Electric Methods in Geophysics
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Questions and Answers

Which of the following factors can produce spontaneous potentials?

  • Geothermal Gradient (correct)
  • Air quality
  • Volcanic eruptions
  • Photosynthesis

Spontaneous potentials are exclusively produced by geothermal activity.

False (B)

Name one application of mapping related to groundwater.

Geological mapping

Spontaneous potentials may arise from _______ differences.

<p>mineralization</p> Signup and view all the answers

Match the following causes of spontaneous potentials with their descriptions:

<p>Electro-chemical action = Interaction between chemical reactions and electricity Geothermal activity = Heat from the Earth’s interior affecting potential Bioelectric generation of vegetation = Electric potentials generated by living plants Mineralization differences = Variations in mineral content leading to potential changes</p> Signup and view all the answers

What is located above the reduction zone in the provided content?

<p>Water Table (C), Oxidation Zone (D)</p> Signup and view all the answers

The mechanism of self-potential is associated with polarizing electrodes.

<p>False (B)</p> Signup and view all the answers

Name the two zones mentioned in the content.

<p>Oxidation Zone and Reduction Zone</p> Signup and view all the answers

The _____ electrodes are mentioned in relation to the mechanism of self-potential.

<p>nonpolarizing</p> Signup and view all the answers

Match the terms with their corresponding descriptions:

<p>Oxidation Zone = Upper layer where oxidation occurs Water Table = Boundary between saturated and unsaturated soil Reduction Zone = Lower layer where reduction takes place Nonpolarizing electrodes = Electrodes that do not polarize during measurements</p> Signup and view all the answers

What is a possible risk associated with managing survey profiles and Cu SO4?

<p>Errors may accumulate (C)</p> Signup and view all the answers

What characteristic should the wire used in SP surveys have?

<p>It should be strong, hardy, and of low resistance. (C)</p> Signup and view all the answers

Electrodes must be leaped over one another when managing survey profiles.

<p>True (A)</p> Signup and view all the answers

The resolution of the millivolt meter can only be 1.0 mV.

<p>False (B)</p> Signup and view all the answers

What substance is mentioned that requires careful management during the survey?

<p>Cu SO4</p> Signup and view all the answers

What should be maintained similar between the two pots to avoid creating an unwanted potential?

<p>temperature</p> Signup and view all the answers

Care must be taken to keep track of the __________ when managing survey profiles.

<p>errors</p> Signup and view all the answers

Match the following terms related to survey profiles with their descriptions:

<p>Electrodes = Fixed potential polarity Cu SO4 = Chemical compound requiring careful management Errors = Can accumulate over time Leap-frog technique = Separates electrodes at a fixed distance</p> Signup and view all the answers

One electrode is kept __________ at a base station on unmineralised ground.

<p>fixed</p> Signup and view all the answers

Match the following characteristics related to the SP survey:

<p>Wire = Strong and low resistance Millivolt meter = Resolution of 0.1 or 1.0 mV Electrode position = One fixed and one movable Temperature = Similar for both pots</p> Signup and view all the answers

What is the primary method of interpretation used in surveys?

<p>Qualitative interpretation (B)</p> Signup and view all the answers

Anomalies in the survey profile are typically found at higher elevations.

<p>False (B)</p> Signup and view all the answers

What does the depth in surveys refer to?

<p>The width of the half-amplitude.</p> Signup and view all the answers

In survey profiles, the anomaly minimum is observed over the ______ body.

<p>anomalous</p> Signup and view all the answers

Match the following terms with their definitions:

<p>Anomaly Minimum = The lowest point of deviation in a survey profile Half-Amplitude = The measure of depth in surveys Qualitative Interpretation = Analysis focused on understanding patterns and meanings Survey Profile = A graphical representation of survey data</p> Signup and view all the answers

What does the shape of the anomaly indicate?

<p>The attitude of the body (C)</p> Signup and view all the answers

A steep slope and positive tail of the anomaly can be found on the up-dip side.

<p>False (B)</p> Signup and view all the answers

What type of anomaly is observed over water flowing out of a fracture zone?

<p>Self Potential anomaly</p> Signup and view all the answers

The steep slope and positive tail of the anomaly are associated with the ______ side.

<p>down-dip</p> Signup and view all the answers

Match the following terms with their descriptions:

<p>Symmetry = Indicates uniformity in anomaly Asymmetry = Indicates irregularities in anomaly Steep slope = Indicates a rapid change in the anomaly Positive tail = A characteristic of anomaly indicating a trend</p> Signup and view all the answers

Flashcards

Spontaneous Potentials

Electrical signals generated naturally, often related to geothermal activity, mineral differences, or biological processes.

Geothermal Gradient

Rate of change in temperature with depth beneath the Earth's surface.

Mineralization Differences

Variations in the concentration or type of minerals in the earth.

Electrochemical Action

Chemical reactions producing electrical signals.

