What is the required sample size for this test?

A right circular cylinder with a length to diameter (L/D) ratio between 2.0 and 2.5. The diameter should not be less than NX size (approx. 54 mm) (Clause 5.1).

Why is the test performed on a saturated specimen?

To measure the resistivity of the combined rock-fluid system, which is more representative of in-situ conditions below the water table. The pore fluid is the primary conductor in most rocks.

What is the formula used to calculate resistivity?

ρ = R * (A / L), where ρ is resistivity in ohm-metres, R is the measured resistance, A is the cross-sectional area, and L is the specimen length (Clause 7.1).

Why is low-frequency AC current used instead of DC?

To minimize electrode polarization and electrolytic effects at the rock-electrode interface, which can cause erroneous voltage readings with a DC source (Clause 4.1).

IS 14436

: 1997

Method of test for laboratory determination of resistivity on rock specimen

CurrentSpecializedTesting MethodBIMGeotechnical · Rock Mechanics

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This standard specifies the laboratory method for determining the electrical resistivity of saturated cylindrical rock specimens. It details the apparatus, specimen preparation, test procedure, and calculation method required for the test.

Method of test for laboratory determination of resistivity on rock specimen

Quick Reference Values

specimen shapeRight circular cylinder

specimen length to diameter ratio2.0 to 2.5

minimum specimen diameterNX size (approx 54 mm)

saturating fluid resistivity5 to 10 ohm.m (tap water) or specimen pore fluid

test frequencyLow frequency AC to minimize polarization effects

Key Formulas

ρ = R * (A / L) — Electrical resistivity, where R is resistance, A is cross-sectional area, L is length

Practical Notes

Ensuring complete saturation of the rock specimen is critical for accurate and repeatable results, as trapped air will significantly increase measured resistivity.

Good, uniform contact between the specimen ends and the electrodes is essential to avoid contact resistance errors. A conductive paste or saturated filter paper is often used.

The resistivity of the saturating fluid must be measured and reported, as it heavily influences the bulk resistivity of the rock specimen.