Airborne Transient Electromagnetic System Research Articles

An efficient 2.5‐D forward algorithm based on the spectral element method for airborne transient electromagnetics data

AbstractThe airborne transient electromagnetics method has been widely used in geophysical exploration in recent years, but it still faces challenges in balancing the accuracy and efficiency of electromagnetic data interpretation. As far as forward modelling is concerned, the higher the dimension of the geophysical model, the higher the accuracy of data interpretation, but, correspondingly, the more computing resources need to be consumed, which will greatly reduce investigation efficiency and practicability. In this paper, the spectral element method is first introduced for solving the 2.5‐dimensional forward modelling of the airborne transient electromagnetic system, which has a smaller computing scale than three‐dimensional modelling and is closer to the actual geological structure than either one‐ or two‐dimensional modelling. In the forward algorithm, a non‐uniform quadrilateral structured mesh is adopted to simplify the computing scale, and the Talbot algorithm rather than Gaver–Stefest algorithm is applied to the inverse Laplace transform to improve the numerical precision of this conversion. Moreover, we use parallel computing technology to improve the algorithm efficiency while keeping satisfactory accuracy. The study shows that, whether a low‐resistivity or high‐resistivity layered geophysical model, the numerical solutions of the proposed spectral element method forward algorithm agree well with the analytical solutions of the corresponding models; furthermore, the key factors affecting the accuracy of the numerical solution are analysed by experiments. Finally, we successfully applied it to the 2.5‐dimensional geoelectric model simulation.

Performance of light fixed‐wing airborne time‐domain electromagnetic system for mapping the near‐surface cover layer in an alluvial plain context: A numerical study

AbstractThe mapping of the vertical and lateral variations in the physical properties of the few‐meter cover layer over near‐surface aquifers is important for hydrogeological modelling, particularly for the quantification of the recharge of groundwater systems. The first ground‐based time‐domain electromagnetic survey over a small catchment (Avesnelles, France) of the watershed of Orgeval (Seine basin) was carried out to determine discontinuities in the first silt layer as well as in the Brie multilevel aquifer limestone horizon. The results highlighted the following: (1) a good sensitivity of the time‐domain electromagnetic survey to the presence of multi‐decametric resistive sand lenses, particularly in a location where they were previously identified and (2) the interest in conducting a survey at a fine sampling step but extending to the meso‐scale. To overcome the sampling issue over a watershed of several hundred square kilometres, we proposed numerically assessing the use of a prototype of low‐cost airborne transient electromagnetic systems towed by light fixed‐wing airplanes (with transmitting and receiving loops in the same plane). The present numerical analysis, in 1D for the vertical (i.e. thickness) variation and in 3D for the lateral extensions of localized sandy and resistive units, showed that a conductive few‐meter cover can be mapped even with a system flying at 50 m with, however, the need of a priori constraint on the resistivity of the first layer to estimate its thickness variation as accurately as possible. Even if it did not bring more sensitivity to the layer thickness and despite the severe difficulty of practical implementation with a decametric emission loop, the vertical co‐planar configuration potentially offered better near‐surface lateral resolution (down to ∼40 m) to delineate the sandy units (discontinuities) within the silt layer (if units are at least 50 m in size) and provided better spatial constraints compared to the classical horizontal co‐planar geometry used in the time‐domain electromagnetic. Even if not aerodynamically in the plane of the emission loop, the measurement of the Hx component with a vertical dipole emission loop (PERPxz geometry for perpendicular) improved the lateral resolution (down to ∼20 m; still with at least 50 m size sand units) and confirmed that a geometry different from the classical horizontal co‐planar configuration could be valuable.

