Apparatus for the measurement of magnetic susceptibility and its anisotropy

52 Pages·2011·14.99 MB·English

GEOLOGINEN TUTKIMUSLAITOS
GEOLOGICAL SURVEY OF FINLAND
TUTKIMUSRAPORTTI N:o 28
REPORT OF INVESTIGATION No. 28
Maunu Puranen and Risto Puranen
Apparatus for the measurement of magnetic
susceptibility and its anisotropy
Espoo 1977
GEOLOGINEN TUTKIMUSLAITOS
GEOLOGICAL SURVEY OF FINLAND
Tutkimusraportti N:o 28
Report of investi gation No. 28
Maunu Puranen and Risto Puranen
APPARATUS FOR THE MEASUREMENT OF MAGNETIC SUSCEPTIBILITY
AND ITS ANISOTROPY
Espoo 1977
Puranen, M. and Puranen, R. 1977. Apparatus for the measure
ment of magnetic susceptibility and its anisotropy. Geologic
al Survey of Finland, Report of Investigation No. 28. 46 pages,
11 figures, 5 tables and one appendix.
The paper describes two apparatuses for measuring mag
netic susceptibility and its anisotropy. Both apparatuses
contain an oscillator (f = 1025 Hz), a bridge unit (two induct
ance coils and two resistors), and a detector system (lock-in
amplifier and output meter). Special attention was paid to
mechanical stability and thermal insulation of the bridge unit
to minimize measurement errors due to mechanical shocks and
variations in external temperature.
The theory of the a.c. bridge is treated in detail. For
mulae are derived for calculating scale constants for samples
of various size. The effects of external and internal sources
of error on measuring accuracy are discussed. The apparatuses
were calibrated by several methods, which are compared. A new
and simple calibration method, applying small ferrite balls,
is introduced.
Apparatus No. 1 is chiefly used in measuring the aniso
tropy of susceptibility. The samples are commonly cylindrical
in shape (height 29 mm, diameter 32 mm). The absolute accuracy
6
of the device is about 1 percent and its sensitivity 1.4 x 10-
SI units. Apparatus No. 2 is employed to measure the suscepti
bility of hand specimens with volumes mostly between 100 and
3
300 cm . This is a more rapid process, since the specimens
need not be prepared into a specific shape before measurements.
Both apparatuses have been in routine use for over a year and
have proved to be reliable.
ISBN 951-690-079-8
CONTENTS
Introduction .......................................... 5
Description of the apparatus .....•...........••••..... 6
Power source and bridge unit ...•..........•.•..•.. 7
Detector system and balancing of the bridge ..•...• 9
Sample rotating system and measurement practice ... 10
Theory of the bridge ...••. ...••...•.•...•...••..•••••• 12
Change in coil inductance caused by a rock sample. 12
Output voltage caused by inductance change ••.•.••• 15
Scale constants for a small and a large sample •••• 17
Small sampIe .................................. 17
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Large sample 18
Demagnetization effect .••••...••.••....•.•..•••....••. 22
Calibration by different methods 26
Paramagnetic s al t s ............................... . 27
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferrite ball 29
A small coil and a variable capacitor •••.......••• 30
Comparison of calibration methods ••..•.•.•••.••••• 33
Discussion on errors in susceptibility measurements ••• 34
Disturbing external fields ....•....•...••.....•..• 35
Temperature variation and mechanical shocks •.••••• 36
Noise in bridge and amplifier ••.••••...•.• ~ ..•..•• 38
Precision of susceptibility measurements •...•...•. 39
Variations in susceptibility caused by inhomogeneity
and anisot ropy of rocks ..•..•....•.••..•••••.••••• 40
Concl usions ........................................... 41
Acknowledgemen ts ...................•.••.•••..........• 43
References ............................................ 43
Appendix 1. Calculation of the magnetic field of a
cylindrical coil
--- - - -----------------------
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INTRODUCTION
Data on the magnetic susceptibility of rocks are es
sential for geophysical and geological investigations, par
ticularly for the geological interpretation of magnetic maps.
In susceptibility measurements, devices are currently used
that are based on various principles (astatic magnetometer,
spinner magnetometer, ballistic methods, a.c. bridge methods,
etc.). These methods have been reviewed, for example by Nagata
(1961), Collinson and Creer (1967) and Col1inson (1975).
In principle, the anisotropy of susceptibility can be
measured with the above instruments, if their sensitivity and
accuracy are sufficient. In anisotropy investigations, it is
often necessary to measure susceptibility differences of a
few percent between various directions in a weakly magnetic
sampIe. Devices used for these measurements have been described
by Girdler (1961), Fuller (1967), Graham (1967), Christie and
Symons (1969) and Jelinek (1973). The anisotropy measurements
give information about the magnetic fabric of rock and soil
specimens. This knowledge can be further used in studies of
the direction and intensity of magma flows (igneous rocks),
deformation (metamorphic rocks), paleocurrents (sedimentary
formations) and of glacial flow (till deposits) . To date, the
method has not been applied very often, but it seems to be
gaining in popularity.
The present paper describes a susceptibility measuring
system, based on a low frequency a.c. bridge, and developed
in recent years in the Geological Survey of Finland. The con
struction of the device was initiated by the lack of a com
mercial instrument suitable for measuring rock sampIes of
various size (dimensions 2 to 10 cm). When the work started
in 1974, one of the chief expectations was that the meter
could be used in the fabric analysis of til1 sampIes
from northern Finland, a hope that was later fulfilled (Pu
ranen 1977). Two a.c. bridge apparatuses, similar in princi
pIe, were constructed for studying sampIes of various size.
