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.

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.

Aftereffects in the Transient Electromagnetic Method: Inductively Induced Polarization

2021 ◽  
Vol 62 (12) ◽  
pp. 1440-1448
Author(s):  
N.O. Kozhevnikov ◽  
E.Yu. Antonov
Keyword(s):  
Transient Response ◽  
Electric Polarization ◽  
Induced Polarization ◽  
Point Of View ◽  
Electromagnetic Method ◽  
Time Range ◽  
General Point ◽  
Transient Electromagnetic Method ◽  
Transient Electromagnetic ◽  
Tem Method

Abstract —Inductively induced electric polarization (IIP) is one of the aftereffects inherent in the geologic materials and affecting results of the transient electromagnetic method. Its effect on the inductive transient response manifests itself as a nonmonotonic EMF decay, including the polarity reversal. The dependence of IIP on many conditions makes it difficult to study the basic regularities in its manifestation. One of the ways to address this problem is to present the simulation results as a normalized transient response. From the most general point of view, the intensity and time range of the IIP manifestation are controlled by the competition between induction and induced polarization phenomena. Induced polarization manifests itself differently, depending on the transmitter used for the excitation of the ground response. Therefore, when studying polarizable ground, the results of the conventional IP method and those of the TEM method do not always correlate.

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.

Vertical and horizontal resistive limit formulas for a rectangular-loop source on a conductive half-space

Geophysics ◽  
2012 ◽  
Vol 77 (1) ◽  
pp. E91-E99 ◽  
Author(s):  
Ralf Schaa ◽  
Peter K. Fullagar
Keyword(s):  
Least Squares ◽  
Half Space ◽  
Host Rock ◽  
Late Time ◽  
Definite Integrals ◽  
Apparent Conductivity ◽  
Transient Electromagnetic ◽  
Analytic Expressions ◽  
Fast Transient

We derived analytic expressions for the time integrals of vertical and horizontal transient B-field responses on a conductive half-space excited by a rectangular-loop source. These formulas were applied in two ways in a fast transient electromagnetic (TEM) inversion scheme. Indefinite integrals were used to extrapolate measured TEM decays to early and late time to convert the observed data to resistive limits. Definite integrals over all time provided estimates for the resistive limit response of host-rock. An apparent conductivity of the host was calculated from the resistive limits via a simple least-squares formulation. These applications of the formulas were tested on synthetic and real TEM data.

Transient electromagnetic response of the Teutonic Bore orebody

Geophysics ◽  
1986 ◽  
Vol 51 (4) ◽  
pp. 957-963 ◽  
Author(s):  
G. Buselli ◽  
K. G. McCracken ◽  
M. Thorburn
Keyword(s):  
Host Rock ◽  
Delay Time ◽  
Free Space ◽  
Time Range ◽  
Late Time ◽  
Mining Operations ◽  
Transient Electromagnetic ◽  
Analog Modeling ◽  
Delay Times ◽  
Over The Top

Transient electromagnetic (TEM) measurements have been made with SIROTEM on four separate surveys over the Teutonic Bore orebody (Western Australia), both before mining operations began and subsequently during different stages of stripping overburden from the mineral deposit. In the late stage of the transient decay the target response was relatively free of the overburden and host‐rock response. Beyond ∼ 6 ms, the maximum anomalous response was a factor of 8 to 10 greater than the combined overburden and host‐rock response. Analog modeling with a copper plate in free space shows that the TEM response of the target consists of a single peak at early delay times, while at delay times beyond ∼ 4.2 ms, the response becomes a double‐peak anomaly with a low directly over the top of the plate. Mathematical modeling of the TEM response with a free‐space infinitely thin plate produces profile characteristics similar to those obtained by analog modeling beyond a delay time of ∼ 4.2 ms. Inversion of premining survey profiles in the delay time range 7.0 to 13.2 ms gives values of 82 m for target depth d, and 86 degrees for dip angle θ. These agree well with the values d = 86 m and θ = 82 degrees derived from drilling data. A target conductance value in the range 250 to 320 S is obtained from the TEM data, indicating that the massive sulfide target is highly conductive. Responses calculated for surveys made during overburden stripping are lower than corresponding field values at early delay times because of the absence of overburden response in the model measurements. At delay times beyond 8.5 ms, the model values are consistent with the field values. These results indicate that for a case similar to the Teutonic Bore orebody, where the maximum anomalous late‐time response is a factor of 8 to 10 times greater than the background response, important target parameters may be derived from free‐space models.

