“Laboratory conductivity measurement using a contactless coil-type probe,” The Log Analyst, pp.82-90, May 1995.
A new apparatus has been developed to measure the conductivity of samples at 100 kHz. The probe consists of coil-type transmitters and receivers and requires no physical contact with the sample. The measurement is based on the induction principle. The conductivity of a sample as a function of its length can be measured with good resolution. The resolution, defined as the thickness of the conductivity inhomogeneity that can be resolved with 90% accuracy of the conductivity, is approximately 2.5 cm when the contrast in conductivity between adjacent layers is 2; it is 5 cm when the contrast is 20. An inversion technique using the maximum cntropy criterion is developed to interpret the measured data. Data processing is accomplished by applying a constrained optimization algorithm with an automatically adjusted Lagrange multiplier. The measurement system is validated by using saline solutions and water-saturated sands of known conductivities. The apparatus is accurate when the sample resistivity is between 0.1 and 20 ohm-m.