Electrode Array Geometry and DOI

Depth of Investigation (DOI)

• DOI indicates how deep into the ground data can provide reliable information about subsurface resistivity.
• DOI is influenced by electrode array geometry.
• Wenner, dipole-dipole, and Schlumberger arrays have DOIs that are approximately 30%, 25%, and 20% of their current electrode separation, respectively.
• Electrode spacing can be manipulated to achieve sufficient depth.

Parameters Affecting the Depth of Investigation

• The length of the current electrodes (AB line) and the distance between the current electrodes (AB) and the potential electrodes (MN) influence the depth of investigation.
• Different electrode configurations have unique advantages and limitations for depth penetration, lateral resolution, and field setup.

Factors Affecting Measurements

• Resistivity of the Ground (ρ): High resistivity materials (like hard rock) produce stronger signals, while low resistivity materials (like clayey soils) produce weaker signals.
• Ground Water: The presence of groundwater can significantly alter resistivity measurements, as water is a good conductor of electricity.
• Soil Moisture: Soil moisture content can influence resistivity, as moist soil generally has lower resistivity than dry soil.
• Instrument Calibration: Proper calibration ensures accuracy and reliability of measurements, including verifying instrument settings, cables, and electrodes.
• Safety Considerations: Identify potential hazards at the site and implement safety protocols to mitigate risks associated with power lines, unstable terrain, or weather conditions.

Survey Design and Layout

• Electrode Configuration Selection: Choose appropriate electrode configurations like Wenner, Schlumberger, or dipole-dipole based on factors like depth and resolution requirements.
• Grid or Line Layout: Use a grid layout for large-area mapping or a line layout for detailed profiling of specific geological features.
• Electrode Spacing and Array Geometry: Plan electrode spacing and array geometry carefully to meet depth and resolution requirements. Wider spacing increases depth but can reduce resolution. Adjust spacing based on geological conditions.
• Survey Line Orientation: Orient survey lines perpendicular to the expected strike of subsurface structures to maximize contrast in resistivity readings.
• Measurement Stations: Distribute measurement stations systematically to ensure full coverage and consistent data acquisition.

Data Acquisition Techniques

• Current and Potential Electrode Placement: Install current electrodes at the ends of survey lines for uniform current injection, and place potential electrodes according to the chosen configuration to measure voltage differences.
• Current Injection: Maintain stable current magnitude throughout the survey to ensure consistent measurements. Fluctuations in current levels can lead to inaccuracies.
• Potential Measurement: Measure the voltage potential between pairs of potential electrodes and record readings at each station.
• Data Recording: Record data with a data logger or digital recording system, documenting relevant parameters like electrode spacing, instrument settings, and environmental conditions.
• Survey Progression: Progress systematically across the layout, maintaining consistent electrode spacing and following the designed configuration.
• Quality Control: Implement quality control measures to identify and correct potential issues, such as checking electrode contact resistance, instrument performance, and data consistency.
• Repeat Measurements: Repeat measurements or use multiple electrode configurations to enhance accuracy and reliability.
• Post-Processing: Use data inversion and interpretation techniques to create a subsurface resistivity model from raw data, allowing for the identification of geological features or anomalies.

Field Effects Upon Measurements

• Effect of a Cliff or Road Cut: Cliffs and road cuts disrupt the flow of electrical current, leading to inaccurate measurements. Place electrodes at least one electrode spacing away from these features.
• Effect of a Wrong A or B Moving Electrodes:
  • Shortened Distance: Abnormally high resistivity values due to concentrated current flow.
  • Overshooting the Distance: Abnormally low resistivity values due to dispersed current flow.
• Effect of Leakage in Cables: Current leaks into the ground at the point of leakage, leading to a drop in the displayed value.
• Effect of Wire-Mesh Fence: Current flows into the ground at the fence instead of the new electrode position, causing unexpected drops in resistivity due to higher current density.

Conclusion

• Electrical resistivity method provides a valuable tool for characterizing subsurface geological conditions.
• Understanding the principles of DOI, electrode configurations, field effects, and data acquisition techniques ensures accurate and reliable results.
• Proper planning, execution, and data analysis are essential for successful resistivity surveys.

Explore how different electrode array geometries influence the Depth of Investigation (DOI) in subsurface resistivity measurements. This quiz covers factors affecting DOI such as electrode spacing and resistivity variations in different ground materials. Test your understanding of these concepts in geophysical methods.