Current Density and Equipotentials Quiz

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Questions and Answers

What is the Resistivity Method used for?

Measuring current flow

Measuring electrical potential

Measuring electromagnetic fields

Studying earth resistivity (correct)

What is Induced Polarization (IP)?

A passive method that measures current flow

An active method that measures transient variations in potential (correct)

A method that measures the presence and quality of pore fluids

A method that measures electromagnetic fields

What phenomena can be measured using electrical methods?

Current flow and electrical potential (correct)

Gravity and magnetism

Temperature and pressure

Chemical composition and crystal structure

What is the advantage of Wenner array over Schlumberger array?

Wenner array requires less sensitive voltmeters Signup and view all the answers

What is the disadvantage of Wenner array?

Wenner array can be more susceptible to near-surface lateral variations in resistivity Signup and view all the answers

What is the limitation of DC soundings?

Limited to simple, horizontally layered structures Signup and view all the answers

What is the empirical relationship between current and voltage defined by Georg Ohm in 1827?

V = IR Signup and view all the answers

What is the constant of proportionality in Ohm's law called?

Resistance Signup and view all the answers

What is resistivity and what is the symbol used to indicate it?

Resistivity, ρ Signup and view all the answers

What controls the measured resistivities of Earth materials?

The movement of charged ions in pore fluids Signup and view all the answers

What can lead to low resistivities in crystalline rock?

Fractures filled with fluids Signup and view all the answers

What is an equipotential line?

A line where the voltage drops are constant along circular lines centered at the electrode Signup and view all the answers

What is current density?

The amount of current passing through a unit area of an equipotential surface Signup and view all the answers

What is the formula for electric potential at distance away from current source on surface?

V(r)=ρI/2πr Signup and view all the answers

What are the most commonly used electrodes for DC resistivity surveys?

Aluminum, copper, or steel rods Signup and view all the answers

What is the simplest geometry of electrode placement for resistivity sounding surveys?

Wenner survey Signup and view all the answers

What is the advantage of using a Schlumberger survey over a Wenner survey?

Schlumberger surveys are faster to conduct than Wenner surveys Signup and view all the answers

What is the equation for computing the apparent resistivity in a Wenner survey?

∆V/i Signup and view all the answers

What is the disadvantage of using a Schlumberger survey for large current electrode spacings?

The voltmeters used need to be very sensitive Signup and view all the answers

What is the maximum value of a that can be used in a Schlumberger survey?

2s/5 Signup and view all the answers

What is the Electromagnetic (EM) method used for?

Locating conductive base-metal deposits, buried pipes and cables, and near-surface geophysical mapping Signup and view all the answers

What is the Magnetotelluric (MT) method used for?

Determining electrical properties of materials at relatively great depths inside the Earth Signup and view all the answers

What is the Self Potential (SP) method used for?

Measuring naturally occurring electrical potentials associated with the weathering of sulfide ore bodies Signup and view all the answers

What equipment is needed for conducting an SP survey?

A high-impedence voltmeter and some means of making good electrical contact with the ground Signup and view all the answers

What is the purpose of the Magnetotelluric (MT) technique?

To estimate subsurface distribution of electrical resistivity Signup and view all the answers

Study Notes

Resistivity Method

Used for subsurface exploration, mapping geological structures, and mineral resource assessment.
Measures resistance to electric current in the ground to infer material properties.

Induced Polarization (IP)

A geophysical method measuring the delayed response of ground materials to applied electrical current.
Useful for detecting the presence of clay, metal ores, and groundwater.

Electrical Methods Phenomena

Measure potential differences, current flow, resistivity, and induced polarization.
Effective for identifying subsurface features and assessing material properties.

Wenner Array Advantage

Provides uniform current distribution and more accurate resistivity measurements.
Better suited for detecting lateral variations in resistivity.

Wenner Array Disadvantage

Can produce more noise and may be less effective in complex geological setups than other methods.

DC Soundings Limitation

Depth of investigation is limited by the spacing between electrodes and the resistivity of subsurface materials.

Ohm's Law

Defines the relationship between current (I) and voltage (V) as V = IR, where R is resistance.
Introduced by Georg Ohm in 1827.

Constant of Proportionality in Ohm's Law

Called resistance (R), measured in ohms.

Resistivity Definition

Indicates a material's ability to resist electric current.
Symbol used for resistivity is ρ (rho).

Factors Affecting Measured Resistivities

Controlled by mineral composition, fluid content, temperature, and porosity of Earth materials.

Low Resistivities in Crystalline Rock

Can be caused by the presence of conductive minerals, water saturation, or alteration minerals.

Equipotential Line

A line along which the electrical potential is constant, indicating equal potential points.

Current Density

Defined as the electric current per unit area of cross-section, indicating how much current flows through a specific area.

Electric Potential Formula

Potential (V) at a distance (r) from a current source: V = I / (2πr) for a line source.

Common Electrodes for DC Resistivity Surveys

Copper and copper-sulfate electrodes are frequently used for reliable measurements.

Simplest Geometry for Resistivity Sounding

The Wenner configuration, which aligns electrodes in a straight line and equidistant spacing.

Schlumberger Survey Advantage

More efficient for deep sounding due to better current penetration and improved depth resolution.

Apparent Resistivity Equation for Wenner Survey

Given by ρa = 2πaΔV/I, where ρa is apparent resistivity, a is electrode spacing, ΔV is potential difference, and I is current.

Schlumberger Survey Disadvantage

Ineffective at large current electrode spacings due to potential measurement errors and increased environmental noise.

Maximum Value of 'a' in Schlumberger Survey

Typically limited to 100 meters, beyond which the configuration may yield unreliable data.

Electromagnetic (EM) Method

Used for geological mapping, mineral exploration, and groundwater investigation through electromagnetic field responses.

Magnetotelluric (MT) Method

A passive geophysical technique measuring the Earth's natural electric and magnetic fields for subsurface conductivity mapping.

Self Potential (SP) Method

Measures natural voltage in the ground related to subsurface fluid movement and mineralization.

Equipment for SP Survey

Typically requires voltmeters, electrodes, and data acquisition systems to measure and record potential differences.

Purpose of Magnetotelluric (MT) Technique

Aims to explore deep geological structures and identify variations in subsurface conductivity related to different rock types and fluids.

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Description

Test your knowledge on current density and equipotentials with this quiz! Learn more about the voltage drop imposed by resistivity in a medium and how it affects the current passing through it. Challenge yourself by answering questions on circular lines and equipotentials. Get familiar with the terminology and concepts related to current density and equipotentials.