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TRAINING ON SURFACE EXPLORATION STUDIES FOR GEOTHERMAL RESOURCES AND DEVELOPMENT
OF CONCEPTUAL MODELS
UNDER THE AUSPICES OF INTERIM PROJECT COORDINATION UNIT OF THE AFRICA GEOTHERMAL
CENTER OF EXCELLENCE
ASMARA ERITREA 8-17 APRIL 2019
Magnetic surveying
Data acquisition, processing and interpretation
Gylfi Páll Hersir, Iceland GeoSurvey
Thursday April 11th 2019
Magnetic methods are widely used in geothermal
exploration and often applied together with other
geophysical methods and geological mapping for
mapping geological structures:
✓ Location and depth of concealed instrusives, dikes and faults
✓ Deriving depth to basement
✓ Paleomagnetism
Direct detection of thermal effects:
✓ Locating hydrothermally-altered areas
✓ Curie point depth analysis
The geomagnetic field of the earth is mostly caused by slowly
varying electric currents in the earth‘s liquid core. Additionally
we have the Rock Magnetic Field (induced and remanent)
which is due to magnetic iron oxides such as magnetite in the
surroundings rocks.
The geomagnetic lines around the earth are similar to those by
a bar magnet (J) situated at its centre aligned semi-parallel to
the rotation axis (ω). Taken from, Geirfinnur Jónsson and Leó
Kristjánsson, 2002.
The components of the
geomagnetic field.
B is the total field vector,
while Z is the vertical and
H the horizontal
component.
Declination (D) is the
horizontal angle between
magnetic and geographic
north. Inclination (I) is the
dip of the field.
A-p.159
The present-day magnetic field of the Earth, with the intensity
shown in thousands of nano Teslas (or ).
The total field vector varies from 25,000 nT at the equator to >
60,000 nT at the poles.
J-p.44
The dip (inclination) of the present-day magnetic field of the
Earth.
J-p.44
The numbers for Iceland:
The total field is 52,399 nT and has increased about 700
nT since 1965. The declination (in Reykjavík) is -13°38'
and changes around 0.25°/year. The inclination is rather
stable, about 75° 30’.
In Kenya the total field is 30,000 to 34,000 nT, the
inclination is 22-30° and the declination is close to 0°.
Eritrea: The total field is little less than 40,000 nT.
The variation of the inclination of the total magnetic field with
latitude.
A-p.160 & J-p.43
The location of the magnetic poles is not stable in time, even in
our time scale. In Iceland the annual changes of the declination
is 0.25 per year.
The magnetic declination is the deviation from the geographic
north. The changes in magnetic declination as observed from
London over four centuries:
Q-p.35
Magnetics – changing location of the magnetic north pole
ICELAND
GEOSURVEY
Diurnal variations (micropulsations): Quiet days (Q): 20-50 γ
and disturbed days (D)-magnetic storms: 100-1000 γ
Secular variations: Slowly changes with time, both in intensity
and location of the poles
Reversals of the magnetic poles;
Major time periods with the same direction of the
magnetic field: Magnetic epoches:
•Brunhes: 0-0.78 m.y. ago normal
•Matuyama: 0.78-2.5 m.y. ago reversed
•Gauss: 2.5-3.3 m.y. ago normal
•Gilbert: 3.3-4.5 m.y. ago reversed
ICELAND GEOSURVEYA simplified geological and geothermal map of Iceland
The first map of magnetic
anomalies (published 1961)
revealed alternating strips of high
and low values of the magnetic
field – a mirror image. The strips
run parallel to the axes of the
mid-ocean ridges, often with
offsets by fracture zones; typical
amplitudes of the anomalies are
±500 nT. This example is from
the Reykjanes ridge.
R-p.428
Unit
H is expressed in A/m.
The cgs unit of magnetic field strength is Gauss (G) = 10-4 T.
The tesla is too large a unit to express magnetic anomalies
therefore the unit nano Tesla is used (1 nT = 10-9T). In c.g.s
units the equivalent gamma () = 10-5G.
Units for magnetic field anomalies: nT = gamma ()
When a material is placed in a
magnetic field it acquires a
magnetization in the same direction as
the field.
It is lost when the field is removed.
This is referred to as induced
magnetization.
Elements of material with elementary
dipoles align in the direction of the
external field.
The magnetic moment M of a dipole
with poles of a strength m at a distance
l apart is given by M=ml
M=ISn (current carrying coil)
A-p.156
The intensity of the induced magnetization Ji of a material, is
the dipole moment per unit volume, Ji = M/(LA), where M is the
magnetic moment.
The induced intensity of magnetization is proportional to the
strength of the magnetization force H, Ji =kH, where k is the
magnetic susceptibility.
Since both Ji, and H have the unit A/m, susceptibility is
dimensionless.
Histogram showing
mean values and range
in susceptibility for the
main rock types.
A-p.159
When a multi-domain grain is placed in a weak external
magnetic field, a growth of domains is caused in the direction of
the field. This will go back as the field is removed.
When a stronger field is applied the changes of some domains
are irreversible. This inherited magnetization remaining is
known as remanent or permanent magnetization Jr.
