Robust estimation of the band‐limited inductive‐limit response from impulse‐response TEM measurements taken during the transmitter switch‐off and the transmitter off‐time: Theory and an example from Voisey’s Bay, Labrador, Canada

Modern transient electromagnetic systems are able to take measurements in the transmitter on‐time. Integrating measurements taken during the transmitter switch‐off and those collected in the transmitter off‐time yield an estimate of the primary field plus the secondary inductive‐limit response. If the transmitter loop position is known and the position and orientation of the receiver dipoles are known, it is possible to calculate the primary field. When the theoretical primary field is subtracted from the measured inductive‐limit‐plus‐primary response, the inductive‐limit response can be isolated. An anomalous inductive‐limit response is a diagnostic feature of highly conductive ore bodies. On‐ and off‐time PROTEM data collected in a drill hole proximal to the Reid Brook Zone (one of the Voisey’s Bay deposits in Labrador, Canada) shows a strong inductive‐limit anomaly corresponding to an off‐hole conductor. A drill hole targeted to test this conductor intersected 20.4 m of mineralization, including 8.25 m of massive sulfide.

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Study of transient electromagnetic method measurements using a superconducting quantum interference device as B sensor receiver in polarizable survey area

Geophysics ◽

10.1190/geo2017-0197.1 ◽

2018 ◽

Vol 83 (2) ◽

pp. E111-E116 ◽

Cited By ~ 6

Author(s):

Shangyu Du ◽

Yi Zhang ◽

Yifeng Pei ◽

Kun Jiang ◽

Liangliang Rong ◽

...

Keyword(s):

Quantum Interference ◽

Electromagnetic Method ◽

Transient Electromagnetic Method ◽

Primary Field ◽

Survey Area ◽

Superconducting Quantum Interference Device ◽

Sign Reversal ◽

Wire Loop ◽

Transient Electromagnetic ◽

Off Time

Time-domain transient electromagnetic method (TEM) measurements sometimes exhibit a sign reversal in the secondary field during the off-time, which is usually attributed to the induced-polarization (IP) effect. In contrast with the conventional IP method, which uses a current source, TEM with an ungrounded transmitting loop operates using a pure voltage source, which is induced by the primary field switching on and off. We performed TEM measurements in a resistive survey area showing an IP effect, and we used a low-temperature superconducting quantum interference device (LT-SQUID) with sensitivity of [Formula: see text] as a magnetic field sensor. A sign reversal in all of our measurements was observed; furthermore, the negative amplitude reached [Formula: see text]. In-depth analysis with an extended version of a wire-filament circuit reveals that the large negative signal may be due to discharging of in-ground capacitance, an IP effect. The conduction response of the ground can be restored by subtracting the fitted discharging response (negative valued) from the observed data. To verify this operation, we compared TEM measurements with and without wire-loop targets, which can induce a conduction field with a known decay time constant during the off-time. The extracted conduction responses of the wire-loop targets match the expected ones well. This research reveals that the primary field switch-off must always be included when interpreting TEM data with sign reversal and an LT-SQUID may be a good alternative sensor for studying the IP effect in TEM.

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Transient electromagnetic surveys in the Okiep district

Geophysics ◽

10.1190/1.1443287 ◽

1992 ◽

Vol 57 (5) ◽

pp. 736-744 ◽

Cited By ~ 2

Author(s):

M. J. Maher

Keyword(s):

