Helicopter Electromagnetic System Research Articles

A Bipolar Half-Sine Current Inverter for Airship-Borne Electromagnetic (AEM) Surveying

A topology of a bipolar half-sine current inverter for geophysical airship-borne transient electromagnetic (EM) surveying is developed. In detail, the RLC series resonance inverter with novelty is designed and analyzed. The key parameters of the output current with very small overshoots and high current amplitude are calculated and simulated, respectively. The results of calculation and simulation agree well with each other. The circuit design and principle analysis of the converter are described clearly. The main reason we choose series resonance current inverter rather than the more common parallel one is that a low-voltage source, such as a standard 28-V dc voltage of the usual aircraft power source, could realize high peak current output through a transient series resonance process efficiently. The proposed current inverter is successfully implemented to the airship-mounted geophysical surveying systems. It achieves a substantial improvement of performance and provides an alternative for the traditional fixed-wing EM systems or helicopter EM systems with much lower cost and higher efficiency. Besides, the experimental results for explorations demonstrate that the measured magnetic field of three axes also agrees well with application requirements of airship-borne EM system.

Power line ground resistance detection using helicopter electromagnetic systems

Power line grounding can be a serious concern for the power supply industry. Only a small amount of ground resistance is acceptable because a good electric contact of power line towers with the earth reduces the possibility of the natural hazard associated with lightning. We investigated the possibility of using helicopter-borne electromagnetic (HEM) apparatus to detect the ground resistances of power line towers. Two induction systems were considered: a transmitter-earth-receiver system and a power line loop-earth-receiver system. The transmitter-earth-receiver system built the background signal in the HEM receiver, whereas the power line loop-earth-receiver system produced the signal used for resolving the power line ground resistances. By ignoring the mutual coupling between the two systems, we calculated the induced electromotive forces in the power line loop above which the HEM bird was flown and its two immediate neighbor loops. We replaced other loops along the power line with the Norton-Thevinin equivalence and solved the current flow in the power line loop by the mesh current method. Using the current in the power line loop as induction source, we calculated the electromagnetic responses in the HEM receiver from the power line loop-earth-receiver system. For identification of the poorly grounded towers, we introduced ratios of in-phase and quadrature to signal amplitude and the HEM phase. We evaluated numerical experiments showing that these parameters can well identify power line towers with high ground resistances, and they were insensitive to the flight altitude and direction. Moreover, we also took the catenaries of the power line into consideration so that the elevation differences between towers resulting from rugged mountains could be easily incorporated into the interpretation. We considered vertical coaxial (VCA) and horizontal coplanar (HCP) coil configurations. It was found that the signal for HCP coil has multiple peaks, whereas the VCA coil signal has only a single peak, so we recommend the VCA coil array for power line ground resistance detection. The inversion of synthetic data showed that the power line ground resistances can be resolved from the HEM survey.

Metalliferous mining geophysics — State of the art after a decade in the new millennium

Mining exploration was very active during the first decade of the twenty-first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration. Development came from different sources: instrumentation improvements, new numerical algorithms, and cross-fertilization with the seismic industry. In gravity, gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration. In potential-field methods in general, numerous techniques have been developed for automatic interpretation, and 3D inversion schemes came into frequent use. These inversions will have even greater use when geologic constraints can be applied easily. In airborne electromagnetic (EM) methods, the development of time-domain helicopter EM systems changed the industry. In parallel, improvements in EM modeling and interpretation occurred; in particular, the strengths and weaknesses of the various algorithms became better understood. Simpler imaging schemes came into standard use, whereas layered inversion seldom is used in the mining industry today. Improvements in ground EM methods were associated with the development of SQUID technology and distributed-acquisition systems; the latter also impacted ground induced-polarization (IP) methods. Developments in borehole geophysics for mining and exploration were numerous. Borehole logging to measure physical properties received significant interest. Perhaps one reason for that interest was the desire to develop links between geophysical and geologic results, which also is a topic of great importance to mining geologists and geophysicists.

