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| Syllabus | |
| 1 History of Mining | |
| 2 Using Maps | |
| 3 Geological Exploration | |
| 4 Geophysical Exploration | |
| 5 Geochemical Exploration | |
| 6 Secondary Exploration | |
| 7 Open Pit Mining | |
| 8 *****Mid Term***** | |
| 9 Strip Mining & Placer Mining | |
| 10 Underground Mining | |
| 11 Permits and Laws | |
| 12 Environment Impact | |
| 13 Jobs and Tools | |
| 14 Mineral Economics | |
| 15 Reclamation | |
| 16**** Final Exam***** | |
| Resources | |
| Staff E-mail | |
Indirect Exploration Part 1
Lesson 4
Objectives:
a) Students will compare the data
collected between land based and airborne surveys.
b) Students will list the benefits of geophysical exploration
for the environment and animals in the prospecting process.
Lesson:
What is Geophysics?
As the name implies, geophysics involves the application of physical theories and measurements to discover the properties of the earth. It began as mainly as a scientific approach to earthquake prediction (a problem still unsolved). Major progress began in the late 1500's with initial work in such areas as magnetism and gravity. Tremendous improvements in instrumentation in the early years of the 20th century generated rapid progress in geophysics and ultimately led, in the 1960's, to the theory of plate tectonics.
Plate tectonics, the study of the interior structure of the earth, and such related areas as global and regional processes are known collectively as solid earth geophysics. The sub discipline known as exploration geophysics involves the use of geophysical theory and instrumentation to locate petroleum and other mineral sources. Unlike solid earth geophysics, exploration geophysics generally concentrates on finding lateral differences in a small part of the earth's crust.
Geophysics has increased man's ability to explore natural resources. Human senses cannot quantify, or even detect many physical phenomena (e.g., magnetism). Humans cannot detect variations in the earth's gravitation field of one part per million, but modern gravity meters can (in fact, to 0.02 parts per million or better). Seismology, the primary method of petroleum exploration, requires exact timing and recording of very low-amplitude vibrators, vibrations (or shaking) that is far below that which a human would sense.(The Society of Exploration of Geophysicists)
For example, in the past, mineral exploration in Manitoba and else where has been done by surface geological reconnaissance, where the ore body was exposed on surface. Geological maps and data were the major tools of experienced prospectors, who closely examined favorable areas. The prospectors were looking for any geological trait which was different from its surroundings in any way. The difference they may notice may be chemical, mineral or even in structure.
Most ore bodies that can be discovered via outcroppings are most likely claimed and we now have turned to other methods of prospecting. Information will need to be gathered from undiscovered hidden deposits below the surface. This is where the geophysics has become an important part of prospecting.
The gravity field of the earth is caused by the earth's mass. The strength of the field is a function of the composition of the mass (iron-rich center with silicate minerals near the surface) and the distance away from it's center. Where we stand on the surface, the gravity field has a field strength, which we can measure (as weight). The earth is not completely round. Instead, the radius in the polar areas is approximately 21 kilometers shorter than at the equator. Since you are closer to the earth's center at the poles, you body weight will be greater there than at the equator. The gravity field is also not perfectly spherical: there are many irregularities.
In spite of the complexities in the shape and strength of the field, geophysicists have created a reliable model of the field (with numerous instrumental readings all over the globe). Using an instrument called a gravimeter, a geophysicist can measure the strength of the gravity field over a buried ore deposit to test for its presence. If the gravity readings show a departure from the model's prediction of the normal field strength, it may indicate an abnormally dense rock mass at depth. The gravity data can be plotted as a vertical profile, or in a map view, to highlight the anomalous readings.
The gravity field also affects our perspective of the way to measure density. Gravity causes objects with a dense mass (or "high density") to feel like they weigh more. A good way to compare different mineral or rock substances is to compare their density, which we refer to as the "specific gravity". The specific gravity for a substance is an expression stating the number of times heavier it is than an equal volume of water. For example, the specific gravity of gold is approximately 19, which means nineteen times heavier than water.
