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Júlia Carvalho 2013/2014
Applied Geophysics for the
Hydrocarbon Exploration
Magnetic Methods
Introduction
Man has been systematically observing the earth's
magnetic field for almost 500 years
Earth’s Magnetic Potential Field
The magnitude and direction
of the magnetic field is
governed by positive (south)
and negative (north) poles.
Magnitude varies by a factor
of two from equator to pole.
Earth’s Potential Fields
Earth’s gravity field is simple compared to the
magnetic field.
Lines of the force for the gravity field are
directed toward the center of the Earth while
magnetic field strength and direction depend on
the Earth’s positive and negative poles.
Geophysical exploration techniques that employ
both gravity and magnetic are passive.
The acquisition, reduction, and interpretation of
gravity and magnetic observations are very
similar.
Qualitative and quantitative assessment of
magnetic anomalies more difficult and less
intuitive than gravity anomalies.
Differences Between Gravity and Magnetics
The fundamental parameter that controls gravity variations of interest to us
is rock density. The densities of rocks and soils vary little from place to
place near the surface of the earth.
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.
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.
Differences Between Gravity and Magnetics
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.
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 2 possible sources:
it can be produced via an induced magnetization
or it can be produced via a remnant 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.
Unlike the gravitational field, which does not change significantly with time, the
magnetic field is highly time dependent.
Bar Magnets
Two poles: “north” and “south”
Like poles repel
Unlike poles attract
Magnetic poles cannot be isolated
No magnetic monopoles exist in
nature
Earth is a Big Magnet
The North pole of a small magnet
points towards geographic North
because Earth’s magnetic South
pole is presently up there.
Earth’s Magnetic Field
It surrounds the Earth
Has north and south magnetic poles
Is detected by compasses
Is recorded in rocks and minerals as they cool
Is generated in the Earth’s liquid outer core as it spins and produces electrical currents
Earth’s field similar to
that for bar magnet (left)
Magnetic N and S is not the same as geographic N and S poles (bar magnet “tilted”)
Earth’s Magnetic Field
Change in magnetic north relative to
true north
1831-2001 migration of magnetic north
1580-1970
Consequence of rotation of outer core
Earth’s Magnetic Field
Reverses over time (north and south poles flip). Magnetic field lines reverse
“reversed” polarity: north is south and south is north
After next reversal,
compass needle will
point south
“normal” polarity: north is north and south is south
Earth’s Magnetic Field
How do rocks and minerals acquire magnetism?
rocks and minerals at high temperatures (e.g. molten) must cool
through their Curie temperatures
• above Curie temperature, atoms are random
• below Curie temperature, atoms align in domains that are independent of each other
• below Curie temperature, atoms align with magnetic field if one is present (e.g. Earth)
Earth’s Magnetic Field
How do rocks and minerals acquire magnetism?
rocks and minerals that cool through Curie temperature and stay below
that temperature through time record magnetic field AT THE TIME OF
THEIR COOLING
Paleomagnetism: study of ancient magnetic
fields in rocks reconstruction of past fields
Earth’s Magnetic Field
Re-construct “normal” and “reversed” for lava sequence
Earth’s Magnetic Field
Magnetic material below
“adds” magnetism and
creates positive anomaly
Magnetic anomalies occur in local field from magnetic rock
below surface (similar to gravity anomalies)
Magnetic rocks include
iron ore, gabbro, granite
Earth’s Magnetic Field
Removal of magnetic material from near surface causes negative anomaly
(example is normal faulting)
Magnetic properties of materials of interest
Basement: tends to be igneous or metamorphic, thus greater magnetic
properties.
Soils and other weathered products: because magnetic minerals tend to
weather rather rapidly compared to quartz, will get reduction of magnetic
materials with weathering.
Man-made objects: iron and steel.
Ore deposits: many economic ores are either magnetic, or associated with
magnetic minerals.
Magnetization
Induced magnetization Ji
When a material is exposed to a magnetic field H, it acquires an induced
magnetization. These are related through the magnetic susceptibility, .
Factors affecting the magnetic susceptibility include:
The electron spin.
Number of electrons within the outer shell - pair or odd?
Remnant magnetization Jr
The remnant of past magnetic field that have acted on the material.
Ji H.
Magnetization
Three types of magnetic materials:
Paramagnetic
Diamagnetic
Ferromagnetic
Magnetization
Diamagnetic substance:
Acquisition of SMALL induced magnetization
OPPOSITE to the applied field.
The magnetization depends linearly on the
applied field and reduces to zero on removal
of the field
Magnetization
Paramagnetic substance:
The susceptibilities of paramagnetic
substances are SMALL and POSITIVE.