Comparison of ground-based and airborne transient electromagnetic methods for mapping glacial and permafrost environments: Cases from McMurdo Dry Valleys, Antarctica

The transient electromagnetic (TEM) method is a non-invasive geophysical tool well-suited for subsurface imaging in cold and polar regions, where common targets are associated with strong contrasts in electrical resistivity. By imaging the electrical properties of the subsurface, the TEM methods can discriminate between geological units such as frozen ground (permafrost), fresh/saline groundwater systems, and bedrock/glacier ice. In this study, we compare TEM data acquired with ground-based and airborne TEM systems. We demonstrate the mapping capabilities of these two approaches in high latitude polar environments with datasets from Taylor Glacier, Lake Vanda, and Canada Glacier in the McMurdo Dry Valleys of Antarctica. The results show a high consistency between the airborne and ground-based TEM data, both with a high resolution and a deep penetration depth down to hundreds of meters due to the resistive background material, which makes both approaches capable of mapping hydrological systems and identifying the base of glaciers. The airborne TEM approach offers an unmatched spatial data coverage in difficult terrain and a far improved lateral resolution compared to the static ground-based system. The ground-based TEM system offers the possibility for using larger transmitter coils and longer stacking times and therefore has potential for reaching deeper penetration depths. The ground-based TEM approach is hence a valuable tool that can provide consistent imaging results while also being far more accessible in terms of cost and field logistics compared to an airborne TEM campaign.

A field programmable gate array controlled high‐current pulsed source for airborne transient electromagnetic system

A field programmable gate array controlled high‐current pulsed source for airborne transient electromagnetic system

Weak Coupling Technology With Noncoplanar Bucking Coil in a Small-Loop Transient Electromagnetic System

Small-loop transient electromagnetic (TEM) method can be used for urban underground space detection. Given the mutual inductance between transmitting and receiving coils, the early secondary field signals are contaminated by primary field signals. To compensate for the primary field, bucking coil technology has been utilized in the airborne TEM system due to attractive features of the technology, which include a compensation effect and a strong coupling strength between the receiving coil and the underground body. However, many problems arise when the bucking coil technology is used directly in a ground small-loop TEM system. One is that the size of the receiving coil is limited, resulting in insufficient bandwidth; the other is that the bucking coil brings the near-field effect, which causes the loss of shallow information. To solve these problems, in this article, we propose a new TEM structure with noncoplanar bucking compensation. Compared with the traditional design, the new structure overcomes not only the near-field effect but also reduces the constraint of the receiving coil size. Furthermore, the approach can suppress the primary field coupling more effectively, and has a better fault tolerance for the installation accuracy. The simulation and field experiment results verify the effectiveness and practicability of the new structure.

Electromagnetic Noise Reduction in GREATEM Signal Using Singular Value Decomposition in a Junkyard

The grounded electrical source airborne transient electromagnetic system (GREATEM) is a popular geophysical method in recent years [1], has advantages such as convenience, high efficiency, large detection range, high signal-to-noise ratio, and good spatial resolution. However, since the GREATEM transient electromagnetic secondary field electromagnetic response belongs to a wide-band signal and a large attenuation amplitude, late-stage data is easily contaminated by multiple noises, which will seriously affect the results of the inversion interpretation of the data. Therefore, it is still meaningful to study the de-noising methods for GREATEM’s data. According to the characteristics of the secondary field electromagnetic response, the singular value decomposition method is used to process the theoretical synthesized signal containing white Gaussian noise. The singular value decomposition and reconstruction of the effective singular value and its corresponding vector are performed, and the analog signal is restored for the purpose of denoising. Then process the measured data of a landfill site. Whether it is viewed from the transient electromagnetic secondary field attenuation curve or the electromagnetic induction profile curve, it can be seen that the singular value decomposition is very useful in denoising. Finally, comparing the effect of singular value decomposition method on the inversion results, the study found that this method can effectively improve the quality of inversion imaging.