A comparative study of in-river geophysical techniques to define variations in riverbed salt load and aid managing river salinization

Increased interest in the character of sediments at the base of waterways, for the purpose of managing river salinization, has led to the application of several geophysical techniques for collecting information from this zone. These instream methods are based on established ground and airborne electrical and electromagnetic technologies, including towed transient electromagnetic systems, towed direct current resistivity array systems, and frequency-domain helicopter electromagnetic systems. Although these systems are individually successful, a systematic examination of their relative effectiveness for identifying variations in substrate conductivity for a common stretch of a river remains lacking. We have compared results obtained from data collected using three instream geophysical techniques for a common stretch of the Murray River in southeastern Australia. The Murray River is an important water resource for drinking and agricultural purposes. Data from these surveys were acquired tolocate areas of significant saltwater accession to the Murray from a saline regional groundwater system that discharges into it. The three methods indirectly inform on those reaches that most likely contribute to higher salt loads in the river, and they do this through the identification of a conductive substrate (a gaining reach). For a [Formula: see text] stretch of the river, the methods identified similar variations in the conductivity structure of sediment substrate, although differences were observed in the modeled response relating to intrinsic differences between each system, including the sampling interval and resolution. The helicopter electromagnetic (EM) system is capable of acquiring hundreds of kil-ometers of data in a day, under any river flow condition, from near the river surface to depths in excess of [Formula: see text]. The other two techniques require safe river flow conditions for acquisition, with as much as 50 river km of data per day possible. The ground-based methods had enhanced lateral and vertical resolving capabilities relative to the helicopter EM system, but their depth of investigation was less (usually only [Formula: see text]).

Sea ice as an evaluation target for HEM modelling and inversion

We investigate helicopter electromagnetic (HEM) inversion schemes applied to synthetic and measured HEM sea ice profiling data. Direct HEM data-to-ice-thickness inversion is compared to three different formal, least squares layered earth inversion algorithms. By making several approximations, it is possible to directly invert a single channel measurement (i.e., the in-phase or quadrature component of a single frequency measurement) to obtain an estimate of sea ice thickness. Measurements from multiple input channels, however, can be used in a layered earth inversion to simultaneously recover several model parameters such as sea ice thickness, sea ice conductivity and sub-ice bathymetry. Synthetic data sets for a particular two-frequency HEM system showed that simple least squares inversion algorithms produce reliable estimates of sea ice thickness in cases where the ice is thicker than 3 m. These methods could also recover acceptable estimates of sea ice thickness when a thin, conductive, partially melted sea ice layer was present, and could determine shallow, sub-ice bathymetry in brackish water. As expected, 1D transformations and inversions of synthetic data for a three-dimensional pressure ridge keel structure contained artifacts, notably broadening of the apparent width of the keel. Prior to inverting a field data set acquired over rather thin (~ 0.5 m) Antarctic sea ice, we found it necessary to recalibrate the phase angle of the measurements using a phasor diagram-based method. Direct transformation of a single channel from the recalibrated data set produced more accurate estimates of sea-ice thickness than formal inversion of multi-channel data. We suggest that the least squares inversion methods are inferior in this situation because of the particular characteristics of the two-frequency HEM system used in this evaluation; the extreme differences in sensitivity of high and low frequency data components, the overall low sensitivity to sea ice conductivity (especially for thin ice), and the partially low signal-to noise ratios of the measurements. The data sets used in this study will be made available to the public to allow alternate inversion approaches to be applied and evaluated. It is suggested that inclusion of parameter bounds and other forms of regularization could help to improve the inversion results.

Direct helicopter EM — Sea-ice thickness inversion assessed with synthetic and field data

Accuracy and precision of helicopter electromagnetic (HEM) sounding are the essential parameters for HEM sea-ice thickness profiling. For sea-ice thickness research, the quality of HEM ice thickness estimates must be better than [Formula: see text] to detect potential climatologic thickness changes. We introduce and assess a direct, 1D HEM data inversion algorithm for estimating sea-ice thickness. For synthetic quality assessment, an analytically determined HEM sea-ice thickness sensitivity is used to derive precision and accuracy. Precision is related directly to random, instrumental noise, although accuracy is defined by systematic bias arising from the data processing algorithm. For the in-phase component of the HEM response, sensitivity increases with frequency and coil spacing, but decreases with flying height. For small-scale HEM instruments used in sea-ice thickness surveys, instrumental noise must not exceed [Formula: see text] to reach ice thickness precision of [Formula: see text] at 15-m nominal flying height. Comparable precision is yielded at 30-m height for conventional exploration HEM systems with bigger coil spacings. Accuracy losses caused by approximations made for the direct inversion are negligible for brackish water and remain better than [Formula: see text] for saline water. Synthetic precision and accuracy estimates are verified with drill-hole validated field data from East Antarctica, where HEM-derived level-ice thickness agrees with drilling results to within 4%, or [Formula: see text].