Application of even stronger magnetization causes magnetic
saturation.
Primary remanent magnetization may by acquired either as igneous
rock solidifies and cools through the Curie temperature
(Thermoremanent Magnetization, TRM) or as magnetic particles are
deposited by sedimentation and they align with the current magnetic
field (Depositional Remanent Magnetization DRM).
Minerals loose their spontaneous magnetization as they reach their
Curie temperature at which point thermal agitation exceeds magnetic
ordering.
Any rock containing magnetic minerals may have both induced
and remanent magnetism Ji and Jr. The Königsberger ratio, or Q-
factor, is the ratio of remanent over induced intensities of
magnetization Q = Jr / Ji.
The direction of this magnetization, Jr, may not necessarily be in
the direction of the induced vector. A vector diagram illustrates the
relationship of induced, remanent and total magnetization
components
A-p.158
Remanent magnetisation in volcanic rocks is usually
stronger than induced magnetization
Q: Plutonic rock: 1-3
Q: Volcanic rock: ~10
Q: Sedimentary, metamorphic <1
Magnetite – most common magnetic mineral
• Most rock-forming minerals do not contribute to the rock magnetism. The Fe-Ti-O group possesses a series of magnetic minerals from magnetite (Fe3O4) to ulvöspinel (Fe2TiO4). The other common iron oxide haematite (Fe2O3) is antiferromagnetic and do not give rise to magnetic anomalies.
• Magnetite the most common magnetic mineral has a Curie temperature of 578°C.
ICELAND
GEOSURVEY
Magnetometers
Modern surveying work is mostly done with the help of the
Proton precession type magnetometer, or the newer “optically
pumped” meters.
Both types measure the strength of the total magnetic field.
An accurcy of 1 nT can be obtained at 1 sec intervals,and better
with advanced aircraft equipment.
Some meters can be connected to GPS postitoning instruments
to record internally both position and magnetic value.
• The principle of the proton magnetometer,
with its sensing device (bottle) filled with a
liquid rich of hydrogen atoms. These acts as
small dipoles and align parallel to the
geomagnetic field Be.
• A current is passed through the coil (Bp)
creating a new magnetic field and aligns the
dipoles in a new direction.
• The current is turned off and the dipoles
return to the Be alignment by spiraling back.
• Measuring the socalled Larmor-frequency
(about 2 kHz) and accurately knowing the
gyro-magnetic ratio (p ), the strength of the
total field can be determined as.
• f= p Be /2πA-p.163
Field instruments provide an
absolute reading of ±1 nT.
The sensor does not have to be
accurately orientated, although it
should ideally lie at an appreciable
angle to the total field vector.
Magnetic
surveying in the
Domes area,
Kenya in
December 2010
Base station magnetometer recordings are used for monitoring
and correcting diurnal variations for accurate work. In land
surveys a fixed base station can be periodically visited during the
day. For aeromagnetic surveys an alternative is many crossover
points of different survey lines (readings at the same location at
different times through the day). Magnetic “storms” may make
operations impossible.
J-p.45
An alternative method of
removing a regional gradient
is the use of a trend analysis,
in small survey areas the
regional field trend is
approximately linear.
Elevation and terrain corrections are rarely applied, as the
vertical gradient is 0.015-0.03 nT/m.
A-p.166
Interpretation of magnetic data can be more complex than
gravimetric data.
Magnetic anomalies are controlled by more parameters, such as
susceptibility, remanent magnetization (Q-factor) and its orientation.
Finding a unique model is difficult as the same anomaly can be
explained with different constellations of bodies and magnetic
parameters (data from the Barbados Ridge).A-p.167
Depth estimations are one of the main objectives of magnetic
interpretation. As demonstrated the amplitude of the anomaly
changes with depth.Simple direct methods exist which can give
an estimate (within 20%) of the depth; this is adequate for a
preliminary assessment.
The straight-slope method – “h” comes from the distance
over which the variation is linear, this is often roughly equal to
the depth.
J-p.58
Reduction to the pole: Conversion of the anomalies
into their equivalent form at the “north pole”; done with
map filtering tecniques. Involves rotating the magnetic
vectors to the vertical.
Upward and downward continuation - filtering
Magnetic methods for geothermal
• Aeromagnetic survey (covering large area)
• Ground magnetic survey (detailed survey of
limited area)
Measure spatial variation of Earth’s magnetic field
Aeromagnetic map
of the Hengill area,
SW Iceland
Magnetic low coincides
with surface
manifestations
(demagnetisation due
to alteration)
Ground magnetic survey from
a low-temperature geothermal
area at Hrafnagilshreppur in
northern Iceland. Measured
with a proton precession
magnetometer. Individual
profiles are shown; 20 m are
between profiles and 5 m
between points. Both faults
and dykes can be seen as
peaks (anomalies) forming
lineations across the profiles. Taken from Bára Björgvinsdóttir,
1982.
Geological interpretation
of the magnetic map from
Hrafnagilshreppur, based
on the contour map on the
previous slide.Taken from
Bára Björgvinsdóttir, 1982.