Airborne Electromagnetic Survey Research Articles

Extending Airborne Electromagnetic Surveys for Regional Active Layer and Permafrost Mapping with Remote Sensing and Ancillary Data, Yukon Flats Ecoregion, Central Alaska

Machine‐learning regression tree models were used to extrapolate airborne electromagnetic resistivity data collected along flight lines in the Yukon Flats Ecoregion, central Alaska, for regional mapping of permafrost. This method of extrapolation (r = 0.86) used subsurface resistivity, Landsat Thematic Mapper (TM) at‐sensor reflectance, thermal, TM‐derived spectral indices, digital elevation models and other relevant spatial data to estimate near‐surface (0–2.6‐m depth) resistivity at 30‐m resolution. A piecewise regression model (r = 0.82) and a presence/absence decision tree classification (accuracy of 87%) were used to estimate active‐layer thickness (ALT) (< 101 cm) and the probability of near‐surface (up to 123‐cm depth) permafrost occurrence from field data, modelled near‐surface (0–2.6 m) resistivity, and other relevant remote sensing and map data. At site scale, the predicted ALTs were similar to those previously observed for different vegetation types. At the landscape scale, the predicted ALTs tended to be thinner on higher‐elevation loess deposits than on low‐lying alluvial and sand sheet deposits of the Yukon Flats. The ALT and permafrost maps provide a baseline for future permafrost monitoring, serve as inputs for modelling hydrological and carbon cycles at local to regional scales, and offer insight into the ALT response to fire and thaw processes. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Paradigm change in 3D inversion of airborne EM surveys: case study for oil sands exploration near Fort McMurray, Alberta

Michael S. Zhdanov, Leif Cox, and Jonathan Rudd demonstrate with a case study of oil sands exploration near Fort McMurray, Alberta how the 3D inversion of airborne electromagnetic survey data can be used to improve near-surface imaging, in their view a potential paradigm change particularly with regard to hydrocarbon exploration.

Detection and correction of SPM effects in airborne EM surveys

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 power-law 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.

The Frome airborne electromagnetic survey, South Australia

The Frome airborne electromagnetic (AEM) survey is the largest of three regional AEM surveys flown under the 5-year Onshore Energy Security Program (OESP) by Geoscience Australia (GA). The aim of the survey is to reduce risk and stimulate exploration investment for uranium by providing reliable pre-competitive data. The Frome AEM survey was flown between 22 May and 2 November 2010, is approximately 95 450 km2 in area and collected 32 317 line km of new data at an average flying height of 100 m. The Frome AEM survey covers the Marree (pt), Callabonna (pt), Copley (pt), Frome (pt), Parachilna (pt), Curnamona, Olary and Chowilla (pt) 1:250 000 standard map sheets in South Australia and was flown largely at 2.5 km line spacing, with the northern portion flown at 5 km line spacing. GA partnered with, the Department of Primary Industries and Resources South Australia and an industry consortium. The survey results indicate a depth of investigation (DOI - depth of reliable signal penetration) of up to 400 m in areas of thin cover and resistive basement (e.g., Adelaidean rocks in the Olary Ranges). In Cenozoic ? Mesozoic sediments in the Frome Embayment and the Murray Basin the DOI is up to 100-150 m. A range of under-cover features are revealed, including (but not limited to): extensions to known palaeovalley networks in the Frome Embayment; the under-cover extent of the Benagerie Ridge; regional faults in the Frome Embayment and Murray Basin; folded and faulted Neoproterozoic rocks in the Adelaide Fold Belt; Cenozoic ? Mesozoic stratigraphy in the Frome Embayment; neotectonic offsets in the Lake Eyre Basin; conductive Neoproterozoic rocks associated with copper-gold mineralisation; and, coal-bearing structures in the Leigh Creek area, as well as groundwater features.

Inversion of Large-scale ZTEM Data

As the number of near surface deposits decreases, it becomes increasingly important to develop geophysical techniques to image at depth. Because of the penetration advantage of plane wave natural sources, these techniques are ideal to answer questions about the deep subsurface to the earth. A ZTEM survey is an airborne electromagnetic survey which records the vertical magnetic field that result from natural sources. The data are transfer functions that relate the local vertical field to orthogonal horizontal fields measured at a reference station on the ground. While the airborne nature of the survey means that large survey areas can be surveyed quickly and economically, the high number of cells required to discretize the entire survey area at reasonable resolution can make the computational costs of inverting the entire data set all at once prohibitively expensive. Here we present a workflow methodology that can be used to invert large natural source surveys by decomposing the large inverse problem into smaller more manageable problems before combining the tiles into a final inversion result. We use the procedure to invert synthetic ZTEM data for the Noranda mining camp as well as a field data example. Both of these data sets were far too large to solve on a single grid even with multiple processors at our disposal.