The magnetization depends linearly on the
applied field and reduces to zero on removal
of the field
Can only be observed at relatively low
temperatures. The temperature above
which paramagnetism is no longer
observed is called the Curie Temperature.
Magnetization
Ferromagnetic substance:
The path of the magnetization as a function
of the applied field is non-linear and is called
hysteresis loop.
Magnetization that can be orders of
magnitude larger than for the paramagnetic
solids.
Upon removal of the magnetizing field,
magnetization does not return to zero but
retains a record of the applied field.
Like paramagnetism, ferromagnetism is
observed only at temperatures below the
Curie temperature.
Natural Remnant Magnetization (NRM)
In situ magnetization of rocks is the vector sum of two
components:
J Ji Jr .
NRM is the remnant magnetization present in a rock sample prior to
laboratory treatment. It depends on the geomagnetic field and geological
processes during rock formation and during the history of the rock.
remnant
induced
J
Ji
Jr
Natural Remnant Magnetization (NRM)
For a rock to acquire remnant magnetization, what type of
material must be present?
Jr Jrprimary Jr
secondary.
Three forms of primary NRM:
Thermo-remnant magnetization: acquired during cooling from high
temperature.
Chemical-remnant magnetization: formed by growth of ferromagnetic
grains below the Curie temperature.
Natural Remnant Magnetization (NRM)
Detrital-remnant magnetization: acquired during accumulation of
sedimentary rocks containing detrital ferromagnetic minerals.
Natural Remnant Magnetization (NRM)
Secondary NRM:
Results from chemical changes affecting ferromagnetic minerals, exposure to
nearby lighting strikes, or long-term exposure to the geomagnetic field
subsequent to rock formation.
NRM - Geological Applications
Fold and conglomerate tests:
(with black arrows indicating directions of NRM)
Question: was NRM acquired prior to or after the conglomerate formation?
Solution: random distribution of NRM indicate that NRM was acquired prior to the
conglomerate.
NRM - Geological Applications
Fold and conglomerate tests:
Question: was NRM acquired prior to or after folding?
Solution: improved grouping of NRM upon restoring the limbs of the fold indicate that
NRM was acquired prior to folding.
(with black arrows indicating directions of NRM)
Field Survey
Strength of magnetic field above
an anomaly in the North Pole.
Field Survey
Strength of magnetic field above
an anomaly in the Equator.
Field Survey
Strength of magnetic field above
an anomaly in the latitude 45
degrees.
Field Survey
In conclusion, it is more difficult
to visually interpret magnetic
anomalies than gravity anomalies.
These visual problems, however,
present no problem for the
computer modeling algorithms
used to model magnetic
anomalies.
Temporal Variations
Magnetic readings taken at the same location at different times
will NOT yield the same results.
Temporal variations are classified according to the rate of occurrence and
source:
Polarity reversal: 103 - 106 years
Secular variations: years
Diurnal variations: hours-days
Magnetic storms: minutes-hours
Temporal Variations: Polarity Reversal
The Cretaceous Superchron
Temporal Variations: Secular Variations
Slow changes in magnetic north over time. Shown below are
the declination and inclination of the magnetic field around
Britain from the years 1500 through 1900.
Temporal Variations: Diurnal Variations
These variations occur over the course of a day, and are related to changes
in the Earth's external magnetic field. Shown below is the typical variations
in the magnetic data recorded at a single location (Boulder, Colorado) over
a time period of two days.
Can be on the order of 20 to 30 nT per day and should be corrected for
when conducting exploration magnetic surveys.
Temporal Variations: Magnetic Storms
Occasionally, magnetic activity in the ionosphere will abruptly increase.
These storms correlates with enhanced sunspot activity. The magnetic field
observed during such times is highly irregular and unpredictable.
In this example, the magnetic field has varied by almost 100 nT in a time
period shorter than 10 minutes!! Exploration magnetic surveys should not
be conducted during magnetic storms.
Temporal Variations: Practical Implications
Unlike the gravitational field, the magnetic field can vary quite erratically
with time.
Most investigators conduct magnetic surveys using two magnetometers.
One is used to monitor temporal variations of the magnetic field
continuously at a chosen base station, and the other is used to collect
observations related to the survey proper.
Unlike gravimeters, magnetometers show no appreciable instrument drift.
By recording the times at which each magnetic station readings are made
and subtracting the magnetic field strength at the base station recorded at
that same time, temporal variations in the magnetic field can be eliminated.
The resulting field then represents relative values of the variation in total
field strength with respect to the magnetic base station.
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