Airborne Transient Electromagnetic Method Research Articles

Airborne transient electromagnetic imaging method based on wavelet neural network

The airborne transient electromagnetic method is a crucial technique for near-surface exploration in rugged terrains. In this study, we introduce a multi-input, single-output double-hidden wavelet neural network design for airborne transient electromagnetic pseudo-resistivity imaging. We construct uniform half-space, stratified stratum, and three-dimensional geoelectric models. By evaluating various performance indicators of neural networks that employ different wavelet basis functions as activation functions, we identify the most suitable wavelet basis functions. The quasi-resistivity is computed using both the wavelet neural network and the backpropagation neural network and then juxtaposed with traditional apparent resistivity. Our findings indicate that the wavelet neural network's quasi-resistivity aligns more closely with the resistivity of the real model. It is also more responsive to low resistivity anomalies than the conventional apparent resistivity translation algorithm. The wavelet approach moderates the undershoot or overshoot occurrences during abrupt stratum changes, offering a more accurate representation of subterranean electrical properties. When compared to the backpropagation neural network, the wavelet neural network provides a superior fit for the model, rendering a smoother quasi-resistivity depth curve. Therefore, it stands out as an improved method for pseudo-resistivity imaging. By processing survey data via the trained wavelet neural network, we find that the all-time apparent resistivity and quasi-resistivity align well with real-world situations. The wavelet neural network prominently showcases the low-resistance calculation results, offering a broader resistivity range. This clarity enhances anomaly detection, confirming the wavelet neural network's applicability and practicality for airborne transient electromagnetic imaging.

Deep-learning-based airborne transient electromagnetic inversion providing the depth of investigation

We develop an integrated workflow that uses deep-learning (DL)-based approaches for processing and inverting airborne transient electromagnetic (ATEM) data. Our novel workflow automates these preprocessing steps and enables real-time inversion in the field. Thus, we develop an entire inversion workflow using three DL networks that cover all steps from preprocessing to imaging. The preprocessing DL network performs interpolation to discard data that are severely noise contaminated and suppress the effects of noise in a late-time channel. We use an inversion DL network and a depth of investigation (DOI) network to generate images of subsurface resistivities exclusively within the DOI range where reliable predictions can be made. To optimize the inversion process, our approach focuses on designing the inversion DL network to simultaneously minimize data misfit and model misfit. By addressing these two aspects, we ensure a more robust outcome in the final resistivity images. The practical applicability of the workflow is verified by comparing the imaging results of the field data with those of conventional inversion and geologic interpretation. Each workflow is nearly automatic and very fast; we expect that our workflow will contribute to the development of real-time imaging software for the ATEM survey, which expands the applications of the ATEM survey in various fields.

Combined wave emission system based on dual resonance method for airborne transient electromagnetic detection

To improve the detection efficiency of airborne transient electromagnetic method (ATEM) and reduce the times of flight experiments, a dual resonance-based combined wave emission circuit is proposed in this paper, which realizes the combined emission of half-sine wave and triangular wave through the selection of different resonant branches. Based on the operating principle of the proposed emission circuit, the model of second-order resistor-inductor-capacitor equivalent circuit is established. Then, the generation process of the combined wave is analyzed, and the segmentation control method is proposed. Finally, the feasibility and effectiveness of the proposed combined wave emission system are verified by simulation model building and prototype testing, and the results show that the emission current has good quality and high stability. Compared with the traditional emission of single wave current, the combined wave current has the advantages of large magnetic moment of half-sine wave and rich high-frequency components of triangular narrow pulse, which can effectively expand the detection range and improve the measurement efficiency of ATEM.