The geophysical study of Drybones kimberlite using 3D Time Domain EM Inversion and 3D ZTEM inversion algorithms

Two airborne EM surveys were conducted over the Drybones kimberlite, NWT, Canada. A VTEM helicopter time domain EM survey was flown by Geotech in 2005, followed by a ZTEM helicopter tipper EM survey flown in 2009. The data sets collected over the kimberlite were inverted and interpreted first by Geotech, Ltd using a 1D TDEM conductivity-depth transform for VTEM data and a 2D inversion for ZTEM data. The data sets were transferred to UBC-GIF for 3D inversions and comparative analysis. Both the VTEM and ZTEM data have now been inverted in 3D and the results compared with the drill data over the kimberlite and with older results of previous geophysical interpretation. The 2D and 3D ZTEM inversions show the same general structure but the 3D inversion has greater electrical conductivity contrast and may have fewer artefacts in zones of low sensitivity. On a large scale the conductivity structure, recovered from the 3D inversion of time domain data, is comparable to that from the CDI transform, but the 3D result shows a more complicated distribution of the conductive properties. The 3D magnetic inversion had not been done previously and it shows susceptible material to depths of approximately 400 m, which is consistent with the known geometry of the kimberlite.

Assessing the presence of hard rock along a gas pipeline alignment with airborne EM

Over the past decades, airborne electromagnetic (AEM) surveys have mostly been used in connection with mineral exploration and a variety of issues in hydrogeophysical mapping. However, increasingly, AEM is used for a wide range of geotechnical purposes: pollution mapping, geotechnical assessment on road and freeway alignments, bathymetry, and depth to bedrock. We present an investigation using a helicopter-borne transient electromagnetic system along the planned trace of a gas pipeline. Oil and gas pipelines are often buried at a depth of a few meters and the cost of construction depends critically on whether the subsurface is composed of soft sediments that can be easily excavated or hard rock formations that require much heavier equipment and possibly have to be blasted. The aim of the AEM survey was to distinguish between the soft, relatively conductive sediments and the hard, relatively resistive bedrock in the upper few meters of the subsurface. Data were collected with a rather small transmitter moment, but a high repetition frequency that simultaneously allowed high acquisition speed, and reliable data quality. Measurements were inverted with 1D models with both vertical and lateral constraints to produce model sections along flight lines. A novel method of statistical analysis of the set of equivalent models for each inverted model, calibrated against boreholes, improved the estimates of the presence of hard rock along the flight lines.

Airborne Electromagnetic Surveys for Groundwater Characterization

Airborne electromagnetic (AEM) surveys provide densely sampled data over large areas (typically several hundred sq. km) that cannot be covered effectively using ground-based methods. AEM data are inverted to estimate the three-dimensional distribution of electrical resistivity structures from shallow depths to several hundred meters. These models convey unparalleled details that are used to make inferences about hydrogeologic properties and processes at the watershed and local scale. This information is being used in groundwater models that are critical to water management decisions, to better understand geologic frameworks, and to improve climate change models. The U.S. Geological Survey (USGS) has been engaged in the application of AEM to many watershed and local scale groundwater projects within United States. We present the results of several frequency- and time-domain AEM surveys acquired by the USGS that have been used for mapping alluvial valleys, buried glacial aquifers, fault-bounded basins, and understanding permafrost distributions.

3D VTEM inversion for delineating sub-vertical shear zones in the West African gold belt

SUMMARY Accurately delineating sub-vertical graphitic thrust faults and shear systems is one of the major strategies followed in West African gold exploration. Due to the dense vegetation and limited outcrops, airborne electromagnetic (AEM) surveys have been routinely flown for geological mapping. We present a case study for practical 3D inversion of VTEM data acquired in Ghana. The geology is very complex, and consists of carbonaceous shear zones containing multiple thrust faults. Certain types of gold mineralization are directly associated with the shear zones which manifest themselves as sub-vertical conductors. We compare results from 3D AEM inversion to those obtained from traditional AEM interpretation methods, including conductivity depth imaging, decay constant analysis, profile anomaly picking, Fraser filtering, and plate modelling. We show that even with no a priori information, 3D AEM inversion accurately delineates the sub-vertical conductors, and recovers the most accurate representation of the geology. Interpretations based on CDIs are shown to be unreliable.

