Magnetic Methods Questions and Answers

Q: The ionospheric magnetic field is because of :

The key gives Solar winds as the cause.The ionosphere's own magnetic field comes from electric currents flowing in the ionised upper atmosphere. That ionisation and the currents are produced by the Sun: its ultraviolet and X-ray radiation ionises the gas, and the solar wind interacting with the magnetosphere drives further currents, especially at high latitudes. These are the external, Sun-driven part of the geomagnetic field.Hydrogen fusion takes place in the Sun's core, not in the ionosphere, and 'helium bombardment on sodium' is not a mechanism for it. Fluctuations of the geomagnetic field are an effect of the ionospheric currents, not their cause.

Q: The current geomagnetic polarity is named as :

The present polarity interval is the Brunhes Normal chron.The earth's magnetic field has reversed many times. The current interval of normal polarity, the Brunhes chron, began about 780,000 years ago at the Brunhes-Matuyama reversal. It is named after Bernard Brunhes, who first found reversely magnetised rocks.Before it came the Matuyama reversed chron, which was mostly reversed, not normal, and before that the Gauss normal and the Gilbert reversed chrons, both older than about 2.6 million years. None of these is the current polarity.

Q: Magnetic survey data needs to be connected for which of the following effects ?

Magnetic survey data must be corrected for the diurnal variation caused by ionospheric changes.Electric currents in the ionosphere, driven by solar heating, make the earth's field at the surface change by tens of nanotesla over a day, and more during magnetic storms. A base-station magnetometer records this variation through the survey, and it is subtracted from the field readings so that the remaining anomalies come from the rocks.Polar wandering and the apparent polar wander path concern palaeomagnetism over millions of years, not a survey made over days. Normalising a 'sample depth' is not a standard magnetic survey correction; the usual others are the removal of the regional (IGRF) field and, in aeromagnetics, levelling.

Q: Which of the following is true statement in relation to proton precession magnetometer ?

The true statement is that it uses a proton-rich liquid such as water surrounded by a magnetising coil as detector.In a proton precession magnetometer, a current in the coil around a bottle of water or kerosene aligns the spins of the hydrogen nuclei (protons). When the current is switched off, the protons precess around the earth's field at a frequency proportional to its strength, about 2 kHz, and the coil picks up this signal. The frequency gives the total field very accurately.The protons are ordinary hydrogen nuclei, not radiation from uranium. The instrument measures the total field strength, so it is a scalar, not a vector, magnetometer, and it uses no permanent 'proton magnets'.

Q: Identify the correct statements from the following :(i) Proton precession magnetometer measures the total field(ii) Fluxgate magnetometer measures the total field(iii) Fluxgate magnetometer measures a Component of the field(iv) Proton precession magnetometer measures component of the field

Statements i and iii are correct, so the answer is (i) and (iii).A proton precession magnetometer measures the frequency at which protons in a liquid such as kerosene precess around the ambient field after a polarising field is switched off. That frequency is proportional to the strength of the field whatever its direction, so the instrument measures the total field (i).A fluxgate magnetometer uses ferromagnetic cores that saturate in an alternating field; the imbalance produced by the external field is sensitive to the field component along the core axis, so it measures a component of the field (iii). Statements ii and iv swap the two instruments.

Q: Which of the following relations between blocking temperature, Tb and Curie temperature Tc is correct ?

The key's answer is that Tb is always less than Tc.The Curie temperature is where a mineral loses its spontaneous magnetisation altogether. The blocking temperature is the temperature, on cooling, below which the magnetisation of a grain becomes fixed (blocked) and can record the ambient field as remanence. Since there is no spontaneous magnetisation to block at or above the Curie temperature, blocking can only happen below it; for large grains the blocking temperature approaches, but stays below, the Curie temperature.Blocking temperatures above the Curie temperature are impossible, so the two options allowing Tb greater than Tc are wrong.

Q: The magnetic inclination, I and magnetic latitude, λ are related by :

The relation is tan I = 2 tan λ.For a geocentric axial dipole, the horizontal and vertical field components at magnetic latitude λ are proportional to cos λ and 2 sin λ. The inclination is the angle whose tangent is their ratio:\[ \begin{aligned} \tan I &= \frac{Z}{H} = \frac{2\sin\lambda}{\cos\lambda} \\ &= 2\tan\lambda \end{aligned} \]This dipole equation is the basis of palaeomagnetism: measuring the inclination of a rock's remanence gives the palaeolatitude where it formed. The other forms lack the factor 2 or put it in the wrong place, and would give the wrong inclination at every latitude except the equator.

Q: The magnetic field at the equator, compared to that at the pole is :

The field at the equator is half that at the pole.For a dipole field the total intensity at magnetic latitude λ is\[ F = B_0 \sqrt{1 + 3\sin^{2}\lambda} \]At the equator, \(\lambda = 0\), so \(F = B_0\); at the pole, \(\lambda = 90^\circ\), so \(F = 2B_0\). The Earth's real field, about 30,000 nT at the equator and 60,000 nT near the poles, shows the same ratio.Twice and four times reverse or exaggerate the ratio, and a uniform field ('the same') would not be a dipole at all.

