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Similarities between gravity and magnetics and application of different geophysical methods
1. Similarities Between Gravity and
Magnetics and Application of different
geophysical methods
• Jyoti Anischit
• MSc in Engineering
Geology
• TU
• Roll 10/2072
2. Applications of the following
Gravity survey
Magnetic survey
Electrical resistivity survey
Seismic survey
4. Definition:
It is a method
of measuring the force of gravity in
drill holes by means of gravimeters in order to
determine the mean density values of rocks at
different depths in their natural bed.
6. Hydrocarbon exploration
Regional geological studies
Isostatic compensation determination
Exploration for mass estimation of mineral
deposits
Detection of subsurface cavities
Location of buried rock valley
Determination of glacier thickness
Shape of earth(geodesy)
Monitoring volcanoes
7. Hydrocarbon exploration
This is also called oil and gas exploration is the
search
by geophysicist for hydrocarbon deposits
beneath the Earth's surface such as oil
and natural gas . Oil and gas exploration are
grouped under the science of petroleum
geology.
9. Accumulations Of Hydrocarbons
Gravity Surveys are used either alone or in
conjunction with magnetic surveys, to
determine the location and size of the major
source structures which contain
accumulations of hydrocarbons.
10. Hydrogeology
Determination of the major structures that control
the location of aquifers.
Additionally, Magneto telluric and Magnetics surveys
may also be used for the same purpose
11. The gravity method is a nondestructive
geophysical technique that measures
differences in the earth’s studies including
locating voids and karst features, buried
stream valleys, water table levels and the
determination of soil layer thickness.
13. Gravity data in engineering and environmental
applications should be collected in a grid or
along a profile with stations spacing 5 meters
or less. In addition, gravity station elevations
must be determined to within 0.2m
Using the highly precise locations and
elevations plus all other quantifiable
disturbing effects, the data are processed to
remove all these predictable effects.
15. Magnetic survey
Magnetic survey is one of a number of
methods used in archaeological geophysics
Magnetic surveys record variation in the
Earth's magnetic field. In archaeology ,
magnetic surveys are used to detect and map
archaeological artifacts and features.
Magnetic surveys are used in both terrestrial
and marine archaeology.
16. • Basic Concepts
• The Earth possesses a magnetic field caused
primarily by sources in the core. The form of
the field is roughly the same as would be
caused by a dipole or bar magnet located near
the Earth's center and aligned sub parallel to
the geographic axis
19. Locating buried Pipes, cables, tank and drums
Fault studies
Ore exploration identification of metalliferous
deposits, for example massive sulphides
Geothermal exploration
Mapping buried utilities
20. Exploration of fossil fuels (oil and gas)
Exploration of ore deposit
Regional and global tectonics
Large scale of geological structures ,volcanology
Engineering construction site investigation
Burried foundation ,fine pits for archeology
Mineral exploration
21. Locating buried Pipes, cables, tank and drums
Magnetic geophysical surveys measure small,
localized variations in the Earth's magnetic field. The
magnetic properties of naturally occurring materials
such as magnetic ore bodies and basic igneous rocks.
Magnetometer surveys find underground storage
tanks, drums, piles and reinforced concrete
foundations by detecting the magnetic anomalies
they produce.
22. Exploration Mineral
Magnetic survey, one of the tools used by
exploration mineral-bearing ore bodies or
even oil-bearing sedimentary structures and
locate on map the remains of buried
structures.
23. Marine Magnetic Methods
Magnetic methods, as employed in marine
applications, differ little in theory from surface
magnetic surveys. The instrument measures the
earth’s total magnetic field at each measurement
location. However, practical applications of field
techniques, field equipment, and geographic control
can vary greatly between surface and marine surveys.
24. Magnetics surveys measure the magnitude
and orientation of the Earth’s magnetic field.
Magnetic field at Earth’s surface depends on
field generated in Earth’s core, magnetic
mineral content of surface materials, and
remnant magnetization of surface rocks.
26. Definition:
Electrical resistance survey also called earth
resistance or resistivity survey is one of a
number of methods used in archaeological
geophysics. In this type of survey electrical
resistance meters are used to detect and map
subsurface archaeological features and
patterning.
29. The presence of certain metallic ore
The temperature of sub surface
Geothermal energy
The presence of archeological feature
Graves
Fire pits
Pot holes .