Superparamagnetic effects in EM surveys for Mineral Exploration

Superparamagnetic effects in soil and rock cause responses in a variety of EM survey types that are routinely used in mineral exploration surveys. The identification of these effects can be difficult and the response may be very similar to that from targeted base metal orebodies. Identification of these effects in TDEM datasets is critical for prioritising ground follow-up of targets and failing to correctly identify SPM anomalies frequently leads to inaccurate inversions, misdirected field work, and unnecessary, deep drill holes. As signal-to-noise ratios in acquisition systems increase, SPM effects will become an increasingly common issue in both airborne and ground EM surveys. This paper presents examples of SPM responses in data collected with the latest airborne, ground, and drill hole EM systems used in minerals exploration. Ground follow-up results confirm the SPM responses. Techniques that have been found to be effective to firstly avoid, and secondly to identify SPM anomalies in survey data are presented. These techniques are based around minimising the primary energising magnetic field at the source of the SPM, and by distancing the sensor so that it is separated from the source. The main technique for recognising SPM effects directly in data remains the fairly predictable power law decay rate. Secondary techniques such as association of anomalies with alluvial sediments, topographical features (e.g. drainage, hills), and palaeochannel patterns also provide indications of SPM. Associations of anomalies with low transmitter loop height in airborne systems is also indicative of SPM, however systems that can better discriminate SPM need to be developed.

3D inversion of SPECTREM and ZTEM data from the Pebble Cu-Au-Mo porphyry deposit, Alaska

This case study compares 3D inversion results from Spectrem Air?s SPECTREM 2000 fixed-wing timedomain airborne electromagnetic (AEM) system, and Geotech?s Z-axis Tipper Electromagnetic (ZTEM) airborne audio-frequency magnetics (AFMAG) system flown over the Pebble Cu-Au-Mo deposit in Alaska. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and with the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The 3D interpretation of both surveys has yielded improved the understanding of the geology, alteration and mineralization of the Pebble system. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation on one system over the other. We do conclude however, that 3D inversions of AEM and ZTEM surveys add significant value to exploration.

AEM Inversion of VTEM Data from a Resistive Terrain

SummaryData from a VTEM™ airborne electromagnetic survey over resistive terrain is examined. Forward modelling and analysis of high-altitude lines shows that the amplitudes of random noise, bucking error, processing corrections and geological signals can be large compared to the geological signal in the resistive terrain. The negative impacts of the low geological signal to noise ratio on conductivity estimates generated by layered-earth inversion and conductivity transformations are demonstrated. The reader is alerted to the degree of uncertainty and non-uniqueness that is inherent in conductivity estimates generated from similar datasets.

Towed transient electromagnetic survey for groundwater investigation - challenges and solutions.

SUMMARY Towed transient electromagnetic survey (TEM) is conducted by towing loops of wire behind a vehicle, one transmitting and several receiving. The loop sizes must be large enough to transmit/receive moments well in excess of EM noise levels while loop inductances must be kept low in order to keep turn-off time and system response low enough to resolve shallow detail. Long systems with the towing vehicle well in front of two separate nonmetallic loop support structures are practical. As such a system must fit through farm gates, between trees, and along road margins without undue traffic disruption, it must be capable of folding to legal road vehicle width. Processing involves most of the normal complications of airborne EM survey but with the additional complication that the system cannot be elevated high above the ground to isolate system response. Additional challenges are identification and removal/minimization of noise and metallic source anomalies: 1. created by movement of coils through the magnetic field of the earth, 2. created by buried telephony cables, and 3. from other sources such as power lines, buried metal pipes and fences. Much of Australia is reasonably navigable and not densely covered with infrastructure and it is in these parts of Australia where towed TEM systems, which can be deployed and manoeuvred flexibly, have a market for groundwater and other exploration.

Flawed Processing of Airborne EM Data Affecting Hydrogeological Interpretation

Airborne electromagnetics (AEMs) is increasingly being used across the globe as a tool for groundwater and environmental management. Focus is on ensuring the quality of the source data, their processing and modeling, and the integration of results with ancillary information to generate accurate and relevant products. Accurate processing and editing of raw AEM data, the topic of this article, is one of the crucial steps in obtaining quantitative information for groundwater modeling and management. In this article, we examine the consequences that different levels of processing of helicopter transient electromagnetic method data have on the resulting electrical models and subsequently on hydrogeological models. We focus on different approaches used in the industry for processing of the raw data and show how the electrical resistivity-depth models, which is the end "geophysical" product (after data inversion) of an AEM survey, change with different levels of processing of the raw data. We then extend the study to show the impact on some of the hydrogeological parameters or models, which can be derived from the geophysical results. The consequences of improper handling of raw data to groundwater and environmental management can be significant and expensive.