3D animated visualization of EM diffusion for a frequency-domain helicopter EM system

The electromagnetic fields inside the earth are calculated by continuation downward of the electromagnetic (EM) solutions at the location of a frequency-domain helicopter electromagnetic (HEM) sensor. Models examined using the continuation approach include a layered isotropic and anisotropic earth. The finite-element approach is used to model 2D structures of a dipping contact or a dipping dike. By incorporating a time factor, we display the EM diffusion in the earth (change in direction and amplitude of the EM field through time) as 3D animated vectors or contours. The propagation of the EM smoke ring, influenced by the resistivity and structure of the earth, is apparent from the dynamic presentations. The current propagates downward and outward with time, becoming wider and more diffuse, and the phase varies with time, depth, and outward distance. The downward propagation of EM fields is slower in more conductive geology. In a layered isotropic earth, the current ring is symmetric with no vertical current flow for both vertical and horizontal dipole transmitters. In anisotropic or 2D structures; however, the current flow is significantly distorted resulting in vertical current flow and nonsymmetric smoke rings.

Pendulum motion in airborne HEM systems

Helicopter electromagnetic systems typically have a transmitter-receiver coil pair in a towed bird that, in flight, behaves as a complex compound pendulum. Pendulum motions of the towed bird create not only a geometric and inductive effect in the measured signal, but they may also contribute to altitude error. The oscillation of the towed-bird pendulum can be observed from both video recordings and GPS positions and is most usefully broken down into two modes: one in the direction of travel (in-line) and one perpendicular to the direction of travel (cross-line). We analyse the electromagnetic and altitude data for a RESOLVE survey on the Chowilla flood plain, near Renmark, South Australia. The in-line and cross-line motions of the towed bird are shown to have different frequencies of oscillation. In-line motions, which cause pitching of the towed bird, and cross-line motions, which generate roll, create a systematic error in data easily evident in the dimensionless in-phase divided by inductive limit (R/G) domain. We show that the bird swing due to both modes of pendulum motion is responsible for altitude error, and that a filter designed to use the observed period of oscillation can largely correct the R/G domain data. The filtering process does not require any more measurements than are usually provided from a typical electromagnetic survey.

Airborne electromagnetics in Europe: recent activities and future goals

Annika Steuer and Uwe Meyer of the Federal Institute for Geosciences and Natural Resources, report on a recent gathering of organizations in Europe using airborne electromagnetics. In numbers, European systems are dominated by frequency domain instruments. The Geological Surveys of Norway, Germany, and Austria, as well as the AWI, use helicopter electromagnetic (HEM) systems, which differ in the number of frequencies and configuration of the coil systems. NGU and GBA use the Geotech Hummingbird, NGU with three horizontal coplanar and two horizontal coaxial coils whereas GBA uses four horizontal coplanar coils. BGR operates a DIGHEM bird from Fugro with five horizontal coplanar coils. AWI developed a dual-frequency HEM system, specially designed for the sounding of seaice thicknesses in Polar Regions. The Geological surveys of the United Kingdom (BGS) and Finland (GTK) established a 'Joint Airborne-geoscience Capability' (JAC) based on a system originally developed by GTK. They operate a fixed wing aircraft carrying a four-frequency system using vertical coplanar coils. The Swedish Geological Survey and the University of Uppsala also use a fixed-wing. Both developed a new VLF instrument working in the wide frequency band of 1-350 kHz. At present, the only time-domain system in Europe is operated by the University of Aarhus. The Aarhus Sky TEM system is a transient electromagnetic technique in a 'central loop'-like configuration, towed by a helicopter.