The Earth has a magnetic field which resembles the magnetic field of a simple bar magnet with axis of the magnet closely aligned with the spin axis of the earth. From a point on the surface where we are standing, we call the angle between the magnetic north pole and true north is called the declination. Since a compass needle points to magnetic north, the compass declination is typically adjusted to compensate and cause north on the compass to read as true north.
In detail, the earth's magnetic field, like the earth's gravity field, is much more complex than the simple bar magnet model would suggest. For one thing, the location of the north pole slowly migrates, and has been slowly migrating throughout geologic time. For example, in England over the last two centuries, the direction towards the north pole has migrated from 15 degrees east of north, to 25 degrees west of north, and then back to 5 degrees west of north.
Certain minerals containing iron or nickel can be magnetic and can have small magnetic fields of their own which are readily measurable. Many minerals have a trace of magnetism at least which can be measured. The strength of the magnetism associated with the mineral is called the "susceptibility". The minerals magnetite and pyrrhotite have strong magnetic susceptibility compared to the mineral zircon, which has a low susceptibility. Where magnetite and other magnetic minerals are present in abundance in a rock, the rock can be measured and traced with sophisticated magnetometers. Geophysicists measure the field strength and orientation on the surface and compare it with predicted values using a model. The magnetic field has been measured in numerous locations to model its shape and strength. When the measured data on the surface shows anomalous values, there may be rocks below the surface causing the anomalies.
Resource:
Adams, Dave, Introduction to Geology, Delta Mining Training Center, 2000
Vocabulary:
The following definitions are from Robert
E. Sheriff's "Encyclopedic Dictionary of Exploration Geophysics".
Geophysics:
The study of the earth by quantitative physical method, especially
by seismic reflection and refraction, gravity, magnetic, electrical,
electromagnetic, and radioactivity methods.
The application of physical principles to studies of the earth.
Includes the branches of (a) seismology (earthquakes and elastic
waves); (b) geothermometry (heating of the earth, heat flow,
volcanology, and hot springs); (c) hydrology (ground and surface
water, sometimes including glaciology); (d) physical oceanography;
(e) meteorology; (f) gravity and geodesy (the earth's gravitational
field and the size and form of the earth); (g) atmospheric electricity
and terrestrial magnetism (including ionosphere, Van Allen belts,
telluric currents, etc.); (h) tectonophysics (geological processes
in the
Physical method, especially by seismic reflection and refraction,
gravity, magnetic, electrical, electromagnetic, and radioactivity
methods.
Exploration geophysics is the use of seismic, gravity, magnetic,
electrical, electromagnetic, etc., methods in the search for
oil, gas, minerals, water, etc., with the objective of economic
exploitation.
Geophysicist
One who studies the physical properties
of the earth or applies physical measurements to geologic problems;
a specialist in geophysics
Prospecting: preliminary exploration
Indirect prospecting: using theories and measurements to discover the properties of the earth.
Prospect: potential ore deposit, based on preliminary exploration.
Ore body or Ore Deposit: naturally occurring materials from which a mineral or minerals of economic value can be recovered at a reasonable profit.
Plate Tectonics: the study of the interior structure of the earth, and such related areas as global and regional processes are known collectively as solid earth geophysics.
Resources:
Geophysical Methods using magnetic surveys.
http://www.digistar.mb.ca/minsci/finding/magnetic.htm
Magnetics-Fast Sampling Land Meter.
http://www.geoinstruments.com.au/html/main.htm
What is new at Arctic Geoscience Airborne
Magnetometer- A good resource for pictures of equipment
for field that can be used for field acquisition data. Compare
the processed data presentations (maps) on page 5 on this document.
www.arcticgeo.com/new_web_pdf/KodiakUXO.pdf
Geophysical Exploration
http://www.mineralswa.asn.au/~cmeminex/page2.html#geophys
Geophysical Exploration
http://www.geology.co.jp/tansa/tansaindex-e.html
Introduction to Geophysical Exploration
http://www.mines.edu/fs_home/tboyd/GP311/introgp.shtm
Delta Mine Training Center
Copyright © 2001 DMTC. All rights reserved.