Calculation and application of full-wave airborne transient electromagnetic data in electromagnetic detection

Airborne electromagnetic transient method enjoys the advantages of high-efficiency and the high resolution of electromagnetic anomalies, especially suitable for mining detection around goaf areas and deep exploration of minerals. In this paper, we calculated the full-wave airborne transient electromagnetic data, according to the result of numerical research, the advantage of switch-off time response in electromagnetic detection was proofed via experiments. Firstly, based on the full-wave airborne transient electromagnetic system developed by Jilin University (JLU-ATEMI), we proposed a method to compute the full-waveform electromagnetic (EM) data of 3D model using the FDTD approach and convolution algorithm, and verify the calculation by the response of homogenous half-space. Then, through comparison of switch-off-time response and off-time response, we studied the effect of ramp time on anomaly detection. Finally, we arranged two experimental electromagnetic detection, the results indicated that the switch-off-time response can reveal the shallow target more effectively, and the full-waveform airborne electromagnetic system is an effective technique for shallow target detection.

Application of the EEMD method for distinction and suppression of motion-induced noise in grounded electrical source airborne TEM system

Airborne electromagnetic (AEM) detection is an important method for obtaining subsurface conductivity distribution. However, the response of observation system includes not only the underground media response but also a variety of noise components. The motion-induced noise is one of the main noise sources of the airborne electromagnetic data, which has a low frequency, large amplitude, non-periodic and other characteristics. In this paper, we will introduce the principle of the ensemble empirical mode decomposition (EEMD) method and use it for decomposing electromagnetic signal of grounded electrical source airborne transient electromagnetic system. The EEMD method will decompose the electromagnetic signal into multi-stage intrinsic mode function (IMF) components and distinguish the IMF component containing the motion-induced noise. Then we can get the noise-free signal by reconstructing remaining IMF components and residual component. We use the EEMD method for the theoretical signal correction and compared with the cubic spline method, the correction result indicates that the EEMD method can fit the motion-induced noise more accurately with a higher signal-to-noise ratio. To verify the effect of the application of the EEMD method, we went to Weifang city, Shandong province, East China, for the concealed fault investigation. The correction result of the time series shows that the EEMD method can suppress the motion-induced noise more effectively than the cubic spline method. Compared with the uncorrected data and the corrected data using the cubic spline method, the result shows that the fake anomaly can be nearly avoided and a more clear geological structure can be obtained through the corrected data with EEMD method. The results also prove that the EEMD method is a practical as well as effective method for the motion-induced noise suppression.

Electromagnetic noise reduction in grounded electrical‐source airborne transient electromagnetic signal using a stationarywavelet‐based denoising algorithm

ABSTRACTThe grounded electrical‐source airborne transient electromagnetic signal is significantly affected by electromagnetic noise (sferics and aircraft engine electromagnetic noise), leading to inferior image formation. We calculate the theoretical response of the grounded electrical‐source airborne transient electromagnetic system and propose a denoising algorithm based on the transient characters of the theoretical electromagnetic response and the measured noise. The proposed algorithm utilises multi‐resolution analysis via a stationary wavelet transform of the data. Primarily, the measured data are decomposed into detailed coefficients and approximated coefficients. Then, the logarithmic slope of the measured data and a threshold are calculated to identify the noise in the detailed coefficients; the corresponding detailed coefficients are processed to reduce the noise. Finally, the undisturbed data are reconstructed using inverse stationary wavelet transform. The algorithm is verified using synthetic signal and is applied for electromagnetic noise suppression in synthetic data and real data collected over the coastal areas in Yanwei Harbor, China. The results confirm that the denoising algorithm can effectively remove the electromagnetic noise from the grounded electrical‐source airborne transient electromagnetic signal. Moreover, the image formations clearly reveal the structure of the Huaiyin–Xiangshui fault zone.

