Time‐domain electromagnetic detection of a hidden target

Geophysics ◽  
1988 ◽  
Vol 53 (4) ◽  
pp. 537-545 ◽  
Author(s):  
David C. Bartel ◽  
A. Becker
Keyword(s):  
Magnetic Field ◽  
Time Domain ◽  
Time Window ◽  
Time Derivative ◽  
Step Response ◽  
Time Range ◽  
Geometrical Parameters ◽  
Target Signal ◽  
Time Domain Electromagnetic ◽  
The Time Domain

Numerical modeling of the time‐domain electromagnetic (EM) step response of a vertical tabular target hidden beneath a thin conductive overburden reveals that the target’s presence may be detected only during a well‐defined time window. In a situation where the secondary magnetic field is sensed by an airborne system equipped with horizontal coaxial dipoles, a conductance contrast of about ten between the target and the overburden is needed to ensure target detection. This value will, of course, vary with the size and depth of the target and, to a lesser extent, with the geometry of the system. In general, the time at which the window opens is a function of the geometrical parameters of the target, the height of the system, and the conductance of the overburden. For a given target, its width (defined as the ratio of the time of closure to the time of opening) is only a function of the conductance contrast between the target and the overburden. While the target signal is visible, one observes a maximum value of the target‐to‐overburden response ratio. The time at which this occurs is mainly controlled by the conductance of the target. The presence of the overburden causes the target signal to build up gradually before decaying toward zero. However, once the target signal dominates the overburden response, the signal can be approximated by a simple exponential decay over the time range of interest. The time constant of this decay is determined by the size and conductance of the target. Using this model, it is easy to relate the magnetic field step response calculated here to the response observable with a conventional EM system that transmits a primary field pulse of finite duration and detects the time derivative of the secondary magnetic field.

scholarly journals Electromagnetic induction in an inhomogeneous conductive thin sheet

Geophysics ◽  
1987 ◽  
Vol 52 (12) ◽  
pp. 1677-1688 ◽  
Author(s):  
Richard S. Smith ◽  
G. F. West
Keyword(s):  
Magnetic Field ◽  
Time Domain ◽  
Time Domain Method ◽  
Step Response ◽  
Scale Model ◽  
Frequency Domain Method ◽  
Vertical Magnetic Field ◽  
Zero Crossing ◽  
Domain Method ◽  
The Time Domain

Distinguishing between the electromagnetic (EM) response of a subsurface conductor and the EM response of an overburden whose conductivity and/or thickness varies laterally requires a capability to calculate the EM response of both types of conductor. While methods for calculating the response of some simple subsurface conductors such as dipping rectangular sheets are already available, methods for computing the response of an irregular overburden are not common. Using Price’s analysis, we have formulated two numerical techniques for calculating the response of a laterally varying overburden which is thin and flat, and which lies on a perfectly resistive subspace. The first technique is a frequency‐domain method in which a large matrix equation is solved to find the horizontal‐wavenumber components of the secondary vertical magnetic field. The method is best suited to calculating the response of the overburden when the EM source and receiver are located above the sheet, such as in airborne EM systems. Helicopter EM profiles calculated using this technique have been checked against a simple scale model. The second method calculates the time‐domain step response of the overburden by time‐stepping the vertical component of the magnetic field. The method is suitable for calculating the response of the overburden when the EM source is a large transmitter loop close to the overburden. Using the time‐domain method to investigate the response of simple conductance structures illustrates that the zero crossing of the vertical magnetic field moves more slowly across conductive regions than across resistive regions. This is because the rate of decay of the vertical field in a region varies in proportion to the resistance of the region. A response profile from a UTEM survey shows a response that could be interpreted as due to a dipping subsurface conductor. This response has been modeled using the time‐domain method, and a geologically acceptable pattern of lateral variations in the overburden conductance yields a response close to the measured EM response. Thus, a subsurface conductor need not lie below the profile line to explain the response.

