Time-domain Airborne Electromagnetic System Research Articles

Inversion for 3D Conductivity and Chargeability Models Using EM Data Acquired by the New Airborne TargetEM System in Ontario, Canada

This paper introduces an original approach to the joint inversion of airborne electromagnetic (EM) data for three-dimensional (3D) conductivity and chargeability models using hybrid finite difference (FD) and integral equation (IE) methods. The inversion produces a 3D model of physical parameters, which includes conductivity, chargeability, time constant, and relaxation coefficients. We present the underlying principles of this approach and an example of a high-resolution inversion of the data acquired by a new active time domain airborne EM system, TargetEM, in Ontario, Canada. The new TargetEM system collects high-quality multicomponent data with low noise, high power, and a small transmitter–receiver offset. This airborne system and the developed advanced inversion methodology represent a new effective method for mineral resource exploration.

The Test Flight of iFTEM-I Fixed-Wing Airborne Time-Domain Electromagnetic System in Binxian, Heilongjiang Province, China

The fixed-wing time-domain airborne electromagnetic method (FTEM) has been widely used in metal mining exploration, groundwater mapping and other fields worldwide, and in recent decades, its use has gradually also become more prevalent in China. The first generation of the fixed-wing time-domain airborne electromagnetic system (iFTEM-I), researched and developed by the Institute of Geophysical and Geochemical Exploration (IGGE), has been demonstrated in recent years. In this article, we introduce the brief working principle and system parameters of iFTEM-I, and we show that the effective prospecting depth of iFTEM-I is up to 350 m, which is indicated by a comparison of the results between iFTEM-I and a ground TEM system (TEM-30B) carried out in Binxian.

Early time data processing in shallow AEM exploration

For shallow exploration, high-frequency pseudo-random binary sequence is used in the time-domain airborne electromagnetic system. This system is working in the on-time and the earth response could be derived from system identification, which is different from the previous time-domain electromagnetic systems. The early time data is crucial for obtaining a detailed resolution. It could be severely disturbed by the primary field, the receiver response and noise. Here we present an approach for removing the primary field by matching subtraction in transformation of correlation calculation. The noise is suppressed as not correlated with the pseudo-random binary sequence. This approach is proved efficient by applying to simulation data based on forward modeling. The receiver influence is analyzed through the response of the equivalent electronic circuit. The 1D inversion results indicate that critical damping state and high cut-off frequency could reduce the adverse effect on the resolution of shallow exploration.

Adaptive finite element for 3D time-domain airborne electromagnetic modeling based on hybrid posterior error estimation

Airborne electromagnetic (AEM) forward modeling has been extensively developed in past years. However, not much attention has been paid to the adaptive numerical algorithms for time-domain electromagnetic modeling. We have created an adaptive method that can generate an effective mesh for time-domain 3D AEM full-wave modeling using an unstructured finite-element method and a backward Euler scheme. For the estimation of the posterior error in the adaptive process, we use a hybrid technique based on the continuity of the normal current density for modeling the off-time channels, and on the continuity of the tangential magnetic field for the on-time channels. To improve the stability of the forward modeling and control the number of grids in the adaptive process, a random grid-selection technique is applied. We check the modeling accuracy of the algorithm by comparing our adaptive results with the semianalytical solution for a time-domain AEM system over a homogeneous half-space. Furthermore, we test the effectiveness of our algorithm for multiple-source time-domain AEM systems by analyzing the meshes generated by the adaptive method and the model results. Finally, we study the topographic effect by calculating time-domain AEM responses over a hill model with an abnormal body embedded.

Low-frequency rotational isolator for airborne exploration

We have determined the performance of a passive rotational vibration isolator for a time-domain airborne electromagnetic (TDEM) receiver. The isolator uses neutrally buoyant flotation to provide very soft suspension and a very low resonant frequency of 0.065 Hz ± 0.005 Hz. One of the limitations of mapping deeper targets in areas of conductive overburden with TDEM systems is that low-frequency coil-vibration noise provides a lower bound to the transmitter base frequency (typically limited to 25 Hz). The purpose of this new isolator is to improve coil vibration related noise between 5 and 20 Hz to allow the transmitter base frequency to be reduced. A fixed-wing flight test determined that a receiver inside the new isolator had five times less rotational noise at 10 Hz than a current commercial system.