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SP Profile

A map showing the pattern or distribution of spontaneous potentials.

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Oxidation Zone

The region above the water table where oxidation of materials occurs.

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Water Table

The level below which the ground is saturated with water.

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Reduction Zone

The region below the water table where reduction reactions take place.

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Self-Potential (S-P)

A method used to determine the electrochemical potential of a subsurface environment.

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Nonpolarizing Electrodes

Electrodes that do not significantly alter the electrochemical potential of the environment when measuring.

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Cu SO4

Copper sulfate, a chemical compound often used in solutions for electrical conductivity measurements.

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Electrode Potential

The voltage difference between an electrode and the solution it is immersed in.

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Two Electrodes

Two conductive materials with different electrical potentials used to measure the conductivity of a solution.

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Fixed Separation

Maintaining a constant distance between two electrodes.

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Leap-Frogging Electrodes

Moving one electrode over the other along a survey line.

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Survey wire material properties

Survey wires need to be strong, durable, and have low resistance.

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Millivolt meter resolution

The millivolt meter should measure potential differences with a precision of 0.1 mV or 1.0 mV.

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Pot temperature control

The temperatures of the measuring pots should be kept similar to avoid unwanted electrical potentials.

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Electrode placement (fixed)

One electrode needs to be kept fixed at a point on unmineralised ground.

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Electrode placement (moving)

The second electrode gets moved to measure the potential difference along the ground.

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Anomaly Symmetry

The symmetry or asymmetry of the anomaly provides information on the attitude of the body, revealing whether it is tilted or upright.

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Down-Dip Anomaly

An anomaly located on the lower side of an inclined rock formation, typically characterized by a steep slope and a positive tail.

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Positive Tail of Anomaly

The part of the anomaly that extends downward and has a higher value compared to the surrounding area.

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Self-Potential Anomaly

An electrical potential difference that occurs naturally due to the movement of fluids, such as water flowing out of a fracture zone.

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Fracture Zone

An area where rocks have cracked or broken, allowing fluid flow.

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Survey Profile

A graphical representation showing the distribution of a geological feature or property measured at various locations.

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Anomaly Minimum

The lowest point in a survey profile where the measured value is significantly different from its surroundings.

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Anomalous Body

A geological feature that causes an anomaly in a survey profile, often due to variations in mineral composition or density.

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Displaced Downhill

When the anomaly minimum is shifted downwards from the expected position due to geological factors.

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Depth (Half-Amplitude)

The distance from the peak of an anomaly in a survey profile to the point where the value is half of its maximum value.

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Study Notes

Electric Methods

  • Self-potential (SP) method
  • Resistivity methods
  • Induced polarization

Self-Potential (SP) Method

  • Based on surface measurement of natural potentials caused by electrochemical reactions in the subsurface.
  • Doesn't require injecting electric currents into the ground.
  • Useful in groundwater and geothermal investigations, engineering applications (e.g., mapping seepage flow), and geological mapping (e.g., delineating shear zones and dams near water tables).
  • Simpler to operate in the field compared to other methods
  • Can be used to locate massive metallic deposits and disseminated ores.

SP Method: Mechanism

  • Spontaneous potentials are generated by:
    • Mineralization differences
    • Electrochemical action
    • Geothermal activity
    • Bioelectric generation of vegetation

SP Method: Equipment

  • Nonpolarizing electrodes: Crucial for accurate measurements without introducing artificial potentials.
  • Survey wire: Needs to be strong, durable, and have low resistance for reliable data transmission.
  • Millivolt meter: Must have high resolution (e.g., 0.1 or 1.0 mV) to detect subtle voltage variations.

SP Method: Survey Procedure

  • Remote Electrode: One electrode is kept fixed at a base station on unmineralised ground.
  • SP Data Recording: The other electrode is moved along the survey profile.
  • Dipole System: Two electrodes with fixed spacing are leap-frogged, one over the other, for mapping.
  • Temperature Control: Maintaining similar temperatures in the electrodes prevents creation of unwanted potentials (e.g., 0.5 mV increase/degree Celcius for Cu/CuSO4).
  • Polarity Tracking: Careful documentation and tracking of the potential polarity is necessary.

SP Method: Interpretation

  • Primarily qualitative, focusing on identifying patterns and anomalies related to subsurface features rather than precise quantitative measurements.
  • Depth: Determined by the width of the half-amplitude of the anomaly .
  • Symmetry: The shape (symmetry or asymmetry) of the anomaly reveals information about the orientation (attitude) of the subsurface body (underground).
  • Tail of the anomaly: The slope of the anomaly's positive tail generally aligns with the down dip side of the body. The steep slope and length of this tail are also used for interpretation.

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Related Documents

Self Potential Method PDF

Description

Explore the electric methods used in geophysics, focusing on the Self-Potential (SP) method, resistivity techniques, and induced polarization. This quiz covers aspects ranging from equipment to mechanisms and applications in geological mapping and groundwater investigations.

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