3D anisotropic TEM modeling with loop source using model reduction method

Abstract For model reduction techniques, there have been relatively few studies performed regarding the forward modeling of anisotropic media in comparison to transient electromagnetic (TEM) forward modeling of isotropic media. The transient electromagnetic method (TEM) responses after the current has been turned off can be represented as a homogeneous ordinary differential equation (ODE) with an initial value, and the ODE can be solved using a matrix exponential function. However, the order of the matrix exponential function is large and solving it directly is challenging, thus this study employs the Shift-and-Invert (SAI-Krylov) subspace algorithm. The SAI-Krylov subspace technique is classified as a single-pole approach compared to the multi-pole rational Krylov subspace approach. It only takes one LU factorization of the coefficient matrix, along with hundreds of backward substitutions. The research in this paper shows that the anisotropic medium has little effect on the optimal shift ${\gamma _{opt}}$ and subspace order m. Furthermore, as compared to the mimetic finite volume method (SAI-MFV) of the SAI-Krylov subspace technique, the method proposed in this paper (SAI-FEM) can further improve the computing efficiency by roughly 13%. In contrast to the standard implicit time step iterative technique, the SAI-FEM method does not require discretization in time, and the TEM response at any moment within the off-time period can be easily computed. Next, the accuracy of the SAI-FEM algorithm was verified by 1D solutions for an anisotropic layer model and a 3D anisotropic model. Finally, the electromagnetic characteristics of the anisotropic anomalous body of the center loop device and separated device of the airborne transient electromagnetic method were analyzed, and it was found that horizontal conductivity has a considerable influence on the TEM response of the anisotropic medium.

Inversion of airborne transient electromagnetic data based on reference point lateral constraint

The airborne transient electromagnetic method (ATEM) has become a popular tool in mineral and resource exploration due to its speed and high efficiency. The distance between two adjacent measured points is minuscule for a flight line because of the oversampling in the ATEM, which results in a sharp change in the horizontal direction. The conventional lateral constraint inversion method accumulates errors and has a significant calculation cost. Therefore, a laterally constrained, segmented inversion method based on a reference point is proposed to reduce errors and improve accuracy. Electrical information about the known logging data is employed as the lateral constraint of the reference point; alternatively, the inversion results of previous point can be used as lateral constraints, which guarantees a continuous inversion profile. The golden-section method is applied to solve the Lagrange multiplier to ensure a quick and effective convergence of inversions. Constraint inversions of a one-dimensional model show that the strategy converges quickly, and the inversion results are closer to the true resistivity of the model. Constraint inversion of a 2D model indicates that lateral constraints considerably reduce the discontinuity of a single-point inversion and smooths the inversion's resistivity pseudo-section. Finally, the effectiveness of the proposed algorithm is verified by the inversion of the airborne transient electromagnetic survey data in Xinjiang province.

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.

Nonlinear Noise Reduction for the Airborne Transient Electromagnetic Method based on Kernel Minimum Noise Fraction

The airborne transient electromagnetic method has become a powerful tool to explore deep resource and tectonic structures. However, aircraft vibrations and flight environments produce very strong and complex nonlinear noise and result in poor data quality compared to ground transient electromagnetic methods. Consequently, the reduction of airborne electromagnetic noises is of vital importance to data inversion and imaging. To suppress and remove the nonlinear noise, we propose using kernel minimum noise fraction (KMNF), which is a nonlinear generalized method of minimum noise fraction. First, an adaptive variable window-width filtering algorithm is used to evaluate the noises and perform the preliminary denoising. Then, we adopt the two filter methods, which are minimum noise fraction (MNF) and KMNF to suppress the noise. The results show that these two methods can both suppress noise and make the decay curves smooth, but kernel MNF is more effective for the nonlinear characteristics of noise and it does not weaken the anomaly. Finally, field data from the Qinling mine area is processed, using the MNF and KMNF methods. The results show that nonlinear noise is suppressed by both methods but the results of KMNF are better than those of the linear MNF method.

Research on the application of UAV airborne transient electromagnetic in the detection of shallow sliding surface

In complex terrain conditions, how to achieve fast and efficient detection of the sliding surface is primary issue for the shallow landslide hazard investigation. In this paper, the UAV (Unmanned Aerial Vehicle) airborne transient electromagnetic method of Lengshui landslide in Fujian province was detecting a sliding surface. The comparison and analysis of resistivity data and borehole data show that the consistency of resistivity data and borehole geological stratification is good, and the three-dimensional distribution characteristics of the landslide can be inferred from resistivity data. Also, this paper determines the acquisition parameters of UAV airborne transient electromagnetic flight state and array types through experimental research, which has certain practical reference and guidance significance.

