Interpretation of large‐loop transmitter transient electromagnetic surveys

1982 ◽  
Author(s):  
J. D. McNeill
Keyword(s):  
Large Loop ◽  
Transient Electromagnetic ◽  
Electromagnetic Surveys

Transient electromagnetic fields of a buried horizontal magnetic dipole

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. E481-E491 ◽  
Author(s):  
Andrei Swidinsky ◽  
Misac Nabighian
Keyword(s):  
Electric Field ◽  
Magnetic Dipole ◽  
Time Domain ◽  
Major Axis ◽  
Secondary Response ◽  
Burial Depth ◽  
Transient Electromagnetic ◽  
The Earth ◽  
Smoke Ring ◽  
Electromagnetic Surveys

Electromagnetic surveys using a vertical transmitter loop are common in land, marine, and airborne geophysical exploration. Most of these horizontal magnetic dipole (HMD) systems operate in the frequency domain, measuring the time derivative of the induced magnetic fields, and therefore a majority of studies have focused on this subset of field measurements. We examine the time-domain electromagnetic response of a HMD including the electric fields and corresponding smoke rings produced in a conductive half-space. Cases of a dipole at the surface and buried within the earth are considered. Results indicate that when the current in the transmitter is rapidly switched off, a single smoke ring is produced within the plane of the vertical transmitter loop, which is then distorted by the air-earth interface. In this situation, the circular smoke ring, which would normally diffuse symmetrically away from the source in a whole space, is approximately transformed into an ellipse, with a vertical major axis at an early time and a horizontal major axis at a late time. As measured from the location of the transmitter, the depth of investigation and lateral footprint of such a system increases with burial depth. It is also observed that the electric field measured in the direction of the magnetic dipole only contains a secondary response related to the charge accumulation on any horizontal conductivity boundaries because the primary field is always absent. This field component can be expressed analytically in terms of a static and time-varying field, the latter term adding spatial complexity to the total horizontal electric field at the earth surface at early times. Applications of this theoretical study include the design of time-domain induction-logging tools, crossborehole electromagnetic surveys, underground mine expansion work, mine rescue procedures, and novel marine electromagnetic experiments.

Understanding LOTEM data from mountainous terrain

Geophysics ◽  
2000 ◽  
Vol 65 (4) ◽  
pp. 1113-1123 ◽  
Author(s):  
Andreas Hördt ◽  
Martin Müller
Keyword(s):  
Field Data ◽  
Electromagnetic Coupling ◽  
Theoretical Data ◽  
Correction Method ◽  
Topographic Effects ◽  
Mountainous Terrain ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Conductivity Structure ◽  
Long Offset

Long‐offset transient electromagnetic (LOTEM) data from the Vesuvius volcano, in Italy, show that the EM response of the topography is a potential cause of data distortions. A modeling study was carried out to simulate the effect of mountainous terrain on vertical magnetic‐field time derivatives using a 3-D finite‐difference code. The objectives were to assess the importance of topographic effects and to help identify them in existing field data. The total effect of topography on the LOTEM response can be considered as a combination of four distortions of the corresponding responses for a flat terrain. First, the receiver is at some height above the flat surface. Second, the mountain acts as a conductive body displacing air. Third, large loop receivers are nonhorizontal and sense a combination of horizontal and vertical magnetic fields. Finally, the electromagnetic coupling between the mountain and deeper‐lying structure modifies the structure response. Each of the effects can be identified in field data recorded at Mount Vesuvius. The topographic induced distortions for the model used in this study are moderate in the sense that 1-D inversions of the theoretical data still recover the gross conductivity structure, albeit with small deviations from the true parameters. Although this result might imply that topography might be ignored during the first stage of an interpretation, no simple correction method is evident, so topography will have to be included in any 2-D or 3-D inversion attempt.

scholarly journals Transient Electromagnetic Surveys for the Determination of Talik Depth and Geometry Beneath Thermokarst Lakes

2018 ◽  
Vol 123 (11) ◽  
pp. 9310-9323 ◽  
Author(s):  
A. L. Creighton ◽  
A. D. Parsekian ◽  
M. Angelopoulos ◽  
B. M. Jones ◽  
A. Bondurant ◽  
...  
Keyword(s):  
Thermokarst Lakes ◽  
Transient Electromagnetic ◽  
Electromagnetic Surveys

scholarly journals SkyTEM helicopter transient electromagnetic surveys of tailings dams

