Electrical Resistivity Methods Questions and Answers

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.

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:

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

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.

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.

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.

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.

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.

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.