The electrical conductivity of sandstone, limestone and granite

summary . The electrical conductivity of cylindrical cores of Westerly granite, Indiana limestone and Nugget, St Peter and Kayenta sandstones was measured at about 25°C in vacuo , in air, and after saturation in distilled water, tap water, and 0.1 MNaCl solution. The three-electrode technique with a guard ring and the twoelectrode technique without a guard ring were used. Core aspect ratio over the range of 2.00 to 0.25, as well as frequency over the range of 50 Hz to 10 kHz, influences the conductivity of all rocks, especially those measured in vacuo . Measurements from water-saturated samples using a guard ring are not appreciably different from those obtained without a guard ring. The conductivity of rocks saturated in 0.1 MNaCl solution changes least with a change in aspect ratio; for these rocks a linear relationship, known as Archie's Law, exists between log porosity and log conductivity. No simple correlation was found between those factors in rocks saturated with tap or distilled water. Thus, it appears Archie's Law is of questionable value for correlating laboratory data from rocks saturated with low-conductivity fluids.     

Comment on 'The use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities' by H. Gurrola, J. B. Minster and T. Owens

In a recent paper, Gurrola, Minster & Owens (1994; hereafter referred to as GMO) introduced a technique for the analysis of seismic data and applied it to the records of station OBN in Russia. GMO claimed that this was a new technique in earthquake seismology, and that their results for OBN provided a new insight into the deep structure of the Russian platform. We have some comments on their techniques and results.     

Comment on 'The magnetotelluric phase tensor' by T. Grant Caldwell, Hugh M. Bibby and Colin Brown

We demonstrate a minor inconsistency in the appendix of 'The magnetotelluric phase tensor' by Caldwell et al. that can lead to incorrect results, and provide a simple solution for this problem.