Time Constant ◽

Massive Sulfide ◽

Sulfide Mineralization ◽

Limited Size ◽

Modern Times ◽

Transient Electromagnetic ◽

Diamond Drilling ◽

Ore Bodies ◽

Sulfide Ore ◽

Massive Sulfide Mineralization

In the Okiep District early miners produced massive sulfide ore from some five deposits. Some of these deposits later contributed to the reserves of disseminated ore mined during modern times. It is unreasonable to assume that all of the massive sulfide bodies present within the area are intersected by the erosion surface and thus were discovered by the early miners. Consequently, blind massive sulfide ore bodies could be present and may have large quantities of disseminated ore associated with them. The transient electromagnetic method is ideally suited to exploring for massive sulfide bodies, and six test surveys were carried out at various sites in the district. Four of these surveys were unsuccessful whereas, at the remaining two sites, excellent anomalies were recorded. At Ezelsfontein East Extension an anomaly was recorded indicative of a massive sulfide body at shallow depth and of generally flat attitude. This anomaly has a time constant of 15 ms and the interpreted body was confirmed by a limited diamond drilling program. A deep, flat‐lying conductor was interpreted from the TEM results at Fonteintjie West Prospect. This anomaly, with a time constant of 0.6 ms, has limited size. Diamond drilling confirmed the presence of submassive to massive sulfide mineralization at this locale. Neither of these two drilled prospects had economic mineralization.

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Transient electromagnetic smoke rings in a halfspace during active transmitter excitation

Geophysics ◽

10.1190/geo2020-0673.1 ◽

2021 ◽

pp. 1-38

Author(s):

Adam Smiarowski ◽

Greg Hodges

Keyword(s):

Current Distribution ◽

Current System ◽

Primary Field ◽

Depth Of Penetration ◽

Near Surface ◽

Surface Sensitivity ◽

Field Coupling ◽

Transient Electromagnetic ◽

Smoke Ring ◽

Off Time

The smoke ring concept is a useful device for understanding how the electromagnetic fields induced in a 1-D earth propagate and diffuse through a medium. Aside from facilitating a physical understanding of field propagation, the smoke ring concept has been used to interpret behavior of vertical and radial magnetic fields at the surface and used to estimate depth of penetration for conductivity-depth transforms. Past studies have focused on the current distribution during the off-time. We calculate and illustrate the current in a halfspace from a half-sine excitation (which provides a continuous induction). In comparison, the current pattern from a continuously excited waveform is more densely distributed near-surface than the off-time current system, suggesting that measurements during a continuously excited on-time are more sensitive to shallow targets. For airborne applications, where the primary field coupling changes and is an important noise source, a primary field-stripping algorithm impacts the current distribution but does not deleteriously affect near-surface sensitivity.

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Double Trapezoidal Wave Transmitting System with Controllable Turn-Off Edge

Applied Sciences ◽

10.3390/app10217932 ◽

2020 ◽

Vol 10 (21) ◽

pp. 7932

Author(s):

Yuan Jiang ◽

Yanju Ji ◽

Yibing Yu ◽

Shipeng Wang ◽

Yuan Wang

Keyword(s):

Polarization Effect ◽

Induced Polarization ◽

Emission Current ◽

Wire Loop ◽

Transient Electromagnetic ◽

Charging Time ◽

The Earth ◽

Electromagnetic Measurement ◽

Induced Polarization Effect ◽

Off Time

For time domain transient electromagnetic measurement, the negative sign often appears in the polarization region, which contains the induced polarization information. It is considered that the polarization effect is caused by the capacitance charge of the earth. Extending the turn-off time of the emission current means increasing the charging time, and reducing the charging voltage, which makes the polarization effect easier to observe. Therefore, a double trapezoidal wave transmitting system with a controllable turn-off edge is designed in this paper. In the process of current transmitting, the turn-off time can be controlled by changing the clamping voltage depending on the passive clamping technology. By cutting into the absorption resistance, the current oscillation can be eliminated under the condition of ensuring linearity. To verify the effectiveness of the system, we designed a polarized wire loop based on the filament model simulating the polarized earth. Comparing the response of the wire loop, the emission current with short and long turn-off times contributes to inducing the induction and polarization fields respectively. The double trapezoidal wave transmitting system with a controllable turn-off edge is suitable for measuring the induced polarization effect.

