Transient electromagnetic responses of 3D polarizable body

Hesham M. El‐Kaliouby; Essam A. Eldiwany

doi:10.1190/1.1707062

Transient electromagnetic responses of 3D polarizable body

Geophysics ◽

10.1190/1.1707062 ◽

2004 ◽

Vol 69 (2) ◽

pp. 426-430 ◽

Cited By ~ 8

Author(s):

Hesham M. El‐Kaliouby ◽

Essam A. Eldiwany

Keyword(s):

The Body ◽

Voltage Response ◽

Negative Part ◽

Electromagnetic System ◽

Body Dimensions ◽

Negative Voltage ◽

Redundant Data ◽

Transient Electromagnetic ◽

Electromagnetic Responses ◽

Polarizable Body

The transient voltage response of a central loop electromagnetic system above a buried polarizable parallelepiped target (such as pyrites, sulfides, clays, etc.) is studied. The voltage response, which includes a positive part and a negative part (related to the polarization properties), is studied by varying the body characteristics, including the Cole‐Cole parameters, body dimensions, depth, and thickness. By varying the loop radius and by using profiling, the lateral body dimensions can be identified, particularly using the negative response for polarizable bodies. There is a clear difference in the dependencies of the positive and negative voltage responses on the body characteristics, which means that the negative voltage is not just redundant data, but we may use it, together with the positive response, to have better understanding for the body dimensions and polarization parameters effects.

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Three‐dimensional transient electromagnetic responses for a grounded source

Geophysics ◽

10.1190/1.1442064 ◽

1986 ◽

Vol 51 (11) ◽

pp. 2117-2130 ◽

Cited By ~ 21

Author(s):

Brian M. Gunderson ◽

Gregory A. Newman ◽

Gerald W. Hohmann

Keyword(s):

Magnetic Field ◽

Half Space ◽

Time Derivative ◽

Three Dimensional ◽

The Body ◽

Vertical Magnetic Field ◽

Horizontal Magnetic Field ◽

One Dimensional ◽

Transient Electromagnetic ◽

Electromagnetic Responses

When the current in a grounded wire is terminated abruptly, currents immediately flow in the Earth to preserve the magnetic field. Initially the current is concentrated near the wire, with a broad zone of return currents below. The electric field maximum broadens and moves downward with time. Currents are channeled into a conductive three‐dimensional body, resulting in anomalous magnetic fields. At early times, when the return currents are channeled into the body, the vertical magnetic field is less than the half‐space field on the far side of the body but is greater than the half‐space field between the source and the body. Later the current in the body reverses; the vertical field is enhanced on the far side of the body and decreased between the source and the body. The horizontal magnetic field has a well‐defined maximum directly over the body at late times, and is a better indicator of the position of the body. The vertical magnetic field and its time derivative change sign with time at receiver locations near the source if a three‐dimensional body is present. These sign reversals present serious problems for one‐dimensional inversion, because decay curves for a layered earth do not change sign. At positions away from the source, the decay curves exhibit no sign reversals—only decreases and enhancements relative to one‐dimensional decay curves. In such cases one‐dimensional inversions may provide useful information, but they are likely to result in fictitious layers and erroneous interpretations.

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Effect of conductive host rock on borehole transient electromagnetic responses

Geophysics ◽

10.1190/1.1442687 ◽

1989 ◽

Vol 54 (5) ◽

pp. 598-608 ◽

Cited By ~ 7

Author(s):

Gregory A. Newman ◽

Walter L. Anderson ◽

Gerald W. Hohmann

Keyword(s):

Magnetic Field ◽

Half Space ◽

Time Derivative ◽

The Body ◽

Galvanic Current ◽

Large Loop ◽

Transient Electromagnetic ◽

Electromagnetic Responses ◽

Pass Through ◽

The Magnetic Field

Transient electromagnetic (TEM) borehole responses of 3-D vertical and horizontal tabular bodies in a half‐space are calculated to assess the effect of a conductive host. The transmitter is a large loop at the surface of the earth, and the receiver measures the time derivative of the vertical magnetic field. When the host is conductive (100 Ω ⋅ m), the borehole response is due mainly to current channeled through the body. The observed magnetic‐field response can be visualized as due to galvanic currents that pass through the conductor and return in the half‐space. When the host resistivity is increased, the magnetic field of the conductor is influenced more by vortex currents that flow in closed loops inside the conductor. For a moderately resistive host (1000 Ω ⋅ m), the magnetic field of the body is caused by both vortex and galvanic currents. The galvanic response is observed at early times, followed by the vortex response at later times if the body is well coupled to the transmitter. If the host is very resistive, the galvanic response vanishes; and the response of the conductor is caused only by vortex currents. The shapes of the borehole profiles change considerably with changes in the host resistivity because vortex and galvanic current distributions are very different. When only the vortex response is observed, it is easy to distinguish vertical and horizontal conductors. However, in a conductive host where the galvanic response is dominant, it is difficult to interpret the geometry of the body; only the approximate location of the body can be determined easily. For a horizontal conductor and a single transmitting loop, only the galvanic response enables one to determine whether the conductor is between the transmitter and the borehole or beyond the borehole. A field example shows behavior similar to that of our theoretical results.

