Hydrogeology 101: Introduction to Resistivity Surveys

Overview

• Instructor: Andreas de Jong
• Topic: Resistivity surveys for locating water wells in the Salouville aquifer.
• Goal: To find a cost-effective method to scout for the ideal drilling sites without extensive drilling.

What is Resistivity?

• Definition: Resistivity is a fundamental physical property of materials, representing resistance per unit volume.
• Ohm's Law: R = V / I
  • R = resistance (Ohms)
  • V = potential difference (Volts)
  • I = current (Amperes)
• Analogy: Electricity flow is likened to water flow in pipes (current = flow rate, resistance = friction losses).
• Resistivity Measurement: Measured in ohm-meters.

Resistivity Values and Aquifer Characteristics

• Resistivity Range:
  • < 10 ohm-meters: Clay-rich or saline water formations.
  • 10 to 500 ohm-meters: Typical for most aquifers.

  • 1000 ohm-meters: Generally indicates bedrock with low productivity.

Equipment Used for Resistivity Surveys

• Common Devices:
  • ABEM Terrameter: Lightweight, robust.
  • Cisco Pro: Used for surveys in various global locations.
• Equipment Requirements:
  • Power source (batteries/generator)
  • Well-insulated cables
  • Current electrodes (A and B)
  • Potential electrodes (M and N)
• Team Requirements:
  • Minimum of two workers, ideally four for efficiency.
  • Water for electrodes and shade for protection.

Conducting a Resistivity Survey

Electrical Resistivity Profile

• Process of plotting apparent resistivity against distance.
• Observations:
  • High resistivity on one side indicates bedrock.
  • Low resistivity indicates a thicker alluvial aquifer.
  • Data points connected to visualize subsurface structure.

Vertical Electrical Sounding (VES)

• Purpose: Determine how deep to drill based on resistivity profiles.
• Conducting VES:
  • Adjust distance between electrodes to gather data from various depths.
  • Record resistivity values to identify layers.

Electrode Arrays

• Schlumberger Array:

  • A and B electrodes spaced at least five times further apart than M and N.
  • Advantages: Faster, more accurate, deeper depth of investigation.

• Venn Array:

  • Electrode distances are constant.
  • More effort required for measurements, less efficient than Schlumberger.

Depth of Investigation

• Effective Depth: Depth at which 50% of the measurement response comes from above and below.
• Rule of thumb: Depth of investigation should be one-third of the AB spacing.
• Example: For 100 meters depth, AB spacing should be at least 300 meters.

Apparent Resistivity Curves

• Illustrates resistivity changes between layers:
  • Typical shapes help identify layers and their resistivities.
  • Important for interpreting subsurface conditions.

Summary of Uses in Resistivity Surveys

• Outcomes:
  • Depth to the base of the aquifer.
  • Resistivity of the aquifer and intermediate layers.
  • Static water level depth is typically obtained from baseline surveys.

Interpretation Software

• Modern software used for data processing:
  • 1D inversion software (e.g., 1x1D by Interpex Limited).
  • 2D inversion software (e.g., Resist2T).
• Historical methods: Master curves (less common now).

Conclusion

• Best methods for groundwater exploration: Resistivity profiling and VES
• Important to ensure data quality:
  • Smooth data curves indicate reliability.
  • Erratic data suggests a need for field re-evaluation.
• Closing remarks: Encouragement to learn more in future videos.