A model study of a thin plate in free space for the EM37 transient electromagnetic system

Geophysics ◽  
1985 ◽  
Vol 50 (6) ◽  
pp. 1002-1019 ◽  
Author(s):  
Peggie R. Gallagher ◽  
Stanley H. Ward ◽  
G. W. Hohmann
Keyword(s):  
Thin Plate ◽  
Free Space ◽  
Time Derivative ◽  
Late Time ◽  
Loop Analysis ◽  
Response Curves ◽  
Electromagnetic System ◽  
Transient Electromagnetic ◽  
Small Depth ◽  
Depth Extent

The computer program PLATE, developed at the University of Toronto, models the electromagnetic (EM) response of an inductively thin plate in free space. We used PLATE to compute two components of the time derivative of the magnetic field for a range of models for the EM37 fixed‐source transient system ([Formula: see text] loop). Analysis of the response curves produced methods of interpretation for obtaining plate geometry and conductance. The overall width of an anomaly, the distance between peaks and the width of the updip lobes, can provide an estimate of depth. Dip has the dominant effect on the ratio of the peak amplitudes. A rough estimate of plate size and the position in time (early or late) of the currents is essential before proceeding with interpretation. Strike length is not obviously reflected in the shape of the curves, but depth extent is indicated by the rate at which the downdip tail returns to the baseline, except for vertical plates. For vertical plates, curve matching may be the only method of obtaining an estimate of depth extent. Varying conductance for a particular model in free space affects whether a channel represents an early, intermediate, or late time response. The shape of a profile varies with the time of measurement. The estimated time constant can be used to calculate the conductance, provided an estimate of the shortest dimension of the plate is available. Extinction angles appear frequently for plates of small depth extent but do not occur for plates which are of infinite strike and depth extent with respect to the size of the transmitting loop.

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.

The influence of a conductive host on two‐dimensional borehole transient electromagnetic responses

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 861-869 ◽  
Author(s):  
Perry A. Eaton ◽  
Gerald W. Hohmann
Keyword(s):  
Electric Field ◽  
Half Space ◽  
Time Window ◽  
Time Derivative ◽  
Finite Width ◽  
Late Time ◽  
Time Behavior ◽  
Two Dimensional ◽  
Peak Response ◽  
Transient Electromagnetic

We have computed transient borehole electromagnetic (EM) responses of two‐dimensional (2-D) models using a direct and explicit finite‐difference algorithm. The program computes the secondary electric field which is defined as the difference between the total field and the primary (half‐space) field. The time derivative of the vertical magnetic field in a borehole is computed by numerical differentiation of the total electric field. These models consist of a thin horizontal conductor with a finite width, embedded in a conductive half‐space. Dual line sources energized by a step‐function current lie on the surface of the half‐space and simulate the long sides of a large rectangular loop. Numerical results substantiate several important features of the transient impulse response of such models. The peak response of the target is attenuated as the resistivity of the host decreases. A sign reversal in the secondary electric field occurs later in time as the resistivity of the host decreases. The peak response and the onset of late‐time behavior are delayed in time as well. Secondary responses for models with different host resistivities (10–1000 Ω-m) are approximately the same at late time. If the target is less conductive, the effects of the host, i.e., the attenuation and time delay, are less. It is readily apparent that there exists a time window within which the target’s response is at a maximum relative to the half‐space response. At late time the shape of the borehole anomaly due to a thin conductive 2-D target appears to be independent of the conductivity of the host. The late‐time secondary decay of the target is neither exponential nor power law, and a time constant computed from the slope of a log‐linear decay curve at late time may be much larger than the actual value for the same target in free space.

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.

Inversions of surface and borehole data from large‐loop transient electromagnetic system over a 1‐D earth

Geophysics ◽  
2001 ◽  
Vol 66 (4) ◽  
pp. 1090-1096 ◽  
Author(s):  
Z. Zhang ◽  
J. Xiao
Keyword(s):  
Early Time ◽  
Late Time ◽  
Inversion Algorithm ◽  
Borehole Data ◽  
Electromagnetic System ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Surface Data ◽  
The Time Domain ◽  
The Magnetic Field

Large‐loop electromagnetic (EM) systems that measure transient EM (TEM) data on the surface or in boreholes have shown increased application in exploration geophysics. Accurate interpretation of borehole TEM data is necessary to discover deep hidden targets that cannot be detected with surface systems. However, the inversion of borehole TEM data has not been fully addressed. In this paper, we study the propagation of the TEM field from a large‐loop EM borehole system inside a layered earth and develop a new inversion algorithm to reconstruct layered conductivity structures from large‐loop TEM data measured with both surface and borehole configurations. The magnetic field and sensitivities are first computed in the frequency domain and are then transformed into the time domain where the inversion is performed. The surface data have a higher S/N ratio at early time channels, while the borehole data have a higher S/N ratio at late time channels. Consequently, the surface data can be inverted to better resolve shallow structures, and the borehole data can be used to better detect deep structures. The merits of joint inversions of borehole and surface data are explored. We test our inversion algorithm using numeric examples.

