids and 2D conductivity depth inversions have typically been provided using the Tzx component.More recent, full 3D inversion of data has begun to be applied which provides concurrent information about cross-line (derived from the Tzx) and along line (derived from the Tzy) data.The exploration value of inversions whether 2D or 3D is not yet well established due to 1) the limited surveys available to date and 2) limited availability of inversion codes and their application. In the present assessment, 2D and 3D inversion results over three deposits will be examined and considered in terms of the geological knowledge available at each site.

An assessment of ZTEM and time domain EM results over three mineral deposits

The ZTEM airborne EM system was introduced by Geotech Ltd. into commercial service in 2008. Based on measuring the tipper ratios of AFMAG frequencies, ZTEM provides conductivity information about the earth's near surface that can assist in the exploration for a variety of mineral deposit styles. The results of a surveys over three deposits will be examined; a skarn-style or carbonate replacement deposit in Mexico, a porphyry copper deposit in British Columbia and an epithermal precious-base metals deposit in the Yukon. For each survey, an helicopter time domain EM (HTEM) data set is available to compare with the ZTEM results. Overall, while similarities were noted between the ZTEM and HTEM results, there were also some significant differences that are not totally understood based on available geological information.