Device No. 1 is suitable for measuring susceptibility in
- 6 -
samples up to 6 cm in diameter and anisotropy in specimens
whose diameter and height do not exceed 3.2 cm. Device No. 2
is mainly employed for the susceptibility determinations of
larger hand specimens (maximum dimensions about 10 cm).
Maunu Puranen is responsible for the general design of
the apparatuses and the treatment of the a.c. bridge theory.
Risto Puranen developed the system for anisotropy determina
tions, calibrated the instruments and wrote the programs
pertinent to the numerical calculations.
Throughout this work, the SI system of units is used.
DESCRIPTION OF THE APPARATUS
The measuring apparatus consists of five units: oscil
lator, bridge unit, lock-in amplifier, digital voltmeter,
sample holder and rotating system (Fig. 1). The lock-in am
plifier and digital voltmeter are regular commercial instru
ments. The other units were constructed in our laboratory.
Lock - in Digital
amplifier voltmeter
Oscit tat or t--E.....;:o:;":s:,,,::,c_--+---J f11I
Ii
f=1025Hzr-----~~ 11 r-----------T-----__~
11 Eo
1 I __- ---''--_. .....
Bridge unit
Fig. 1. Block diagram of measurement system.
- 7 -
The principle of operation is as follows. First the
bridge is balanced without a sample. Then a sample ·is in
serted into the coil of the bridge, which becomes unbalanced.
The susceptibility of the sample in the direction of the
coil axis is proportional to the output voltage. Hence, the
measurement is based on the deviation method, in which the
bridge is not rebalanced during the measurement. The other
method using rebalancing is that of compensation. The devi
ation method is generally considered quicker, but less ac
curate. We chose this method because of the large sets of
samples to be treated. The compensation method is applied
in measuremens of anisotropy requiring high precision.
Power source and bridge unit
The first power source was a model 200 CD Hewlett
Packard oscillator. It was not, however, altogether satis
factory and was later replaced by an LC-oscillator construct
ed by Mr. K. Sulkanen. This oscillator has a very stable fre
quency (1025 Hz) and output voltage, which was adjusted to
give E = 10 V for the feeding voltage of the bridge. The
o
value E = 1 V can also be selected for strongly magnetic
o
samples. The components of the bridge unit are mounted into
an inner box made of PVC sheets (thickness 1 cm). The box
together with aseparate thick base plate is placed inside
an outer box. The space between the boxes is filled with a
layer of styrox (25 mm thick) for thermal insulation (Fig. 2).
The bridge unit includes a small ferrite core trans
former, which is electrostatically shielded. The bridge is
the regular symmetric Maxwell bridge (Hague 1946) with two
inductances and two resistances. The resistances R and R
l 2
(5000 ohms each) are wound bifilarly using manganine wire
with a low temperature coefficient of resistance (20 ppm/oc) .
The copper wire coils L and L of the bridge are of as sim
l 2
ilar construction as possible. Apparatus 1 uses 0.35 mm
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1
6
5
Fig. 2. Bridge unit box of apparatus No. 1: coi1s 1 and 2,
inner box 3 with rubber 1egs 4, base p1ate 5, styrox 1ayer
6 and outer box 7. Dimensions in mm.
B
3
A ~
r-.
('I
3 ['I
1 • I"
~ 4 ('I
I ~r- \- i'
2' i'
/ i'
...... 1
I 1\
4 I i'
.......... i'
,
i' N
/,2 I i' N
i' N
1\
i'
I ~
I ~ ('I
'r" I"-
I'
I'
I
~l
I
I ~~l J~
~
l-
1~-116 I
76 ..
Fig. 3. Coi1s of A) apparatus No. 1 and B) apparatus No. 2:
quartz glass tube 1, pyrex glass tube 1', winding 2 with 5184
turns, winding 2' with 3990 turns, p1astic ring 3 and p1astic
cover 4. Dimensions in mm.
- 9 -
enamelled copper wire and apparatus 2 0.40 mm wire. The di
mensions of the coils are shown in Fig. 3. The distances be
tween the coil axes (26 cm in apparatus 1 ana 32 cm in appa
ratus 2) are long enough to minimize the mutual effect of
their magnetic fields. To eliminate external electric dis
turbances the coils and the entire system were shielded with
a coating of silver paint, which made hand capacity effects
negligible. To reduce the magnetic disturbances, the axes of
the coils were oriented parallel to each other and the coils
connected "astatically" so that the disturbance voltages tend
to cancel each other.
Detector system and the balancing of the bridge
The signal voltage 6E and the reference voltage E
ref
are fed through separate coaxial cables to the lock-in am
plifier (Type 401A, Brookdeal Electronics Ltd). The amplifier
is characterized by a very narrow band width and correspond
ing low noise voltage. It can be used for measuring both the
in-phase and out-of-phase components of the signal voltage.
The amplifier includes several measuring ranges (1 ]JV - 100 mV) "
a variable time constant (10 ms - 100 s) and a variable phase
0 0
angle (0 - 180 ). The high input impedance of the amplifier
does not affect the balance of the bridge. To increase read-
ing accuracy, the output voltage of the amplifier is measured
with a separate digital voltmeter (Data Precision, Model 245
Digital Multimeter).
Despite the high symmetry of the bridge constructed
(LI = L and R = R within an accuracy of 0.1 %), there are
2 l 2
slight differences between the components owing to parasitic
capacitances and inductances. Therefore, to balance the bridge
small additional adjustments are needed. These can be done
by coupling, parallel to the bridge resistance R or R , a
l 2
large resistance (about 1 Mn) and a small capacitance (about
100 pF), whose values can be varied. The values are adjusted