Geophysics- Lecture 3 Resistivity Flashcards
Resistance equation
R=V/I
What is resistance determined by?
Path length ( in 3D Resistance increases with object length L but decreases with
cross-sectional area A)
Resistivity in 3D equation
p=RA/L
Which rock type has the highest resistivity and which has the lowest?
- Igneous= highest resistivity
- Sedimentary- lowest
Relationship between conductivity and resistivity
Conductivity= the inverse of resistivity C=1/R
Potassium chloride and sodium chloride
Strong electrolytes meaning even low concentrations are important
Archie’s law
Describes the relationship between resistivity, porosity, and fluid saturation in porous media.
How does porosity (𝜙)
affect resistivity (𝜌) according to Archie’s Law?
Higher porosity decreases resistivity, as more conductive fluid occupies the pore spaces.
How does water saturation (Sw) influence resistivity?
Lower water saturation increases resistivity, as pore spaces contain less conductive water.
Practical use of Archie’s law?
Estimating hydrocarbon saturation in reservoir rocks during subsurface exploration.
Apparent resistivity
= resistivity x a geometric factor
Determined by measured resistance and a geometric factor dependent on electrode array
What does greater electrode spacing mean when measuring resisitivity?
Greater penetration depth
Wenner electrode configuration
4 electrodes equally spaced in a straight line
How is the Wenner array resistivity calculated?
ρ=2πaV/I, where a is the electrode spacing, 𝑉 is voltage, and I is current
What is the main advantage of the Wenner array?
High signal-to-noise ratio, ideal for shallow subsurface investigations.
Schlumberger electrode arrangement
Outer electrodes gradually moved away from the inner electrodes to probe deeper
What is the main advantage of the Schlumberger array?
Greater depth penetration with fewer electrode movements compared to Wenner.
What is the dipole-dipole electrode configuration used for?
Measuring resistivity by separating current electrodes and potential electrodes into distinct pairs
What is the main advantage of the dipole-dipole array?
High lateral resolution, suitable for identifying horizontal and vertical subsurface features. Popular in 2D and 3D resistivity surveys
Tripotential test wenner array
Involves measuring three potential differences between electrodes to enhance resolution and reduce ambiguity.
