Applicability of frequency-domain and time-domain electromagnetic methods for mountain permafrost studies

2001 ◽  
Vol 12 (1) ◽  
pp. 39-52 ◽  
Author(s):  
Christian Hauck ◽  
Mauro Guglielmin ◽  
Ketil Isaksen ◽  
Daniel Vonder Mühll
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Electromagnetic Methods ◽  
Mountain Permafrost ◽  
Time Domain Electromagnetic

Electromagnetic mapping of saline contamination at an active brine pit

1996 ◽  
Vol 33 (2) ◽  
pp. 309-323 ◽  
Author(s):  
I J Ferguson ◽  
W J Taylor ◽  
K Schmigel
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Power Lines ◽  
Late Time ◽  
Signal Distortion ◽  
Electromagnetic Methods ◽  
Time Domain Electromagnetic ◽  
Survey Results ◽  
Limited Degree ◽  
Cultural Noise

Frequency-domain and time-domain electromagnetic methods were used to investigate groundwater contamination at an active brine pit in southwestern Manitoba, Canada. The objectives of the survey were to delineate contamination suspected to be occuring at the site and to compare frequency-domain electromagnetic (FDEM) and time-domain electromagnetic (TDEM) measurements in a survey area containing pipelines, fences, and power lines. The survey successfully delineated a region of high conductivity around brine pit, confirming that leakage is occurring from the pit. Modelling of the FDEM results suggests the contamination is spreading within a series of shallow sand units. Comparison of FDEM and TDEM survey results indicate that small-separation FDEM systems are much more useful for mapping in a developed area containing sources of cultural noise. The FDEM systems permit rapid mapping of spatial variations in conductivity, are affected to only a limited degree by cultural features, and provide some resolution of the depth variation of conductivity at shallow depth. It was not possible to obtain useful TDEM measurements anywhere near the active brine pit because of the signal distortion in the late-time response. Key words: geophysics, electromagnetic, brine pit, saline contamination.

Three effective inverse Laplace transform algorithms for computing time-domain electromagnetic responses

Geophysics ◽  
2016 ◽  
Vol 81 (2) ◽  
pp. E113-E128 ◽  
Author(s):  
Jianhui Li ◽  
Colin G. Farquharson ◽  
Xiangyun Hu
Keyword(s):  
Laplace Transform ◽  
Frequency Domain ◽  
Time Domain ◽  
Computing Time ◽  
Inverse Laplace Transform ◽  
Double Precision ◽  
Horizontal Electric Dipole ◽  
Time Domain Electromagnetic ◽  
Double Precision Arithmetic ◽  
Efficient Option

The inverse Laplace transform is one of the methods used to obtain time-domain electromagnetic (EM) responses in geophysics. The Gaver-Stehfest algorithm has so far been the most popular technique to compute the Laplace transform in the context of transient electromagnetics. However, the accuracy of the Gaver-Stehfest algorithm, even when using double-precision arithmetic, is relatively low at late times due to round-off errors. To overcome this issue, we have applied variable-precision arithmetic in the MATLAB computing environment to an implementation of the Gaver-Stehfest algorithm. This approach has proved to be effective in terms of improving accuracy, but it is computationally expensive. In addition, the Gaver-Stehfest algorithm is significantly problem dependent. Therefore, we have turned our attention to two other algorithms for computing inverse Laplace transforms, namely, the Euler and Talbot algorithms. Using as examples the responses for central-loop, fixed-loop, and horizontal electric dipole sources for homogeneous and layered mediums, these two algorithms, implemented using normal double-precision arithmetic, have been shown to provide more accurate results and to be less problem dependent than the standard Gaver-Stehfest algorithm. Furthermore, they have the capacity for yielding more accurate time-domain responses than the cosine and sine transforms for which the frequency-domain responses are obtained by interpolation between a limited number of explicitly computed frequency-domain responses. In addition, the Euler and Talbot algorithms have the potential of requiring fewer Laplace- or frequency-domain function evaluations than do the other transform methods commonly used to compute time-domain EM responses, and thus of providing a more efficient option.

Three-dimensional inversion of airborne time-domain electromagnetic data with applications to a porphyry deposit

Geophysics ◽  
2012 ◽  
Vol 77 (2) ◽  
pp. B23-B34 ◽  
Author(s):  
Dikun Yang ◽  
Douglas W. Oldenburg
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Large Scale ◽  
Small Scale ◽  
3D Inversion ◽  
Porphyry Deposit ◽  
Layered Earth ◽  
Fine Mesh ◽  
Conductivity Structure ◽  
Time Domain Electromagnetic

We inverted airborne time-domain electromagnetic (ATEM) data over a porphyry deposit in central British Columbia, Canada and recovered the 3D electrical conductivity structure. Full 3D inversion was required because of the circular geometry of the deposit. Typical analysis, which assumes a homogeneous or layered earth, produces conductive artifacts that are contrary to geologic expectations. A synthetic example showed that those misleading artifacts arise by assuming a 1D layered earth and that a 3D inversion can successfully solve the problem. Because of the computational challenges of solving the 3D inversion with many transmitters of airborne survey, we introduced a work flow that uses a multimesh strategy to handle the field data. In our inversion, a coarse mesh and a small number of soundings are first used to rapidly reconstruct a large-scale distribution of conductivity. The mesh is then refined and more soundings are incorporated to better resolve small-scale features. This strategy significantly speeds up the 3D inversion. The progressive refinement of the mesh also helps find the resolution limit of the data and an appropriate mesh for inversion, thus overcomputing on an unnecessarily fine mesh can be avoided. The final conductivity structure has features that emulate the expected geologic structure for a porphyry system and this substantiates the need and capability for working in 3D. However, the necessity for using 3D can depend upon the EM system used. A previous 1D interpretation of frequency-domain EM data at Mt. Milligan indicated a resistive stock. We reconciled this result with the present by computing the footprints of the frequency and time-domain surveys. The distribution of currents for the frequency-domain system was smaller than the length scale of the geologic target while the opposite was true for the time-domain data.

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