Resistivity-depth Imaging with the Airborne Transient Electromagnetic Method Based on an Artificial Neural Network

We developed an artificial neural network to map the distribution of geologic conductivity in the earth subsurface using the airborne transient electromagnetic method. The artificial neural network avoids the need for complex derivations of electromagnetic field formulas and requires only input and transfer functions to obtain a quasi-resistivity image. First, training sample set from the airborne transient electromagnetic response of homogeneous half-space models with different resistivities was formed, and network model parameters, including the flight altitude, time constant, and response amplitude, were determined. Then, a double-hidden-layer back-propagation (BP) neural network was established based on the mapping relationship between quasi-resistivity and airborne transient electromagnetic response. By analyzing the mean square error curve, the training termination criterion of the BP neural network was determined. Next, the trained BP neural network was used to interpret the airborne transient electromagnetic responses of various typical layered geo-electric models, and the results were compared with that from the all-time apparent resistivity algorithm. The comparison indicated that the resistivity imaging from the BP neural network approach was much closer to the true resistivity of the model, and the response to anomalous bodies was better than that from an all-time apparent resistivity. Finally, this imaging technique was used to process field data acquired by employing the airborne transient method from the HuaYin survey area. Quasi-resistivity depth sections calculated with the BP neural network and the actual geological situation were in good.

Detecting Permafrost in Plateau and Mountainous Areas by Airborne Transient Electromagnetic Sensing

Transportation has become a key bottleneck which restricts economic development in Western China. However, during the construction of the western railway, the permafrost problem has plagued railway construction on the Qinghai–Tibet Plateau, and has not yet been resolved. Accurately identifying permafrost by geophysical method is the most effective means to solve this problem. However, the mountainous and plateau terrain in Western China impose huge challenges in collecting geophysical data. To address this issue, this paper proposes an airborne transient electromagnetic method to collect geophysical electromagnetic data to identify permafrost in the mountains and plateaus of Western China. Based on Maxwell’s equations, the forward model of the airborne electromagnetic was derived, and the finite element method was used to calculate the two-dimensional (2D) space electromagnetic responses of different permafrost geo-electrical models. Furthermore, a coupling function was constructed to estimate the distribution of the resistivity of the permafrost by the least-squares fitting algorithm. Comparison between inversion resistivity distribution and the geo-electrical model showed that the proposed airborne transient electromagnetic method was valid for exploring the permafrost in the mountains and the Qinghai–Tibet Plateau in Western China.

A finite‐element time‐domain forward‐modelling algorithm for transient electromagnetics excited by grounded‐wire sources

ABSTRACTA three‐dimensional finite‐element time‐domain forward‐modelling algorithm is developed to simulate transient electromagnetics excited by grounded‐wire sources. The main advantage of this finite‐element time‐domain algorithm is that full transmitting‐current waveforms and complex‐shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half‐space model, a stratified‐medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half‐space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step‐off, step‐on and direct current waveforms. For the stratified‐medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non‐negligible topography, the effects of which on the shapes of the grounded‐wire sources must be taken into account when implementing the finite‐element time‐domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.

The Progress of the Helicopter-Borne Transient Electromagnetic Method and Technology in China

The central and western regions of China have appropriate conditions for mining development, but there are many deserts, swamps and forest-covered areas in this vast land. As a result, the traditional on-ground methods are difficult to carry out efficiently in these areas. The airborne transient electromagnetic method can implement detection over large areas rapidly and efficiently, and play an important role in terms of resource and environment detection. In recent years, two Helicopter-borne Transient electromagnetic (HTEM) systems, CHTEM-I and CAS-HTEM, were developed in China. In this paper, the difficult issues in the development of HTEM hardware and software will be analyzed, firstly. By tracing the specific technical paths to solve these difficult issues, the latest research achievements in the development process of CHTEM-I and CAS-HTEM will be expounded. Finally, we would like to briefly discuss the research and development direction of HTEM technology in China to promote further development of the research and application of this technology.

