Handbook of Geophysics & Geomagnetism

149 Pages·2014·4.219 MB·English

by  Boles, Otilia Hsu Reese

First Edition, 2012

ISBN 978-81-323-0773-0

© All rights reserved.

Published by:
Academic Studio
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Ansari Road, Darya Ganj,
Delhi - 110002
Email: [email protected]
Table of Contents

Chapter 1 - Geophysics
Chapter 2 - Physical Concepts in Geophysics
Chapter 3 - Regions of the Earth
Chapter 4 - Geochronology
Chapter 5 - Earth's Magnetic Field
Chapter 6 - North Magnetic Pole
Chapter 7 - Paleomagnetism
Chapter 8 - Magnetic Field
Chapter 9 - Magnetosphere
Chapter 10 - Geomagnetic Reversal
Chapter- 1
Geophysics

Geophysics is the physics of the Earth and its environment in space. Its subjects include the
shape of the Earth, its gravitational and magnetic fields, the dynamics of the Earth as a whole
and of its component parts, the Earth's internal structure, composition and tectonics, the
generation of magmas, volcanism and rock formation, the hydrological cycle including snow and
ice, all aspects of the oceans, the atmosphere, ionosphere, magnetosphere and solar-terrestrial
relations, and analogous problems associated with the Moon and other planets.
Geophysics is also applied to societal needs, such as mineral resources, mitigation of natural
hazards and environmental protection. Geophysical survey data are used to analyze potential
petroleum reservoirs and mineral deposits, to locate groundwater, to locate archaeological finds,
to find the thicknesses of glaciers and soils, and for environmental remediation.
History
Ancient and classical eras
The magnetic compass existed in China back as far as the fourth century BC. It was used as
much for feng shui as for navigation on land. It was not until good steel needles could be forged
that compasses were used for navigation at sea; before that, they could not retain their magnetism
for long. The first mention of a compass in Europe was in 1190.
In circa 240 BC, Erastothenes of Cyrene deduced that the Earth was round and measured the
circumference of the Earth, using trigonometry and the angle of the Sun at more than one latitude
in Egypt. He developed a system of latitude and longitude and measured the tilt of the Earth's
axis.
Perhaps the earliest contribution to seismology was the invention of a seismoscope by the
prolific inventor Zhang Heng in 132 CE. This instrument was designed to drop a bronze ball
from the mouth of a dragon into the mouth of a toad. By looking at which of eight toads had the
ball, one could determine the direction of the earthquake. It was 1571 years before the first
design for a seismoscope was published in Europe, by Jean de la Hautefeuille. It was never built.
Replica of Zhang Heng's seismoscope
Beginnings of modern science
One of the publications that marked the beginning of modern science was William Gilbert's De
Magnete (1600), a report of a series of meticulous experiments in magnetism. Gilbert deduced
that compasses point north because the Earth itself is magnetic.
In 1687 Isaac Newton published his Principia, which not only laid the foundations for classical
mechanics and gravitation but also explained a variety of geophysical phenomena such as the
tides and the precession of the equinox.
The first seismometer, an instrument capable of keeping a continuous record of seismic activity,
was built by James Forbes in 1844.
Other fields and related disciplines
Fields
• Geodesy, measurement of the Earth: GPS, vertical and horizontal motions of the Earth's
surface, navigation, the study of the Earth's gravitational field, and the size and form of
the Earth
• The study of large-scale motions of the Earth's surface and interior, including:
• Tectonophysics, the study of the physical processes that cause and result from
plate tectonics
• Geodynamics, the study of modes of transport deformation within the Earth: rock
deformation, mantle flow and convection, heat flow, lithosphere dynamics
• Shallow seismology is used in exploration geophysics (to find oil and gas) and for
environmental characterization of the subsurface
• Geomagnetism, the study of the Earth's magnetic field, including its origin, telluric
currents driven by the magnetic field, the Van Allen belts, and the interaction between the
magnetosphere and the solar wind. This field is associated with paleomagnetism, or the
measurement of the orientation of the Earth's magnetic field over the geologic past.
• Mathematical geophysics, The development and applications of mathematical methods
and techniques for the solution of geophysical problems.
• Geophysical surveying:
• Exploration and engineering geophysics, using surface methods to detect or infer
the presence and position of concentrations of ore minerals and hydrocarbons
• Archaeological geophysics, for archaeological imaging or mapping
• Environmental and Engineering Geophysics, for locating underground storage
tanks (USTs) or utilities, Unexploded ordnance (UXO), delineating landfills,
locating voids or potential subsidence, finding depth to, P-wave or S-wave
velocity of, or rippability of bedrock, or the pathway of groundwater movement
Related disciplines
• Volcanology, the study of volcanoes, volcanic features (hot springs, geysers, fumaroles),
volcanic rock, and heat flow related to volcanoes
• Atmospheric sciences, which includes:
• Atmospheric electricity and the ionosphere
• Aeronomy, the study of the physical structure and chemistry of the atmosphere.
• Meteorology and Climatology, which both involve studies of the weather.
• The study of water on the Earth, hydrology, physical oceanography and glaciology
• Geological and geophysical engineering and Engineering geology, applying geophysics
to the engineering design of facilities including roads, tunnels, and mines
• The study of the rocks and minerals, including petrophysics and aspects of mineralogy
such as physical mineralogy and crystal structure
Methods of geophysics
Space probes
Space probes made it possible to collect data not only the visible light region, but in other areas
of the electromagnetic spectrum. The planets can be characterized by their force fields: gravity
and their magnetic fields, which are studied through geophysics and space physics.
Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine
details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity
field disturbances above lunar maria were measured through lunar orbiters, which lead to the
discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium,
Nectaris and Humorum basins.
In 2002, NASA launched the Gravity Recovery and Climate Experiment, wherein two twin
satellites map variations in Earth's gravity field by making measurements of the distance between
the two satellites using GPS and a microwave ranging system. Gravity variations detected by
GRACE include those caused by changes in ocean currents; runoff and ground water depletion;
melting ice sheets and glaciers.
Chapter- 2
Physical Concepts in Geophysics

