Spectral induced polarization as well as complex electrical measurements are used to estimate, on a non‐invasive basis, hydraulic permeability in aquifers. Basic laboratory measurements on a variety of shaly sands, silts and clays showed that the main feature of their conductivity spectra in the frequency range from 10‐3 to 103 Hertz is a nearly constant phase angle. Thus, a constant‐phase‐angle model of electrical conductivity is applied to interpret quantitatively surface and borehole spectral induced polarization measurements. The model allows for the calculation of two independent electrical parameters from only one frequency scan and a simple separation of electrical volume and interface effects. The proposed interpretation algorithm yields the true formation factor, the cation exchange capacity and the surface‐area‐to‐porosity ratio, which corresponds to the inverse hydraulic radius. Using a Kozeny–Carman‐like equation, the estimation of hydraulic permeability is possible.

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BORNER, F., GRUHNE, M. and SCHON, J. 1993. Contamination indications derived from electrical properties in the low frequency range. Geophysical Prospecting 41,83-98.Electrical measurements are an important and integrated component of geophysical investigations connected with environmental problems. As a result of an analysis of the electrical conductivity, basic experiments on sandstones at frequencies below 10 kHz show that the complex behaviour of conductivity is caused exclusively by a complex interface conductivity. Its value is determined mainly by the internal rock interface to porosity ratio, the composition of the pore fluid and connected matrix-water interactions resulting in a specific microstructure of the interface. Therefore, it can be expected that the interface region of a soil or rock material is very sensitive to changes in composition caused by contamination. Contaminated sandstone and clay samples were investigated using a low-frequency measurement system. The investigations are directed at the influence of different contaminants and their concentration. Results show that the complex electrical conductivity (real and imaginary parts) is influenced by properties of the pore-filling contaminant. This influence results in a change of the level of both parts and the shape of their frequency dependence. The imaginary part in particular seems to provide important secondary information ; in some cases this part alone allows a differentiation of the various contaminants. The different behaviour of various rock types shows that the effects observed are the result of interactions between pore fluid properties and the internal pore surface structure.

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Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by 13C and 15N dipolar chemical shift NMR spectroscopy and ab initio calculations. Employing a combination of dipolar chemical shift NMR, selective isotope labeling and ab initio gas phase calculations, possible molecular structures and chemical shielding tensors of all 15N nuclei and of two out of the three 13C nuclei were revealed. Four possible stable configurations of the molecule with different energies were found in the calculations. The CSA tensors were calculated for these configurations. While the calculated 13C(urea) CSA tensor orientation of the configuration with the lowest energy is in good agreement with the experimental tensor orientation, there are pronounced differences between calculated and experimental tensor eigenvalues. These differences are a clear indication of the presence of intermolecular hydrogen bonds in the experimental sample, which are neglected in the gas phase calculations. Four different possible orientations of the experimental 13C(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the 13C CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where δ11 points approximately in direction of N(imide), δ22 approximately in direction of the C=O bond, and δ33 is oriented perpendicular to the molecular frame, is adopted in the molecule.

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