The effect of near‐surface superparamagnetic material on electromagnetic measurements

Geophysics ◽  
1982 ◽  
Vol 47 (9) ◽  
pp. 1315-1324 ◽  
Author(s):  
G. Buselli
Keyword(s):  
Transient Response ◽  
Soil Cover ◽  
Lateritic Soil ◽  
Near Surface ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Electromagnetic Measurements ◽  
Delay Times ◽  
Residual Response ◽  
Made In

The development of an instrument that enables transient electromagnetic (TEM) measurements to be made to voltage levels of 1 μV/A and less has enabled the detection of an anomalous transient response in some areas with lateritic soil cover. This anomalous transient causes apparent resistivity values derived from the measured transient decay to decrease at late delay times in areas where the known geology indicates the values should increase with delay time toward the resistivity value of the basement. The main cause of the anomalous transient has been identified as the response of superparamagnetic material in the lateritic soil cover. Both field and laboratory measurements of the voltage M induced by this transient, show a [Formula: see text] time dependence. This is the same behavior reported previously for magnetic viscosity over a longer time scale. Measurements of magnetic susceptibility of material separated magnetically from soil samples taken at areas where a residual response is measured, show that over a wide temperature range (from −196°C to 590°C) the susceptibility increases with temperature, confirming the presence of superparamagnetic particles. The anomalous transient response is localized to within 3 m of the transmitter loop; it is consequently detected only by loop configurations where the receiver loop is in proximity to the transmitter loop. The effects caused by the presence of a superparamagnetic response within 3 m of the transmitter loop apply to all electromagnetic methods, whether the measurements are made in the time or frequency domain.

The use of magnetotellurics for mineral exploration: an experiment in the Chibougamau region of Quebec

1992 ◽  
Vol 29 (4) ◽  
pp. 621-635 ◽  
Author(s):  
Ping Zhang ◽  
Michel Chouteau
Keyword(s):  
Mineral Exploration ◽  
Three Dimensional ◽  
Central Section ◽  
Southern Boundary ◽  
Near Surface ◽  
Metasedimentary Rocks ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Abitibi Subprovince ◽  
Geophysical Information

Ten magnetotelluric (MT) soundings were recorded in the range 300 Hz to 200 s along an 11 km profile across a long, well-defined, east–west sub vertical conductor in the vicinity of Chibougamau. This conductor had been recognized by the airborne transient electromagnetic technique and confirmed by other ground electromagnetic methods. The primary purpose of the MT survey was to further understand and constrain the structure of the conductor, especially its extent at depth. This study is the first to demonstrate the utility of the MT method for mineral exploration in Abitibi subprovince.The subsurface of the survey area can be roughly divided into a resistive northern section, a conductive central section (containing the conductive sheet), and a resistive southern boundary. Although some data are distorted by static shifts or three-dimensional effects, there is evidence that the data from the central conductive section are relatively free of static distortions. Therefore, taking into account existing geological and geophysical information and with extensive two-dimensional modelling, it is concluded that a conductive sheet with conductivity of 2 S∙m−1 and a width of 25 m should extend to a depth of no more than 750 m; an additional conductive block is required below this sheet. Cores from boreholes have shown that the conductivity of the near-surface sheet is mainly caused by sulfide minerals and graphite. It is also believed that graphite in the metasedimentary rocks under the central section may be responsible for the conductivity at depth.There is a clear boundary in terms of geoelectric characteristics between the northern part of the MT profile which is located in the Optica subprovince and the southern MT profile located in the Abitibi subprovince. Under the Optica subprovince, there are no structures other than a very resistive block with a thickness of at least 20 km. In contrast, conductive layers are found in the upper crust in the western and central parts of the Abitibi subprovince.

scholarly journals Inverting magnetotelluric data with distortion correction—stability, uniqueness and trade-off with model structure

