Magnetic Studies in Northern Lake Superior

1972 ◽  
Vol 9 (11) ◽  
pp. 1349-1367 ◽  
Author(s):  
H. C. Halls
Keyword(s):  
Lake Superior ◽  
Magnetic Polarity ◽  
Magnetic Data ◽  
Aeromagnetic Data ◽  
Total Field ◽  
Magnetic Studies ◽  
Late Precambrian ◽  
Thunder Bay ◽  
Basement Ridge ◽  
Volcanic Series

Analysis of total field magnetic data, together with other geophysical evidence has led to the following conclusions concerning the structure and stratigraphy of late Precambrian Keweenawan rocks that underlie and border the northern part of Lake Superior:(1) The uppermost few hundred feet of the Osler Volcanic Series has a normal remanent magnetization. The normally magnetized mafic flows are separated from older, reversely magnetized ones which form the bulk of the Osler Series, by a thin zone of felsic igneous rocks with unknown magnetic polarity. The upper mafic unit appears from aeromagnetic data to outcrop on some of the outer islands at the mouth of Nipigon Strait, and represents the lowest part of a normally magnetized volcanic sequence that lies offshore, beneath the waters of Lake Superior.(2) The contact between the upper mafic unit and older rocks of the Osler Series is a stratigraphic rather than a faulted one. Aeromagnetic data show that mafic volcanics above and below this contact have the same strike, except toward the Thunder Bay and Schreiber regions. In these areas there is a convergence in strike between the two mafic units which may indicate an unconformity between them or a thinning of flows in the older unit. Ultimately the older, reversely magnetized unit appears to pinch out beneath the younger one to both the east and west.(3) The younger volcanics and overlying sediments occur in a large basin centered about 30 mi (50 km) WSW of the Slate Islands. Whether the older, reversely magnetized volcanics accumulated in this basin or in a smaller more northerly one is not entirely clear. The eastern margin of the main basin appears to be formed by a basement ridge extending from the Slate Islands southward to Superior Shoal, over which volcanics and younger sediments are relatively thin.

A Paleomagnetic Reversal in the Osler Volcanic Group, Northern Lake Superior

1974 ◽  
Vol 11 (9) ◽  
pp. 1200-1207 ◽  
Author(s):  
H. C. Halls
Keyword(s):  
Volcanic Activity ◽  
Lake Superior ◽  
Aeromagnetic Data ◽  
Magnetic Reversal ◽  
Geological Evidence ◽  
Late Precambrian ◽  
Quiescent Period ◽  
Clastic Sediments ◽  
Paleomagnetic Study ◽  
Reversed Polarity

A geological and paleomagnetic study in the Nipigon Strait area, northern Lake Superior, has confirmed previous conclusions drawn from aeromagnetic data, that an unconformity occurs within the Late Precambrian Keweenawan Osler volcanics and separates normally magnetized lavas from older ones with reversed polarity. The new data, together with other paleomagnetic and geological evidence from Keweenawan rocks, suggest that the magnetic reversal occurred when there was a temporary halt or decline in volcanic activity throughout the Lake Superior region. During the quiescent period, coarse clastic sediments were deposited by marginal erosion of the subsiding Keweenawan basin; sinking of the basin with corresponding uplift of the margins may have been greater in the south where the thickest sections of conglomerate and sandstone are preserved.

Constrained 3D inversion of magnetic data with structural orientation and borehole lithology: A case study in the Macheng iron deposit, Hebei, China

Geophysics ◽  
2019 ◽  
Vol 84 (2) ◽  
pp. B121-B133 ◽  
Author(s):  
Shida Sun ◽  
Chao Chen ◽  
Yiming Liu
Keyword(s):  
Remanent Magnetization ◽  
Magnetic Data ◽  
Iron Deposit ◽  
Reference Field ◽  
Total Field ◽  
Equivalent Source ◽  
Structural Orientation ◽  
Amplitude Data ◽  
Ore Bodies