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Three-dimensional Modeling of Transient Electromagnetic Responses of Water-bearing Structures in Front of a Tunnel Face

Journal of Environmental and Engineering Geophysics ◽

10.2113/jeeg19.1.13 ◽

2014 ◽

Vol 19 (1) ◽

pp. 13-32 ◽

Cited By ~ 27

Author(s):

S. Li ◽

H. Sun ◽

X. Lu ◽

X. Li

Keyword(s):

Three Dimensional ◽

Tunnel Face ◽

Three Dimensional Modeling ◽

Transient Electromagnetic ◽

Dimensional Modeling ◽

Electromagnetic Responses

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The use and misuse of apparent resistivity in electromagnetic methods

Geophysics ◽

10.1190/1.1442194 ◽

1986 ◽

Vol 51 (7) ◽

pp. 1462-1471 ◽

Cited By ~ 78

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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Using an induction coil sensor to indirectly measure the B-field response in the bandwidth of the transient electromagnetic method

Geophysics ◽

10.1190/1.1444837 ◽

2000 ◽

Vol 65 (5) ◽

pp. 1489-1494 ◽

Cited By ~ 34

Author(s):

Richard S. Smith ◽

A. Peter Annan

Keyword(s):

Magnetic Field ◽

Field Data ◽

Indirect Measurement ◽

Rate Of Change ◽

Induction Coil ◽

Voltage Response ◽

Magnetic Field Data ◽

Transient Electromagnetic ◽

Different Characteristics ◽

The Magnetic Field

The traditional sensor used in transient electromagnetic (EM) systems is an induction coil. This sensor measures a voltage response proportional to the time rate of change of the magnetic field in the EM bandwidth. By simply integrating the digitized output voltage from the induction coil, it is possible to obtain an indirect measurement of the magnetic field in the same bandwidth. The simple integration methodology is validated by showing that there is good agreement between synthetic voltage data integrated to a magnetic field and synthetic magnetic‐field data calculated directly. Further experimental work compares induction‐coil magnetic‐field data collected along a profile with data measured using a SQUID magnetometer. These two electromagnetic profiles look similar, and a comparison of the decay curves at a critical point on the profile shows that the two types of measurements agree within the bounds of experimental error. Comparison of measured voltage and magnetic‐field data show that the two sets of profiles have quite different characteristics. The magnetic‐field data is better for identifying, discriminating, and interpreting good conductors, while suppressing the less conductive targets. An induction coil is therefore a suitable sensor for the indirect collection of EM magnetic‐field data.

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Genotype, gestation length, season, parity and sex effects on growth traits of two rabbit breeds and their crosses

Biotechnology in Animal Husbandry ◽

10.2298/bah1404717e ◽

2014 ◽

Vol 30 (4) ◽

pp. 717-729 ◽

Cited By ~ 1

Author(s):

S.S.A. Egena ◽

G.N. Akpa ◽

I.C. Alemede ◽

A. Aremu

Keyword(s):

Growth Traits ◽

Tail Length ◽

The Body ◽

Gestation Length ◽

Body Measurements ◽

Season Of Birth ◽

Trunk Length ◽

Body Dimensions ◽

Sex Effects ◽

Foundation Stock

One hundred and thirty rabbits were used to evaluate the effect of genotype, gestation length, season, parity and sex on growth traits of two breeds of rabbit and their crosses. The rabbit used for the experiment were breeds of the New Zealand White (NZW) and Chinchilla (CH) breed. Six breeding bucks (three/breed) and eighteen breeding does (nine/breed) served as the foundation stock. Traits measured include: body weight (BW), nose to shoulder length (NTS), shoulder to tail length (STL), heart girth (HG), trunk length (TL) and length of ear (LE). Results revealed that, BW of the rabbits were influenced (p<0.05) by genotype, gestation length and season. CH x (CH x NZW) progenies had better BW at 35-d and 49-d of age while NZW x CH progenies had better BW at 21-d of age. Kittens born late (32-34 days) had better BW at 21-d, 35-d and 49-d while kittens kindled during early dry season had better BW at 21-d, 35-d and 49-d. Genotype affected (p<0.05) all the body measurements at 21-d, 35-d and 49-d. Gestation length affected (p<0.05) all the body measurements except for NTS at 21-d and HG at 49-d respectively. Season of birth also influenced (p<0.05) all the body measurements except for LE 21-d. Parity and sex had no effect (p>0.05) on BW, NTS, STL, HG, TL and LE. It was concluded that genotype, gestation length and season influenced BW and body dimensions of the two breeds of rabbit and their crosses while parity and sex had no effect.

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