CHAPTER 10 GEOPHYSICAL METHODS OF EXPLORATION
| Jurisdiction | United States |
(Apr 1980)
GEOPHYSICAL METHODS OF EXPLORATION
Director of Geophysics (Retired) Newmont Exploration Ltd.
TUCSON, Arizona
CONTENTS
I. Background and General Comments
1. Re Resources
2. Re Exploration
3. Re Geophysics
4. Re Geophysics
II. Fundamental Concepts of Various Geophysical Exploration Methods
1. Magnetics
2. Gravity
3. Resistivity and Induced Polarization Methods
A. Resistivity
B. Induced Polarization
C. Magnetic Induced Polarization
D. Resistivity—I.P. in Drill Holes
4. Electromagnetic Induction "E.M." Systems
A. Basic Concepts and Measurement Systems
B. Two Additional "E.M." Induction Systems
C. Time Domain E.M. Induction Systems
D. E.M. Measurements in Drill Holes
5. Long Grounded Cable Methods
A. Magnetic Resistivity
B. Audio Magneto Tellurics
6. Methods Based on Natural Earth Electromagnetic Fields
A. Afmag Method
B. Magneto Tellurics
7. Miscellaneous Electrical Methods
A. So-Called VLF
B. Radar
8. Radioactive Exploration Methods
9. Seismic Methods
10. Geochemistry
11. Summary
III. Figures Illustrating the Geophysical Techniques Listed and Discussed in Section II
Figure 1—Examples of Aeromagnetics 10-29
Figure 2—Examples of Ground Magnetics 10-34
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Figure 3—Example of a Drill Hole Mag Survey 10-37
Figure 4—Examples of Gravity Data 10-40
Figure 5—Examples of Resistivity and Induced Polarization (I.P.) Arrays and Procedures 10-44
Figure 6—Examples of Induced Polarization (I.P.), Magnetic I.P., and Drill Hole Work 10-50
Figure 7—Ground EM Induction Responses 10-54
Figure 8—Airborne EM Systems 10-58
Figure 9—EM Induction, Time Domain 10-68
Figure 10—EM Induction Methods in Drill Holes 10-71
Figure 11—Long Grounded Cable Methods 10-75
Figure 12—Methods Using Natural Earth EM Fields 10-79
Figure 13—Examples of VLF Response 10-82
Figure 14—Examples of Radioactive Survey Data 10-85
Figure 15—Examples of Seismic Surveys 10-92
Figure 16—Examples of Geochemical Surveys 10-96
Figure 17—Example of Drill Hole Logs Across Coal Seams (shaded) 10-100
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This presentation is divided into three parts:
I. Background and General Comments
II. Fundamental concepts of Various Geophysical Exploration Methods
III. Examples of Field Results
I. General Comments
1. Resources are occurrences and discoveries that prove to have sufficient grade and tonnages to permit profitable exploitation under the existing conditions of taxation, labor costs, and political risks relative to market processes. Thus the higher taxation, and the greater the political risks, the higher are the tonnage-grade requirements. Therewith ensue reduced probability of discovery, increased cost of discovery, and increase of marginal resources never developed.
Resource shortages do not result from lack of occurrences, but from inability to exploit under the existing tax, political, cost, versus price, framework.
Thus taxation, direct and indirect, within the United States, contributes slightly more than 50 percent to the cost of indigenous copper production.
Political risk versus long lead times of six to twelve years from discovery to cash flow, and high forward capital investment, not to mention government to government foreign aid, the worst conceivable, have stymied third world mineral development and the capital generation historically resulting therefrom.
We are presently a have-not in many critical minerals, cobalt, chrome, manganese, tin, brought over long and susceptible supply lines. Historically the twilight of empire has coincided with material shortages and centralized, non-productive overheads unable to be sustained.
2. Exploration consists of geologic conception (increasingly important), observation by geologic and technical means, analysis, then drilling.
In these, geophysical techniques, including geochemistry and remote sensing, play a part, perhaps 20 to 25 percent of oil exploration (pre-discovery, not development) costs, and 10 percent of mineral exploration costs, contrasted to some 40 percent for drilling.
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Oil and mining companies spend about 3 percent of gross on pre-discovery exploration. Costs per discovery, depending on commodity, can run from $10 to $100 million.
