Helicopter Time-domain Electromagnetic Research Articles

Integrated geophysical signatures and structural geometry of the Kabinakagami Lake greenstone belt, Superior Province, Ontario, Canada: Exploration implications for concealed Archean orogenic gold deposits

Geophysical exploration for concealed orogenic gold deposits is challenging because petrophysical contrasts between the gold lodes and their host rocks are normally subtle. Most greenstone-hosted lode‑gold deposits do not contain strong conductors as they normally contain <5 vol% of poorly conductive disseminated arsenopyrite and pyrite. Where there is no outcrop, reliance must be placed on lithological and structural interpretations from aeromagnetic data.Airborne Induced Polarization (AIP) Cole-Cole parameters from helicopter time-domain electromagnetic (HTEM) surveys are demonstrated to be effective in geophysical exploration for concealed gold deposits in the Kabinakagami Lake greenstone belt in Ontario, Canada. Interpretation reveals that most AIP positive responses are associated with clays and all known gold deposits in the study area are in resistive rock formations. Hence, resistivity-scaled chargeability (RSC) can be used to detect weakly chargeable zones within these resistive units.Deep Neural Network (DNN) predictive targeting analysis is utilized to integrate the AIP apparent resistivity, resistivity-scaled chargeability (RSC), EM induction time-constant, and magnetic data. The DNN results imply that the known gold occurrences coincide with high DNN probabilities, and hence exploration targets can be identified based on DNN analysis. From a geological perspective, gold prospects also occur at triple-point junctions between granite intrusions and at probable intersections between potentially hosting shear zones and oblique cross faults or dykes, both common structural geometries for orogenic gold deposits. Consequently, DNN predictive targeting results based both on HTEM and magnetic data, and consistent with expected structural geometries, are proposed as a vectoring tool in the generation of drill targets during exploration for concealed orogenic gold deposits in greenstone belts such as those of the Superior Province, Canada.

The Helicopter Time-Domain Electromagnetic Technology Advances in China

The helicopter time-domain electromagnetic (HTEM) method is a method that uses geophysical exploration technology and a helicopter platform to carry electromagnetic detection equipment. HTEM has developed rapidly, in both theoretical and technological terms, in recent years because of its high efficiency, ability to drape steep and variable terrain, large detection depth, and high resolution. This work briefly introduces the HTEM technology principle and forward and inversion theory, summarizes the main performance parameters of international representative HTEM systems, and analyzes the performance and structural characteristics of VTEM, SkyTEM and HeliTEM systems. To provide a technical reference for scholars engaged in HTEM instrument technology, we analyze the factors affecting the detection capability of HTEM systems and the main difficulties in system design. Taking China’s HTEM systems as the object of analysis, we introduce the key technologies behind the systems in detail, including high-power transmitting, receiving sensor design, and motion-noise-suppression technologies. This work also discusses the development trend of HTEM and the existing technical problems, including detection depth, measurement accuracy, and noise refinement classification, providing ideas for the further development of HTEM technology in the future.

Airborne electromagnetic signature of a paleochannel uranium deposit

SUMMARY Reptile Uranium, a wholly owned subsidiary of Deep Yellow Ltd., has been actively exploring four exclusive prospecting licenses (EPLs) in Namibia since 2007. Prior exploration of this area in the 1970’s and 80’s resulted in the discovery of extensive Tertiary uranium mineralization hosted by paleochannels. Detailed airborne radiometric data acquired in 2007 shows good correlation with known surficial mineralization, but provides no depth information about the known channels, and does not identify potential buried channels. In 2008, Aeroquest Surveys completed an AeroTEM helicopter time domain electromagnetic (HTEM) survey over the four EPLs. The electromagnetic (EM) data proved to be an excellent tool for mapping the conductive material associated with both near-surface and buried paleochannels. There is generally good correlation between the stronger AeroTEM responses and the known mineralization. The HTEM data were inverted using a 1D layered earth algorithm to model the distribution of conductivity laterally and with depth. The modeled data effectively describe the lateral extent, thickness and relative conductivity of the paleochannels and have been found to be very useful in directing the ongoing drill program both for the delineation of known paleochannels and for the drilling of previously unknown paleochannels. Successful trenching and drill results from the Tubas-Oryx-Tumas paleochannel system clearly illustrate the important role that the HTEM data has played in the ongoing exploration on this project.