The harmonic and transient electromagnetic response of a thin dipping dike

Geophysics ◽  
1983 ◽  
Vol 48 (7) ◽  
pp. 934-952 ◽  
Author(s):  
P. Weidelt
Keyword(s):  
Formal Solution ◽  
Harmonic Excitation ◽  
Electromagnetic Response ◽  
Approximate Determination ◽  
Finite Conductivity ◽  
Circular Loop ◽  
Decay Modes ◽  
Transient Electromagnetic ◽  
Free Decay

An exact solution is given for the electromagnetic induction in a dipping dike of finite conductivity, represented as a thin half‐sheet in a nonconducting surrounding. The problem is formulated for arbitrary dipole or circular loop [Formula: see text] configurations. The formal solution obtained by the Wiener‐Hopf technique is cast into a rapidly convergent triple integral suitable for an effective numerical treatment. A good agreement is found between numerical results and analog measurements available for harmonic excitation. The transient response is obtained as a superposition of the half‐sheet free‐decay modes and is illustrated by some numerical examples for coincident loops, including a diagram for the approximate determination of conductance and depth of a vertical dike.

Influences of the Earth’s Magnetic Field on the Transient Electromagnetic Process in the Geoelectric Field: an Experimental Study

2021 ◽  
Vol 62 (12) ◽  
pp. 1430-1439
Author(s):  
V.S. Mogilatov ◽  
V.V. Potapov ◽  
A.N. Shein ◽  
V.A. Gur’ev
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Field Experiments ◽  
Geoelectric Field ◽  
Earth’S Magnetic Field ◽  
Electromagnetic Process ◽  
Transient Electromagnetic ◽  
Schematic Layout ◽  
Earth's Magnetic Field ◽  
Tem Method

Abstract —A mathematical model of the influence of the Earth’s magnetic field (the Hall effect) on results of the controlled source transient electromagnetic (TEM) method has been elaborated. For identification of this effect, we propose a schematic layout of the experimental grounded system with a pulsed loop source and signals recording by radial receive lines equally spaced relative to the loop. The 2018–2019 special field experiments were conducted in the Tatar region of the West Siberian Lowland with an aim to estimate the Hall effect contributions to the TEM method. To detect the Hall effect, transient electromagnetic responses were measured mainly by four receive lines radiating from a 500×500 m square loop. Analysis of the TEM results processing aimed at improving the signal quality and reducing the interference revealed a great similarity in signals from the radial lines, which is theoretically possible only under the Hall effect. Comparison of the field signals with the theoretical ones enabled estimation of the components caused by the Hall effect, in particular, conductivity at ~0.002 S/m.

Study of water salinity in the Chtouka plain (Morocco) using TEM geophysical method, chemical and isotopes tracers

2021 ◽  
Author(s):  
Henrik Schreiber ◽  
Saadou Oumarou Danni ◽  
Amine Touab ◽  
Fatima Abourig ◽  
Nelly Montcoudiol ◽  
...  
Keyword(s):  
Seawater Intrusion ◽  
Monitoring Network ◽  
Water Salinity ◽  
Geophysical Method ◽  
Transient Electromagnetic ◽  
Rock Water Interaction ◽  
Groundwater Overexploitation ◽  
Local Water ◽  
Stable Isotopes Of Water ◽  
Tem Method

<p>The Chtouka plain in Morocco suffers from groundwater overexploitation and a significant increase in water salinity. In this study, a multidisciplinary approach combining water chemistry, stable isotopes of water (18O, 2H) and Transient Electromagnetic (TEM) method was used. The main objective was to identify the water salinity sources and the extension of the marine intrusion. Water samples were collected from wells and boreholes, springs, the Massa river and the main source of freshwater in the region, the Youssef Ibn Tachfine Dam. Geophysical (TEM) measurements (12 profiles comprising 83 measurement points) were carried out along the coastal zone and around the northern bank of the Massa river. The results show a spatial variability of water salinity, indicating rock-water interaction, seawater intrusion and anthropogenic influence. The interpretation of the TEM soundings allow to draw the front line of the marine intrusion in the aquifer. The results, compared to previous numerical simulations, show a significant progress of the marine intrusion into the coastal aquifer. The intrusion indeed reaches a distance of 2.5 km from the coast, far beyond models’ predictions. The local water authorities can use these results to improve their monitoring network and better assess the progress of the seawater intrusion.<br>Keywords: Water salinity, TEM geophysical method, chemical and isotopes tracers, marine intrusion</p>

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