Match the following Resistivity curve types :III(a) \(P_{1} \gt P_{2} \lt P_{3}\) High-Low-High(i) Q type(b) \(P_{1} \lt P_{2} \lt P_{3}\) Low-Low-High(ii) A type(c) \(P_{1} \lt P_{2} \gt P_{3}\) Low-High-Low(iii) K type(d) \(P_{1} \gt P_{2} \gt P_{3}\) High-Low-Low(iv) H type

The correct matching is (a)-(iv), (b)-(ii), (c)-(iii), (d)-(i).Three-layer sounding curves are named by the resistivity of the middle layer relative to the outer ones:\(\rho_1 \gt \rho_2 \lt \rho_3\), a conductive layer in the middle, is the H type.\(\rho_1 \lt \rho_2 \lt \rho_3\), resistivity rising steadily with depth, is the A type.\(\rho_1 \lt \rho_2 \gt \rho_3\), a resistive layer in the middle, is the K type.\(\rho_1 \gt \rho_2 \gt \rho_3\), resistivity falling steadily, is the Q type.Option B swaps A and Q, and options C and D use some types twice, which a one-to-one matching cannot do.

Electrical Resistivity Methods Questions and Answers

Q: Six electrode configuration, termed axial parallel, perpendicular, Radial, Azimuthal and equatorial spreads are associated with which of the following array.

The six spreads belong to the dipole array.In dipole electrode configurations the current electrodes form one closely spaced pair and the potential electrodes another, separated by a much larger distance. Depending on how the potential dipole is placed and oriented relative to the current dipole, the arrangements are named axial (or polar), parallel, perpendicular, radial, azimuthal and equatorial spreads.The Wenner and Schlumberger arrays are collinear four-electrode arrays with fixed geometric rules and no such family of orientations. The Lee partitioning method is a modification of the Wenner array with an extra central electrode.

Q: In which of the following methods of electrical resistivity exploration, all four electrodes are placed in a line, but the distance between current electrodes is maintained equal to or more than five times the distance between potential electrodes ?

The arrangement described is the Schlumberger configuration (printed Schlamberger).In the Schlumberger array the four electrodes are in line and symmetrical about the centre, and the potential electrodes are kept close together: the current electrode spacing AB is at least five times the potential spacing MN. For a sounding, the current electrodes are moved outward while MN stays fixed for as long as the signal allows.In the Wenner array all four electrodes are equally spaced, so AB is exactly three times MN. In the dipole-dipole array the current pair and potential pair are separated as two dipoles, not with the potential pair between the current electrodes. The Orellana-Mooney name refers to published master curves for interpreting soundings, not to an array.

Q: The average electrical conductivity of the Earth is about :

The key gives about \(10^{-3}\) S/m.The conductivity of the Earth's surface materials varies widely, from about 4 S/m in sea water to less than \(10^{-4}\) S/m in dry crystalline rock. For average ground, the value used in radio-wave propagation and in many geophysics texts is of the order of \(10^{-3}\) S/m (resistivity of about 1000 ohm-m).Values of 10 and 1 S/m are typical of sea water and highly saline material, not of the Earth on average, and \(10^{-9}\) S/m is close to an insulator such as dry quartz or the atmosphere.

Q: In general, electrical methods are commonly employed for :

Electrical methods are used mainly for relatively shallow subsurface exploration.Resistivity, induced polarisation and self-potential surveys send currents or measure natural potentials near the surface, and their depth of investigation is limited to a fraction of the electrode spread, usually tens to a few hundred metres. They are widely used for groundwater, engineering site investigation, mineral prospecting and environmental work.Deep and very deep targets are studied by seismic, gravity and magnetotelluric methods. Groundwater is a major application but not the only one, so option D is too narrow.

Q: In Wenner configuration the electrodes are :

In the Wenner configuration the electrodes are equally spaced.The four electrodes are placed in a line in the order current-potential-potential-current, with the same spacing a between each adjacent pair. The apparent resistivity is then:\[ \rho_{a} = 2\pi a \frac{\Delta V}{I} \]Option C describes the Schlumberger array, in which the current electrodes are spread far apart while the potential electrodes stay close together; option D describes no standard array. Since the Wenner spacing is uniform, 'unequally spaced' is also wrong.