Direct time‐domain calculation of the transient response for a rectangular loop over a two‐layer medium

Geophysics ◽  
1987 ◽  
Vol 52 (7) ◽  
pp. 997-1006 ◽  
Author(s):  
Mark M. Goldman ◽  
David V. Fitterman
Keyword(s):  
Magnetic Field ◽  
Time Domain ◽  
Electric Dipole ◽  
Late Stage ◽  
Apparent Resistivity ◽  
Time Derivative ◽  
Vertical Magnetic Field ◽  
First Case ◽  
Layer Medium ◽  
The Time Domain

The time derivative of the vertical magnetic field due to an electric dipole on the surface of a two‐layer half‐space is computed directly in the time domain by applying the residue theorem to the analytic field expressions. The second layer must be either insulating [Formula: see text] or perfectly conducting [Formula: see text]. The first case can be used to estimate the response of a conductive overburden for mining exploration problems. The second case is useful in explaining the overshoot seen in transient sounding voltage apparent‐resistivity curves when a conductive basement underlies a resistive first layer. In the late stage, the time derivative of the vertical magnetic field decays as [Formula: see text] and the late‐stage apparent resistivity increases as t for [Formula: see text], while for [Formula: see text], these quantities behave as [Formula: see text] and [Formula: see text], respectively, where [Formula: see text], [Formula: see text], is the first‐layer conductivity, [Formula: see text] is the first‐layer thickness, and [Formula: see text]. The electric dipole expressions are integrated to obtain solutions for rectangular loops. Numerical results for a rectangular loop on a layer over an insulating basement (overburden case) show that the overburden response is initially positive inside the loop and negative outside the loop. At later times, the response outside the loop becomes positive. The thinner the overburden layer, the greater the maximum response.

scholarly journals Lightweight Unmanned Aerial System for Time-Domain Electromagnetic Prospecting—The Next Stage in Applied UAV-Geophysics

2021 ◽  
Vol 11 (5) ◽  
pp. 2060 ◽  
Author(s):  
Alexander Parshin ◽  
Ayur Bashkeev ◽  
Yuriy Davidenko ◽  
Marina Persova ◽  
Sergey Iakovlev ◽  
...  
Keyword(s):  
Time Domain ◽  
Gamma Ray ◽  
Pulse Generator ◽  
Measuring System ◽  
Electromagnetic Sounding ◽  
Mountainous Regions ◽  
Time Domain Electromagnetic ◽  
Unmanned System ◽  
Classical Triad ◽  
The Time Domain

Nowadays in solving geological problems, the technologies of UAV-geophysics, primarily magnetic and gamma surveys, are being increasingly used. However, for the formation of the classical triad of airborne geophysics methods in the UAV version, there was not enough technology for UAV-electromagnetic sounding, which would allow studying the geological environment at depths of tens and hundreds of meters with high detail. This article describes apparently the first technology of UAV-electromagnetic sounding in the time domain (TDEM, TEM), implemented as an unmanned system based on a light multi-rotor UAV. A measuring system with an inductive sensor—an analogue of a 20 × 20 or 50 × 50 m receiving loop is towed by a UAV, and a galvanically grounded power transmitter is on the ground and connected to a pulse generator. The survey is carried out along a network of parallel lines at low altitude with a terrain draping at a speed of 7–8 m/s, the maximum distance of the UAV’s departure from the transmitter line can reach several kilometers, thus the created technology is optimal for performing detailed areal electromagnetic soundings in areas of several square kilometers. The results of the use of the unmanned system (UAS) in real conditions of the mountainous regions of Eastern Siberia are presented. Based on the obtained data, the sensitivity of the system was simulated and it was shown that the developed technology allows one to collect informative data and create geophysical sections and maps of electrical resistivity in various geological situations. According to the authors, the emergence of UAV-TEM systems in the near future will significantly affect the practice of geophysical work, as it was earlier with UAV-magnetic prospecting and gamma-ray survey.

scholarly journals Megasource time-domain electromagnetic sounding at Poggi del Sasso - Cinigiano (GR)