The Development and Applications of the Helicopter-borne Transient Electromagnetic System CAS-HTEM

2019 ◽  
Vol 24 (4) ◽  
pp. 653-663 ◽  
Author(s):  
Xin Wu ◽  
Guangyou Fang ◽  
Guoqiang Xue ◽  
Lihua Liu ◽  
Leisong Liu ◽  
...  
Keyword(s):  
Dynamic Range ◽  
Early Time ◽  
Time Signal ◽  
Late Time ◽  
Time Data ◽  
Electromagnetic System ◽  
Transient Electromagnetic ◽  
Chinese Academy Of Sciences ◽  
Band Limited ◽  
Academy Of Sciences

Over the past decade, helicopter-borne transient electromagnetic (HTEM) systems have been rapidly developed. A new HTEM prototype (referred to as a CAS-HTEM) has been developed by the Chinese Academy of Sciences. In terms of hardware, the CAS-HTEM system uses an inflatable structure to carry the transmitting loop, which significantly reduces the weight of the system and makes it easier to transport. A dual gain receiver was innovated to extend the dynamic range of the system. In addition, an observation circuit for transmitting voltage waveform is introduced, so that the derivative waveform of transmitting current with higher SNR could be calculated. In terms of data processing, more reliable early time data could be obtained by band-limited effect removal; a weighted stacking algorithm is introduced to reduce the narrow band noise more effectively and increase the sensitivity of data to the anomaly location; a method based on τ-domain transform is used for late time signal processing. The results of the field test which was carried out in Inner Mongolia were found to be consistent with the drill data, which effectively verified the performance of this HTEM prototype.

tTEM — A towed transient electromagnetic system for detailed 3D imaging of the top 70 m of the subsurface

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. E13-E22 ◽  
Author(s):  
Esben Auken ◽  
Nikolaj Foged ◽  
Jakob Juul Larsen ◽  
Knud Valdemar Trøllund Lassen ◽  
Pradip Kumar Maurya ◽  
...  
Keyword(s):  
High Resolution ◽  
Raw Materials ◽  
Geophysical Methods ◽  
Central Valley ◽  
Aquifer Vulnerability ◽  
Late Time ◽  
Electromagnetic System ◽  
Aquifer Storage ◽  
Transient Electromagnetic ◽  
Subsurface Layers

There is a growing need for detailed investigation of the top 30–50 m of the subsurface, which is critical for infrastructure, water supply, aquifer storage and recovery, farming, waste deposits, and construction. Existing geophysical methods are capable of imaging this zone; however, they have limited efficiency when it comes to creating full 3D images with high resolution over dozens to hundreds of hectares. We have developed a new and highly efficient towed transient electromagnetic (tTEM) system, which is capable of imaging the subsurface up to depth of 70 m at a high resolution, horizontally and vertically. Towed by an all-terrain vehicle, the system uses a [Formula: see text] transmitter coil and has a [Formula: see text]-component receiver placed at 9 m offset from the transmitter. The tTEM uses dual transmitter moment (low and high moment) measurement sequence to obtain the early and late time gates corresponding to shallow and deep information about the subsurface layers. The first bias-free gate is as early as [Formula: see text] from beginning of the ramp ([Formula: see text] after end of ramp). Data are processed and inverted using methods directly adopted from airborne electromagnetics. The system has been successfully used in Denmark for various purposes, e.g., mapping raw materials, investigating contaminated sites, and assessing aquifer vulnerability. We have also used the tTEM system in the Central Valley of California (United States) for locating artificial recharge sites and in the Mississippi Delta region, to map complex subsurface geology in great detail for building hydrogeologic models.

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.

scholarly journals A parallel finite‐difference approach for 3D transient electromagnetic modeling with galvanic sources

Geophysics ◽  
2004 ◽  
Vol 69 (5) ◽  
pp. 1192-1202 ◽  
Author(s):  
Michael Commer ◽  
Gregory Newman
Keyword(s):  
Magnetic Field ◽  
Finite Difference ◽  
Electric Field ◽  
Electric Fields ◽  
Time Derivative ◽  
Stable Solution ◽  
Parallel Computer ◽  
Staggered Grid ◽  
Electromagnetic Modeling ◽  
Transient Electromagnetic

A parallel finite‐difference algorithm for the solution of diffusive, three‐dimensional (3D) transient electromagnetic field simulations is presented. The purpose of the scheme is the simulation of both electric fields and the time derivative of magnetic fields generated by galvanic sources (grounded wires) over arbitrarily complicated distributions of conductivity and magnetic permeability. Using a staggered grid and a modified DuFort‐Frankel method, the scheme steps Maxwell's equations in time. Electric field initialization is done by a conjugate‐gradient solution of a 3D Poisson problem, as is common in 3D resistivity modeling. Instead of calculating the initial magnetic field directly, its time derivative and curl are employed in order to advance the electric field in time. A divergence‐free condition is enforced for both the magnetic‐field time derivative and the total conduction‐current density, providing accurate results at late times. In order to simulate large realistic earth models, the algorithm has been designed to run on parallel computer platforms. The upward continuation boundary condition for a stable solution in the infinitely resistive air layer involves a two‐dimensional parallel fast Fourier transform. Example simulations are compared with analytical, integral‐equation and spectral Lanczos decomposition solutions and demonstrate the accuracy of the scheme.

Rapid Mapping of Hydrological Systems in Tanzania Using a Towed Transient Electromagnetic System

Ground Water ◽  
2021 ◽  
Author(s):  
Denys Grombacher ◽  
Pradip Kumar Maurya ◽  
Johan Christensen Lind ◽  
John Lane ◽  
Esben Auken
Keyword(s):  
Electromagnetic System ◽  
Rapid Mapping ◽  
Transient Electromagnetic
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