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Spatial averaging of the vertical magnetic field in central‐loop configuration

Geophysics ◽

10.1190/1.1443365 ◽

1993 ◽

Vol 58 (10) ◽

pp. 1507-1510 ◽

Cited By ~ 2

Author(s):

Wei Qian ◽

Laust B. Pedersen

Keyword(s):

Tensor Decomposition ◽

Data Interpretation ◽

Impedance Tensor ◽

Logarithmic Scale ◽

Resistivity Structure ◽

One Dimensional ◽

Transient Electromagnetic ◽

The Earth ◽

Band Limited ◽

Central Loop

Local resistivity heterogeneities can cause static shifts in the magnetotelluric (MT) impedance tensor that severely complicate data interpretation; the apparent resistivity is shifted on a logarithmic scale across the recorded frequency range while the phase has a band‐limited response. Different techniques such as electromagnetic array profiling (EMAP) (Torres‐Verdín and Bostick, 1992) and tensor decomposition (Zhang et al., 1987; Groom and Bailey, 1989; 1991) have been developed in the MT community to recognize and remove static shifts. Sternberg, et al. (1988) and Pellerin and Hohmann (1990) suggest that central‐loop transient electromagnetic (TEM) soundings can obtain an unbiased estimate of the regional resistivity structure of the earth and thereby correct for magnetotelluric static shifts. The regional resistivity structure of the earth must be one‐dimensional (1-D) for this method to work well.

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Seismic reflection profiling of the Pyhäsalmi VHMS-deposit: A complementary approach to the deep base metal exploration in Finland

Geophysics ◽

10.1190/geo2011-0240.1 ◽

2012 ◽

Vol 77 (5) ◽

pp. WC15-WC23 ◽

Cited By ~ 16

Author(s):

Suvi Heinonen ◽

Marcello Imaña ◽

David B. Snyder ◽

Ilmo T. Kukkonen ◽

Pekka J. Heikkinen

Keyword(s):

Contact Zone ◽

Seismic Data ◽

Seismic Reflection ◽

Volcanic Rocks ◽

Volcanic Belt ◽

Massive Sulfide ◽

Drill Hole ◽

High Grade ◽

Seismic Reflection Profiling ◽

Vhms Deposit

In the Pyhäsalmi case study, the seismic data is used in direct targeting of shallowly dipping mineralized zones in a massive sulfide ore system that was deformed in complex fold interference structures under high-grade metamorphic conditions. The Pyhäsalmi volcanic-hosted massive sulfide (VHMS) deposit ([Formula: see text]) is located in a Proterozoic volcanic belt in central Finland. Acoustic impedance of Pyhäsalmi ore ([Formula: see text]) is distinct from the host rocks ([Formula: see text]), enabling its detection with seismic reflection methods. Drill-hole logging further indicates that the seismic imaging of a contact zone between mafic and felsic volcanic rocks possibly hosting additional mineralizations is plausible. Six seismic profiles showed discontinuous reflectors and complicated reflectivity patterns due to the complex geology. The most prominent reflective package at 1–2 km depth was produced by shallowly dipping contacts between interlayered felsic and mafic volcanic rocks. The topmost of these bright reflections coincides with high-grade zinc mineralization. Large acoustic impedances associated with the sulfide minerals locally enhanced the reflectivity of this topmost contact zone which could be mapped over a wide area using the seismic data. Seismic data enables extrapolation of the geologic model to where no drill-hole data exists; thus, seismic reflection profiling is an important method for defining new areas of interest for deep exploration.

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FINITE‐ELEMENT COMPUTATION OF SEISMIC ANOMALIES FOR BODIES OF ARBITRARY SHAPE

Geophysics ◽

10.1190/1.1440597 ◽

1976 ◽

Vol 41 (1) ◽

pp. 145-150 ◽

Cited By ~ 18

Author(s):

Bruce A. Bolt ◽

Warwick D. Smith

Keyword(s):

Finite Element ◽

Arbitrary Shape ◽

Plane Waves ◽

Surface Profile ◽

Massive Sulfide ◽

Spectral Ratios ◽

Element Analysis ◽

S Waves ◽

Ore Bodies ◽

Upward Moving

A method which uses observed frequency spectral ratios of seismic plane waves for exploration of ore bodies is now available. The new method is based on the numerical solution of the response of a two‐dimensional shallow structural anomaly to an upward‐moving seismic wave from a distant earthquake or explosion. Finite‐element analysis is used for both P- and S-waves. Solutions to the direct problem for bodies of arbitrary shape have not previously been available. Results in the time and frequency domains are discussed here for a salt ridge and for a massive sulfide body. For the inverse problem, interpretation using contours of spectral ratios along a surface profile is suggested.

