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.

A single apparent resistivity expression for long‐offset transient electromagnetics

Geophysics ◽  
1986 ◽  
Vol 51 (6) ◽  
pp. 1291-1297 ◽  
Author(s):  
Yang Sheng
Keyword(s):  
Apparent Resistivity ◽  
Early Time ◽  
Point Of View ◽  
Time Range ◽  
Late Time ◽  
Careful Study ◽  
Physical Point ◽  
Layered Earth ◽  
Transient Electromagnetic ◽  
Long Offset

Early‐time and late‐time apparent resistivity approximations have been widely used for interpretation of long‐offset transient electromagnetic (LOTEM) measurements because it is difficult to find a single apparent resistivity over the whole time range. From a physical point of view, Dr. C. H. Stoyer defined an apparent resistivity for the whole time range. However, there are two problems which hinder its use: one is that there is no explicit formula to calculate the apparent resistivity, and the other is that the apparent resistivity has no single solution. A careful study of the two problems shows that a numerical method can be used to calculate a single apparent resistivity. A formula for the maximum receiver voltage over a uniform earth, when compared with the receiver voltage for a layered earth, leads to the conclusion that, in some cases, a layered earth can produce a larger voltage than any uniform earth can produce. Therefore, our apparent resistivity definition cannot be applied to those cases. In some other cases, the two possible solutions from our definition do not merge, so that neither of them is meaningful for the whole time range.

The transient EM step response of a dipping plate in a conductive half‐space

Geophysics ◽  
1992 ◽  
Vol 57 (9) ◽  
pp. 1116-1126 ◽  
Author(s):  
James E. Hanneson
Keyword(s):  
Half Space ◽  
Free Space ◽  
Early Time ◽  
Step Response ◽  
Current Loop ◽  
Late Time ◽  
Electromagnetic System ◽  
University Of Toronto ◽  
Transient Electromagnetic ◽  
Induction Process

An algorithm for computing the transient electromagnetic (TEM) response of a dipping plate in a conductive half‐space has been developed. For a stationary [Formula: see text] current loop source, calculated profiles simulate the response of the University of Toronto electromagnetic system (UTEM) over a plate in a 1000 Ω ⋅ m half‐space. The objective is to add to knowledge of the galvanic process (causing poloidal plate currents) and the local induction process (causing toroidal currents) by studying host and plate currents with respect to surface profiles. Both processes can occur during TEM surveys. Plates are all [Formula: see text] thick with various depths, dips, and conductances. Calculated host and plate currents provide quantitative examples of several effects. For sufficiently conductive plates, the late time currents are toroidal as for a free‐space host. At earlier times, or at all times for poorly conducting plates, the plate currents are poloidal, and the transitions to toroidal currents, if they occur, are gradual. At very late times, poloidal currents again dominate any toroidal currents but this effect is rarely observed. Stripped, point‐normalized profiles, which reflect secondary fields caused by the anomalous plate currents, illustrate effects such as early time blanking (caused by noninstantaneous diffusion of fields into the target), mid‐time anomaly enhancement (caused by galvanic currents), and late time plate‐in‐free‐space asymptotic behavior.

Possibilities of electromagnetic methods in the investigations of talik under lake (numerical modeling)

2020 ◽  
Author(s):  
Alexandr N. Shein ◽  
◽  
Vladimir V. Olenchenko ◽  
Yaroslav K. Kamnev ◽  
◽  
...  
Keyword(s):  
Numerical Modeling ◽  
Field Station ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Tem Method

The paper presents the results of numerical modeling of electroresistivity tomography (ERT) and transient electromagnetic (TEM) signals for the model of a freezing talik under khasyrei near the "Parisento" field station (Gydan peninsula). The inversion of ERT data results in artifacts that interpreted as an increase of the talik with depth. It is shown that TEM method is useable to determine the bottom of the talik and the base of the permafrost at a depth of 300 m with an accuracy of 10 m.

Response characteristics of coincident loop transient electromagnetic systems

Geophysics ◽  
1982 ◽  
Vol 47 (9) ◽  
pp. 1325-1330 ◽  
Author(s):  
P. Weidelt
Keyword(s):  
Step Function ◽  
Voltage Response ◽  
Complex Frequency ◽  
Induced Voltage ◽  
Response Characteristics ◽  
Transient Electromagnetic ◽  
Driving Current ◽  
Occasional Occurrence ◽  
Frequency Independent ◽  
Independent Distribution

The occasional occurrence of persistent sign reversals in coincident loop transient electromagnetic (TEM) measurements stimulates an investigation of possible causes for this effect. By examining the response in the complex frequency plane near the spectrum of freely decaying current modes, it is shown that for any physically reasonable frequency‐independent distribution of electrical conductivity and magnetic permeability the voltage response to a step function driving current is of one sign only. Moreover, under the conditions mentioned above, the logarithm of the induced voltage is a decreasing convex function of time. These characteristics are retained for more general time functions of the driving current. The conservation of sign for frequency‐independent material parameters supports the assumption of IP effects as a possible mechanism for sign reversals. The latter point is illustrated by a simplified example.