Influence of displacement currents on the response of helicopter electromagnetic systems

For the purpose of shallow-earth geophysical mapping, progressively higher frequencies have been developed for helicopter electromagnetic (HEM) systems. However, concern has been expressed about the vulnerability of high-frequency EM signals to the influence of the displacement current, especially the phase shift of the HEM signal resulting from the finite speed of light that describes the propagation of the EM wave in free space. In this paper we investigate the influence of the displacement current and the finite speed of light on HEM responses, based on a full solution of the EM field for a conductive, magnetically, and dielectrically polarizable earth half-space and an overlying half-space of air with free-space dielectric permittivity. We calculate the amplitude change and the phase shift of the HEM signal and the change in the apparent resistivity. We find that the displacement current, when both the air and the earth half-space assume the free-space dielectric permittivity, has a small influence on the HEM signal, while substantial influence may occur when the earth is dielectrically polarizable. The finite speed of the EM propagation in free space does not result in significant phase changes in the HEM signal.

SkyTEM–a New High-resolution Helicopter Transient Electromagnetic System

SkyTEM is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation. Developed as a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 × 40 m2 system. Independent of the helicopter, the entire system is carried as an external sling load. In the present system, the transmitter, mounted on a lightweight wooden lattice frame, is a four-turn 12.5 x 12.5 m2 square loop, divided into segments for transmitting a low moment with one turn and a high moment with all four turns. The low moment uses about 30 A with a turn-off time of about 4 |as; the high moment draws approximately 50 A, and has a turn-off time of about 80 |J,s. The shielded, overdamped, multi-turn receiver loop is rigidly mounted on the side of the transmitter loop. This is essentially a central-loop configuration with a 1.5 m vertical offset.In vertical hover mode the SkyTEM responses were within 2% of those from a conventional ground-based system. Instrument bias level is not a concern as high-altitude tests showed that the background noise level is higher than the instrument bias level. By inverting a sounding from a test site to a standard model and then applying the SkyTEM system parameters to compute the forward response, conventional measurements were within 5% of SkyTEM responses for flight heights of 7.25, 10, and 20 m. Standard field operations include establishment of a repeat base station in the survey area where data are acquired approximately every 1.5 hours, when the helicopter is refuelled, to monitor system stability. Data acquired in a production survey were successful in detecting and delineating a buried-valley structure important in hydrogeophysical investigations.

The effect of the electrical anisotropy on the response of helicopter‐borne frequency‐domain electromagnetic systems

ABSTRACTHelicopter electromagnetic (HEM) systems are commonly used for conductivity mapping and the data are often interpreted using an isotropic horizontally layered earth model. However, in regions with distinct dipping stratification, it is useful to extend the model to a layered earth with general anisotropy by assigning each layer a symmetrical 3 × 3 resistivity tensor.The electromagnetic (EM) field is represented by two scalar potentials, which describe the poloidal and toroidal parts of the magnetic field. Via a 2D Fourier transform, we obtain two coupled ordinary differential equations in the vertical coordinate. To stabilize the numerical calculation, the wavenumber domain is divided into two parts associated with small and large wavenumbers. The EM field for small wavenumbers is continued from layer to layer with the continuity conditions. For large wavenumbers, the EM field behaves like a DC field and therefore cannot be sensed by airborne EM systems. Thus, the contribution from the large wavenumbers is simply ignored.The magnetic fields are calculated for the vertical coaxial (VCX), horizontal coplanar (HCP) and vertical coplanar (VCP) coil configurations for a helicopter EM system. The apparent resistivities defined from the VCX, VCP and HCP coil responses, when plotted in polar coordinates, clearly identify the principal anisotropic axes of an anisotropic earth. The field example from the Edwards Aquifer recharge area in Texas confirms that the polar plots of the apparent resistivities identify the principal anisotropic axes that coincide well with the direction of the underground structures.