An investigation of the hidden precious water resources of Dampier Peninsula using airborne electromagnetic method

SUMMARY 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.

MULTIPULSE – high resolution and high power in one TDEM system

An airborne time domain electromagnetic (TEM) system with high resolution and great depth of exploration is desired for geological mapping as well as for mineral exploration. The MULTIPULSE technology enables an airborne TEM system to transmit a high power pulse (a half-sine, for instance) and one or multiple low power pulse(s) (trapezoid or square) within a half-cycle. The high power pulse ensures good depth of exploration and the low power pulse allows a fast transmitter current turn off and earlier off-time measurement thus providing higher frequency signals, which allows higher near-surface resolution and better sensitivity to weak conductors. The power spectrum of the MULTIPULSE waveform comprising a half-sine and a trapezoid pulse clearly shows increased power in the higher frequency range (> ~2.3 kHz) compared to that of a single half-sine waveform. The addition of the low power trapezoid pulse extends the range of the sensitivity 10-fold towards the weak conductors, expanding the geological conductivity range of a system and increasing the scope of its applications. The MULTIPULSE technology can be applied to standard single-pulse airborne TEM systems on both helicopter and fixed-wing. We field tested the HELITEM MULTIPULSE system over a wire-loop in Iroquois Falls, demonstrating the different sensitivity of the high and low power pulses to the overburden and the wire-loop. We also tested both HELITEM and GEOTEM MULTIPULSE systems over a layered oil sand geologic setting in Fort McMurray, Alberta, Canada. The results show comparable shallow geologic resolution of the MULTIPULSE to that of the RESOLVE system while maintaining superior depth of exploration, confirming the increased geological conductivity range of a system employing MULTIPULSE compared to the standard single-pulse systems.

Airborne electromagnetic detection of shallow seafloor topographic features, including resolution of multiple sub-parallel seafloor ridges

The HoistEM helicopter time-domain electromagnetic (TEM) system was flown over waters in Backstairs Passage, South Australia, in 2003 to test the bathymetric accuracy and hence the ability to resolve seafloor structure in shallow and deeper waters (extending to ~40 m depth) that contain interesting seafloor topography. The topography that forms a rock peak (South Page) in the form of a mini-seamount that barely rises above the water surface was accurately delineated along its ridge from the start of its base (where the seafloor is relatively flat) in ~30 m water depth to its peak at the water surface, after an empirical correction was applied to the data to account for imperfect system calibration, consistent with earlier studies using the same HoistEM system. A much smaller submerged feature (Threshold Bank) of ~9 m peak height located in waters of 35 to 40 m depth was also accurately delineated. These observations when checked against known water depths in these two regions showed that the airborne TEM system, following empirical data correction, was effectively operating correctly. The third and most important component of the survey was flown over the Yatala Shoals region that includes a series of sub-parallel seafloor ridges (resembling large sandwaves rising up to ~20 m from the seafloor) that branch out and gradually decrease in height as the ridges spread out across the seafloor. These sub-parallel ridges provide an interesting topography because the interpreted water depths obtained from 1D inversion of TEM data highlight the limitations of the EM footprint size in resolving both the separation between the ridges (which vary up to ~300 m) and the height of individual ridges (which vary up to ~20 m), and possibly also the limitations of assuming a 1D model in areas where the topography is quasi-2D/3D.

Estimating Penetration Depth of the CHTEM‐I System by the Diffusive Electric Field Method

AbstractPenetration depth is one of key technical parameters in the airborne time‐domain electromagnetic system (ATEM), which plays an important role in system design as well as data interpretation. When the difference of conductivity between the target layer and host rock is small or the anomaly response of the target layer is not obvious, the conventional method for estimating penetration depth is no longer applicable. To overcome this drawback, this study suggests a new approach. It simulates the diffusion processes of the induced electric field in a uniform half‐space model or a layering model and determines the transient position of the field with the largest amplitude in the subsurface. Then it defines that the depth of this position is the penetration depth of the system, where the measured response value is equal to the given noise level. Taking the China helicopter time‐domain electromagnetic system I (CHTEM‐I), developed independently by our own forces, as the example, this work presents the application of this new method. It compares calculation results under various conditions, produces relation curves of penetration depth, flight height and noise level, and explores how to enhance the penetration depth of the system. The results show that the system can probe down depth 300 m in the case that noise level is proportional to t−05 when the half‐space model has conductivity in 0.000295∼0.0422 S/m. This depth estimation method is not affected by factors such as layer thickness and magnitude of conductivity difference between the target layer and host rock, and thus very applicable. It is of significance to design including estimation of penetration depth for various time‐domain airborne electromagnetic systems.

