Broadband electromagnetic measurements over a massive sulfide prospect

Geophysics ◽  
1979 ◽  
Vol 44 (10) ◽  
pp. 1677-1699 ◽  
Author(s):  
D. F. Pridmore ◽  
S. H. Ward ◽  
J. W. Motter
Keyword(s):  
Tilt Angle ◽  
Host Rock ◽  
Massive Sulfide ◽  
Massive Sulfides ◽  
Homogeneous Half Space ◽  
Electromagnetic Measurements ◽  
Electric And Magnetic Fields ◽  
Dipole Sources ◽  
The Magnetic Field ◽  
Frequency Distance

This paper presents the results of measurements of the tilt angle and ellipticity of the polarization ellipse of the magnetic field, made at 14 frequencies in the range 10.5 Hz to 86 kHz, over a massive sulfide prospect in the foothills copper belt of California. As is typical of this class of deposit, the massive sulfides grade laterally into the disseminated sulfides of a tuff unit. The host rock resistivities are in the range 80 to 500 ο-m. The interpretation of the results is carried out using crude dike models in free space, although an attempt is made to account for the conductive host rock by examining the electric and magnetic fields generated by magnetic dipole sources over a homogeneous half‐space. The utility of broadband multifrequency electromagnetic (EM) measurements over a complex earth is shown clearly by the results over the prospect. The response of the massive sulfides, disseminated sulfides, and the host rock can all be distinguished when the tilt angle and ellipticity values are contoured in frequency‐distance space. A comparison is made of the results obtained using vertical‐loop, rotating vertical‐loop, and horizontal‐loop transmitter configurations. It is found at this mineral prospect that (1) the results from the vertical‐loop transmitter are easier to interpret, in terms of simple models, than those produced by the horizontal‐loop transmitter, (2) although the rotating vertical‐loop transmitter gives a larger response from the massive sulfides, the results show no more fine structure in the massive sulfides than that shown by the response from the vertical‐loop transmitter; and (3) moving the vertical‐loop transmitter does not give significantly more information about the geology.

Electrical and magnetometric fields in a layered earth containing buried electrodes

Geophysics ◽  
1990 ◽  
Vol 55 (12) ◽  
pp. 1605-1612 ◽  
Author(s):  
D. Veitch ◽  
M. W. Asten ◽  
E. H. van Leeuwen
Keyword(s):  
Half Space ◽  
Field Data ◽  
Massive Sulfide ◽  
Magnetic Field Measurement ◽  
Layered Earth ◽  
Homogeneous Half Space ◽  
Electric And Magnetic Fields ◽  
Borehole Logs ◽  
Two Layer Model ◽  
The Magnetic Field

The number of analytical magnetometric resistivity (MMR) results available for basic earth geometries is limited compared to that of electrical resistivity methods, which is unfortunate since MMR has advantages for certain classes of problems. This paper extends the list of MMR results by deriving the response for the homogeneous half‐space and the multilayered earth. Both are calculated for arbitrary source and receiver positions. We show how the result for the layered earth reduces to that of the half‐space when there is only one layer. Necessary procedures for successful numerical evaluation of results for the layered case are given. Sample borehole logs of electric and magnetic fields for an example of a two‐layer model illustrate an advantage of the magnetic‐field measurement; namely, that it is sensitive to the position of layer boundaries rather than to the position of transmitter electrodes. The algorithm is also applied to interpretation of MMR field data from two boreholes drilled near massive sulfide conductors. The magnetometric influence (or background) due to borehole geometry in an electrically layered earth may be computed and subtracted from the field data, after which it is possible to perform quantitative modeling of the residual MMR anomaly to define the location of ore‐related conductors.