Odds of an economic discovery per significant exploration test are about 50 to 100 to 1 against. The more trials you make, the better chance of at least one success. (The probability function, "gambler's ruin," is logarithmic.) Hence the tendency to joint venturing.
3. As regards geophysical techniques, the post-war period 1945-60 saw the development of airborne magnetics, of new ground electrical systems, and of electromagnetic induction systems both in the air and on the ground. Also in the same period, the initial and considerable work was done in the mathematical area, whereby the responses expected could be evaluated for many idealized geologic conditions. This is exceedingly important if field data is to be more capably analyzed, and method basis understood.
Geochemical techniques have broadly evolved and become more esoteric from about 1950 on, but particularly in the last ten years.
With the ramification of computers, and in-the-field mini computers, repetitive measurements, data treatment, extensive analysis and model fitting have become commonplace.
More important in the last 10 to 15 years geologic conceptions as to ore origins, emplacement and precipitation controls, particularly as regards sedimentary deposits, have led to significant discoveries. Of all, this is the most important development.
4. All geophysical exploration techniques are based on the measurement of some physical or chemical property of the earth's at and near surface material. Thus the geophysical techniques are tailored to the geologic and surface conditions and nature of occurrence presumed.
None of the systems detect economically desirable minerals directly, though in certain cases they may through association. An exception is geochemistry which may suggest economically desirable minerals, but not as to grade or tonnage.
All methods have limited ability to recognize a target at depth, with the exception of seismic. The factor involved is the size of the target to its depth, somewhere in a ratio range of 1:1 to 1:5. This inability to detect to depth is a major geophysical limitation.
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All geophysical methods respond in greater or less degree to surface material, surface formations and conditions. Thus either corrections or awareness must be applied, and/or methods used not influenced by these conditions.
Interpretation consists in analyzing the measurement data, into the effects of a series of bodies and conditions (including surface), and then transfering these into a geologic framework, where geologic formations and conditions are assigned to the inferred geophysically derived bodies. This interpreted picture must make sense geologically. This step is in fact a translation from a physical property derived, to a geologic compatible condition. Many mistakes occur at this step. A considerable knowledge of both physics and geology is necessary and is not always present.
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II. Fundamental Concepts of Various Geophysical Exploration Methods
1. Magnetics
This is the most widely used, and generally most applicable method.
The earth acts as a magnet, due to material flow currents in the core. The crustal rocks appear magnetized to greater or less degree, depending on their physical property of "susceptibility", determined to a first degree by the amount of accessory magnetite present.
In general basic rocks are more magnetic than acid, plutonics and intrusives more magnetic than sediments.
Measurements may be made on the ground over a grid network of stations, or by flying along parallel spaced lines in the air.
The original airborne magnetometer was an adaption of the World War II, anti-submarine magnetic detection device. Now there are several devices, based on various physical phenomena, in use both in the air and on the ground.
The earth's magnetic field is approximately one-half Oersted, a basic unit of magnetic field strength. Exploration measurements are made in "gamma" where 1 Oersted=105 gamma. Present day magnetometers are sensitive to a few gamma or better. Anomolies may range from a few gamma for oil work to tens and hundreds of gamma for mineral exploration.
Procedure is to present the data as contour "egg" maps, and to plot also selected profiles of values.
The contour maps present plan data as regards strike, folds, offsets and faults. The profile data is used to derive geologic cross sections as to contacts, dips, etc.
Mathematical procedures now permit the data to be "continued" to what it would be at a higher or lower elevation, to be presented as the first or second horizontal derivative of the data, and to be reduced to what the data would be were the earth's magnetizing field oriented vertically rather than obliquely, so called "reduction to the pole."
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In mineral exploration magnetic association of desired geology is sought. Thus the porphyry type, copper bearing intrusions are magnetic from Utah northward, but not southward into Arizona, Central America and Peru. Many magnetic intrusives of Nevada and northward are not copper bearing.
In Canada, the combination of a magnetic anomoly (pyrrhotite and magnetite) plus airborne electromagnetic induction (conductivity) response, suggesting massive sulphides, is sought. However, the significant Kidd Creek, zinc, copper, silver body yielded no magnetic anomoly, and was almost not drilled, in fact not till the last gasp effort.
In South Africa, the kimberlites (not all are diamondiferous, and there are many similar type occurrences of non-kimberlitic rocks) are quite magnetic and detectable from the air. In the recently discovered diamond area of...
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