Test-site calibration and validation of airborne and ground-based TEM systems

Validation and calibration of airborne and ground-based time-domain electromagnetic (TEM) systems are important to obtain high-quality data and thereby reliable and reproducible results. Validation and calibration become even more important when the TEM systems are to be used for low-anomaly groundwater and environmental applications. The recent extension of the Danish TEM test site has made it possible to perform detailed validation of airborne TEM systems and calibrate airborne TEM systems that cannot make hovering measurements. We evaluated the Danish TEM test site and used a test site calibration scheme recommended for ground-based as well as airborne TEM systems. Furthermore, we discovered an extended data set from the airborne TEM system, SkyTEM, from the test site, used for an extensive validation of the SkyTEM system. This validation included repeatability tests at different heights and comparisons with the reference sections obtained with ground-based measurements. The validation and comparison were performed directly on the inversion results and in data space, down to the single data gate values. The extensive validation of the SkyTEM system at the TEM test site revealed a very stable and reliable system. The data repeatability of the SkyTEM system at different heights and directions was, in general, well within the standard deviation (STD) of the data. The agreement between the ground-based reference model sections and the SkyTEM model section from different recording heights was very good. Likewise, the match between the ground-based reference data and the SkyTEM data was good and, in general, within 1.5 times the STD on the data. The positive outcome of the extensive validation also confirmed that data processing and modeling were performed at the highest standard.

Review of three airborne EM systems

Time Domain Airborne Electromagnetic (TDEM) systems are defined by a set of technical specifications, which include dipole moment, bandwidth, transmitter waveform and transmitter-receiver geometry. Comprehensive analysis of these specifications is fundamental in understanding how they define the target response. For example, a system optimised for mapping deep, discrete ore bodies is not necessarily the ideal solution for mapping regolith where good vertical resolution may be required. Data acquired by three TDEM systems developed by Fugro Airborne Surveys are used to demonstrate the effects that different system specifications have on the response of an exploration target.

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.

Historical Development and Performance of Airborne Magnetic and Electromagnetic Systems for Mapping and Detection of Unexploded Ordnance

Over the past fifteen years, notable progress has been made in the performance of airborne geophysical systems for mapping and detection of unexploded ordnance in terrestrial and shallow marine environments. For magnetometer systems, the most significant improvements include development of boom-mounted platforms, and implementation of higher sample rates, denser magnetometer arrays, and vertical gradient configurations. Nine magnetometer-based systems are described and their performance summarized. In prototype analyses and recent U.S. Department of Defense Environmental Security Technology Certification Program (ESTCP) assessments using new production systems, the best performance has been achieved with a vertical gradient configuration. As effective as magnetometer systems have proven to be at many sites, they are inadequate at sites where basalts and other ferrous geologic formations or soils produce anomalies that approach or exceed those of target ordnance items. Additionally, magnetometer systems are ineffective where detection of non-ferrous ordnance items is of primary concern. We discuss the development of airborne time-domain electromagnetic systems over the past ten years. Overall, improvements in airborne geophysical systems have led to more consistent detection of smaller ordnance. These trends should continue as additional technological advances are made.

Quantification of modeling errors in airborne TEM caused by inaccurate system description

Being able to recover accurate and quantitative descriptions of the subsurface electrical conductivity structure from airborne electromagnetic data is becoming more and more crucial in many applications such as hydrogeophysical and environmental mapping, but also for mining exploration. The effect on the inverted models of inaccurate system description in the 1D forward modeling of helicopter time-domain electromagnetic (TEM) data was studied. The most important system parameters needed for accurate description of the subsurface conductivity were quantified using a nominal airborne TEM system and three different reference models to ensure the generality of the conclusions. By calculating forward responses, the effect of changing the system transfer function of the nominal airborne TEM system was studied in detail. The data were inverted and the consequences of inaccurate modeling of the system transfer function were studied inthe model space. Errors in the description of the transfer function influence the inverted model differently. The low-pass filters, current turn-off, and receiver-transmitter (Rx-Tx) timing issues primarily influenced the early time gates. The waveform repetition, gate integration, altitude, and geometry mainly influenced the late time gates. Depth of investigation is highly model dependent, but in general the early times hold information on the shallower parts of the model and the late times hold information on the deeper parts of the model. Amplitude, gain, and current variations affect the entire sounding and therefore the entire model. The results showed that all of these parameters should be measured and modeled accurately during inversion of airborne TEM data. If not, the output model can differ quite dramatically from the true model. Layer boundaries can be inaccurate by tens of meters, and layer resistivities by as much as an order of magnitude. In the worst cases, the measured data simply cannot be fitted within noise level.

