CHP 7

• The electrical method in geophysics measures the resistivity of soils and rocks
• Ohm's Law gives the relationship between voltage (V), current (I), and resistance (R), V = R * I
• In geophysics, the current spreads in all directions in the earth, not passing through a single resistor
• The equation for Ohm's Law for current flow through a continuous medium is given by ρ = φ * I, where ρ is the resistivity of the medium and φ is the potential due to a current source I
• In a field survey, the resistivity of the subsurface is measured by passing a current through the ground using four metal electrodes
• Electric current flows in the earth through electronic conduction and electrolytic conduction
• The resistivity of a soil or rock depends on the nature of the solid matrix, porosity, and pore fluid
• The resistivity of most sediments and rocks depends on the water content
• The resistivity of groundwater depends largely on the concentration of dissolved salts
• Metals, such as iron, have extremely low resistivity values
• Array types with their respective configurations and geometric factors are used in resistivity surveys
• Wenner array configurations:

  • Wenner-Alpha: C1 P1 P2 C2
  • Wenner-Beta: C2 C1 P1 P2
  • Wenner-Gamma: C1 P1 C2 P2
• Wenner arrays are highly sensitive to vertical changes in resistivity below the center of the array
• Wenner arrays are less sensitive to lateral/horizontal resistivity changes like sills and sedimentary structures
• Schlumberger array is similar to the Wenner array but with closely spaced P1-P2 electrodes
• Schlumberger array has slightly better horizontal coverage compared to the Wenner array
• Pole-dipole array has stronger signal strength compared with the dipole-dipole array, but lower than Wenner and Wenner-Schlumberger arrays
• Pole-dipole array electrodes are configured as C1 P1 P2 with C2 at a sufficiently large distance
• Wenner-Schlumberger array is a hybrid between Wenner-Alpha and Schlumberger arrays
• Wenner-Schlumberger array has smaller signal strength and slightly wider horizontal data coverage than the Wenner array
• Wenner-Schlumberger array has higher signal strength and narrower horizontal data coverage than the dipole-dipole array
• Models with noisy and less noisy data sets are adjusted during inversion by changing the damping factor value
• Cell width with half electrode spacing gives better resolution and a smoothened model than cell width of normal electrode spacing
• For achieving RMS or absolute error less than 10%, the iteration subroutine is usually set to 5 iterations for convergence