Lecture 5a - Electrical Resistivity Method - Principles

Flashcards covering the principles of DC resistivity methods for subsurface investigations.

What is the typical depth range for near-surface investigations using electrical geophysical techniques?

Within the top 50 meters.

Why are electrical techniques often used for near-surface investigations?

Because they are sensitive to water content and related earth properties.

What does 'DC' stand for in DC resistivity?

Direct Current, meaning a steady current flowing in one constant direction.

What is the relationship between voltage, current, and resistance according to Ohm's Law?

Voltage (V) equals Current (I) times Resistance (R), or V = IR.

What is the difference between electrical resistance and resistivity?

Resistance is an absolute quantity, while resistivity is a material property that accounts for the geometry of the medium.

What is conductivity?

Conductivity is the reciprocal of resistivity.

Name four factors that the bulk resistivity of geomaterials depends on.

Solid particles, pore space, fluids filling the pore spaces, salinity, and temperature.

Why is temperature important in electrical resistivity measurements?

Because many materials vary their electrical resistivity with temperature.

How does electrical current flow through geomaterials?

Mostly within the pore space as an electrolytic current through the liquid phase.

What is the 'formation factor'?

The conductivity of the electrolyte fluid divided by the conductivity of the bulk material.

What is tortuosity?

A measure of how complicated a path is that electric charge has to travel to get through a material.

What parameters does Archie's Law for saturated media describe?

The bulk conductivity in terms of the conductivity of the fluid, the effective porosity, and the cementation.

What does the saturation term (Sw) represent in Archie's Law for unsaturated media?

The degree of saturation, varying from zero (fully dry) to one (complete saturation).

What is the double layer of charge around mineral grains in electrolytic fluids?

A fixed layer tightly bound to the mineral surface and a more diffuse layer outside of that, facilitating surface conduction.

What are the two components included in the equation for the bulk conductivity of a material?

Electrolytic and surface conductivity components.

How are subsurface variations in resistivity or conductivity measured?

By injecting current into the ground using electrodes and measuring potential differences at the surface.

What is the typical setup for a DC resistivity measurement?

Inject current (I) through current electrodes (C1, C2) and measure the potential difference (ΔV) through potential electrodes (P1, P2).

What is the shape of the equipotential surface around a current electrode in a homogenous subsurface?

Hemispherical.

What is 'apparent resistivity'?

The resistivity of a homogenous subsurface that would give the same voltage and current measurements as the actual subsurface.

What is the method used to calculate the apparent resistivity of the subsurface in DC resistivity?

By dividing the potential difference by the current and multiplying by a geometric factor.