Predictive filter calculation of primary fields in a fixed-wing time-domain AEM system

High-altitude data are used to calibrate a least-squares recursion filter that estimates the continually changing primary field of an airborne electromagnetic (AEM) system. The coupling changes in fixed-wing towed-bird systems generate “geometry noise” that in the on-time can be much larger than the ground secondary response. The LSQ filter accurately predicts the high-altitude primary field of a fixed-wing system. The filter is then applied to survey-altitude data to estimate the primary field for subsequent subtraction. After removing the primary field, a spatially consistent difference is detected over a range of delay times, as would be expected from geologic responses. A map of decay constants is produced for the survey area using the data corrected by the predicted primary field. Comparing these time constants with those computed from the conventional method, the maximum decay constant detectable was seven times larger. Thus, the new process can characterize conductors that are seven times more conductive than the conventional processing method. The residual primary field occurs at relatively high frequencies compared to targets of interest. At low frequencies [Formula: see text], we estimate that 28% of the survey-altitude primary field remains whereas only 1% of calibration flight primary is not predicted.

Constrained inversion of AEM data for mapping of bathymetry, seabed sediments and aquifers

A shallow (depth < 20 m) layer of water, fresh, brackish or saline, covers millions of square kilometres of sediments and bedrock along the world?s coastlines, rivers, lakes, and lagoons. These geological units are extremely important, both environmentally and economically. Airborne electromagnetic (AEM) has been used with some success to obtain the bathymetry of shallow surface water ((Macnae et al., 2004; Vrbancich et al., 2000; Vrbancich et al., 2005). Some attempts have also been made to retrieve information about the sediments under the water bottom (Vrbancich et al., 2000 ). The limited research that has been carried out so far tells us that we need to make improvements and further developments, both in hardware, as well as in data processing and modelling. This manuscript aims at making a contribution at the data inversion level, by applying constrained inversion methodology to different AEM datasets flown over water. In constrained inversion, adjacent model parameters are regularized through lateral constraints that allow information to flow from soundings that contain more information to those that contain less. We present results from constrained inversion (smooth and few layers) of a portion of SkyTEM survey (a helicopter borne time domain AEM system) flown in Denmark, and of a RESOLVE survey ( a helicopter borne frequency domain AEM system) carried out along the Murray River in Australia. In both cases bird height was included as an inversion parameter, allowing the inversion to compensate for errors in laser altimeter readings over water.

Bathymetry and sediment depth investigation in Broken Bay using a prototype AEM time domain system (SeaTEM)

The use of airborne electromagnetic (AEM) methods for measuring water depth and estimating sediment thickness has been demonstrated using commercial AEM equipment that is not optimized for marine surveying. A new prototype helicopter time domain AEM system, SeaTEM(0), is under development for bathymetric surveying. The first sea trial of the SeaTEM(0) system took place over Broken Bay, NSW, in shallow water up to ~ 30 m in depth. The SeaTEM(0) system was untested and the Broken Bay survey identified instrumentation problems that will be addressed in future surveys. Broken Bay was chosen because the separate paleodrainage systems for the Hawkesbury River, Brisbane Waters and Pittwater which join in Broken Bay give rise to paleo-valleys infilled with unconsolidated sediments, ranging in thickness between 0 m (bedrock outcrop) and ~ 200 m. Sediment thickness and water depth is predicted from stitched 1D inversion of data based on a simplified two-layer model that represents seawater and sediment overlying a resistive half-space basement (bedrock). The resulting bathymetric profiles show agreement typically to within ~ ±1 m with known water depths in areas less than 20 m deep. The inverted depth profile of the second (sediment) layer is noisy; however, the profiles reveal coarse topographic features of paleochannels to depth limits of ~ 80 m below sea level in 30 m water depth.

