A bit of a read for some further research to help the...

A bit of a read for some further research to help the discussion.

Dipolar effect is what you need to look at...
here's a hint read from start to finish and you may be enlightened...


FS¡V236¡V95 OCTOBER 1997
Introduction to Potential Fields: Magnetics
Introduction
Magnetic and gravity exploration, also
referred to as ¡§potential fields¡¨ exploration, is
used to give geoscientists an indirect way to
¡§see¡¨ beneath the Earth¡¦s surface by sensing different
physical properties of rocks (magnetization
and density, respectively). Gravity and magnetic
exploration can help locate faults, mineral and
petroleum resources, and ground-water reservoirs.
Potential-field surveys are relatively inexpensive
geophysical methods and can quickly
cover large areas of ground.
What is magnetism?
The force a magnet exerts on an iron filing or
the force the Earth¡¦s magnetic field exerts on the
needle of a compass are two common examples
of magnetism. A magnetic field has both intensity
and direction. The strength of the magnetic force
depends on the amount of magnetic material
present and its distance and direction relative to
the detector. The Earth¡¦s magnetic field probably
is caused by movement of partially molten iron in
the Earth¡¦s outer core. The magnetic field
strength increases from 25,000 nanoteslas (nT) at
the magnetic equator to 70,000 nT at the magnetic
poles. (One nanotesla equals 1 gamma, and
105 gammas equals 1 oersted.) The Earth¡¦s magnetic
field changes in intensity and direction
slowly over time.
Like all dipole magnets, the Earth has a magnetic
field (also called the core or main field) that
has a North and South Pole. The angle between a
compass needle and true north is called the magnetic
declination. The north-seeking end of a
compass needle that is free to orient itself in an
up-down direction will point down in the Northern
Hemisphere and up in the Southern Hemisphere.
The angle between the needle and horizontal
is called the magnetic inclination.
How do scientists measure the magnetic field?
Geoscientists measure the Earth¡¦s magnetic
field intensity to an accuracy of 0.1 nT using
magnetometers. Magnetic surveys usually are
conducted from an aircraft. Ground surveys
also can be made and are especially useful for
locating buried metallic objects such as waste
barrels.
An aeromagnetic survey is flown using an
aircraft (airplane or helicopter) to which a
magnetometer is attached. The most common
aircraft magnetometers measure the total
intensity of the magnetic field, but not its
direction, along continuous flight lines that are
a fixed distance apart. The aircraft can be
flown at a constant barometric elevation (such
as 9,000 ft above sea level) or at a constant
distance above the ground (such as 500 ft
above terrain, also called a ¡§draped¡¨ survey).
Magnetometers measure all effects of the
Earth¡¦s magnetic field. Because the field
changes slowly over time, models of this field,
called the International Geomagnetic Reference
Field (IGRF), are updated every 5 years.
The IGRF for the time and location of a magnetic
survey is calculated and removed. The
magnetic field is also subject to complex shortterm
variations such as magnetic storms. For
purposes of correcting aeromagnetic survey
data, a base magnetometer records magnetic
levels at a fixed location within the study area,
and these variations are removed from the airborne
magnetic data. What remains is the magnetic
field largely associated with magnetic
minerals in crustal rocks.
What is a magnetic anomaly?
Although the force of the Earth¡¦s magnetic
field is not very strong, it is large enough to
magnetize certain kinds of rocks that contain
iron or other magnetite-bearing minerals. Magnetic
anomalies, therefore, are the differences
between measured magnetic values and the
values predicted from the model of the Earth¡¦s
core field. They are caused by variations in
magnetization of crustal rocks. Measurements
of many rock samples show that most sediƒn
mentary rocks are generally not magnetic, whereas
igneous rocks rich in iron minerals often are
very magnetic.
Because of the dipolar nature of magnetism, a
single magnetic body can cause either a positive
or negative magnetic anomaly or both (especially
if the Earth¡¦s magnetic field at the time of rock
formation is reverse of the present-day field), or
it can cause a more complex magnetic pattern
caused by tilting of the magnetic body itself.
What is a magnetic anomaly map?
A magnetic anomaly map is made from
recorded flight-line measurements across the area
of interest from which the Earth¡¦s field has been
removed. These data are then gridded so that the
flight-line magnetic data are converted to a representation
of the magnetic field at equally spaced
locations along and between the flight lines.
Magnetic anomaly maps can be shown as color
images¡Xwith warm colors (reds and oranges)
showing areas of higher magnetic values and cool
colors (blues and greens) showing lower values¡X
or as contour line maps, where each contour
line follows a constant magnetic value.
What is rock magnetism?
Magnetic susceptibility is a rock property
describing the amount of magnetizable material
in a rock. It is a dimensionless unit, and 1 electromagnetic
unit (emu) in the cgs (centimeter-gramsecond)
system equals 4„jƒnSI (System International)
units.
Rocks containing magnetic minerals may
have two kinds of magnetization: induced and
remanent. Induced magnetization exists only in
the presence of an external magnetic field.
Remanent magnetization, however, is frozen
within the rock, and the rock remains magnetized
in a field-free area. Sometimes the direction of
the Earth¡¦s field at the time of rock formation or
alteration is preserved. The study of rock paleomagnetism
is based on this property and, in some
places, can be used to show rock movement
through time. Studies of remanently magnetized
rock show that the magnetic North and South
Poles have reversed through geologic time.
Remanent magnetization, therefore, can also give
some indication of the age of magnetization. Both
induced and remanent magnetization vanish
above the Curie temperature (about 580„aC for
magnetite).
What is a derivative magnetic map?
A magnetic map contains information about
both rock magnetization changes across an area
and depth to the source of the anomaly. Maps
can be derived from the original magnetic
anomaly grid by using tools to enhance parts of
the magnetic field.
In general, the deeper the magnetic source,
the broader and gentler the gradients of the
resulting anomaly will be. Also, in general, the
shallower the magnetic object, the sharper and
narrower the resulting anomaly. Derivative
maps can show anomalies that have been filtered
for size and shape to emphasize either
shallow or deep sources. Another type of
derivative map, called ¡§reduced to the pole,¡¨
can correct the anomalies for inclination and
declination differences caused by location and
produce the magnetic field of the bodies as
though the area were moved to the North Pole.
This simplifies complex anomaly shapes
caused by dipole effects of the Earth¡¦s magnetic
field and centers the anomaly over its
source. Another derivative method can magnify
magnetic gradients, places where the magnetic
field changes from high to low¡Xthese
places often mark edges of rock units or faults.
All of these maps can be used together to make
a geologic interpretation.
Additional information
U.S. Geological Survey Open-File Report
95¡V77 lists many USGS computer programs
and databases used to create magnetic maps:
http://minerals.er.usgs.gov
Information on the availability of magnetic
maps and data in specific areas and other general
information on USGS airborne coverage
can be obtained from:
Pat Hill
U.S. Geological Survey
Box 25046, MS 964, Denver Federal Center
Denver, CO 80225
(303) 236-1343
[email protected]
(136.177.80.14)
Viki Bankey (same address)
(303) 236-1348
[email protected]
Vicki Langenheim
U.S. Geological Survey
345 Middlefield Road, MS 939
Menlo Park, CA 94025
(415) 329-5313
[email protected]
(130.118.4.68)