130513=GAPH_Magnetic

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.