Q: The apparent resistivity sounding curve representing the resistivity structure p1 > p2 < p3 < p4 of a four-layer Earth is :

The curve is an HA-type curve.A four-layer curve is named by splitting it into two overlapping three-layer sections and naming each with the standard letters: H for a minimum in the middle (\(\rho_{1} > \rho_{2} < \rho_{3}\)), A for a steady rise (\(\rho_{1} < \rho_{2} < \rho_{3}\)), K for a maximum in the middle and Q for a steady fall.Here the first three layers, \(\rho_{1} > \rho_{2} < \rho_{3}\), form an H curve, and layers 2 to 4, \(\rho_{2} < \rho_{3} < \rho_{4}\), form an A curve, so the curve is HA (the paper writes \(\rho\) as p). HK would need \(\rho_{3} > \rho_{4}\). KH and KQ both need \(\rho_{2}\) to be a maximum, which it is not.

Q: The Schlumberger configuration is used in :

The Schlumberger configuration is used in electric profiling, that is, electrical resistivity surveys.In the Schlumberger array, four electrodes lie in a line: two outer current electrodes and two closely spaced inner potential electrodes. The current electrodes are moved apart step by step for vertical electrical sounding, and the array is also used for profiling. It is one of the standard arrays of resistivity work, along with the Wenner and dipole-dipole arrays.Magnetic and geomagnetic surveys use magnetometers, not electrodes. Seismic profiling uses an energy source and geophones. None of these uses an electrode configuration.

Six electrode configuration, termed axial parallel, perpendicular, Radial, Azimuthal and equatorial spreads are associated with which of the following array.

A.Wenner array
B.Schlumberger array
C.Dipole array
D.Lee-Partitioning

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

The six spreads belong to the dipole array.

In dipole electrode configurations the current electrodes form one closely spaced pair and the potential electrodes another, separated by a much larger distance. Depending on how the potential dipole is placed and oriented relative to the current dipole, the arrangements are named axial (or polar), parallel, perpendicular, radial, azimuthal and equatorial spreads.

The Wenner and Schlumberger arrays are collinear four-electrode arrays with fixed geometric rules and no such family of orientations. The Lee partitioning method is a modification of the Wenner array with an extra central electrode.

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Match the following Resistivity curve types :

I	II
(a) \(P_{1} \gt P_{2} \lt P_{3}\) High-Low-High	(i) Q type
(b) \(P_{1} \lt P_{2} \lt P_{3}\) Low-Low-High	(ii) A type
(c) \(P_{1} \lt P_{2} \gt P_{3}\) Low-High-Low	(iii) K type
(d) \(P_{1} \gt P_{2} \gt P_{3}\) High-Low-Low	(iv) H type

A.(a)-(iv), (b)-(ii), (c)-(iii), (d)-(i)
B.(a)-(iv), (b)-(i), (c)-(ii), (d)-(iii)
C.(a)-(i), (b)-(ii), (c)-(i), (d)-(ii)
D.(a)-(ii), (b)-(iii), (c)-(iv), (d)-(ii)

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

The correct matching is (a)-(iv), (b)-(ii), (c)-(iii), (d)-(i).

Three-layer sounding curves are named by the resistivity of the middle layer relative to the outer ones:

\(\rho_1 \gt \rho_2 \lt \rho_3\), a conductive layer in the middle, is the H type.
\(\rho_1 \lt \rho_2 \lt \rho_3\), resistivity rising steadily with depth, is the A type.
\(\rho_1 \lt \rho_2 \gt \rho_3\), a resistive layer in the middle, is the K type.
\(\rho_1 \gt \rho_2 \gt \rho_3\), resistivity falling steadily, is the Q type.

Option B swaps A and Q, and options C and D use some types twice, which a one-to-one matching cannot do.

Discuss in Forum

In which of the following methods of electrical resistivity exploration, all four electrodes are placed in a line, but the distance between current electrodes is maintained equal to or more than five times the distance between potential electrodes ?

A.Wenner configuration
B.Dipole-Dipole array
C.Schlamberger configuration
D.Orellana-money configuration

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The arrangement described is the Schlumberger configuration (printed Schlamberger).

In the Schlumberger array the four electrodes are in line and symmetrical about the centre, and the potential electrodes are kept close together: the current electrode spacing AB is at least five times the potential spacing MN. For a sounding, the current electrodes are moved outward while MN stays fixed for as long as the signal allows.

