Albrecht Hahn; Wilhelm Bosum:


Selected examples and case histories

Ed.: S. Saxov; H. C. Soffel

1986. VIII, 166 pages, 91 figures, 8 tables, 17x24cm, 800 g
Language: English

(Geoexploration Monographs, Number 10)

ISBN 978-3-443-13016-9, bound, price: 49.80 €

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Content Description top ↑

In the geophysical scientific literature there is no lack in elementary textbooks where geomagnetism is treated together with other subjects like gravimetry seismics, seismology, electrical methods etc. There are even some, however few, books which are entirely devoted to subdisciplines of geomagnetism. Most of them are of high physical and mathematic standard.

The various sub-disciplines of geomagnetism (main field theory, palaeomagnetism, rock magnetism, magnetic induction, magnetic anomalies) are commonly developed on the basis of the potential theory, Maxwell's equations or solid state physics. Only few authors try to complete their often quite theoretical deductions with practical examples. The authors of this volume try to fill in this gap in the case of applied geomagnetics, i.e. the interpretation of geomagnetic anomalies. Both authors have worked in this field during the last 30 years and have contributed a lot to the development of the method itself. They have now so to say "opened their files" for the scientific community to show what the reality of interpretation of magnetic anomalies really is. They selected various case histories from many places of the world (anomalies of basic and acid intrusions, ore deposits, basement rocks, thermal anomalies, even anomalies due to magnetised sediments) to show the general problems of their interpretation.

The crucial unknown quantity in the interpretation of geomagnetic anomalies is — and will always be — the direction and intensity of the magnetisation of deep seated, i.e. not outcropping layers. Much effort has therefore been devoted to extract this information from the correlation between ground based, airborne and satellite magnetic measurements. Although the problem is far from being solved, some general constraints can be given for the magnetic properties of various crustal types.

Contents top ↑

1 Introduction 1
2 Magnetite Ore Anomalies — Ore Prospecting 2
2.1 Aeromagnetic Anomalies of the Magnetite Ore Deposit near Kiruna, Sweden 2
2.2 Ground Magnetic Survey of the Iron Ore Deposit at Tschogart, Iran 3
3 Anomalies of Magmatic Rocks 10
3.1 Magnetic Anomalies above Basalt Flows and Pipes 11
3.2 Anomalies above Magmatic Rocks Forming Dipping Layers — Example from the
Saar-Nahe Syncline, F.R. Germany 13
Occasion of Survey — Geological Setting, Ground Survey Data and Results, Paleomagnetic Studies, Final Remark
4 Basement Anomalies 19
4.1 Magnetic Basement Mapping on the West African Shield in Benin (formerly Dahomey) 19
Technical Data, Outline of Geology, Correlations of Magnetic Anomalies and Rock Magnetism with Geology, A Structural Map
4.2 Magnetic Investigations in the Spessart Mountains, F.R. Germany 32
Geological Setting, Technical Data of Airborne and Ground Survey —
Magnetisation of Rocks, Results of the Ground Survey, Interpretation, Conclusions
4.3 Aeromagnetic Mapping in Northern Sweden 41
Survey Characteristics, Evaluation of Results
4.4 Magnetic Basement Mapping at Moderate Depths — an Aeromagnetic Survey in the Massif Centra], France 46
Survey Data, Main Features of the Magnetic Anomalies and their Relation to the Geology, Interpretation of Anomalies
4.5 Basement at Great Depth 61
The Magnetic Basement in NW Germany, Aeromagnetic Surveying as an Initial Part of Hydrocarbon Exploration — Survey of Southern Afghanistan
5 Magnetic Anomalies of Sedimentary Rocks 92
5.1 Position and Shape of a Sedimentary Anomaly in NW Germany 92
Further Investigations of the Magnetic Layer in the Albian, Conclusions
6 Magnetic Anomalies Occuring in Areas with Geothermal Anomalies 98
6.1 Area with an Ancient Geothermal Anomaly — Magnetic Anomaly of Soest, F.R. Germany 98
Measurements, Geological Description, Interpretation, A Drillhole Test, Comparison between the Model Body and the Drillhole Results — Consequences
6.2 An Area with a Recent Geothermal Anomaly — Olkaria, Kenya, in the East African Rift System 108
Geological and Geothermal Situation, Magnetic Survey, Intepretation, Evidence from Boreholes
7 Investigation of the Structure of the Earth's Crust in an Old Shield Area — Ukrainian Shield 113
7.1 Geological Situation 113
7.2 Geophysical Investigations 113
7.3 Interpretation of the Geophysical Results 116
Results of Seismic Investigations, Magnetisation of the Blocks Obtained by Modelling of Field Anomalies, Magnetisation Values Obtained by Rock Magnetic Studies, Comparison of the Results of Rock Sample Investigations with those of Model Calculations
7.4 The Blocks of the Ukrainian Shield 125
The Novograd — Volhynian Block, The Podolian Block, The Belaya Tserkov — Odessa Block and the Golovanevsk Intermediate Zone, The Kirovograd Block, The West-Ingulets Zone, The Peri-Dnieper Block, The Orekhov — Pavlograd Zone and the Peri-Azovian Block
7.5 Conclusions 129
8 Large Scale Anomalies Observed by Satellites 130
8.1 Global Magnetic Surveys 130
8.2 The Magsat Mission 131
Technical Characteristics, Anomaly Maps from Magsat
8.3 A Model of the Magnetisation Distribution within the Earth's Crust 137
Principles of the Model Composition and the Calculation of its Anomaly Field, The Magnetisation of the Crustal Model in Comparison with the Magsat Results, Relationship of the Magnetisation of the Model to the Composition of the Earth's Crust, Distribution
8.4 Conclusions 155
Origins of Anomalies on the Magsat Anomaly Maps, Average Magnetisation of Large Sections of the Crust, Average Remanent Magnetisation of Large Parts of the Crust, Spatial Distribution of Magnetisation within the Earth's Crust, Final Remark
Bibliography / Subject Index / Geographical Index 158