1994 ◽  
Vol 37 (5 Sup.) ◽  
Author(s):  
G. V. Keller ◽  
P. Cantini ◽  
R. Carrara ◽  
O. Faggioni ◽  
E. Pinna
Keyword(s):  
Depth Profile ◽  
Time Domain ◽  
Environmental Conditions ◽  
Dipole Source ◽  
Electromagnetic Noise ◽  
Electromagnetic Sounding ◽  
Piecewise Constant ◽  
Time Domain Electromagnetic ◽  
The Time Domain ◽  
Single Set

An experiment was carried out in the vicinity of the “I Terzi” area in Southeastern Tuscany (fig. 1) to evaluate the applicability of the Time Domain Electromagnetic (TDEM) sounding method under the geological and environmental conditions prevailing in that area. An electromagnetic source was established using a motor-generator set and heavy cable. Measurements were attempted at four sites. Numerous samples of electromagnetic noise were recorded at each of these sites. At one site, signals transmitted for a grounded dipole source at 1.6 km distance were also recorded with the noise. The single set of observations has been processed and inverted to yield a six-layer piecewise constant resistivity depth-profile to a depth of about 2 km. The primary achievement of the experiment was demonstration of the praeticability of TDEM methods under the conditions prevailing in the site.

scholarly journals Convergence of the uniaxial PML method for time-domain electromagnetic scattering problems

2021 ◽  
Author(s):  
Changkun Wei ◽  
Jiaqing Yang ◽  
Bo Zhang
Keyword(s):  
Time Domain ◽  
Electromagnetic Scattering ◽  
Scattering Problem ◽  
Three Dimensional ◽  
Scattering Problems ◽  
Laplace Transform Technique ◽  
The Laplace Transform ◽  
Time Domain Electromagnetic ◽  
Numer Anal ◽  
The Time Domain

In this paper, we propose and study the uniaxial perfectly matched layer (PML) method for three-dimensional time-domain electromagnetic scattering problems, which has a great advantage over the spherical one in dealing with problems involving anisotropic scatterers. The truncated uniaxial PML problem is proved to be well-posed and stable, based on the Laplace transform technique and the energy method. Moreover, the $L^2$-norm and $L^{\infty}$-norm error estimates in time are given between the solutions of the original scattering problem and the truncated PML problem, leading to the exponential convergence of the time-domain uniaxial PML method in terms of the thickness and absorbing parameters of the PML layer. The proof depends on the error analysis between the EtM operators for the original scattering problem and the truncated PML problem, which is different from our previous work (SIAM J. Numer. Anal. 58(3) (2020), 1918-1940).

scholarly journals Reevaluation of Performance of Electric Double-layer Capacitors from Constant-current Charge/Discharge and Cyclic Voltammetry

2016 ◽  
Vol 6 (1) ◽  
Author(s):  
Anis Allagui ◽  
Todd J. Freeborn ◽  
Ahmed S. Elwakil ◽  
Brent J. Maundy
Keyword(s):  
Cyclic Voltammetry ◽  
Double Layer ◽  
Time Domain ◽  
Electric Double Layer ◽  
Constant Current ◽  
Step Response ◽  
Current Voltage ◽  
The Time Domain ◽  
Double Layer Capacitors ◽  
Electric Double Layer Capacitors

Abstract The electric characteristics of electric-double layer capacitors (EDLCs) are determined by their capacitance which is usually measured in the time domain from constant-current charging/discharging and cyclic voltammetry tests, and from the frequency domain using nonlinear least-squares fitting of spectral impedance. The time-voltage and current-voltage profiles from the first two techniques are commonly treated by assuming ideal S s C behavior in spite of the nonlinear response of the device, which in turn provides inaccurate values for its characteristic metrics. In this paper we revisit the calculation of capacitance, power and energy of EDLCs from the time domain constant-current step response and linear voltage waveform, under the assumption that the device behaves as an equivalent fractional-order circuit consisting of a resistance R s in series with a constant phase element (CPE(Q, α), with Q being a pseudocapacitance and α a dispersion coefficient). In particular, we show with the derived (R s , Q, α)-based expressions, that the corresponding nonlinear effects in voltage-time and current-voltage can be encompassed through nonlinear terms function of the coefficient α, which is not possible with the classical R s C model. We validate our formulae with the experimental measurements of different EDLCs.

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