Field tests of an experimental helicopter time‐domain electromagnetic system for unexploded ordnance detection

Field trials of a low‐flying time‐domain helicopter electromagnetic system designed for detection of unexploded ordnance have yielded positive and encouraging results. The system is able to detect ordnance as small as 60‐mm rounds at 1‐m sensor height. We examined several transmitter and receiver configurations. Small loop receivers gave superior signal‐to‐noise ratios in comparison to larger receiver loops at low heights. Base frequencies of 90 Hz and 270 Hz were less affected than other base frequencies by noise produced by proximity to the helicopter and by vibration of the support structure. For small ordnance, a two‐lobed, antisymmetric transmitter loop geometry produced a modest signal‐to‐noise enhancement compared with a large single rectangular loop, presumably because the antisymmetric transmitter produces smaller eddy currents in the helicopter body, thereby reducing this source of noise. In most cases, differencing of vertically offset receivers did not substantially improve signal‐to‐noise ratios at very low sensor altitudes. Signal attenuation from transmitter to target and from target to receiver causes signals from smaller ordnance to quickly become indistinguishable from geological background variations, so that above a sensor height of about 3 m only large ordnance items (e.g., bombs and large caliber artillery rounds) were consistently detected.

Attitude corrections of helicopter EM data using a superposed dipole model

The analysis of helicopter‐borne electromagnetic (EM) survey data commonly assumes that the EM sensor (bird) has flown straight and levelly, whereupon the coils are either horizontal or vertical. The EM data typically are transformed, under this assumption, to yield the electrical properties (conductivity, permeability, permittivity) of a homogeneous earth, or they are inverted to a horizontally layered earth. In actual fact, the bird exhibits some roll, pitch, and yaw rotation, which will generate changes to the data that we call bird attitude effect. On the basis of a superposed dipole model, we distinguish the geometric part of the bird attitude effect from the inductive part. Theoretical investigation shows that the geometric effect results from the reorientation of the bird's coil axes related to the earth. It is independent of the system frequency and the electrical properties of the earth but dependent only on the bird's attitude. In contrast, the inductive effect results from the coupling change between the coils and the conductive earth. It depends not only on the attitude of the bird but also on the system frequency and the earth's electrical properties. We calculate ratios of the EM responses when the bird is rotated to those when the bird flies straight and levelly. These ratios, reflecting the purely geometric effect of the bird's rotation, are simple trigonometric functions of the rotation angles and thus can be easily incorporated into our processing of the EM data. We show that, in the general case of helicopter EM systems, the majority (>95%) of the attitude error results from the geometric effect rather than from the inductive effect. Sufficient accuracy is obtained by using the simple geometric technique for attitude correction, without requiring information about the earth's electrical properties. We demonstrate the usefulness of the method, using an example from a test survey.

Developing recharge reduction strategies in the Riverland of South Australia using airborne electromagnetic data – a case study in tailoring airborne geophysics given a particular target and a desired set of outcomes.

The Riverland area, located on the southern bank of the River Murray in South Australia, is a priority area for intervention under the National Action Plan. As part of the South Australian Salinity Mapping and Management Support Project, airborne geophysics was recognised as having potential to provide valuable biophysical data relevent to the management of irrigation development and groundwater recharge reduction in the area. We examine this potential, giving particular regard to the process involved in understanding the target, defining an appropriate geophysical tool and testing whether the desired output could be delivered economically and at an appropriate resolution. The target in the Riverland area is the near surface Blanchetown Clay unit. Forward modelling suggested that a frequency domain helicopter electromagnetic (HEM) system could map spatial variability associated with this unit. A test survey was conducted using the RESOLVE HEM system further demonstrated that potential. CDI?s of these data were compared with shallow drilling, EM31, EM34 and broadband ground TEM data. Results confirmed that near surface conductivity variations mapped by the airborne EM system were associated with the clay and that a product which could be incorporated with hyrogeological models to help predict rechage and influence management decisions in the area could be generated for the whole area.