30. Amount of ground water present
I. Amount of dissolved salt
II.Presence of contaminants
III.% porosity and permeability
31. Specific Applications
Determine electrical characteristics of surrounding
area.
Determine depth of water table
Detect contaminant plumes
Determine extent of saltwater intrusion
Detect fracture zones and faults
Detect areas of potentially unstable ground, e.g.
mine shafts, sinkholes, voids
32. Assist in dam stability analysis
Determine overburden depth
Locate buried mafic dikes
Provide data for corrosion control design
Determine lithology/structure.
33. Electrical resistivity method is also useful for
simultaneously detecting lateral and vertical
changes in subsurface electrical properties
35. Definition
Seismic survey, method of investigating
subterranean structure, particularly as related
to exploration for petroleum, natural gas,
and mineral deposits.
A seismic survey is a technique similar to an
ultrasound that is used to develop images of
the rock layers below ground.
38. • Seismic surveys are used to locate and estimate the size of
offshore oil and gas reserves.
• To carry out such surveys, ships tow multiple air gun arrays
that emit thousands of high-decibel explosive impulses to
map the seafloor.
• The auditory assault from seismic surveys has been found to
damage or kill fish eggs and larvae and to impair the hearing
and health of fish and marine mammals.
39. Hydrocarbon exploration
It is used by the hydrocarbon industry to provide a high
resolution map of acoustic impedance contrasts at depths of
up to 10 km within the subsurface. This can be combined
with seismic attribute analysis and other exploration
geophysics tools and used to help geologists build a geological
model of the area of interest.
40. Land survey
Land seismic surveys tend to be large entities,
requiring hundreds of tons of equipment and
employing anywhere from a few hundred to a few
thousand people, deployed over vast areas for many
months.
41. Marine survey
Traditional marine seismic surveys are conducted
using specially-equipped vessels that tow one or more
cables containing a series of hydrophones at constant
intervals
42. Seismic surveying is also play vital part of
exploring for oil and gas. That makes it critical
to producing the energy we need to power
our homes and businesses.
43. Similarities Between Gravity and
Magnetics
1) Geophysical exploration techniques that
employ both gravity and magnetic are passive.
By this, we simply mean that when using these
two methods we measure a naturally occurring
field of the earth. Collectively, the gravity and
magnetic methods are often referred to as
potential methods and the gravitational and
magnetic fields that we measure are referred to
as potential fields.
44. 2) Identical physical and mathematical
representations can be used to
understand magnetic and gravitational
forces.
For example, the fundamental element used to define the
gravitational force is the point mass. An equivalent
representation is used to define the force derived from the
fundamental magnetic element. Instead of being called a
point mass, however, the fundamental magnetic element is
called a magnetic monopole.
45. 3) The acquisition ,reduction, and
interpretation of gravity and magnetic
observations are very similar.
4) Vary from place to place.
5) Both are Vector fields
6) Both fields are force fields.
7) Drift correction is applied in both
methods.
8) Both fields exert force with a speed
equal to the speed of light.
46. Differences Between Gravity and
Magnetics
1) The fundamental parameter that controls gravity
variations of interest to us as exploration geophysicists
is rock density.
The highest densities we typically observe are about 3.0 gm/cm3and
the lowest densities are about 1.0 gm/cm3.
The fundamental parameter controlling the
magnetic field variations of interest to us, is
magnetic susceptibility.
This variation is not only present amongst different rock types, but
wide variations in susceptibility also occur within a given rock type.
47. 2) Gravitational field is always
perpendicular to the surface of earth.
While Direction of Magnetic field
changes from place to place (it might be
same at different places but unlike g it is
not always perpendicular).
48. 3) Value of g is less near the equator than
at the poles (about 5 gals) for two reasons
(1-The distance to the earth's center is
greater 2-The rotation of the earth
introduces centripetal acceleration V2/r ).
Magnetic field varies from the equator
to the poles in a range that goes from
20000 nT to 70000 nT.
49. 4) Gravity (mass) is monopolar it always attracts.
Magnetism is dipolar; like poles repel
while unlike poles attract.
5) All matter has mass therefore contributes
to g.
While main magnetic field is due to
convection currents in the core; it effects on
some minerals not all.
50. 6) Gravitational field is measured by
Mechanical Instrument.
While magnetic field is measured by purely
electronic Instruments.