3D inversion for multipulse airborne transient electromagnetic data

Multipulse airborne transient electromagnetic (ATEM) systems transmit one high-power pulse and one low-power pulse containing more high-frequency EM signals. Such systems have better near-surface resolutions while maintaining the depth of exploration of other conventional systems. ATEM systems are especially suitable for geologic mapping and mineral exploration. The inversion of multipulse ATEM data has been mainly limited to 1D modeling, which is not suitable for complex underground structures. We have investigated an algorithm for 3D multipulse ATEM data inversion based on direct Gauss-Newton optimization with quite-fast convergence. The forward problems were solved in the frequency-domain based on the secondary scattered electrical field equation, and then the inverse Fourier transform and the convolution with transmitting waveform were applied to calculate the arbitrary waveform response and sensitivity matrix in the time domain. To optimize the number of computations and memory, we further used an EM “footprint” concept in our inversions to reduce the forward model size and sparse the sensitivity matrix. The inversion results of synthetic data showed that our 3D algorithm is very effective for inverting the multipulse data with results combining advantageous resolutions of different transmitting pulses. Finally, we applied our algorithm to invert real survey data obtained at McMurray, Alberta, Canada, to further test its effectiveness.

A de-noising algorithm based on wavelet threshold-exponential adaptive window width-fitting for ground electrical source airborne transient electromagnetic signal

The ground electrical source airborne transient electromagnetic system (GREATEM) on an unmanned aircraft enjoys considerable prospecting depth, lateral resolution and detection efficiency, etc. In recent years it has become an important technical means of rapid resources exploration. However, GREATEM data are extremely vulnerable to stationary white noise and non-stationary electromagnetic noise (sferics noise, aircraft engine noise and other human electromagnetic noises). These noises will cause degradation of the imaging quality for data interpretation. Based on the characteristics of the GREATEM data and major noises, we propose a de-noising algorithm utilizing wavelet threshold method and exponential adaptive window width-fitting. Firstly, the white noise is filtered in the measured data using the wavelet threshold method. Then, the data are segmented using data window whose step length is even logarithmic intervals. The data polluted by electromagnetic noise are identified within each window based on the discriminating principle of energy detection, and the attenuation characteristics of the data slope are extracted. Eventually, an exponential fitting algorithm is adopted to fit the attenuation curve of each window, and the data polluted by non-stationary electromagnetic noise are replaced with their fitting results. Thus the non-stationary electromagnetic noise can be effectively removed. The proposed algorithm is verified by the synthetic and real GREATEM signals. The results show that in GREATEM signal, stationary white noise and non-stationary electromagnetic noise can be effectively filtered using the wavelet threshold-exponential adaptive window width-fitting algorithm, which enhances the imaging quality.

지상 송신원 항공 전자 탐사 1차원 모델링

Airborne transient electromagnetic (ATEM) surveying was introduced several decades ago in the mining industry to detect shallow conductive targets. However, conventional ATEM systems have limited depth of investigation because of weak signal strength. Recently, the grounded electrical source airborne transient electromagnetic (GREATEM) system was proposed to increase the depth of investigation. The GREATEM is a semi-airborne transient electromagnetic system because a long grounded wire is used as the transmitter. Traditionally, ATEM sounding data have been interpreted with 1D earth models to save the computing time because modern ATEM systems generally collect large data sets. However, the GREATEM 1D modeling requires numerical integration along the wire, so it takes much more time than the 1D modeling of conventional ATEM. In this study, the adaptive Born forward mapping (ABFM) was applied to the ATEM 1D modeling because the ABFM is incommensurably faster than the ordinary GREATEM 1D modeling. Comparing the results from ordinary and ABFM 1D modeling, it was confirmed that the ABFM can be applied to the 1D modeling of GEATEM.

The mutual inductance calculation between circular and quadrilateral coils at arbitrary attitudes using a rotation matrix for airborne transient electromagnetic systems

Performance testing and calibration of airborne transient electromagnetic (ATEM) systems are conducted to obtain the electromagnetic response of ground loops. It is necessary to accurately calculate the mutual inductance between transmitting coils, receiving coils and ground loops to compute the electromagnetic responses. Therefore, based on Neumann's formula and the measured attitudes of the coils, this study deduces the formula for the mutual inductance calculation between circular and quadrilateral coils, circular and circular coils, and quadrilateral and quadrilateral coils using a rotation matrix, and then proposes a method to calculate the mutual inductance between two coils at arbitrary attitudes (roll, pitch, and yaw). Using coil attitude simulated data of an ATEM system, we calculate the mutual inductance of transmitting coils and ground loops at different attitudes, analyze the impact of coil attitudes on mutual inductance, and compare the computational accuracy and speed of the proposed method with those of other methods using the same data. The results show that the relative error of the calculation is smaller and that the speed-up is significant compared to other methods. Moreover, the proposed method is also applicable to the mutual inductance calculation of polygonal and circular coils at arbitrary attitudes and is highly expandable.