Coincident loop transient electromagnetic master curves for interpretation of two‐layer earths

Geophysics ◽  
1981 ◽  
Vol 46 (1) ◽  
pp. 53-64 ◽  
Author(s):  
A. P. Raiche ◽  
B. R. Spies
Keyword(s):  
Time Range ◽  
Late Time ◽  
Sufficient Time ◽  
Master Curves ◽  
Layer Depth ◽  
Apparent Conductivity ◽  
Transient Electromagnetic ◽  
Tem Method ◽  
Earth Conductivity

A set of apparent conductivity master curves has been calculated for the coincident loop transient electromagnetic (TEM) method used over a two‐layer earth. Conductivity contrasts range from 0.001 to 1000. Loop radius/layer depth ratios range from 0.01 to 100. The time range is sufficient to see the entire shape of the curves from the early to the late time asymptotes. These curves allow the determination of the parameters of a two‐layer earth for accurate data over a sufficient time range. Examples using the curves to interpret multilayered earths are given. The curves are also used to show the limitations placed on interpretation by existing TEM equipment.

scholarly journals On coincident loop transient electromagnetic induction logging

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. E211-E220 ◽  
Author(s):  
Andrei Swidinsky ◽  
Chester J. Weiss
Keyword(s):  
Apparent Resistivity ◽  
Earth Model ◽  
Late Time ◽  
Concentric Cylinder ◽  
Transient Electromagnetic ◽  
Water Based ◽  
Resistivity Logs ◽  
Invasion Models ◽  
Whole Space ◽  
Formation Resistivity

Coincident loop transient induction wireline logging is examined as the borehole analog of the well-known land and airborne time-domain electromagnetic (EM) method. The concept of whole-space late-time apparent resistivity is modified from the half-space version commonly used in land and airborne geophysics and applied to the coincident loop voltages produced from various formation, borehole, and invasion models. Given typical tool diameters, off-time measurements with such an instrument must be made on the order of nanoseconds to microseconds — much more rapidly than for surface methods. Departure curves of the apparent resistivity for thin beds, calculated using an algorithm developed to model the transient response of a loop in a multilayered earth, indicate that the depth of investigation scales with the bed thickness. Modeled resistivity logs are comparable in accuracy and resolution with standard frequency-domain focused induction logs. However, if measurement times are longer than a few microseconds, the thicknesses of conductors can be overestimated, whereas resistors are underestimated. Thin-bed resolution characteristics are explained by visualizing snapshots of the EM fields in the formation, where a conductor traps the electric field while two current maxima are produced in the shoulder beds surrounding a resistor. Radial profiling is studied using a concentric cylinder earth model. Results found that true formation resistivity can be determined in the presence of either oil- or water-based mud, although in the latter case, measurements must be taken several orders of magnitude later in time. The ability to determine true formation resistivity is governed by the degree that the EM field heals after being distorted by borehole fluid and invasion, a process visualized and particularly evident in the case of conductive water-based mud.

Transient electromagnetic modeling of shallow A‐type sections with 3-D inhomogeneities

Geophysics ◽  
1992 ◽  
Vol 57 (6) ◽  
pp. 774-780 ◽  
Author(s):  
M. Poddar ◽  
Walter L. Anderson
Keyword(s):  
Apparent Resistivity ◽  
Middle Layer ◽  
Late Time ◽  
Geoelectric Section ◽  
Resistivity Sounding ◽  
Transient Electromagnetic ◽  
Target Layer ◽  
Weathered Layer ◽  
Target Depth ◽  
Transient Electric Field

A hard rock area underlain by granitic/gneissic or basaltic rocks often has an A‐type three‐layer geoelectric section in which resistivity increases with depth. The middle layer of moderate resistivity caused by fracturing/fissuring that lies between the surface‐weathered layer and the substratum of unfractured rock is not a good target for a direct current (DC) resistivity sounding since it is generally suppressed in the observations. Moreover, its definition requires expanding the electrode spacing to a length several times the depth of the target layer, and this may be a drawback if the target layer is either laterally variable or limited in its horizontal extent. We first studied the transient electric field of a horizontal electric dipole (HED) source excited by a step turn‐off current for a 1-D model of an A‐type geoelectric section. The results of this theoretical study are presented as graphs of normalized apparent resistivity versus a time‐related dimensionless parameter. Irrespective of the separation between the transmitter and receiver dipoles, these transient sounding curves become similar to the corresponding Schlumberger sounding curves at late time. Hence the transient electric field measurement offers the possibility of sounding at a fixed transmitter‐receiver spacing that may be shorter than the target depth. Also, at early times, for a certain ratio of the dipole separation to the target depth, there is a dramatic increase in the resolution of the response. Thus, it is possible to resolve suppressed layers of an A‐type section in this type of sounding. A study of the effects of transmitter ramp time and receiver bandwidth on the transient apparent resistivity curves shows that a very fast current shut‐off and wideband measurement are required to realize all the possibilities suggested by this modeling. Some 3-D transient electromagnetic (TEM) modeling was also done to simulate (1) a lateral variation in the resistivity of the middle layer of an A‐type section and (2) a weak zone of limited horizontal extent in the substratum of a two‐layer section. We observed that the 3-D inclusion has less effect at late time but is more pronounced at early time. In view of the above results, we conclude that the transient E‐field sounding with a grounded wire source can be used in place of a conventional DC resistivity sounding to overcome the problem of poor resolution due to the suppression of the intermediate layer in a geoelectric section where the resistivity increases with depth. As such, it has a potential application in groundwater as well as geotechnical surveys, because together with the overlying weathered layer, the fractured rock constitutes the aquifer in hard rocks.