7) Gravimeter is relative instrument;
magnetometer is absolute.
51. 8) tides are only external effect on value of g
and can be corrected; effect of magnetic
storms cannot be removed
9) A properly reduced gravitational field is always
generated by subsurface variations in rock density.
;
A properly reduced magnetic field, however, can
have as its origin at least two possible sources. It
can be produced via an induced magnetization, or
it can be produced via a remanent magnetization.
52. 10) Unlike the gravitational field, which does
not change significantly with
time; the magnetic field is highly time
dependent.
53. 53
Geophysical Surveying Using Magnetic Methods
Introduction
Historical Overview
Unlike the gravitational observations described in the previous section, man has been
systematically observing the earth's magnetic field for almost 500 years. Sir William
Gilbert published the first scientific treatise on the earth's magnetic field entitled De
magnate. In this work, Gilbert showed that the reason compass needles point toward the
earth's north pole is because the earth itself appears to behave as a large magnet. Gilbert
also showed that the earth's magnetic field is roughly equivalent to that which would be
generated by a bar magnet located at the center of the earth and oriented along the
earth's rotational axis. During the mid-nineteenth century, Karl Frederick Gauss confirmed
Gilbert's observations and also showed that the magnetic field observed on the surface of
the earth could not be caused by magnetic sources external to the earth, but rather had
to be caused by sources within the earth.
54. 54
Similarities Between Gravity and Magnetic
Geophysical investigations employing observations of the earth's magnetic field have much
in common with those employing observations of the earth's gravitational field. Thus, you
will find that your previous exposure to, and the intuitive understanding you developed
from using, gravity will greatly assist you in understanding the use of magnetic. In
particular, some of the most striking similarities between the two methods include:
1- Geophysical exploration techniques that employ both gravity and magnetic are passive.
By this, we simply mean that when using these two methods we measure a naturally
occurring field of the earth: either the earth's gravitational or magnetic fields. Collectively,
the gravity and magnetic methods are often referred to as potential methods and the
gravitational and magnetic fields that we measure are referred to as potential fields.
2- Identical physical and mathematical representations can be used to understand
magnetic and gravitational forces. For example, the fundamental element used to define
the gravitational force is the point mass. An equivalent representation is used to define the
force derived from the fundamental magnetic element. Instead of being called a point
mass, however, the fundamental magnetic element is called a magnetic monopole.
Mathematical representations for the point mass and the magnetic monopole are identical.
3- The acquisition, reduction, and interpretation of gravity and magnetic
observations are very similar.
55. 55
Differences Between Gravity and Magnetics
Unfortunately, despite these similarities, there are several significant differences between
gravity and magnetic exploration. By-in-large, these differences make the qualitative and
quantitative assessment of magnetic anomalies more difficult and less intuitive than gravity
anomalies.
1- The fundamental parameter that controls gravity variations of interest to us as
exploration geophysicists is rock density. The densities of rocks and soils vary little from
place to place near the surface of the earth.
The highest densities we typically observe are about 3.0 gm/cm3 ,
and the lowest densities are about 1.0 gm/cm3. The fundamental parameter controlling the
magnetic field variations of interest to us, magnetic susceptibility, on the other hand, can
vary as much as four to five orders of magnitude. This variation is not only present amongst
different rock types, but wide variations in susceptibility also occur within a given rock type.
Thus, it will be extremely difficult with magnetic prospecting to determine rock types on the
basis of estimated
susceptibilities.
2- Unlike the gravitational force, which is always attractive, the magnetic force can be either
attractive or repulsive. Thus, mathematically, monopoles can assume either positive or
negative values.
3- Unlike the gravitational case, single magnetic point sources (monopoles) can never be
found alone in the magnetic case. Rather, monopoles always occur in pairs. A pair of
magnetic monopoles, referred to as a dipole, always consists of one positive monopole and
one negative monopole.
56. 56
4- A properly reduced gravitational field is always generated by subsurface variations in
rock density. A properly reduced magnetic field, however, can have as its origin at least
two possible sources. It can be produced via an induced magnetization, or it can be
produced via a remanent magnetization. For any given set of field observations, both
mechanisms probably contribute to the observed field. It is difficult, however, to
distinguish between these possible production mechanisms from the field observations
alone.
5- Unlike the gravitational field, which does not change significantly with
time, the magnetic field is highly time dependent.