Hydrostratigraphic modeling using multiple-point statistics and airborne transient electromagnetic methods

Abstract. Creating increasingly realistic groundwater models involves the inclusion of additional geological and geophysical data in the hydrostratigraphic modeling procedure. Using multiple-point statistics (MPS) for stochastic hydrostratigraphic modeling provides a degree of flexibility that allows the incorporation of elaborate datasets and provides a framework for stochastic hydrostratigraphic modeling. This paper focuses on comparing three MPS methods: snesim, DS and iqsim. The MPS methods are tested and compared on a real-world hydrogeophysical survey from Kasted in Denmark, which covers an area of 45 km2. A controlled test environment, similar to a synthetic test case, is constructed from the Kasted survey and is used to compare the modeling results of the three aforementioned MPS methods. The comparison of the stochastic hydrostratigraphic MPS models is carried out in an elaborate scheme of visual inspection, mathematical similarity and consistency with boreholes. Using the Kasted survey data, an example for modeling new survey areas is presented. A cognitive hydrostratigraphic model of one area is used as a training image (TI) to create a suite of stochastic hydrostratigraphic models in a new survey area. The advantage of stochastic modeling is that detailed multiple point information from one area can be easily transferred to another area considering uncertainty. The presented MPS methods each have their own set of advantages and disadvantages. The DS method had average computation times of 6–7 h, which is large, compared to iqsim with average computation times of 10–12 min. However, iqsim generally did not properly constrain the near-surface part of the spatially dense soft data variable. The computation time of 2–3 h for snesim was in between DS and iqsim. The snesim implementation used here is part of the Stanford Geostatistical Modeling Software, or SGeMS. The snesim setup was not trivial, with numerous parameter settings, usage of multiple grids and a search-tree database. However, once the parameters had been set it yielded comparable results to the other methods. Both iqsim and DS are easy to script and run in parallel on a server, which is not the case for the snesim implementation in SGeMS.

Robust cancellation of minimum shift keying-encoded radio interference in data from the transient electromagnetic method

Interference from radio signals in the very low frequency (VLF, 3–30 kHz) band constitutes a significant part of the noise in data, measured using the transient electromagnetic method (TEM). Most radio signals in this frequency band convey streams of binary data, encoded using minimum shift keying or variants thereof. We have developed a framework for cancellation of these signals by modeling and subtracting the interfering signals from the recorded data. The framework uses several techniques to ensure robust and accurate modeling of the radio signals in a typical airborne TEM environment. These include combination of signals from multiple receiver coils, tracking of time-varying parameters for each receiver coil, use of a priori knowledge pertaining to the TEM primary field, and use of a high-performance decoding algorithm. Examples using recorded noise data from the field combined with synthetic TEM signals indicate a significant reduction in noise power. This results in a reduction of the standard deviation of approximately 1.5–4 times in the early to intermediate-time response, after common processing steps have been applied. In the late-time part of the response, little to no improvement is seen. Although the attainable improvement in signal quality is highly case dependent, we expect that significant noise reduction can be achieved in most cases. Despite not being tested, our framework should be directly applicable in the low frequency (LF, 30–300 kHz) band, which has relevance for wide-band systems.

Airborne TEM for the recovery of basin scale solute distribution; Perth Basin, Western Australia.

The distribution of groundwater salinity is a key input for management of water resources. Estimates of the three dimensional distribution of groundwater salinity below areas spanning thousands of square kilometres may be required. Airborne transient electromagnetic methods provide the possibility of recovering first pass large scale solute concentration distributions provided lithological influences on electrical conductivity distribution are not dominant. The Allanooka airborne TEM survey is located in the northern most portion of the Perth Basin in Western Australia. We use data from this Airborne TEM survey combined with data recovered from monitoring wells to highlight the steps used to construct a first pass large scale solute distribution model. We provide a method for converting airborne TEM datasets to estimates of solute concentration distribution for sandstone dominated sediments at a basin scale. For the Allanooka monitoring well network, base line empirical relationships are developed between laboratory derived total dissolved solids and formation conductivity derived from wire line logs. This relationship is then extended to include airborne TEM derived formation conductivities. Appropriate layer discretisation of input seed models for inversion of the airborne TEM data set are based on analysis of resistivities derived from wire-line logs. The interpretation of the inverted airborne TEM was assisted by geological constraints and high resolution seismic reflection transects. Selected inversion statistics were also mapped throughout the 3D volume to provide a quick method for assessing the ?importance? of particular layers to the outcome of the inversion. An approximate volume of low solute concentration sandstone dominated formation below the regional water table was extracted from the airborne TEM data. Our first pass basin-scale Airborne TEM derived 3D solute concentration provides a starting point for more detailed interpretation to commence.