Characterizing the Spiritwood Valley Aquifer, North Dakota, using helicopter time-domain EM

Buried valley aquifers, consisting of permeable sand and gravel deposits in eroded bedrock valleys, are important sources of groundwater supply in many regions of the United States and Canada. Investigations of the Spiritwood aquifer in southern Manitoba by the Geological Survey of Canada and other workers, have demonstrated the value of helicopter time domain electromagnetic (HTEM) surveys in aquifer mapping and characterization using the contrasts between Quaternary glacio-lacustrine sand-gravels (high resistivity) that are relatively permeable and clay-tills (low resistivity) that are relatively impermeable, as well as the deeper, much less resistive Cretaceous Pierre Formation Shale basement rocks. This success provided the impetus for the North Dakota State Water Commission to fly a VTEM helicopter EM survey in the Jamestown, ND region in October, 2016. The VTEM data collected over the Spiritwood-JT block allowed for geological mapping from near surface to depth, in spite of relatively weak resistivity contrasts (<10X). These data were inverted with a layered-earth algorithm to produce resistivity-depth models. These models were able to resolve the location and depths to the top and bottom of the Spiritwood aquifer throughout the central portion of the block providing more detailed pictures of the aquifer’s geometry. In addition to resolving the main aquifer as well as its deeper channels, the VTEM data and models highlighted several smaller, previously undiscovered aquifers that cross-cut/branch-off from the main Spiritwood channel. These are interpreted as probable transverse low-K barriers that were apparent from the existing test drilling and aquifer testing.

Integrated interpretation of helicopter and ground-based geophysical data recorded within the Okavango Delta, Botswana

Integration of information from the following sources has been used to produce a much better constrained and more complete four-unit geological/hydrological model of the Okavango Delta than previously available: (i) a 3D resistivity model determined from helicopter time-domain electromagnetic (HTEM) data recorded across most of the delta, (ii) 2D models and images derived from ground-based electrical resistance tomographic, transient electromagnetic, and high resolution seismic reflection/refraction tomographic data acquired at four selected sites in western and north-central regions of the delta, and (iii) geological details extracted from boreholes in northeastern and southeastern parts of the delta. The upper heterogeneous unit is the modern delta, which comprises extensive dry and freshwater-saturated sand and lesser amounts of clay and salt. It is characterized by moderate to high electrical resistivities and very low to low P-wave velocities. Except for images of several buried abandoned river channels, it is non-reflective. The laterally extensive underlying unit of low resistivities, low P-wave velocity, and subhorizontal reflectors very likely contains saline-water-saturated sands and clays deposited in the huge Paleo Lake Makgadikgadi (PLM), which once covered a 90,000km2 area that encompassed the delta, Lake Ngami, the Mababe Depression, and the Makgadikgadi Basin. Examples of PLM sediments are intersected in many boreholes. Low permeability clay within the PLM unit seems to be a barrier to the downward flow of the saline water. Below the PLM unit, freshwater-saturated sand of the Paleo Okavango Megafan (POM) unit is distinguished by moderate to high resistivities, low P-wave velocity, and numerous subhorizontal reflectors. The POM unit is interpreted to be the remnants of a megafan based on the arcuate nature of its front and the semi-conical shape of its upper surface in the HTEM resistivity model. Moderate to high resistivity subhorizontal layers are consistent with this interpretation. The deepest unit is the basement with very high resistivity, high P-wave velocity, and low or complex reflectivity. The interface between the POM unit and basement is a prominent seismic reflector.

Incorporating ancillary data into the inversion of airborne time-domain electromagnetic data for hydrogeological applications

Abstract Helicopter time-domain electromagnetic (HTEM) surveys often suffer from significant inaccuracies in the early-time or near-surface data—a problem that can lead to errors in the inverse model or limited near-surface resolution in the event that early time gates are removed. We present an example illustrating the use of seismic data to constrain the model recovered from an HTEM survey over the Spiritwood buried valley aquifer in Manitoba, Canada. The incorporation of seismic reflection surfaces results in improved near-surface resistivity in addition to a more continuous bedrock interface with a sharper contact. The seismic constraints reduce uncertainty in the resistivity values of the overlying layers, although no a priori information is added directly to those layers. Subsequently, we use electrical resistivity tomography (ERT) and borehole data to verify the constrained HTEM models. Treating the ERT and borehole logs as reference information, we perform an iterative time-shift calibration of the HTEM soundings to achieve regional-scale consistency between the recovered HTEM models and the reference information. Given the relatively small time-shifts employed, this calibration procedure most significantly affects the early-time data and brings the first useable time gate to a time earlier than the nominal first gate after ramp off. Although time shifts are small, changes in the model are observed from the near-surface to depths of 100 m. Calibration is combined with seismic constraints to achieve a model with the greatest level of consistency between data sets and, thus, the greatest degree of confidence. For the Spiritwood buried valley, calibrated and constrained models reveal more structure in the valley-fill sediments and increased continuity of the bedrock contact.

Processing and inversion of commercial helicopter time-domain electromagnetic data for environmental assessments and geologic and hydrologic mapping

Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semiautomatically identified and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. For example, the average resistivity of the thick surface layer decreased from [Formula: see text] to [Formula: see text], the depths to the layer boundaries were reduced by 15%–23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.

