Yanju Ji

A wavelet-based baseline drift correction method for grounded electrical source airborne transient electromagnetic signals

A grounded electrical source airborne transient electromagnetic (GREATEM) system on an airship enjoys high depth of prospecting and spatial resolution, as well as outstanding detection efficiency and easy flight control. However, the movement and swing of the front-fixed receiving coil can cause severe baseline drift, leading to inferior resistivity image formation. Consequently, the reduction of baseline drift of GREATEM is of vital importance to inversion explanation. To correct the baseline drift, a traditional interpolation method estimates the baseline ‘envelope’ using the linear interpolation between the calculated start and end points of all cycles, and obtains the corrected signal by subtracting the envelope from the original signal. However, the effectiveness and efficiency of the removal is found to be low. Considering the characteristics of the baseline drift in GREATEM data, this study proposes a wavelet-based method based on multi-resolution analysis. The optimal wavelet basis and decomposition levels are determined through the iterative comparison of trial and error. This application uses the sym8 wavelet with 10 decomposition levels, and obtains the approximation at level-10 as the baseline drift, then gets the corrected signal by removing the estimated baseline drift from the original signal. To examine the performance of our proposed method, we establish a dipping sheet model and calculate the theoretical response. Through simulations, we compare the signal-to-noise ratio, signal distortion, and processing speed of the wavelet-based method and those of the interpolation method. Simulation results show that the wavelet-based method outperforms the interpolation method. We also use field data to evaluate the methods, compare the depth section images of apparent resistivity using the original signal, the interpolation-corrected signal and the wavelet-corrected signal, respectively. The results confirm that our proposed wavelet-based method is an effective, practical method to remove the baseline drift of GREATEM signals and its performance is significantly superior to the interpolation method.

A novel CFS-PML boundary condition for transient electromagnetic simulation using a fictitious wave domain method: Novel CFS-PML for TEM FD-FWD Modeling

In this study, we apply fictitious wave domain (FWD) methods, based on the correspondence principle for the wave and diffusion fields, to finite difference (FD) modeling of transient electromagnetic (TEM) diffusion problems for geophysical applications. A novel complex frequency shifted perfectly matched layer (PML) boundary condition is adapted to the FWD to truncate the computational domain, with the maximum electromagnetic wave propagation velocity in the FWD used to set the absorbing parameters for the boundary layers. Using domains of varying spatial extent we demonstrate that these boundary conditions offer significant improvements over simpler PML approaches, which can result in spurious reflections and large errors in the FWD solutions, especially for low frequencies and late times. In our development, resistive air layers are directly included in the FWD, allowing simulation of TEM responses in the presence of topography, as is commonly encountered in geophysical applications. We compare responses obtained by our new FD-FWD approach and with the spectral Lanczos decomposition method on 3-D resistivity models of varying complexity. The comparisons demonstrate that our absorbing boundary condition in FWD for the TEM diffusion problems works well even in complex high-contrast conductivity models.

Development of induction current acquisition device based on ARM

We design an induction current acquisition device based on ARM in order to realize high resolution and high sampling rate of acquisition for the induction current in wire-loop. Considering its characteristics of fast attenuation and small signal amplitude, we use the method of multi-path fusion for noise suppression. In the paper, the design is carried out from three aspects of analog circuit and device selection, independent power supply structure and the electromagnetic interference suppression of high frequency. DMA and ping-pong buffer, as a new data transmission technology, solves real time storage problem of massive data. The performance parameters of ARM acquisition device are tested. The comparison test of ARM acquisition device and cRIO acquisition device is performed at different time constants. The results show that it has 120dB dynamic range, 47kHz bandwidth, 96kHz sampling rate, 5μV the smallest resolution, and its average error value is not more than 4%, which proves the high accuracy and stability of the device.

Meshfree Method in Geophysical Electromagnetic Prospecting: The 2D Magnetotelluric Example

As an important supplement and development of traditional methods, the meshfree method has received a great deal of attention in the field of engineering calculation, and has been successfully used to solve many problems which traditional methods have difficulty in solving. However, the application of meshfree method is relatively less in the area of geophysics. In this paper, we apply the meshfree method to the numerical simulation of geophysical electromagnetic prospecting, taking the 2D magnetotelluric as an example and deduce the corresponding meshfree radial point interpolation method (RPIM) equivalent linear equations in detail. The high-efficiency and accurate solutions of large-scale sparse linear equations are solved by the quasi-minimal residual method based on Krylov subspace. The optimal values of the shape parameters are given by numerical experiments. The correctness of the meshfree method is verified by a layered model. The root mean square error of the calculation results is no more than 0...

Reduction of Electromagnetic Reflections in 3D Airborne Transient Electromagnetic Modeling: Application of the CFS-PML in Source-Free Media

To solve the problem of electromagnetic reflections caused by the termination of finite-difference time-domain (FDTD) grids, we apply the complex frequency-shifted perfectly matched layer (CFS-PML) to airborne transient electromagnetic (ATEM) modeling in a source-free medium. To implement the CFS-PML, two important aspects are improved. First, our method adopts the source-free Maxwell’s equations as the governing equations and introduces the divergence condition, consequently, the discrete form of Maxwell’s third equation is derived with regard to the CFS-PML form. Second, because our method adopts an inhomogeneous time-step, a recursive formula composed of convolution items based on a nonuniform time-step is proposed. The proposed approach is verified via a calculation of the electromagnetic response using homogeneous half-space models with different conductivities. The results show that the CFS-PML can reduce a 60 dB relative errors in late times. Moreover, this approach is also applied to 3D anomalous models; the results indicate that the proposed method can reduce reflections and substantially improve the identification of anomalous bodies. Consequently, the CFS-PML has good implications for ATEM modeling in a source-free medium.