Extracting Pseudo Wave Fields from Transient Electromagnetic Fields Using a Weighting Coefficients Approach

2019 ◽  
Vol 24 (3) ◽  
pp. 351-359
Author(s):  
Junjie Xue ◽  
Jiulong Cheng ◽  
Guoqiang Xue ◽  
Hai Li ◽  
Dongyang Hou ◽  
...  
Keyword(s):  
Electromagnetic Field ◽  
Singular Value Decomposition ◽  
Early Time ◽  
Singular Value ◽  
Late Time ◽  
Decomposition Algorithms ◽  
Transient Electromagnetic ◽  
L2 Norm ◽  
Value Decomposition ◽  
Weighting Coefficients

The diffusive electromagnetic field can be transformed into the wave domain by means of mathematical conversion. The transformed field can then be interpreted with the tools in seismic data processing so that the identification to the underground targets can be effectively improved. However, the conversion is typically an ill-posed problem that needs to be solved using regularization tools. Based on the conventional regularization with smooth constraints in the L2 norm, the inversion result is of low resolution, while that obtained using truncated singular value decomposition (TSVD) methods is typically accurate, but has poor stability. To obtain a stable and accurate transformed electromagnetic field value, this study proposed to combine conventional regularization tools and singular value decomposition algorithms by incorporating a set of weighting coefficients. The proposed method is validated on both synthetic and observed data. The results from the proposed method are more accurate at the early time, and at the late time are more stable compared with the other methods. Furthermore, the example of field data shows that the proposed method could potentially further improve the interpretation accuracy of future mining explorations.

The Transient Electromagnetic Response Characteristics of Unfavorable Geological Bodies in Tunnels

2011 ◽  
Vol 261-263 ◽  
pp. 1494-1498
Author(s):  
Lu Bo Meng ◽  
Tian Bin Li ◽  
Zheng Duan
Keyword(s):  
Apparent Resistivity ◽  
Electromagnetic Response ◽  
High Resistivity ◽  
Karst Cave ◽  
Geological Body ◽  
Response Characteristics ◽  
Finite Element Software ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Geological Conditions

A numerical model of Transient Electromagnetic Methods, which used to detect geological conditions in tunnels, was created by ANSYS finite element software, and the transient electromagnetic response characteristics of typical unfavorable geological bodies were discussed. The results indicate that the apparent resistivity isoline almost outputs in straight structure, and augments equably with the depth increasing when no unfavorable geological body is in front of the tunnel workface. The apparent resistivity isoline obviously reduces when water-rich karst cave, fault, or fracture exist in the front of the tunnel workface, where water-rich karst cave exists, the apparent resistivity isoline becomes close together, and there is to be an ordered pyramid resistivity-reduced area which can be an evidence of rich water fault. However, it will increase when dry fault or karst cave exist, and the high resistivity area corresponding to dry cave shows some triangles, but this phenomena is hard identified, which need detail analyses according to hydrogeologic data.

Comparison of apparent resistivity functions for transient electromagnetic methods

Geophysics ◽  
1983 ◽  
Vol 48 (6) ◽  
pp. 787-789 ◽  
Author(s):  
A. P. Raiche
Keyword(s):  
Magnetic Field ◽  
Apparent Resistivity ◽  
Time Derivative ◽  
Mineral Resources ◽  
Transient Electromagnetic ◽  
Research Organizations ◽  
Electromagnetic Methods ◽  
Direct Measurements ◽  
The World ◽  
The Magnetic Field

The use of transient electromagnetic (TEM) methods is increasing throughout the world because of their success in finding conductive anomalies in regions previously thought inappropriate for EM techniques. Most TEM systems now in use (SIROTEM UTEM, Crone Pulse EM, etc.) employ voltage measurements, i.e., measurements of the time derivative of the magnetic field. However, the success of SQUID based receiving systems in many fields has led research organizations (such as the Bureau of Mineral Resources, Australia) to investigate the use of SQUIDs in TEM systems to obtain direct measurements of the magnetic field.

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