Transboundary geophysical mapping of geological elements and salinity distribution critical for the assessment of future sea water intrusion in response to sea level rise

Abstract. Geophysical techniques are increasingly being used as tools for characterising the subsurface, and they are generally required to develop subsurface models that properly delineate the distribution of aquifers and aquitards, salt/freshwater interfaces, and geological structures that affect groundwater flow. In a study area covering 730 km2 across the border between Germany and Denmark, a combination of an airborne electromagnetic survey (performed with the SkyTEM system), a high-resolution seismic survey and borehole logging has been used in an integrated mapping of important geological, physical and chemical features of the subsurface. The spacing between flight lines is 200–250 m which gives a total of about 3200 line km. About 38 km of seismic lines have been collected. Faults bordering a graben structure, buried tunnel valleys, glaciotectonic thrust complexes, marine clay units, and sand aquifers are all examples of geological structures mapped by the geophysical data that control groundwater flow and to some extent hydrochemistry. Additionally, the data provide an excellent picture of the salinity distribution in the area and thus provide important information on the salt/freshwater boundary and the chemical status of groundwater. Although the westernmost part of the study area along the North Sea coast is saturated with saline water and the TEM data therefore are strongly influenced by the increased electrical conductivity there, buried valleys and other geological elements are still revealed. The mapped salinity distribution indicates preferential flow paths through and along specific geological structures within the area. The effects of a future sea level rise on the groundwater system and groundwater chemistry are discussed with special emphasis on the importance of knowing the existence, distribution and geometry of the mapped geological elements, and their control on the groundwater salinity distribution is assessed.

3D inversion of airborne electromagnetic data

Time-domain airborne surveys gather hundreds of thousands of multichannel, multicomponent samples. The volume of data and other complications have made 1D inversions and transforms the only viable method to interpret these data, in spite of their limitations. We have developed a practical methodology to perform full 3D inversions of entire time- or frequency-domain airborne electromagnetic (AEM) surveys. Our methodology is based on the concept of a moving footprint that reduces the computation requirements by several orders of magnitude. The 3D AEM responses and sensitivities are computed using a frequency-domain total field integral equation technique. For time-domain AEM responses and sensitivities, the frequency-domain responses and sensitivities are transformed to the time domain via a cosine transform and convolution with the system waveform. We demonstrate the efficiency of our methodology with a model study relevant to the Abitibi greenstone belt and a case study from the Reid-Mahaffy test site in Ontario, Canada, which provided an excellent practical opportunity to compare 3D inversions for different AEM systems. In particular, we compared 3D inversions of VTEM-35 (time-domain helicopter), MEGATEM II (time-domain fixed-wing), and DIGHEM (frequency-domain helicopter) data. Our comparison showed that each system is able to image the conductive overburden and to varying degrees, detect and delineate the bedrock conductors, and, as expected, that the DIGHEM system best resolved the conductive overburden, whereas the time-domain systems most clearly delineated the bedrock conductors. Our comparisons of the helicopter and fixed-wing time-domain systems revealed that the often-cited disadvantages of a fixed-wing system (i.e., response asymmetry) are not inherent in the system, but rather reflect a limitation of the 1D interpretation methods used to date.

Regional-scale airborne electromagnetic surveying of the Yucatan karst aquifer (Mexico): geological and hydrogeological interpretation

Geometry and connectivity of high-permeability zones determine groundwater flow in karst aquifers. Efficient management of karst aquifers requires regional mapping of preferential flow paths. Remote-sensing technology provides tools to efficiently map the subsurface at such scales. Multi-spectral remote sensing imagery, shuttle radar topography data and frequency-domain airborne electromagnetic (AEM) survey data were used to map karst-aquifer structure on the Yucatan Peninsula, Mexico. Anomalous AEM responses correlated with topographic features and anomalous spectral reflectance of the terrain. One known preferential flow path, the Holbox fracture zone, showed lower bulk electrical resistivity than its surroundings in the AEM surveys. Anomalous structures delineated inland were sealed above by a low-resistivity layer (resistivity: 1–5 Ωm, thickness: 5–6 m). This layer was interpreted as ejecta from the Chicxulub impact (Cretaceous/Paleogene boundary), based on similar resistivity signatures found in borehole logs. Due to limited sensitivity of the AEM survey, the subsurface configuration beneath the low-resistivity layer could not be unambiguously determined. AEM measurements combined with remote-sensing data analysis provide a potentially powerful multi-scale methodology for structural mapping in karst aquifers on the Yucatan Peninsula and beyond.