Q: Most sensitive instrument for magnetic survey is :

The most sensitive instrument is the optically pumped magnetometer.Optically pumped magnetometers, such as caesium vapour instruments, measure the field from the frequency of electron transitions in atoms aligned by light. They reach sensitivities of about 0.01 nT or better and sample rapidly, which makes them standard for high-resolution airborne and gradiometer surveys.The proton precession magnetometer is robust and absolute but less sensitive, about 0.1 to 1 nT. The fluxgate is a vector instrument of similar or lower sensitivity, and the mechanical magnetic field balance is the oldest and least sensitive of the four.

Q: Non-compensated spin moments and the positive exchange interaction of the co-directed spins results in :

The result is a ferromagnetic material.In ferromagnets such as iron, nickel and cobalt, atoms carry uncompensated (unpaired) spin moments, and a positive exchange interaction makes neighbouring spins line up parallel (co-directed). Large regions, the domains, are therefore spontaneously magnetised even without an external field, below the Curie temperature.In antiferromagnets the exchange interaction is negative, so neighbouring spins align antiparallel and cancel. Paramagnets have unpaired spins but no ordering interaction, so their moments point at random unless a field is applied. Diamagnets have no net atomic moment at all and are weakly repelled by a field.

The ionospheric magnetic field is because of :

A.Hydrogen ions fusion
B.He bombardment on Na
C.Solar winds
D.Fluctuation in Geomagnetic fields

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The key gives Solar winds as the cause.

The ionosphere's own magnetic field comes from electric currents flowing in the ionised upper atmosphere. That ionisation and the currents are produced by the Sun: its ultraviolet and X-ray radiation ionises the gas, and the solar wind interacting with the magnetosphere drives further currents, especially at high latitudes. These are the external, Sun-driven part of the geomagnetic field.

Hydrogen fusion takes place in the Sun's core, not in the ionosphere, and 'helium bombardment on sodium' is not a mechanism for it. Fluctuations of the geomagnetic field are an effect of the ionospheric currents, not their cause.

Discuss in Forum

The current geomagnetic polarity is named as :

A.Matuyama Normal
B.Gauss Normal
C.Gilbert Reversal
D.Brunhes Normal

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: D

The present polarity interval is the Brunhes Normal chron.

The earth's magnetic field has reversed many times. The current interval of normal polarity, the Brunhes chron, began about 780,000 years ago at the Brunhes-Matuyama reversal. It is named after Bernard Brunhes, who first found reversely magnetised rocks.

Before it came the Matuyama reversed chron, which was mostly reversed, not normal, and before that the Gauss normal and the Gilbert reversed chrons, both older than about 2.6 million years. None of these is the current polarity.

Discuss in Forum

Magnetic survey data needs to be connected for which of the following effects ?

A.Variation in magnetic indication due to polar wandering
B.Variation in magnetic declination with reference to distance from APW curve path.
C.Diurnal variation due to variation in Earth's magnetic field due to ionospheric changes resulting in variation in Earth's magnetic field intensity at its surface during course of day.
D.Variation in sample depth to be normalised to NSL.

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

Magnetic survey data must be corrected for the diurnal variation caused by ionospheric changes.

Electric currents in the ionosphere, driven by solar heating, make the earth's field at the surface change by tens of nanotesla over a day, and more during magnetic storms. A base-station magnetometer records this variation through the survey, and it is subtracted from the field readings so that the remaining anomalies come from the rocks.

Polar wandering and the apparent polar wander path concern palaeomagnetism over millions of years, not a survey made over days. Normalising a 'sample depth' is not a standard magnetic survey correction; the usual others are the removal of the regional (IGRF) field and, in aeromagnetics, levelling.

Discuss in Forum

Which of the following is true statement in relation to proton precession magnetometer ?

A.It uses protons from radioactive material such as uranium to detect magnetic fields
B.It is a vector magnetometer
C.It uses proton-rich liquid such as water surrounded by magnetising coil as detector.
D.It uses special proton magnets arranged perpendicular to each other.

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The true statement is that it uses a proton-rich liquid such as water surrounded by a magnetising coil as detector.

In a proton precession magnetometer, a current in the coil around a bottle of water or kerosene aligns the spins of the hydrogen nuclei (protons). When the current is switched off, the protons precess around the earth's field at a frequency proportional to its strength, about 2 kHz, and the coil picks up this signal. The frequency gives the total field very accurately.

The protons are ordinary hydrogen nuclei, not radiation from uranium. The instrument measures the total field strength, so it is a scalar, not a vector, magnetometer, and it uses no permanent 'proton magnets'.