We have developed a case study on the use of constrained inversion of magnetic data for recovering ore bodies quantitatively in the Macheng iron deposit, China. The inversion is constrained by the structural orientation and the borehole lithology in the presence of high magnetic susceptibility and strong remanent magnetization. Either the self-demagnetization effect caused by high susceptibility or strong remanent magnetization would lead to an unknown total magnetization direction. Here, we chose inversion of amplitude data that indicate low sensitivity to the direction of magnetization of the sources when constructing the underground model of effective susceptibility. To reduce the errors that arise when treating the total-field anomaly as the projection of an anomalous field vector in the direction of the geomagnetic reference field, we develop an equivalent source technique to calculate the amplitude data from the total-field anomaly. This equivalent source technique is based on the acquisition of the total-field anomaly, which uses the total-field intensity minus the magnitude of the reference field. We first design a synthetic model from a simplified real case to test the new approach, involving the amplitude data calculation and the constrained amplitude inversion. Then, we apply this approach to the real data. The results indicate that the structural orientation and borehole susceptibility bounds are compatible with each other and are able to improve the quality of the recovered model to obtain the distribution of ore bodies quantitatively and effectively.

Using airborne vector magnetic data to calculate the projection of magnetic anomaly vector onto normal geomagnetic field

Geophysics ◽  
2021 ◽  
pp. 1-47
Author(s):  
Rukuan Xie ◽  
Shengqing Xiong ◽  
Shuling Duan ◽  
Jinlong Wang ◽  
Ping Wang ◽  
...  
Keyword(s):  
Magnetic Anomaly ◽  
Geomagnetic Field ◽  
Approximation Error ◽  
Magnetic Data ◽  
Total Field ◽  
Projection Formula ◽  
Total Magnetization ◽  
Vector Formula ◽  
Relationship Of ◽  
Anomaly Formula

The total-field magnetic anomaly [Formula: see text] is an approximation of the projection [Formula: see text] of the magnetic anomaly vector [Formula: see text] onto the normal geomagnetic field [Formula: see text]. However, for highly magnetic sources, the approximation error of [Formula: see text] cannot be ignored. To reduce the error, we have developed a method for calculating [Formula: see text] by using airborne vector magnetic data based on the vector relationship of geomagnetic field [Formula: see text]. The calculation uses the magnitude of the vectors [Formula: see text], [Formula: see text], and [Formula: see text] through a simple approach. To ensure that each magnitude has the same level, we normalize the magnitude of [Formula: see text] using the total-field magnetic data measured by the scalar magnetic sensor. The method is applied to the measured airborne vector magnetic data at the Qixin area of the East Tianshan Mountains in China. The results indicate that the calculated [Formula: see text] has high precision and can distinguish the approximation error less than 3.5 nT. We also analyze the characteristics of the approximation error that are caused by the effects of different total magnetization inclinations. These error characteristics are used to predict the total magnetization inclination of a 2D magnetic source based on the measured airborne vector magnetic data.

Parameters for a Roll On-Roll Off Marine Intermodal Service on Lake Superior

2003 ◽  
Vol 1820 (1) ◽  
pp. 46-54
Author(s):  
Richard D. Stewart ◽  
Terry LaVoie ◽  
Nathan Shutes
Keyword(s):  
Lake Superior ◽  
The United States ◽  
Future Application ◽  
Spreadsheet Model ◽  
Thunder Bay ◽  
Application Potential ◽  
Terminal Design ◽  
The Baltic ◽  
Terminal Operations ◽  
Cost Factors

The feasibility of reestablishing roll on–roll off (RO–RO) service linking Thunder Bay, Ontario, and the Twin Ports of Duluth, Minnesota, and Superior, Wisconsin, was examined. An analysis of similar services both on the Great Lakes and overseas was undertaken. Previous operations on the same route on Lake Superior and short sea service in the Baltic region were analyzed, and the attributes of an appropriate vessel type, such as ice strengthened scantlings and adequate speeds, were determined. A route schedule for a typical vessel was developed. A profile of a suitable vessel was prepared, and the market availability of the vessels was assessed. Issues in terminal design and operation were applied to potential docks in the ports of call to determine the adequacy of shoreside facilities. Cost factors for vessel operations and terminal operations were determined, and a spreadsheet model was developed for future application. Potential revenue streams from vessel and terminal operations were also explored. Revenue generators included day passengers as well as trailers and containers. The feasibility of linking the RO–RO service to a larger intermodal system serving Canada and the United States was also assessed.