Near-surface electromagnetic induction — Introduction

There has been explosive growth in near-surface electromagnetic (EM) induction geophysics in the past several years (Everett, 2012). New and experienced practitioners are achieving great success in applying the method to an increasing variety of problems. Moreover, theorists are becoming better able to exploit the rich information content that is available in electromagnetic induction data sets. The EM induction method, with its broad opportunities to design new transmitters, receivers, and interpretation tools, continues to offer wide avenues to capture the spatial complexity of the subsurface. This special Geophysics issue brings forward the latest achievements, which should stimulate interest across a broad spectrum of geophysicists, as well as set the tone for continuing developments in this field. This special issue contains advances in theory, instrumentation, data processing and interpretation, and innovative applications of near-surface applied EM induction geophysics. The range of topics is varied and, as described below, includes modeling and inversion, airborne electromagnetics, hydrogeophysics, soil science, audiomagnetotellurics, unexploded ordnance (UXO) discrimination, archaeology, geothermal mapping, and joint inversion of EM and magnetic resonance sounding data. Vrbancich analyzes helicopter time-domain EM data acquired over shallow seawater overlying reefs and sediment-filled paleovalleys. A favorable comparison is shown of water depths and sediment thickness derived from 1D inversion with known bathymetry and independent marine seismic …

Airborne electromagnetic signature of a paleochannel uranium deposit

SummaryReptile Uranium, a wholly owned subsidiary of Deep Yellow Ltd., has been actively exploring four exclusive prospecting licenses (EPLs) in Namibia since 2007. Prior exploration of this area in the 1970’s and 80’s resulted in the discovery of extensive Tertiary uranium mineralization hosted by paleochannels. Detailed airborne radiometric data acquired in 2007 shows good correlation with known surficial mineralization, but provides no depth information about the known channels, and does not identify potential buried channels. In 2008, Aeroquest Surveys completed an AeroTEM helicopter time domain electromagnetic (HTEM) survey over the four EPLs. The electromagnetic (EM) data proved to be an excellent tool for mapping the conductive material associated with both near-surface and buried paleochannels. There is generally good correlation between the stronger AeroTEM responses and the known mineralization. The HTEM data were inverted using a 1-D layered earth algorithm to model the distribution of conductivity laterally and with depth. The modeled data effectively describe the lateral extent, thickness and relative conductivity of the paleochannels and have been found to be very useful in directing the ongoing drill program both for the delineation of known paleochannels and for the drilling of previously unknown paleochannels. Successful trenching and drill results from the Tubas-Oryx-Tumas paleochannel system clearly illustrate the important role that the HTEM data has played in the ongoing exploration on this project.

The emerging role of helicopter time domain EM systems

Helicopter time domain electromagnetic (H-TEM) systems are an emerging technology. The ability to integrate the great depth penetration of fixed-wing systems with the spatial resolution and conductance discrimination of helicopter frequency domain systems is an important development for the detection of discrete conductors. H-TEM systems can be configured with multiple receiver coils to make their responses more diagnostic, such as the discrimination of thick from thin conductors. The use of a rigid geometry allows these systems to measure during the transmitter on-time where high conductance targets produce a stronger response. In areas of conductive overburden, it is possible to remove much of the high amplitude response due to surface conductivity from deeper bedrock conductors by subtracting the off-time response from the corresponding on-time response. The resultant profiles are more diagnostic of bedrock conductors than are the late off-time channels from the same system at a lower base frequency.

Kentucky coal plant gets SCR retrofit

Integration of information from the following sources has been used to produce a much better constrained and more complete four-unit geological/hydrological model of the Okavango Delta than previously available: (i) a 3D resistivity model determined from helicopter time-domain electromagnetic (HTEM) data recorded across most of the delta, (ii) 2D models and images derived from ground-based electrical resistance tomographic, transient electromagnetic, and high resolution seismic reflection/refraction tomographic data acquired at four selected sites in western and north-central regions of the delta, and (iii) geological details extracted from boreholes in northeastern and southeastern parts of the delta. The upper heterogeneous unit is the modern delta, which comprises extensive dry and freshwater-saturated sand and lesser amounts of clay and salt. It is characterized by moderate to high electrical resistivities and very low to low P-wave velocities. Except for images of several buried abandoned river channels, it is non-reflective. The laterally extensive underlying unit of low resistivities, low P-wave velocity, and subhorizontal reflectors very likely contains saline-water-saturated sands and clays deposited in the huge Paleo Lake Makgadikgadi (PLM), which once covered a 90,000 km2 area that encompassed the delta, Lake Ngami, the Mababe Depression, and the Makgadikgadi Basin. Examples of PLM sediments are intersected in many boreholes. Low permeability clay within the PLM unit seems to be a barrier to the downward flow of the saline water. Below the PLM unit, freshwater-saturated sand of the Paleo Okavango Megafan (POM) unit is distinguished by moderate to high resistivities, low P-wave velocity, and numerous subhorizontal reflectors. The POM unit is interpreted to be the remnants of a megafan based on the arcuate nature of its front and the semi-conical shape of its upper surface in the HTEM resistivity model. Moderate to high resistivity subhorizontal layers are consistent with this interpretation. The deepest unit is the basement with very high resistivity, high P-wave velocity, and low or complex reflectivity. The interface between the POM unit and basement is a prominent seismic reflector.