Full transmitter waveform transient electromagnetic modeling and inversion for soundings over coal measures

Geophysics ◽  
1987 ◽  
Vol 52 (3) ◽  
pp. 279-288 ◽  
Author(s):  
Michael W. Asten
Keyword(s):  
Case Depth ◽  
Earth Model ◽  
Electromagnetic Modeling ◽  
Resistive Layer ◽  
Homogeneous Half Space ◽  
Full Waveform ◽  
Transient Electromagnetic ◽  
Electromagnetic Soundings ◽  
And Inversion ◽  
Coal Measures

Modeling of transient electromagnetic soundings requires consideration of the full bipolar transmitter waveform in order to obtain full measurement accuracy from field instruments. The full‐waveform transient may be computed using existing impulse‐response forward algorithms followed by a convolution procedure with truncation and residual estimation after an even number of terms, which results in average increases in run time of about 80 percent. Simple calculations based on approximations for the behavior of transient decay over (a) a conductive layer over a resistive basement, (b) a homogeneous half‐space, and (c) a resistive layer over a conductive basement show that the effect of the full waveform is greatest at sample times near the end of the transmitter off‐time, and for the case of a conductive basement. Failure to consider the full waveform is expected to yield errors in the amplitude of the computed transient in the range of 4 to 100 percent or more, depending upon the sample time and type of earth model. Examples of soundings over moderately conductive coal measures and highly conductive cindered coal show that failure to use the full waveform yields depths and resistivities biased to erroneously high values. The bias is small in the former case, but produces substantial errors in the latter case. Depth to the electrical basement of 300 to 400 m may be estimated to accuracies of approximately 5 percent by computer inversion to three‐layer or four‐layer models.

Airborne Transient Electromagnetic Modeling and Inversion Under Full Attitude Change

2017 ◽  
Vol 14 (9) ◽  
pp. 1575-1579 ◽  
Author(s):  
Youzheng Qi ◽  
Ling Huang ◽  
Xucun Wang ◽  
Guangyou Fang ◽  
Gang Yu
Keyword(s):  
Attitude Change ◽  
Electromagnetic Modeling ◽  
Transient Electromagnetic ◽  
And Inversion

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

2018 ◽  
Vol 2018 ◽  
pp. 1-14 ◽  
Author(s):  
Yanju Ji ◽  
Xuejiao Zhao ◽  
Jiayue Gu ◽  
Dongsheng Li ◽  
Shanshan Guan
Keyword(s):  
Recursive Formula ◽  
Electromagnetic Response ◽  
Complex Frequency ◽  
Electromagnetic Modeling ◽  
Free Medium ◽  
Time Step ◽  
Homogeneous Half Space ◽  
Transient Electromagnetic ◽  
Anomalous Bodies ◽  
Relative Errors

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.

scholarly journals On electric fields produced by inductive sources on the seafloor

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. E297-E313 ◽  
Author(s):  
Roxana Safipour ◽  
Sebastian Hölz ◽  
Marion Jegen ◽  
Andrei Swidinsky
Keyword(s):  
Electric Fields ◽  
Mineral Exploration ◽  
Massive Sulfide ◽  
Loop System ◽  
Earth Model ◽  
Volcanic Complex ◽  
Tyrrhenian Sea ◽  
Resistive Layer ◽  
Transient Electromagnetic ◽  
Tem Method

The transient electromagnetic (TEM) method has recently been proposed as a tool for mineral exploration on the seafloor. Similar to airborne TEM surveys conducted on land, marine TEM systems can use a concentric or coincident wire-loop transmitter and receiver towed behind a ship. Such towed-loop TEM surveys can be further augmented by placing additional stationary receivers on the seafloor throughout the survey area. We examine the electric fields measured by remote receivers from an inductive source transmitter within a 1D layered earth model. At sea, it is conceivable to deploy either a horizontal transmitter (such as the analogous standard airborne configuration) or a more exotic vertical transmitter. Therefore, we study and compare the sensitivity of the vertical and horizontal towed-loop systems with a variety of seafloor conductivity structures. Our results indicate that the horizontal loop system is more sensitive to the thickness of a buried conductive layer and would be advantageous over the vertical loop system in characterizing the size of a shallowly buried mineralized zone. The vertical loop system is more sensitive to a resistive layer than the horizontal loop system. The vertical electric field produced by the vertical loop transmitter is sensitive to greater depths than the horizontal fields, and measuring the vertical field at the receivers would therefore be advantageous. We also conducted a novel test of a towed horizontal loop system with remote dipole receivers in a marine setting. The system was tested at the Palinuro volcanic complex in the Tyrrhenian Sea, a site of known massive sulfide mineralization. Preliminary results are consistent with shallowly buried material in the seafloor of conductivities [Formula: see text].

2-D electromagnetic modeling by finite‐element method with a dipole source and topography

Geophysics ◽  
2000 ◽  
Vol 65 (2) ◽  
pp. 465-475 ◽  
Author(s):  
Yuji Mitsuhata
Keyword(s):  
Finite Element ◽  
Delta Function ◽  
Dipole Source ◽  
Electromagnetic Modeling ◽  
Secondary Field ◽  
Controlled Source ◽  
Transient Electromagnetic ◽  
Isoparametric Finite Element ◽  
Anomalous Bodies ◽  
Element Technique

I present a method for calculating frequency‐domain electromagnetic responses caused by a dipole source over a 2-D structure. In modeling controlled‐source electromagnetic data, it is usual to separate the electromagnetic field into a primary (background) and a secondary (scattered) field to avoid a source singularity, and only the secondary field caused by anomalous bodies is computed numerically. However, this conventional scheme is not effective for complex structures lacking a simple background structure. The present modeling method uses a pseudo‐delta function to distribute the dipole source current, and does not need the separation of the primary and the secondary field. In addition, the method employs an isoparametric finite‐element technique to represent realistic topography. Numerical experiments are used to validate the code. Finally, a simulation of a source overprint effect and the response of topography for the long‐offset transient electromagnetic and the controlled‐source magnetotelluric measurements is presented.

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