Fast 3D magnetic inversion of a surface relief in the space domain

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. J57-J67 ◽  
Author(s):  
Marlon C. Hidalgo-Gato ◽  
Valéria C. F. Barbosa
Keyword(s):  
Hessian Matrix ◽  
Computational Cost ◽  
Magnetization Vector ◽  
Forward Modeling ◽  
Magnetic Data ◽  
Total Field ◽  
Inversion Algorithm ◽  
Sensitivity Matrix ◽  
Magnetic Inversion ◽  
Magnetic Basement

We have developed a fast 3D regularized magnetic inversion algorithm for depth-to-basement estimation based on an efficient way to compute the total-field anomaly produced by an arbitrary interface separating nonmagnetic sediments from a magnetic basement. We approximate the basement layer by a grid of 3D vertical prisms juxtaposed in the horizontal directions, in which the prisms’ tops represent the depths to the magnetic basement. To compute the total-field anomaly produced by the basement relief, the 3D integral of the total-field anomaly of a prism is simplified by a 1D integral along the prism thickness, which in turn is multiplied by the horizontal area of the prism. The 1D integral is calculated numerically using the Gauss-Legendre quadrature produced by dipoles located along the vertical axis passing through the prism center. This new magnetic forward modeling overcomes one of the main drawbacks of the nonlinear inverse problem for estimating the basement depths from magnetic data: the intense computational cost to calculate the total-field anomaly of prisms. The new sensitivity matrix is simpler and computationally faster than the one using classic magnetic forward modeling based on the 3D integrals of a set of prisms that parameterize the earth’s subsurface. To speed up the inversion at each iteration, we used the Gauss-Newton approximation for the Hessian matrix keeping the main diagonal only and adding the first-order Tikhonov regularization function. The large sparseness of the Hessian matrix allows us to construct and solve a linear system iteratively that is faster and demands less memory than the classic nonlinear inversion with prism-based modeling using 3D integrals. We successfully inverted the total-field anomaly of a simulated smoothing basement relief with a constant magnetization vector. Tests on field data from a portion of the Pará-Maranhão Basin, Brazil, retrieved a first depth-to-basement estimate that was geologically plausible.

THE INTERPRETATION OF AEROMAGNETIC SURVEYS FOR HYDROCARBON EXPLORATION

1999 ◽  
Vol 39 (1) ◽  
pp. 494
Author(s):  
I. Kivior ◽  
D. Boyd
Keyword(s):  
Spectral Analysis ◽  
Sedimentary Basins ◽  
Hydrocarbon Exploration ◽  
Petroleum Exploration ◽  
Magnetic Data ◽  
Curve Matching ◽  
Aeromagnetic Data ◽  
Profile Data ◽  
Aeromagnetic Survey ◽  
New Approach

Aeromagnetic surveys have been generally regarded in petroleum exploration as a reconnaissance tool for major structures. They were used commonly in the early stages of exploration to delineate the shape and depth of the sedimentary basin by detecting the strong magnetic contrast between the sediments and the underlying metamorphic basement. Recent developments in the application of computer technology to the study of the earth's magnetic field have significantly extended the scope of aeromagnetic surveys as a tool in the exploration for hydrocarbons. In this paper the two principal methods used in the analysis and interpretation of aeromagnetic data over sedimentary basins are: 1) energy spectral analysis applied to gridded data; and, 2) automatic curve matching applied to profile data. It is important to establish the magnetic character of sedimentary and basement rocks, and to determine the regional magnetic character of the area by applying energy spectral analysis. Application of automatic curve matching to profile data can provide results from the sedimentary section and deeper parts of a basin. High quality magnetic data from an experimental aeromagnetic survey flown over part of the Eromanga/Cooper Basin has recently been interpreted using this new approach. From this survey it is possible to detect major structures such as highs and troughs in the weakly magnetic basement, as well as pick out faults, and magnetic layers in the sedimentary section. The results are consistent with interpretation from seismic and demonstrate that aeromagnetic data can be used to assist seismic interpretation, for example to interpolate between widely spaced seismic lines and sometimes to locate structures which can not be detected from seismic surveys. This new approach to the interpretation of aeromagnetic data can provide a complementary tool for hydrocarbon exploration, which is ideal for logistically difficult terrain and environmentally sensitive areas.

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