Discuss in Forum

Identify the correct statements from the following :

(i) Proton precession magnetometer measures the total field
(ii) Fluxgate magnetometer measures the total field
(iii) Fluxgate magnetometer measures a Component of the field
(iv) Proton precession magnetometer measures component of the field

A.(i) and (iv)
B.(ii) and (iv)
C.(i) and (iii)
D.(iii) and (iv)

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

Statements i and iii are correct, so the answer is (i) and (iii).

A proton precession magnetometer measures the frequency at which protons in a liquid such as kerosene precess around the ambient field after a polarising field is switched off. That frequency is proportional to the strength of the field whatever its direction, so the instrument measures the total field (i).

A fluxgate magnetometer uses ferromagnetic cores that saturate in an alternating field; the imbalance produced by the external field is sensitive to the field component along the core axis, so it measures a component of the field (iii). Statements ii and iv swap the two instruments.

Discuss in Forum

Which of the following relations between blocking temperature, T_b and Curie temperature T_c is correct ?

A.\(T_{b}\) is always greater than \(T_{c}\)
B.\(T_{b}\) is always less than \(T_{c}\)
C.\(T_{b}\) is equal to or less than \(T_{c}\)
D.\(T_{b}\) is equal to or greater than \(T_{c}\)

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The key's answer is that T_b is always less than T_c.

The Curie temperature is where a mineral loses its spontaneous magnetisation altogether. The blocking temperature is the temperature, on cooling, below which the magnetisation of a grain becomes fixed (blocked) and can record the ambient field as remanence. Since there is no spontaneous magnetisation to block at or above the Curie temperature, blocking can only happen below it; for large grains the blocking temperature approaches, but stays below, the Curie temperature.

Blocking temperatures above the Curie temperature are impossible, so the two options allowing T_b greater than T_c are wrong.

Discuss in Forum

The magnetic inclination, I and magnetic latitude, λ are related by :

A.tan I = tan λ
B.tan I = 2 tan λ
C.tan I = ½ tan λ
D.tan I = tan 2λ

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

The relation is tan I = 2 tan λ.

For a geocentric axial dipole, the horizontal and vertical field components at magnetic latitude λ are proportional to cos λ and 2 sin λ. The inclination is the angle whose tangent is their ratio:

\[ \begin{aligned} \tan I &= \frac{Z}{H} = \frac{2\sin\lambda}{\cos\lambda} \\ &= 2\tan\lambda \end{aligned} \]

This dipole equation is the basis of palaeomagnetism: measuring the inclination of a rock's remanence gives the palaeolatitude where it formed. The other forms lack the factor 2 or put it in the wrong place, and would give the wrong inclination at every latitude except the equator.

Discuss in Forum

The magnetic field at the equator, compared to that at the pole is :

A.4 times
B.Twice
C.The same
D.Half

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

The field at the equator is half that at the pole.

For a dipole field the total intensity at magnetic latitude λ is

\[ F = B_0 \sqrt{1 + 3\sin^{2}\lambda} \]

At the equator, \(\lambda = 0\), so \(F = B_0\); at the pole, \(\lambda = 90^\circ\), so \(F = 2B_0\). The Earth's real field, about 30,000 nT at the equator and 60,000 nT near the poles, shows the same ratio.

Twice and four times reverse or exaggerate the ratio, and a uniform field ('the same') would not be a dipole at all.

Discuss in Forum

Most sensitive instrument for magnetic survey is :

A.Optically pumped magnetometer
B.Proton precession magnetometer
C.Magnetic field balance
D.Fluxgate magnetometer

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

The most sensitive instrument is the optically pumped magnetometer.

Optically pumped magnetometers, such as caesium vapour instruments, measure the field from the frequency of electron transitions in atoms aligned by light. They reach sensitivities of about 0.01 nT or better and sample rapidly, which makes them standard for high-resolution airborne and gradiometer surveys.

The proton precession magnetometer is robust and absolute but less sensitive, about 0.1 to 1 nT. The fluxgate is a vector instrument of similar or lower sensitivity, and the mechanical magnetic field balance is the oldest and least sensitive of the four.

Discuss in Forum

10.

Non-compensated spin moments and the positive exchange interaction of the co-directed spins results in :

A.Antiferromagnetic material
B.Diamagnetic material
C.Paramagnetic material
D.Ferromagnetic material

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

The result is a ferromagnetic material.

In ferromagnets such as iron, nickel and cobalt, atoms carry uncompensated (unpaired) spin moments, and a positive exchange interaction makes neighbouring spins line up parallel (co-directed). Large regions, the domains, are therefore spontaneously magnetised even without an external field, below the Curie temperature.

In antiferromagnets the exchange interaction is negative, so neighbouring spins align antiparallel and cancel. Paramagnets have unpaired spins but no ordering interaction, so their moments point at random unless a field is applied. Diamagnets have no net atomic moment at all and are weakly repelled by a field.

Discuss in Forum

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