Transmission line sanitary protective zones. Electromagnetic safety problems

2020 ◽  
pp. 736-736
Author(s):  
N. B. Rubtsova ◽  
A. Y. Tokarskiy
Keyword(s):  
Magnetic Field ◽  
Risk Factor ◽  
Transmission Lines ◽  
General Public ◽  
Field Component ◽  
Regulatory Requirements ◽  
Protection Zones ◽  
Electric And Magnetic Fields ◽  
Health Risk Factor ◽  
The Magnetic Field

The main problems of overhead and cable transmission lines with voltage >=110 kV electric and magnetic fields general public protection are presented. It is shown that it is necessary to develop regulatory requirements for these lines’ sanitary protection zones organization, taking into account the magnetic field component, because its possible health risk factor, up to carcinogenic.

On: “Effect of tropical weathering on electrical and electromagnetic measurements” by G. J. Palacky and K. Kadekaru (GEOPHYSICS, January 1979, p. 69–88).

Geophysics ◽  
1980 ◽  
Vol 45 (7) ◽  
pp. 1205-1208
Author(s):  
B. B. Bhattacharya ◽  
P. K. Hazra
Keyword(s):  
Host Rock ◽  
Vertical Electrical Soundings ◽  
Electromagnetic Measurements ◽  
Tropical Weathering ◽  
Electrical Soundings

In this paper, the authors showed a series of electromagnetic (EM) profiles (Figure 20, p. 86) obtained from the ground follow‐up using a slingram system with a frequency of 1600 Hz and coil separation of 120 m. Measurements along all the profiles (A to E) show considerable in‐phase and quadrature anomaly. The host rock in this case is amphibolite. The vertical electrical soundings over the amphibolite (Figure 16, p. 82) indicate that the weathered amphibolite has mean thickness and resistivity of 8.5 ± 2.1 m and 29.7 ± 6.5 Ω-m, respectively. It would, therefore, be reasonable to assume that the in‐phase and quadrature anomalies have been influenced due to the presence of conducting overburden, and the response of the target in such a situation can be altogether different compared to the anomaly of the target in the presence of an insulating or highly resistive overburden.

The face‐current concept and its application to survey design in electrical exploration

Geophysics ◽  
2003 ◽  
Vol 68 (3) ◽  
pp. 900-910 ◽  
Author(s):  
Carlos A. Mendonça
Keyword(s):  
Survey Design ◽  
Electrical Potential ◽  
Ground Surface ◽  
Electrode Array ◽  
Homogeneous Half Space ◽  
The Face ◽  
Geoelectrical Structure ◽  
Dipole Sources ◽  
Conductivity Contrast

This paper presents a new method to identify the regions over a 3D geoelectrical structure that produce major contributions to the electrical potential established in response to a dc source at the ground surface. The measured potential is represented by a sum of a known primary potential (due to a homogeneous half space) plus an unknown potential caused by conductivity inhomogeneities. Because the primary potential is continuous everywhere, the interfaces with a conductivity contrast act as sources or sinks of currents in order to maintain the continuity of the current density related to the primary flux. These disturbing face currents are responsible for the generation of the secondary potential, and mapping them over a given structure allows us to assess the regions where the secondary potential is generated. In general, the face currents vanish away from the source according to the decay of the primary electric field. For this reason, deeper investigations can be expected when using pole sources because its primary field decays with the inverse of the squared distance, instead of the cubed distance as for dipole sources. For thin sheets, the polarization decay with distance is one order higher than that for large 3D bodies, which makes the detection of a sheet yet more difficult. The quantification of the total face current over the structure for different positions along a profile helps one choose the proper electrode array and determine its optimum length. This is done in two steps: (1) identification of the offset where the dc source provides the highest polarization (face current) on the targeted structure, and (2) determination of the array length by locating the potential electrodes closest to the region with the highest polarization. This second criterion came from an analogy between the face‐current and artificial current sources, where it is intuitively seen that the resulting potential is highest close to the source. The proposed survey design technique is applied to three models commonly used in electrical exploration: a shallow conductive heterogeneity, a buried contact, and a thin conductive sheet.

scholarly journals Comparative Study on Planetary Magnetosphere in the Solar System

