DC Resistivity (M-3)

1. Q: How does it work?
   A: An electrical current is impressed into the ground through two electrodes. Voltages on the surface are measured revealing the direction and amount of current flow in the subsurface. These data are interpreted in terms of the resistivity (bulk resistance to current flow) of the earth materials.
   

2. Q: What is the geological model?
   A: Variations in amount of pore water, pore water conductivity, rock matrix conductivity, shape and connectivity of pores, amount of clay minerals, temperature, and several other properties of a rock determine its resistivity. One or two dimensional variations in the resistivity are sensed.
   

3. Q: What are the requirements?
   A: A powerful source:
   Special purpose transmitters put up to 3 kilowatts of power into the ground
   
   A low-noise receiver:
   Multichannel voltmeters capable of resolving a few microvolts
   
   Low interference from:
   Grounded fences
   Operating power plants or power lines
   Shockable people or animals
   
   Surface Access:
   electrodes driven or drilled into ground
   
   Ground truth:
   Well logs
   Drill holes
   
   
4. Q: What are the pitfalls?
   A: Interpretation is one-dimensional (electric drilling) or two-dimensional (electric trenching).
   Ambiguity: conductivity-thickness equivalence.
   Current channeling in conductive environments.
   
5. Q: What logistics are needed?
   A: Crew size usually 2 or 3 people.
   Portable if source can be carried.
   
6. Q: What are the deliverables?
   A: Plan maps, data presentations, inverse models, forward calculations of the same model or cross-section, and a narrative description of the work done.
   
7. More detailed information.