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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.