Sensors ◽  
2020 ◽  
Vol 20 (6) ◽  
pp. 1673
Author(s):  
Ching-Ming Lai ◽  
Jean-Fu Kiang
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Solar System ◽  
Comparative Study ◽  
Magnetic Reconnection ◽  
Tilt Angle ◽  
Planetary Magnetosphere ◽  
Field Distributions ◽  
Mhd Simulations ◽  
The Magnetic Field

The magnetospheric responses to solar wind of Mercury, Earth, Jupiter and Uranus are compared via magnetohydrodynamic (MHD) simulations. The tilt angle of each planetary field and the polarity of solar wind are also considered. Magnetic reconnection is illustrated and explicated with the interaction between the magnetic field distributions of the solar wind and the magnetosphere.

The oldest seafloor massive sulfide deposits at the Mid-Atlantic Ridge: 230Th/U chronology and composition

2015 ◽  
Vol 42 (1) ◽  
Author(s):  
Vladislav Kuznetsov ◽  
Eriks Tabuns ◽  
Kathrine Kuksa ◽  
Georgy Cherkashov ◽  
Fedor Maksimov ◽  
...  
Keyword(s):  
Massive Sulfide ◽  
Hydrothermal Activity ◽  
Ocean Floor ◽  
Hydrothermal Field ◽  
Sulfide Deposits ◽  
Massive Sulfides ◽  
Geochemical Properties ◽  
Geochemical Study ◽  
Mid Atlantic Ridge ◽  
Seafloor Massive Sulfide

Abstract A geochronological and geochemical study on 10 samples of seafloor massive sulfides (SMS) from the inactive Peterburgskoye hydrothermal field at the Mid-Atlantic Ridge (MAR) was carried out. The 230Th/U ages of the SMS are the oldest for the Quaternary hydrothermal ores ever found at the ocean floor. According to them the hydrothermal activity at Peterburgskoye field started at least 170 ka and continued down to 63 ka. The oldest hydrothermal ores from this field consist mainly of pyrite and chalcopyrite and have geochemical properties typical for SMS associated with basalts.

Contribution of magnetic modeling to the discovery of a hidden massive sulfide body at Hajar, Morocco

Geophysics ◽  
1991 ◽  
Vol 56 (7) ◽  
pp. 983-991 ◽  
Author(s):  
A. Bellott ◽  
J. Corpel ◽  
R. Millon
Keyword(s):  
Gravity Data ◽  
Massive Sulfide ◽  
Magnetic Survey ◽  
Massive Sulfides ◽  
Magnetic Modeling ◽  
Gravity And Magnetic ◽  
Gravity Response ◽  
Modeling Techniques ◽  
Magnetic Model ◽  
D Model

The Hajar prospect is located in the Guemassa Paleozoic massif, about 30 km southwest of Marrakesh, Morocco. Visean volcano‐sedimentary formations are present in this massif and in the Jebilets massif north of Marrakesh. In these formations, syngenetic massive sulfides occur, and one of these bodies, Kettara in the Jebilets, has been partially mined. Using the Kettara magnetic anomaly to establish the magnetization parameters, we performed a preliminary interpretation of the Hajar anomaly. Our 2.5-D model determined the depth of the structure to be about 150 to 200 m below the surface. This relatively shallow depth was a decisive factor in siting a reconnaissance drillhole, which encountered massive sulfides between 158 and 276 m. After completing a systematic gravity and magnetic survey, 3-D magnetic modeling was attempted, constrained by the results of four drillholes. Now that more than 20 holes have been drilled, this magnetic model still conforms to the newly revealed geology. Unfortunately, it was not possible to extract useful information concerning the orebody from modeling and interpreting the gravity data. The gravity response is masked by disturbances such as faults and variations in depth and nature of the Visean basement. The discovery of the Hajar deposit shows that magnetic investigations, improved by pertinent modeling techniques, can be used at various stages of exploration to help recognize and define massive sulfide bodies.

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