Study of transient electromagnetic method measurements using a superconducting quantum interference device as B sensor receiver in polarizable survey areaTEM measurements using SQUID

Time-domain transient electromagnetic method (TEM) measurements sometimes exhibit a sign reversal in the secondary field during off-time, which is usually attributed to the induced polarization effect. In contrast with the conventional induced polarization method, which employs a current source, TEM with an ungrounded transmitting loop operates using a pure voltage source, which is induced by the primary field switching on and off. We performed TEM measurements in a resistive survey area showing an induced polarization effect, and used a low-temperature superconducting quantum interference device (LT-SQUID) with sensitivity ≈10 fT/√Hz as a magnetic field sensor. A sign reversal in all of our measurements was observed; furthermore, the negative amplitude reached ≈10 pT. In-depth analysis with an extended version of a wire-filament circuit reveals that the large negative signal may be due to discharging of in-ground capacitance, an induced polarization effect. The conduction response of the ground can be re...

Study of transient electromagnetic method measurements using a superconducting quantum interference device as B sensor receiver in polarizable survey area

Time-domain transient electromagnetic method (TEM) measurements sometimes exhibit a sign reversal in the secondary field during off-time, which is usually attributed to the induced polarization effect. In contrast with the conventional induced polarization method, which employs a current source, TEM with an ungrounded transmitting loop operates using a pure voltage source, which is induced by the primary field switching on and off. We performed TEM measurements in a resistive survey area showing an induced polarization effect, and used a low-temperature superconducting quantum interference device (LT-SQUID) with sensitivity ≈10 fT/√Hz as a magnetic field sensor. A sign reversal in all of our measurements was observed; furthermore, the negative amplitude reached ≈10 pT. In-depth analysis with an extended version of a wire-filament circuit reveals that the large negative signal may be due to discharging of in-ground capacitance, an induced polarization effect. The conduction response of the ground can be re...

Full-waveform associated identification method of ATEM 3D anomalies based on multiple linear regression analysis

This article studies full-waveform associated identification method of airborne time-domain electromagnetic method (ATEM) 3-d anomalies based on multiple linear regression analysis method. By using convolution algorithm, full-waveform theoretical responses are computed to derive sample library including switch-off-time period responses and off-time period responses. Extract full-waveform attributes from theoretical responses to derive linear regression equations which are used to identify the geological parameters. In order to improve the precision ulteriorly, we optimize the identification method by separating the sample library into different groups and identify the parameter respectively. Performance of full-waveform associated identification method with field data of wire-loop test experiments with ATEM system in Daedao of Changchun proves that the full-waveform associated identification method is feasible practically.

STUDY OF TEM MEASUREMENTS UTILIZING SQUID AS B SENSOR RECEIVER IN POLARIZABLE SURVEY AREA

Time-domain transient electromagnetic method (TEM) measurements sometimes exhibit a sign reversal in the secondary field during off-time, which is usually attributed to the induced polarization effect. In contrast with the conventional induced polarization method, which employs a current source, TEM with an ungrounded transmitting loop operates using a pure voltage source, which is induced by the primary field switching on and off. We performed TEM measurements in a resistive survey area showing an induced polarization effect, and used a low-temperature superconducting quantum interference device (LT-SQUID) with sensitivity ≈10 fT/√Hz as a magnetic field sensor. A sign reversal in all of our measurements was observed; furthermore, the negative amplitude reached ≈10 pT. In-depth analysis with an extended version of a wire-filament circuit reveals that the large negative signal may be due to discharging of in-ground capacitance, an induced polarization effect. The conduction response of the ground can be re...

Noise reduction of grounded electrical source airborne transient electromagnetic data using an exponential fitting-adaptive Kalman filter

The grounded electrical source airborne transient electromagnetic (GREATEM) system, which uses a grounded electrical transmitter and an aircraft for the receiver, offers deep exploration capability and detection efficiency. However, GREATEM field data usually includes mixed varied noises (white noise, sferics noise and human noise), which make identifying the exponential decaying signal too difficult. Traditional filtering methods mainly focus on suppressing specific noise types, which may cause the distortion of GREATEM signal, especially when the signal is affected by high residual sferics noise. This paper presents an exponential fitting-adaptive Kalman filter (EF-AKF) to remove mixed electromagnetic noises, while preserving the signal characteristics. The EF-AKF consists of an exponential fitting procedure and an adaptive scalar Kalman filter (SKF). The adaptive SKF uses the exponential fitting results in the weighting coefficients calculation. The EF-AKF is verified on an analytical three-layer model. It is compared with the SKF and wavelet threshold-exponential adaptive window width-fitting denoising algorithm (WEF) in synthetic data. The results showed that the EF-AKF outperformed the other methods in the noise reduction of GREATEM data. The EF-AKF is also tested on a synthetic quasi-2D earth model and applied to GREATEM field data in Huaide, Jilin province, China. Application of the EF-AKF allowed considerable improvement of the quality of the GREATEM field data. GREATEM field data usually includes a mixed variety of noises, which makes the exponential decaying signal too difficult to identify. This paper presents an exponential fitting-adaptive Kalman filter (EF-AKF) to remove mixed electromagnetic noises, while preserving the signal characteristics. As a new method, the EF-AKF can be used for denoising exponential decaying signals.