Method of test for laboratory determination of resistivity on rock specimen
1997 الطبعة

IS 14436 advises selecting rock specimens that accurately represent the average rock type, sourced from the same rock block and oriented consistently. Typical specimen dimensions, as per IS 9179, include cylindrical samples with diameters of 50 mm or 54 mm and lengths of 100 mm or 200 mm, maintaining length-to-diameter ratios of 2:1 or 4:1. Specimens should be cut or core-drilled with smooth, parallel ends, safeguarded against extreme temperatures during handling, and immersed in formation water for a minimum of 24 hours before testing unless the rock disintegrates upon saturation.

The equipment for resistivity testing per IS 14436 should comprise a commutator to supply pulsed current to the sample and a receiver to detect voltage drop, preferably with digital readouts. Platinum electrodes are recommended for current application, with sponge-filled hollow Plexiglass tubes for electrode positioning. The apparatus must possess controls such as current selectors, voltage range selectors, polarity indicators, and battery charge status lamps. The use of alternating current is crucial to minimize surface impedance effects, and the system should allow for automatic data acquisition to enhance accuracy and ease of interpretation.

IS 14436 prescribes maintaining rock samples submerged in site or distilled water for at least 24 hours prior to testing. Tests should be conducted in a controlled environment with temperature stabilized at 27 ± 2°C. Additionally, testing sites must be free from significant electromagnetic interference, especially from 50/60 Hz mains power, to prevent measurement noise. Equipment calibration using a standard resistance test box is mandatory before testing, and automatic data acquisition systems are preferred to ensure consistent and reliable results.

Several factors impact the precision and consistency of resistivity measurements: the sample holder must provide stable, leak-free contact; specimen preparation following IS 9179 ensures surface quality and dimensional accuracy; consistent measurement procedures and electrode placement improve reproducibility; electrolyte saturation levels and its chemical composition affect conductivity; and rock porosity influences resistivity values, with higher porosity typically lowering resistivity. Calibration with a standard resistance box and use of appropriate electrodes, such as platinum, are essential for trustworthy results.

Resistivity (C1) is computed using the formula C1 = (V / I) × (A / L), where V is the voltage across the specimen, I is the current through it, A is the mean cross-sectional area, and L is the specimen length. Values must be reported to two significant figures, with averages presented for multiple specimens. Calibration of instruments is crucial, and rounding should follow IS 2:1960 standards. These practices ensure uniformity and reliability in resistivity data for rock classification and engineering analysis.