Fast approximate inversion of SkyTem airborne electromagnetic data

Transient electromagnetic (TEM) soundings have become one of the standard methods of environmental geophysics (Fittermann and Stewart, 1986; Buselli et al., 1990; Hoekstra and Blohm, 1990; Christensen and Sørensen, 1998; Auken et al., 2006; Lane and Pracilio, 2000). Over the past decades, airborne TEM methods have found widespread use in hydrogeophysical investigations, making it possible to cover large areas in a cost-effective way. Several helicopterborne TEM systems have been developed and they now represent the most up-to-date method of airborne hydrogeophysical investigations, collecting 100,000s of data sets. We present a fast approximate method for one-dimensional inversion of time domain electromagnetic data and apply it to SkyTEM helicopterborne data from the Toolibin Lake area of West Australia. The method is based on fast approximate forward computation of transient electromagnetic step responses and their derivatives with respect to the model parameters of a 1D model. The inversion is carried out with multi-layer models in a state-of-the-art formulation of a least-squares iterative inversion scheme including explicit formulation of the model regularization through a model covariance matrix. The method is 50 times faster than conventional inversion for a layered earth model and produces model sections of concatenated 1D models and contoured maps of mean conductivity in elevation intervals almost indistinguishable from those of conventional inversion. To ensure lateral smoothness of the model sections and to avoid spurious artifacts in the mean conductivity maps, the Lateral Parameter Correlation method is applied to the results of the individual inversion models. In this way, well determined parameters are allowed to influence the more poorly determined parameters in the survey area. Applied to the SkyTEM Toolibin data set, the inversion produces model sections and conductivity maps that reveal the distribution of conductivity in the area and thereby the distribution of salinity. This information is crucial for any remediation effort aimed at alleviating the salinity problems. The results are in accordance with previous results from other investigators.

A quantitative appraisal of airborne and ground‐based transient electromagnetic (TEM) measurements in Denmark

The last decade has seen growing use of ground‐based transient electromagnetic (TEM) methods in Denmark for hydrogeological purposes. Due to an intensified mapping campaign, airborne TEM methods were proposed as a possible tool for mapping large areas. The first test flights were flown in June 2000 using the GEOTEM system.Traditional approximate interpretation tools for airborne data are insufficient in hydrogeological investigations where a quantitative model specifying model parameter reliability is needed. We have carried out full nonlinear one‐dimensional inversion on the field amplitude of airborne synthetic and field data and compared the airborne method with the traditional ground‐based PROTEM 47 system that has found extensive use in Denmark. An improved measuring procedure for airborne systems is suggested to facilitate the estimation of noise that is necessary in a quantitative inversion.The analyses of synthetic data demonstrate the differences in resolution capability between ground‐based and airborne data. Ground‐based data typically resolve three‐ or four‐layer models and occasionally up to five layers. Airborne data resolve three layers as a maximum, one or two layers being common. The airborne GEOTEM system detects layers to depths of more than 300 m, bearing only little information about the top 50–70 m. The ground‐based PROTEM 47 system has a maximum penetration of approximately 170 m, with higher resolution capabilities in the top 100 m.Coupling to man‐made conductors is a serious problem for all TEM methods in densely populated areas and results in distorted data. Coupling influences the airborne data from Denmark on two‐thirds of the area covered. These data must be eliminated to avoid misinterpretation.