The Frome Airborne Electromagnetic Survey: Uncovering 10% of South Australia

The Frome airborne electromagnetic (AEM) survey was acquired by Geoscience Australia (GA) under the Australian Government’s Onshore Energy Security Program (OESP). This area is considered to have great potential for uranium mineralisation and includes Australia’s only two producing in situ leach uranium mines, Beverley and Honeymoon. In contrast to deposit-scale investigations carried out by industry, the Frome AEM survey was designed to reveal new geological information at a regional scale, reducing exploration risk, stimulating exploration investment and enhancing the prospectivity within the region primarily for uranium, but also for other commodities including copper, gold, silver, lead, zinc, iron ore, coal and groundwater. The Frome AEM survey (Figure 1) was flown by Fugro Airborne Surveys for GA, using the TEMPESTTM time-domain AEM system.

3D inversion of SPECTREM and ZTEM airborne electromagnetic data from the Pebble Cu–Au–Mo porphyry deposit, Alaska

Geological, geochemical, and geophysical surveys have been conducted in the area of the Pebble Cu–Au–Mo porphyry deposit in south-west Alaska since 1985. This case study compares three-dimensional (3D) inversion results from Anglo American’s proprietary SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and Geotech’s ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble deposit. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The high grade CuEqn 0.6% ore shell is not consistently following the high conductive trend, suggesting that the SPECTREM and ZTEM responses correspond in part to the sulphide distribution, but not directly with the ore mineralization. As in any exploration project, interpretation of both surveys has yielded an improved understanding of the geology, alteration and mineralization of the Pebble system and this will serve well for on-going exploration activities. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation for either system. We can conclude however, that 3D inversion of both AEM and ZTEM surveys is now a practical consideration and that it has added value to exploration at Pebble.

High-resolution magnetotelluric studies of the Archaean-Proterozoic border zone in the Fennoscandian Shield, Finland

SUMMARY The Archaean–Proterozoic collisional zone is a complex mixture of the Archaean complexes [e.g. Iisalmi Complex (IC)], Proterozoic supracrustal belts [e.g. Kainuu Belt (KB) and Savo Belt (SB)] and oceanic arc lithologies in the central Fennoscandian Shield. The zone was formed in the Savo orogeny when the Keitele microcontinent collided with the Archaean Karelian craton in the Palaeoproterozoic time. The crustal architecture of this palaeosuture is studied using new broad-band magnetotelluric data from 104 sites. 2-D conductivity models across the border zone between the Palaeoproterozoic Svecofennian Domain and the Archaean Karelian province are constrained using the recent, partly collocated reflection seismic data from the Finnish Reflection Experiment (FIRE). Dimensionality analyses, in particular the Q-function analysis, show that magnetotelluric data represent reasonably well regional 2-D structure at periods <100 s, which is the longest period used in this study. Strike determinations gave a stable strike of N15W. For the inversions, the data are projected into three parallel profiles with an azimuth of N75E. The determinant inversion is selected as the most suitable method for the data set. Especially the phase data are useable only from the determinant since one of the polarizations have the out-of-quadrant phase at several sites. The interpreted final, geological more appropriate models, where smoother thick conductive areas are replaced by thinner layers, are constructed from the results of the unconstrained smooth inversions with the help of forward modelling, synthetic and prior model inversions and reflection seismic models. The two major sets of crustal conductors are identified. They have an opposite dip and together they form a bowl-shaped conductor. In the west, the eastward dipping SB conductors are located at the bottom of the formation underlain by the Keitele microcontinent. The SB conductors extend to the east possibly cutting the westward dipping conductors of the KB. The conductive KB is flanked above and below by the resistive Archaean IC/Rautavaara and Eastern Finland Complexes and represents the remnants of a basin that was squeezed between two Archaean blocks in the collision. The crustal conductors revealed by the inversion extend to the surface, where they can be associated with the near-surface conductors mapped by airborne electromagnetic surveys. Geological mapping has shown that the near-surface conductors are caused by graphite and/or sulphide-bearing metasedimentary rocks. Thus the deep conductors related with the Proterozoic KB and SB were formed on the surface and transported into the upper and middle crustal levels in the Savo orogeny. The crust in the Karelian Craton is highly resistive having only minor resistivity variations in our research area. Also the Archaean lower crust is resistive on the contrary to the conductive lower crust of the Proterozoic SB and the Keitele microcontinent.