2012 ◽  
Vol 2012 (1) ◽  
pp. 1-4 ◽  
Author(s):  
James Reid ◽  
Niels Christensen ◽  
Kate Godber
Keyword(s):  
Transient Electromagnetic ◽  
Tailings Dams ◽  
Electromagnetic Surveys

Transient electromagnetic surveys for highresolution near-surface exploration: basics and case studies

First Break ◽  
2017 ◽  
Vol 35 (9) ◽  
Author(s):  
M.V. Sharlov ◽  
I.V. Buddo ◽  
N.V. Misyurkeeva ◽  
I.A. Shelokhov ◽  
Yu.A. Agafonov
Keyword(s):  
Case Studies ◽  
Near Surface ◽  
Transient Electromagnetic ◽  
Surface Exploration ◽  
Electromagnetic Surveys

Limitations of large‐loop transient electromagnetic surveys in conductive terrains

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 902-912 ◽  
Author(s):  
Brian R. Spies ◽  
Patricia D. Parker
Keyword(s):  
Decay Constant ◽  
Eddy Currents ◽  
Electrical Contact ◽  
Scale Model ◽  
Geometric Condition ◽  
Model Studies ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Inductive Effects ◽  
Tem Method

Conventional interpretation aids used in transient electromagnetic (TEM) exploration assume that the anomalous response is due to induction in a simple model such as a plate, sphere, or uniform layer. Estimates of conductivity, depth, and size are based on analyzing the profile shape and transient decay constant. In regions with conductive overburden or conductive bedrock, TEM responses obtained with the large‐loop configuration can often be very dependent upon the location of the transmitter loop, and the results can be easily misinterpreted. A series of scale‐model studies was carried out to investigate the influence of current channeling and gathering phenomena with the large fixed loop and moving single‐loop configurations of the TEM method. Models studied included resistive and step discontinuities in a horizontal conductive slab, and a vertical plate in electrical contact with conductive overburden. Large TEM anomalies can be observed at the edge of a conductive unit, when diffusing eddy currents migrating through surface conductors are channeled and become spatially localized. Current gathering occurs when the diffusing eddy currents are gathered into a locally more conductive zone. The most common geometric condition under which these effects are observed is when a conductor of long strike length is located outside a large transmitter loop. The TEM response is enhanced by channeling and gathering, and it is often stronger than simple inductive effects. Current channeling and current gathering phenomena are relatively unimportant when measurements are made within a large transmitter loop, or when a single‐ or in‐loop configuration is used. It is therefore recommended that multiple transmitter loop locations be used when surveying in conductive terrains with the largeloop configuration, or alternatively that the single‐ or in‐loop configuration be employed for additional geophysical control.

scholarly journals Application of Transient Electromagnetic Method for Investigating the Water-Enriched Mined-Out Area

2018 ◽  
Vol 8 (10) ◽  
pp. 1800 ◽  
Author(s):  
Chuantao Yu ◽  
Xinyue Liu ◽  
Jishan Liu ◽  
Enguo Li ◽  
Peng Yue ◽  
...  
Keyword(s):  
Coal Mining ◽  
Electromagnetic Method ◽  
Transient Electromagnetic Method ◽  
Survey Area ◽  
The North ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Geological Information ◽  
North Part ◽  
Rugged Topography

The water-enriched mined-out area, which led to frequent accidents and the serious destruction of ecological environment and serious threat to coal-mining practices. To ensure safe mining practices and to avoid the intrusion of water from the bases of coal deposits, the detection of the mined-out areas, especially the water-filled mined-out area in advanced, is one of the most important issues. This research focuses on detecting mined-out area enriched by water with the large-loop transient electromagnetic method. Data acquisition system was arranged along 23 survey lines, and the total of 1975 survey points were recorded. An inversion technique was used to interpret the TEM data. The inversion results suggested that the mined-out areas enriched by water always exhibit a very low-resistivity, the resistivity contour present closed circle sharp. There are three main water enriched mined-out areas, named JS1, JS2, and JS3 in the north part of the survey area, as well as three main water weak-enriched areas named RJS1, RJS2 and RJS3 in the west and southwest part of the survey area can be inferred by TEM, the interpretation results were verified by drilling. Large loop TEM is proved as an efficient way to investigate water enriched mined-out areas under the rugged topography conditions, and this work provides more detailed geological information to the coal mining enterprise for further coal-mining practices safety arrangement.

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