Inversion of helicopter electromagnetic data to a magnetic conductive layered earth

Inversion of airborne electromagnetic (EM) data for a layered earth has been commonly performed under the assumption that the magnetic permeability of the layers is the same as that of free space. The resistivity inverted from helicopter EM data in this way is not reliable in highly magnetic areas because magnetic polarization currents occur in addition to conduction currents, causing the inverted resistivity to be erroneously high.A new algorithm for inverting for the resistivity, magnetic permeability, and thickness of a layered model has been developed for a magnetic conductive layered earth. It is based on traditional inversion methodologies for solving nonlinear inverse problems and minimizes an objective function subject to fitting the data in a least‐squares sense. Studies using synthetic helicopter EM data indicate that the inversion technique is reasonably dependable and provides fast convergence. When six synthetic in‐phase and quadrature data from three frequencies are used, the model parameters for two‐ and three‐layer models are estimated to within a few percent of their true values after several iterations. The analysis of partial derivatives with respect to the model parameters contributes to a better understanding of the relative importance of the model parameters and the reliability of their determination.The inversion algorithm is tested on field data obtained with a Dighem helicopter EM system at Mt. Milligan, British Columbia, Canada. The output magnetic susceptibility‐depth section compares favorably with that of Zhang and Oldenburg who inverted for the susceptibility on the assumption that the resistivity distribution was known.

Improved and new resistivity-depth profiles for helicopter electromagnetic data

The calculation of apparent resistivities, based on the model of a homogeneous half-space, is commonly the first step in order to evaluate helicopter electromagnetic (HEM) data. Due to the increase in frequencies used in HEM systems, survey results are not only displayed as apparent resistivity maps but also as cross-sections that require resistivity and depth information. After a brief description and discussion of the basic approaches, improved HEM resistivity–depth profiles are derived. The apparent resistivities ρ a are calculated more accurately when better approximations are used. The corresponding depth value, the centroid depth z p*, is newly defined as the sum of the apparent depth and the half of the apparent skin depth. The resulting profile is referred to hereafter as the improved standard sounding curve ρ a( z p*). Several algorithms for deriving “enhanced” resistivity–depth profiles, which are more sensitive to resistivity variations with respect to depth, are presented. Two of these enhanced resistivity–depth profiles are based on algorithms used for the interpretation of MT data. The ρ NB( z s*) sounding curve is derived from the Niblett–Bostick algorithm. It requires multifrequency data because the enhancing is achieved by differentiating the ρ a( f) sounding curve with respect to frequency. The other one, the ρ*( z d*) sounding curve, is computed from each frequency independently of the other frequencies because no differentiation is involved. It is similar to Schmucker's ρ*– z* scheme and it uses the apparent depth d a to enhance the sensitivity of the apparent resistivity. Furthermore, both novel algorithms are able to increase the depth of exploration. All enhanced resistivity–depths profiles are compared with the improved standard sounding curve ρ a( z p*) and with the differential parameter method, ρ Δ( z Δ), published by Huang and Fraser. Since being robust and easy to calculate, the proved ρ a( z p*) method should be used for the standard calculation of resistivity–depth profiles. Besides, it is the basis for the enhanced methods, which, in addition, can be used to derive more sensitive resistivity–depth profiles. Among all methods discussed, only ρ a and ρ NB are relatively independent of the measured sensor height h, which is a serious problem in survey areas with dense forests. The corresponding centroid depth values, z p* and z s*, are not distorted by the vegetation, if they are displayed with respect to the elevation of the HEM sensor which should be given in m a.s.l.

Sources of calibration errors in helicopter EM data

The procedures used to calibrate helicopter electromagnetic systems are analysed to determine the errors produced both by mispositioning the Q-coil and by the effects of conducting ground at the calibration site. In general, errors produced by mispositioning the Q-coil can be greater than 2 percent/cm of displacement for movement in-line with the bird axis. Displacement in other directions produces smaller errors. The use of calibration jigs to stabilise Q-coil position is strongly recommended. The effect of conductive ground at the calibration site depends upon the conductivity of the ground, the transmitter frequency, and the coil geometry. As transmitter frequency increases, conductivity effects appear at higher resistivities for typical five-coil-pair birds with frequencies ranging from 900 Hz to 56 kHz. In general, these effects start to appear in the quadrature channels as resistivities drop below 100 ohm-m. Inphase channels are not affected until resistivities are less than 10 ohm-m. The horizontal coplanar coil geometry is more susceptible to conductivity effects than the vertical coaxial coil geometry, which is relatively immune to these effects at resistivities greater than 10 ohm-m for frequencies up to at least 7200 Hz.