In the Wenner array all four electrodes are equally spaced, so AB is exactly three times MN. In the dipole-dipole array the current pair and potential pair are separated as two dipoles, not with the potential pair between the current electrodes. The Orellana-Mooney name refers to published master curves for interpreting soundings, not to an array.

Discuss in Forum

The average electrical conductivity of the Earth is about :

A.10 s/meter
B.1 s/meter
C.\(10^{-3}\) s/meter
D.\(10^{-9}\) s/meter

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

The key gives about \(10^{-3}\) S/m.

The conductivity of the Earth's surface materials varies widely, from about 4 S/m in sea water to less than \(10^{-4}\) S/m in dry crystalline rock. For average ground, the value used in radio-wave propagation and in many geophysics texts is of the order of \(10^{-3}\) S/m (resistivity of about 1000 ohm-m).

Values of 10 and 1 S/m are typical of sea water and highly saline material, not of the Earth on average, and \(10^{-9}\) S/m is close to an insulator such as dry quartz or the atmosphere.

Discuss in Forum

In general, electrical methods are commonly employed for :

A.Relatively shallow subsurface exploration
B.Deep subsurface exploration
C.Very deep subsurface exploration
D.Groundwater exploration only

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

Electrical methods are used mainly for relatively shallow subsurface exploration.

Resistivity, induced polarisation and self-potential surveys send currents or measure natural potentials near the surface, and their depth of investigation is limited to a fraction of the electrode spread, usually tens to a few hundred metres. They are widely used for groundwater, engineering site investigation, mineral prospecting and environmental work.

Deep and very deep targets are studied by seismic, gravity and magnetotelluric methods. Groundwater is a major application but not the only one, so option D is too narrow.

Discuss in Forum

In Wenner configuration the electrodes are :

A.Equally spaced
B.Unequally spaced
C.Space between current electrodes is more than the potential electrodes
D.Space between current electrodes is less than the potential electrodes

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

In the Wenner configuration the electrodes are equally spaced.

The four electrodes are placed in a line in the order current-potential-potential-current, with the same spacing a between each adjacent pair. The apparent resistivity is then:

\[ \rho_{a} = 2\pi a \frac{\Delta V}{I} \]

Option C describes the Schlumberger array, in which the current electrodes are spread far apart while the potential electrodes stay close together; option D describes no standard array. Since the Wenner spacing is uniform, 'unequally spaced' is also wrong.

Discuss in Forum

The apparent resistivity sounding curve representing the resistivity structure p1 > p2 < p3 < p4 of a four-layer Earth is :

A.HA-type
B.HK-type
C.KH-type
D.KQ-type

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: A

The curve is an HA-type curve.

A four-layer curve is named by splitting it into two overlapping three-layer sections and naming each with the standard letters: H for a minimum in the middle (\(\rho_{1} > \rho_{2} < \rho_{3}\)), A for a steady rise (\(\rho_{1} < \rho_{2} < \rho_{3}\)), K for a maximum in the middle and Q for a steady fall.

Here the first three layers, \(\rho_{1} > \rho_{2} < \rho_{3}\), form an H curve, and layers 2 to 4, \(\rho_{2} < \rho_{3} < \rho_{4}\), form an A curve, so the curve is HA (the paper writes \(\rho\) as p). HK would need \(\rho_{3} > \rho_{4}\). KH and KQ both need \(\rho_{2}\) to be a maximum, which it is not.

Discuss in Forum

The Schlumberger configuration is used in :

A.Electric profiling
B.Magnetic profiling
C.Seismic profiling
D.Geomagnetic profiling

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: A

The Schlumberger configuration is used in electric profiling, that is, electrical resistivity surveys.

In the Schlumberger array, four electrodes lie in a line: two outer current electrodes and two closely spaced inner potential electrodes. The current electrodes are moved apart step by step for vertical electrical sounding, and the array is also used for profiling. It is one of the standard arrays of resistivity work, along with the Wenner and dipole-dipole arrays.

Magnetic and geomagnetic surveys use magnetometers, not electrodes. Seismic profiling uses an energy source and geophones. None of these uses an electrode configuration.

Discuss in Forum

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