Approaching 10 Microsec (and Earlier) With The Skytem System

In many applications the resolution of near surface geological layers are of great importance. A number of groundbased transient electromagnetic systems (TEM) provide sufficient early time data enabling such a resolution whereas this has not been possible with airborne TEM systems. Until now. In this abstract we describe the latest development of the SkyTEM system where early time data from as early as 8 microseconds (from begin of ramp) can be used even with a 500 m2 transmitter frame and a maximum magnetic moment of approx. 160,000 Am2. The system itself provides unbiased data from around 14 microseconds, but because the bias response at earlier times is 1) dependent of the transmitter coil geometry only, 2) it is constant from flight to flight and 3) it can be estimated from high altitude measurements, the amplitude of the bias response can be determined during the inversion process and thereby modeled. Using the Laterally Constrained Inversion algorithm to correlate the amplitude from sounding to sounding we show that data from as early as 8 microseconds from begin of ramp can be included in the inversion process. The inverted sections have been successfully validated against ERT measurements on the ground and 6 boreholes to a depth of 25 meters with both geological descriptions, resistivity logs and gamma logs. In the larger perspective the method can be used to accurately estimate near surface layers thus avoiding expensive and logistically challenging groundbased geophysical campaigns.

On the airborne transient EM response of a magnetic anomaly

Occasionally, airborne magnetic anomalies can have amplitudes as large as 70 000 nT and, for shallow dyke-like bodies, it is possible for the amplitude to vary by more than 70 000 nT over a distance of about 200 m. In such cases, the horizontal spatial gradients can be as large as 700 nT/m. Moving an induction coil sensor through a zone where the spatial gradient is large will, by Faraday’s law, result in a voltage being induced in the coil. Because airborne transient electromagnetic (ATEM) systems normally use induction coils, it is possible for voltages as large as a few volts to be induced when a system crosses a very large magnetic gradient. For a sensor height of 70 m, the wavelength of this type of voltage anomaly is typically about 240 m, which corresponds to a frequency of 0.25 Hz for a flying speed of 60 ms-1. The stacking algorithms employed in ATEM systems are specifically designed to reject signals with frequencies this low.In order to assess the efficacy of ATEM stacking algorithms, the response of a thin shallow highly magnetic body has been simulated and processed using the signal processing algorithms implemented in the GEOTEM ATEM system. The stacking algorithm essentially removes the low frequency signal. The minuscule residual signal that is not removed is at least an order of magnitude smaller than the current noise levels. Hence, flying an EM system through a large horizontal spatial gradient will not have an observable effect on the stacked EM response that is measured by the GEOTEM system.

A Comparison of Airborne and Ground Transient EM Systems in their Resolving Power against Geological Noise

Airborne electromagnetic (EM) methods are often used as a reconnaissance tool in mineral exploration. With the improvement of airborne transient EM (TEM) systems and interpretational techniques, more and more quantitative information can be obtained from airborne EM data. Thus we pose the question is it possible to develop airborne TEM technology to be as effective in orebody detection as ground TEM techniques in the future? Recently, we developed a fast approximate solution of transient EM response to a thin target buried under a thin conductive overburden. Using this solution, the signal-to-geological-noise ratios are computed for an airborne and a ground moving-loop configuration over a range of values of overburden conductance, target conductance and target depths. The results indicate that airborne TEM systems have potentially the same resolving capability as low or moderate power ground TEM systems for discrete targets. The key design features required to achieve this potential capability in airborne TEM systems are large system bandwidth (i.e. range of time windows) and large transmitter moment.