A comparative study of inline and broadside time-domain controlled-source electromagnetic methods for mapping resistive targets on land

Geophysics ◽  
2019 ◽  
Vol 84 (4) ◽  
pp. B235-B246 ◽  
Author(s):  
Hai Li ◽  
Qing-Yun Di ◽  
Guo-Qiang Xue
Keyword(s):  
Time Domain ◽  
Joint Inversion ◽  
Granite Porphyry ◽  
Resistive Layer ◽  
Controlled Source ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Molybdenum Deposit ◽  
Subsurface Resistivity ◽  
Conductivity Contrast

The land-based controlled-source electromagnetic (CSEM) method is an important tool in mapping subsurface resistivity contrast, especially for conductive target embedded in a resistive environment. For resistive targets on land, choosing an appropriate configuration to a specific field observation is quite confusing, due to the lack of systematic comparisons of different methods. We have conducted a comparison between the broadside and inline time-domain CSEM methods, using the short-offset transient electromagnetic (SOTEM) method and multitransient electromagnetic (MTEM) method as representatives respectively. We first compared the resolution of these methods by analyzing the relative target response and the misfit space of the designed models. We found that the inline MTEM method had advantages over the SOTEM method in resolving the thin resistive layer. We have developed a weighted joint inversion scheme to enhance the vertical resolution of the MTEM method. We then applied the methods to the investigation of a giant molybdenum deposit. The orebody, which is the real target of the exploration, does not have a conductivity contrast with its host. However, it sits on top of the granite porphyry, which is resistive compared with its surroundings, and so the granite porphyry becomes the resistive target for EM exploration. The results indicated that both methods are effective in locating large resistive target, yet the MTEM method outperforms the SOTEM method when a thick conductive overburden is presented.

Finite-Element Modeling of Time-Domain Controlled-Source Electromagnetic Survey with Weighted Laguerre Polynomials

Geophysics ◽  
2021 ◽  
pp. 1-43
Author(s):  
Qingtao Sun ◽  
Runren Zhang ◽  
Yunyun Hu
Keyword(s):  
Finite Element ◽  
Time Domain ◽  
Laguerre Polynomials ◽  
Time Integration ◽  
Sampling Interval ◽  
Controlled Source ◽  
Time Integration Method ◽  
Transient Electromagnetic ◽  
Backward Euler ◽  
Land Survey

To facilitate the modeling of time-domain controlled-source electromagnetic survey, we propose an efficient finite-element method with weighted Laguerre polynomials, which shows a much lower computational complexity than conventional time integration methods. The proposed method allows sampling the field at arbitrary time steps and also its accuracy is determined by the number of polynomials, instead of the time sampling interval. Analysis is given regarding the optimization of the polynomial number to be used and the criterion of selecting the time scale factor. Two numerical examples in marine and land survey environments are included to demonstrate the superiority of the proposed method over the existing backward Euler time integration method. The proposed method is expected to facilitate the modeling of transient electromagnetic surveys in the geophysical regime.

scholarly journals Step‐on versus step‐off signals in time‐domain controlled source electromagnetic methods using a grounded electric dipole

2020 ◽  
Vol 68 (9) ◽  
pp. 2825-2844
Author(s):  
Amir Haroon ◽  
Andrei Swidinsky ◽  
Sebastian Hölz ◽  
Marion Jegen ◽  
Bülent Tezkan
Keyword(s):  
Time Domain ◽  
Electric Dipole ◽  
Controlled Source ◽  
Electromagnetic Methods

A comparison of loop time-domain electromagnetic and short-offset transient electromagnetic methods for mapping water-enriched zones — A case history in Shaanxi, China

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. B201-B208 ◽  
Author(s):  
Weiying Chen ◽  
Guoqiang Xue ◽  
Afolagboye Lekan Olatayo ◽  
Kang Chen ◽  
Muhammad Younis Khan ◽  
...  
Keyword(s):  
Time Domain ◽  
Mining Depth ◽  
Transient Electromagnetic ◽  
Resolution Capability ◽  
Electromagnetic Methods ◽  
Detection Depth ◽  
Reliable Detection ◽  
Time Domain Electromagnetic ◽  
Tem Method ◽  
And Inversion

Increases in the mining depth of coal pose a significant challenge to the conventional loop source time-domain electromagnetic (TEM) method that requires significant enlargement of the loop size and transmitting current to realize the deeper sounding results required. As an alternative, TEM devices based on a grounded wire source are generally used to solve detections deeper than several hundred meters. To map the water-enriched zones buried underneath approximately 1000 m at a coal mine in Shaanxi, China, loop TEM and short-offset transient electromagnetic (SOTEM) measurements were conducted. We carried out 1D forward modeling and inversion constrained by drilling informa-tion, and the results reveal that the resolution capability of loop TEM and SOTEM is almost the same in detecting a conductive layer in the absence of any noise. However, for a given noise level and decay time, the SOTEM method provides a deeper investigation than loop TEM without compromising sensitivity. The field examples validated the synthetic results. The loop TEM with dimensions of [Formula: see text] realized a maximum depth of 1000 m, whereas the reliable detection depth of 1500 m was achieved by using a 723 m long grounded wire source using the SOTEM method. Moreover, the labor required is significantly reduced, and the efficiency is dramatically raised using the SOTEM method. Our results predict that the SOTEM method should play a more important role in deep hydrogeophysical investigations.

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.

The use and misuse of apparent resistivity in electromagnetic methods

Geophysics ◽  
1986 ◽  
Vol 51 (7) ◽  
pp. 1462-1471 ◽  
Author(s):  
Brian R. Spies ◽  
Dwight E. Eggers
Keyword(s):  
Apparent Resistivity ◽  
Early Time ◽  
Asymptotic Formulas ◽  
Voltage Response ◽  
Late Time ◽  
Induced Voltage ◽  
Resistivity Curve ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Tem Method

Problems and misunderstandings arise with the concept of apparent resistivity when the analogy between an apparent resistivity computed from geophysical observations and the true resistivity structure of the subsurface is drawn too tightly. Several definitions of apparent resistivity are available for use in electromagnetic methods; however, those most commonly used do not always exhibit the best behavior. Many of the features of the apparent resistivity curve which have been interpreted as physically significant with one definition disappear when alternative definitions are used. It is misleading to compare the detection or resolution capabilities of different field systems or configurations solely on the basis of the apparent resistivity curve. For the in‐loop transient electromagnetic (TEM) method, apparent resistivity computed from the magnetic field response displays much better behavior than that computed from the induced voltage response. A comparison of “exact” and “asymptotic” formulas for the TEM method reveals that automated schemes for distinguishing early‐time and late‐time branches are at best tenuous, and those schemes are doomed to failure for a certain class of resistivity structures (e.g., the loop size is large compared to the layer thickness). For the magnetotelluric (MT) method, apparent resistivity curves defined from the real part of the impedance exhibit much better behavior than curves based on the conventional definition that uses the magnitude of the impedance. Results of using this new definition have characteristics similar to apparent resistivity obtained from time‐domain processing.

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