Towards remote sensing of sediment thickness and depth to bedrock in shallow seawater using airborne TEM

Following a successful bathymetric mapping demonstration in a previous study, the potential of airborne EM for seafloor characterisation has been investigated. The sediment thickness inferred from 1D inversion of helicopter-borne time-domain electromagnetic (TEM) data has been compared with estimates based on marine seismic studies. Generally, the two estimates of sediment thickness, and hence depth to resistive bedrock, were in reasonable agreement when the seawater was ~20 m deep and the sediment was less than ~40 m thick. Inversion of noisy synthetic data showed that recovered models closely resemble the true models, even when the starting model is dissimilar to the true model, in keeping with the uniqueness theorem for EM soundings. The standard deviations associated with shallow seawater depths inferred from noisy synthetic data are about ± 5% of depth, comparable with the errors of approximately ± 1 m arising during inversion of real data. The corresponding uncertainty in depth-to-bedrock estimates, based on synthetic data inversion, is of order of ± 10%. The mean inverted depths of both seawater and sediment inferred from noisy synthetic data are accurate to ~1 m, illustrating the improvement in accuracy resulting from stacking. It is concluded that a carefully calibrated airborne TEM system has potential for surveying sediment thickness and bedrock topography, and for characterising seafloor resistivity in shallow coastal waters.

Comparison of Airborne and Ground TEM systems for a conductor beneath conductive cover - an example from North-West Queensland, Australia

Magnetite-pyrrhotite mineralisation at the Bull Creek Prospect in North-West Queensland is both magnetic and conductive. The mineralisation of Proterozoic age is buried beneath 30-50m of conductive younger cover. Overburden conductance values of 10 to 30 S make this a challenging area for airborne electromagnetic systems. The presence of this conductive cover and availability of ground EM and drilling information make this a valuable case study for evaluating the performance of airborne electromagnetic systems. This paper presents data from both ground and airborne time domain electromagnetic systems and compares conductivity sections derived from the data with drilling information. The moving loop ground EM system produced a very clear response from the basement conductor. This conductor was also detected by the 25 Hz GEOTEM and TEMPEST systems flown over the prospect in 1997 and 1998 respectively. Conductivity sections derived from measured response data were useful for visualising the response variations. Due to the width of the discrete conductor and the continuity of the conductive cover, conductivity sections based on a 1-dimensional approximation provided reasonable quantitative estimates for the thickness of conductive cover and the depth to the conductor.

A helicopter time-domain EM system applied to mineral exploration: system and data

David Fountain, Richard Smith, Tom Payne, and Jean Lemieux (Fugro Airborne Surveys) describe recent progress in developing helicopter systems for time-domain airborne electromagnetic surveys, once the ‘domain’ of fixed-wing aircraft. The first operational fixed-wing airborne electromagnetic (AEM) system was introduced in 1948 and was followed by the first operational helicopter AEM system in 1955 (Fountain, 1998; Fountain and Smith, 2003). There has been parallel development of both fixedwing and helicopter AEM since that time and, up to the early 1960s, all these systems were frequency-domain. However, since the late 1970s, fixed-wing AEM systems have been primarily time-domain while the helicopter systems remained primarily frequency-domain. With the new millennium, and especially since 2002/ 2003, there has been significant development and introduction of helicopter time-domain AEM systems (Boyko, et al., 2001; Balch et al., 2003; Eaton et al., 2004; Sorenson and Auken, 2004; Vrbancich and Fullagar, 2004; Witherly et al., 2004). The pace of introduction has been such that, today, the number of time-domain systems in use is equal to the number of frequency-domain systems. Fugro Airborne Surveys and its predecessor companies have been leaders in the continuing development of fixedwing AEM systems, including the introduction of the GEOTEM, MEGATEM, and TEMPEST systems (Smith and Annan, 1997; Lane et al., 2000; Smith, Fountain and Allard, 2003). By the same token, Fugro has been involved in the continuing development of helicopter frequency-domain AEM systems including DIGHEM and RESOLVE. Based upon this combined experience and seeing the need for a broader band, high power helicopter time-domain AEM system, Fugro has introduced the HeliGEOTEM system in 2005. This system brings together the proven GEOTEM/ MEGATEM technology with the greater operational flexibility and improved lateral resolution of helicoptermounted AEM systems.