2020 ◽  
Vol 222 (3) ◽  
pp. 1620-1638 ◽  
Author(s):  
M Moorkamp ◽  
A Avdeeva ◽  
Ahmet T Basokur ◽  
Erhan Erdogan
Keyword(s):  
Real Data ◽  
Distortion Correction ◽  
Impedance Tensor ◽  
Strong Impact ◽  
Magnetotelluric Data ◽  
Data Set ◽  
Near Surface ◽  
Transient Electromagnetic ◽  
Electromagnetic Measurements ◽  
Galvanic Distortion

SUMMARY Galvanic distortion of magnetotelluric (MT) data is a common effect that can impede the reliable imaging of subsurface structures. Recently, we presented an inversion approach that includes a mathematical description of the effect of galvanic distortion as inversion parameters and demonstrated its efficiency with real data. We now systematically investigate the stability of this inversion approach with respect to different inversion strategies, starting models and model parametrizations. We utilize a data set of 310 MT sites that were acquired for geothermal exploration. In addition to impedance tensor estimates over a broad frequency range, the data set also comprises transient electromagnetic measurements to determine near surface conductivity and estimates of distortion at each site. We therefore can compare our inversion approach to these distortion estimates and the resulting inversion models. Our experiments show that inversion with distortion correction produces stable results for various inversion strategies and for different starting models. Compared to inversions without distortion correction, we can reproduce the observed data better and reduce subsurface artefacts. In contrast, shifting the impedance curves at high frequencies to match the transient electromagnetic measurements reduces the misfit of the starting model, but does not have a strong impact on the final results. Thus our results suggest that including a description of distortion in the inversion is more efficient and should become a standard approach for MT inversion.

Negative transient voltage and magnetic field responses for a half‐space with a Cole‐Cole impedance

Geophysics ◽  
1983 ◽  
Vol 48 (6) ◽  
pp. 790-791 ◽  
Author(s):  
A. P. Raiche
Keyword(s):  
Magnetic Field ◽  
Asymptotic Formula ◽  
Half Space ◽  
Transient Response ◽  
Mineral Deposits ◽  
Transient Electromagnetic ◽  
Delay Times ◽  
Transient Voltage

In a recent paper, Lee (1981) developed an asymptotic formula for the coincident loop transient electromagnetic (TEM) response of a polarizable half‐space having a Cole‐Cole impedance. By using parameters corresponding to three different mineral deposits, Lee showed that negative transients would be obtained for delay times of 0.4 to 1.1 msec. The method developed by Knight and Raiche (1982) to calculate the transient response of layered earths was used to check these results for three reasons.

Soil and ecological division into districts of the Baikal region

2018 ◽  
Vol 940 (10) ◽  
pp. 54-64 ◽  
Author(s):  
I.A. Belozertseva ◽  
A.A. Sorokovoj
Keyword(s):  
Baikal Region ◽  
Soil Cover ◽  
Soil Formation ◽  
Forest Steppe ◽  
Leading Role ◽  
Soil Cover Structure ◽  
Reflecting Surface ◽  
Heat And Moisture ◽  
Made In

On the basis of long-term researches of soils in the territory of Russia and Mongolia soil and ecological division into districts of the Baikal region is carried out. At division into districts the whole set of an environment of soil formation was considered. On the map of soil and ecological division into districts 13 mountain, mid-mountain, low-mountain taiga, foothill, hollow-valley, forest-steppe and steppe provinces reflecting surface device originality as the ratio of balance of heat and moisture forming a basis to zoning is shown against the background of difficult orography are allocated. In total 42 districts on lithologic-geomorphological features are allocated. In formation of distinctions of a soil cover of these provinces the leading role is played by bioclimatic factors and inside them the lithologic-geomorphological ones. In the view of structural approach of the district they are considered as territories with a certain natural change of several types of the soil cover structure caused by features of a relief and the parent rock. The map is made in the MapInfo program. It is revealed that on ill-defined width zoning of soils the vertical one which has a greater influence on soils of this region is imposed. Soils of the Baikal region are not similar to the soils located at the same latitude of the flat European territory of Russia. Zone soils of this territory are specific and original.

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.

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