5.2 Resistivity and induced polarization methods

Resistivity and induced polarization methods are powerful tools for peering into the Earth's subsurface. These techniques measure electrical properties of rocks and minerals, revealing hidden structures and resources beneath our feet.

From groundwater exploration to mineral prospecting, these methods offer valuable insights. By analyzing how electricity flows through different materials, geophysicists can map out underground features and identify potential areas of interest for further investigation.

Electrical Resistivity Surveys

Principles and Factors Affecting Resistivity

Top images from around the web for Principles and Factors Affecting Resistivity

• 

  HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original

  Is this image relevant?

• 

  Typical Values for Rocks — Electromagnetic Geophysics View original

  Is this image relevant?

• 

  SE - Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ... View original

  Is this image relevant?

• 

  HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original

  Is this image relevant?

• 

  Typical Values for Rocks — Electromagnetic Geophysics View original

  Is this image relevant?

1 of 3

Top images from around the web for Principles and Factors Affecting Resistivity

• 

  HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original

  Is this image relevant?

• 

  Typical Values for Rocks — Electromagnetic Geophysics View original

  Is this image relevant?

• 

  SE - Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ... View original

  Is this image relevant?

• 

  HESS - Exploring the regolith with electrical resistivity tomography in large-scale surveys ... View original

  Is this image relevant?

• 

  Typical Values for Rocks — Electromagnetic Geophysics View original

  Is this image relevant?

1 of 3

• Electrical resistivity surveys measure the electrical resistance of subsurface materials by injecting an electric current into the ground and measuring the resulting potential difference between two points
• The resistance of subsurface materials depends on factors such as:
  • Rock type (sedimentary, igneous, metamorphic)
  • Porosity (fraction of void space in a rock)
  • Fluid content (water, hydrocarbons)
  • Temperature (higher temperatures generally decrease resistivity)
• Resistivity is the inverse of conductivity and is measured in ohm-meters ($\Omega \cdot m$)

Applications and Detection Capabilities

• Resistivity surveys are based on the principle that different subsurface materials have different electrical properties, allowing the detection of variations in lithology, fluid content, and geological structures
• Applications of resistivity surveys include:
  • Groundwater exploration (identifying aquifers)
  • Environmental studies (detecting contaminant plumes)
  • Geotechnical investigations (characterizing soil and rock properties)
  • Mineral exploration (locating ore bodies)
• Resistivity surveys can help identify:
  • Aquifers (high resistivity zones indicating freshwater-bearing formations)
  • Contaminant plumes (low resistivity anomalies suggesting the presence of conductive fluids)
  • Geological boundaries (resistivity contrasts between different rock units)
  • Ore bodies (low resistivity anomalies associated with conductive minerals like sulfides)

Electrode Configurations

Common Electrode Arrays

• Electrode configurations refer to the arrangement of current and potential electrodes used in resistivity surveys
• The choice of configuration depends on the desired depth of investigation, resolution, and sensitivity to lateral and vertical variations
• Common electrode arrays include:
  • Wenner array
  • Schlumberger array
  • Dipole-dipole array
  • Pole-pole array

Characteristics and Applications of Electrode Arrays

• The Wenner array consists of four equally spaced electrodes, with the outer two electrodes injecting current and the inner two measuring potential
  • Provides good vertical resolution but limited depth of investigation
  • Suitable for shallow investigations and horizontal layering
• The Schlumberger array also uses four electrodes, but the spacing between the potential electrodes is much smaller than the spacing between the current electrodes
  • Offers greater depth of investigation compared to the Wenner array
  • Sensitive to vertical variations in resistivity
• The dipole-dipole array uses two pairs of closely spaced electrodes, with one pair injecting current and the other measuring potential
  • Sensitive to lateral variations and provides good resolution of near-surface features
  • Useful for detecting vertical structures and lateral changes in resistivity
• The pole-pole array uses two electrodes, one for current injection and one for potential measurement, with the other two electrodes placed at a theoretically infinite distance
  • Offers the greatest depth of investigation but lower resolution
  • Requires a large survey area and can be affected by noise and stray currents

Induced Polarization for Exploration

Principles and Measurements

• Induced polarization (IP) is a geophysical method that measures the capacitive properties of subsurface materials in addition to their resistivity
• IP occurs when an electric current is applied to the ground, causing the accumulation of charged particles at the interfaces between different materials, such as mineral grains and pore fluids
• The IP effect is measured by the chargeability, which is the ratio of the secondary voltage (measured after the current is switched off) to the primary voltage (measured during current injection)
  • Chargeability is usually expressed in milliseconds (ms) or as a percentage (%)

Applications in Mineral Exploration

• IP is particularly useful for mineral exploration because certain minerals, such as sulfides and clays, exhibit strong IP effects due to their electronic or membrane polarization properties
• The presence of disseminated sulfide minerals, even in small quantities, can produce significant IP anomalies, making IP surveys valuable for detecting and delineating mineral deposits
  • Examples of sulfide minerals detectable by IP include pyrite, chalcopyrite, and galena
• IP data can provide information about the type, concentration, and distribution of polarizable minerals, aiding in the identification of potential ore bodies and guiding drilling programs
• IP surveys are commonly used in the exploration of porphyry copper, disseminated gold, and massive sulfide deposits

Interpreting Resistivity and Induced Polarization Data

Data Representation and Inversion

• Interpretation of resistivity and IP data involves converting the measured apparent resistivity and chargeability values into a model of the subsurface electrical properties
• Apparent resistivity pseudosections are constructed by plotting the measured resistivity values against the electrode spacing and position
  • Pseudosections provide a qualitative representation of the subsurface resistivity distribution
• Inversion techniques, such as least-squares or robust inversion, are used to create a quantitative model of the true subsurface resistivity and chargeability from the apparent values
  • The inversion process seeks to minimize the difference between the measured and modeled data
• The resulting resistivity and chargeability models can be visualized as 2D or 3D sections or volumes, showing the spatial distribution of electrical properties in the subsurface

Integration and Interpretation

• Interpretation of the models involves identifying resistivity and chargeability anomalies, which may indicate variations in lithology, fluid content, or the presence of mineral deposits
  • Low resistivity anomalies may suggest the presence of conductive materials (clays, saline water, sulfides)
  • High chargeability anomalies often indicate the presence of polarizable minerals (sulfides, graphite)
• Integration of resistivity and IP data with other geological and geophysical information helps to constrain the interpretation and improve the understanding of subsurface structures and processes
  • Borehole logs provide direct information about lithology and mineralization
  • Seismic data can reveal structural features and stratigraphic boundaries
  • Geological maps provide a regional context for interpreting the geophysical data
• Combining multiple datasets allows for a more comprehensive and reliable interpretation of the subsurface geology and potential mineral resources