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SELECTION OF A SUITABLE MODEL FOR QUANTITATIVE INTERPRETATION OF TOWED‐BIRD AEM MEASUREMENTS

Geophysics ◽

10.1190/1.1440838 ◽

1978 ◽

Vol 43 (3) ◽

pp. 576-587 ◽

Cited By ~ 5

Author(s):

G. J. Palacky

Keyword(s):

Half Plane ◽

Time Domain ◽

Massive Sulfide ◽

Suitable Model ◽

Dual Frequency ◽

Plane Model ◽

Short Delay ◽

Ore Bodies ◽

Delay Times ◽

The Time Domain

Many steeply dipping massive sulfide ore bodies have a dike‐like shape, and this has led to wide acceptance of the vertical half‐plane model in the interpretation of electromagnetic data. This model assumes that the conductor is thin, but the restriction has not been considered critical and in practice has frequently been disregarded. Conductance and conductor depth estimates based on the results of towed‐bird AEM surveys have been observed to be lower and less accurate than those obtained from helicopter EM and ground EM measurements. In order to explain the low reliability of the towed‐bird estimates, AEM responses over 17 Canadian ore bodies were analyzed. In the study, field results obtained by the time‐domain Input system and two dual‐frequency quadrature systems were interpreted. Error in conductance and depth estimates results from the frequency‐dependent, diffusive behavior of thick geologic conductors. This dependence makes invalid the basic assumption made in the interpretation of dual‐frequency quadrature EM data, that of the equivalence of response parameters at two frequencies. The estimated conductance and depth are too small when applying the current interpretation procedure based on amplitude ratios at two high and widely separated frequencies. The error is smaller in the case of Input time‐domain measurements, because the delay times are relatively long and the channels narrowly spaced. The vertical half‐plane model has been found to hold for ore bodies less than 10 m wide. In the case of wide mineralized zones, which are more important economically, the vertical half‐plane model could be successfully applied only at long delay times. Applying the vertical half‐plane nomogram at short delay times, the conductance and depth were underestimated, and better values could only be achieved by fitting the field data to a horizontal ribbon model. The consistently low conductance values interpreted from towed‐bird measurements for wide conductive zones have probably resulted in not selecting many potential massive sulfide targets for ground followup.

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Effects of full transmitting-current waveforms on transient electromagnetics: Insights from modeling the Albany graphite deposit

Geophysics ◽

10.1190/geo2018-0573.1 ◽

2019 ◽

Vol 84 (4) ◽

pp. E255-E268 ◽

Cited By ~ 4

Author(s):

Sihong Zeng ◽

Xiangyun Hu ◽

Jianhui Li ◽

Colin G. Farquharson ◽

Peter C. Wood ◽

...

Keyword(s):

Real Data ◽

Forward Modeling ◽

Northern Ontario ◽

Data Set ◽

Rock Types ◽

Full Waveform ◽

Transient Electromagnetic ◽

Steady Stage ◽

Geologic Model ◽

Off Time

In transient electromagnetic (TEM) methods, the full transmitting-current waveform, not just the abrupt turn-off, can have effects on the measured responses. A 3D finite-element time-domain forward-modeling solver was used to investigate these effects. This was motivated by an attempt to match, via forward-modeling, real data from the Albany graphite deposit in northern Ontario, Canada. Initial modeling results for homogeneous half-spaces illustrate the effects that a full waveform can have on TEM responses, especially the durations of the steady stage and turn-off time. For the Albany data set, a geophysical conductivity model was developed from a geologic model that itself had been constructed predominantly from drillhole information. The conductivities of the various geologic units in the model were first estimated based on typical conductivity values for the respective rock types, then adjusted to fit the measured TEM data as closely as possible. We found that the TEM responses differed significantly from the pure step-off response and that incorporating the effects of the full waveform (particularly the linear ramp turn-off) greatly improved the match between observed and computed responses, especially for the early measurement times. In addition, this Albany example illustrates the presence of sign changes in TEM data caused primarily by localized conductivity targets.

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