Gravity anomalies covering the Southern Ocean are shown here in false-color relief. Amplitudes
range between -30 mGal (magenta) to +30 mGal (red). This image has been normalized to
remove variation due to differences in latitude
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used
when either the magnitude of gravitational field or the properties of matter responsible for its
creation are of interest. The term gravimetry or gravimetric is also used in chemistry to define a
class of analytical procedures, called gravimetric analysis relying upon weighing a sample of
material.
Units of measurement
Gravity is usually measured in units of acceleration. In the SI system of units, the standard unit
of acceleration is 1 metre per second squared (abbreviated as m/s2). Other units include the gal
(sometimes known as a galileo, in either case with symbol Gal), which equals 1 centimetre per
second squared, and the g (g ), equal to 9.80665 m/s2. The value of the g approximately equals
n n
the acceleration due to gravity at the Earth's surface (although the actual acceleration g varies
fractionally from place to place).
How gravity is measured
An instrument used to measure gravity is known as a gravimeter, or gravitometer. Since general
relativity regards the effects of gravity as indistinguishable from the effects of acceleration,
gravimeters may be regarded as special purpose accelerometers. Many weighing scales may be
regarded as simple gravimeters. In one common form, a spring is used to counteract the force of
gravity pulling on an object. The change in length of the spring may be calibrated to the force
required to balance the gravitational pull. The resulting measurement may be made in units of
force (such as the newton), but is more commonly made in units of gals.
More sophisticated gravimeters are used when precise measurements are needed. When
measuring the Earth's gravitational field, measurements are made to the precision of microgals to
find density variations in the rocks making up the Earth. Several types of gravimeters exist for
making these measurements, including some that are essentially refined versions of the spring
scale described above. These measurements are used to define gravity anomalies.
Besides precision, also stability is an important property of a gravimeter, as it allows the
monitoring of gravity changes. These changes can be the result of mass displacements inside the
Earth, or of vertical movements of the Earth's crust on which measurements are being made:
remember that gravity decreases 0.3 mGal for every metre of height. The study of gravity
changes belongs to geodynamics.
The majority of modern gravimeters use specially-designed quartz zero-length springs to support
the test mass. Zero length springs do not follow Hooke's Law, instead they have a force
proportional to their length. The special property of these springs is that the natural resonant
period of oscillation of the spring-mass system can be made very long - approaching a thousand
seconds. This detunes the test mass from most local vibration and mechanical noise, increasing
the sensitivity and utility of the gravimeter. The springs are quartz so that magnetic and electric
fields do not affect measurements. The test mass is sealed in an air-tight container so that tiny
changes of barometric pressure from blowing wind and other weather do not change the
buoyancy of the test mass in air.
Spring gravimeters are, in practice, relative instruments which measure the difference in gravity
between different locations. A relative instrument also requires calibration by comparing
instrument readings taken at locations with known complete or absolute values of gravity.
Absolute gravimeters provide such measurements by determining the gravitational acceleration
of a test mass in vacuum. A test mass is allowed to fall freely inside a vacuum chamber and its
position is measured with a laser interferometer and timed with an atomic clock. The laser
wavelength is known to ±0.025 ppb and the clock is stable to ±0.03 ppb as well. Great care must
be taken to minimize the effects of perturbing forces such as residual air resistance (even in
vacuum) and magnetic forces. Such instruments are capable of an accuracy of a few parts per
billion or 0.002 mGal and reference their measurement to atomic standards of length and time.
Their primary use is for calibrating relative instruments, monitoring crustal deformation, and in
geophysical studies requiring high accuracy and stability. However, absolute instruments are
somewhat larger and significantly more expensive than relative spring gravimeters, and are thus
relatively rare.
Gravimeters have been designed to mount in vehicles, including aircraft, ships and submarines.
These special gravimeters isolate acceleration from the movement of the vehicle, and subtract it
from measurements. The acceleration of the vehicles is often hundreds or thousands of times
stronger than the changes being measured. A gravimeter (the Lunar Surface Gravimeter) was
also deployed on the surface of the moon during the Apollo 17 mission, but did not work due to a
design error. A second device (the Traverse Gravimeter Experiment) functioned as anticipated.
Microgravimetry
Microgravimetry is a rising and important branch developed on the foundation of classical
gravimetry. Microgravity investigations are carried out in order to solve various problems of
engineering geology, mainly location of voids and their monitoring. Very detailed measurements
of high accuracy can indicate voids of any origin, provided the size and depth are large enough to
produce gravity effect stronger than is the level of confidence of relevant gravity signal.
History
The modern gravimeter was developed by Lucien LaCoste and Arnold Romberg in 1936.
They also invented most subsequent refinements, including the ship-mounted gravimeter, in
1965, temperature-resistant instruments for deep boreholes, and lightweight hand-carried
instruments. Most of their designs remain in use (2005) with refinements in data collection and
data processing.