Andreas Junge

Dielectric permittivity of fine-grained fractions of soil samples from eastern Spain at 200 MHz

To provide a database for interpreting GPR field data by means of small-scale laboratory studies, we have determined the real and imaginary parts of the dielectric permittivity of fine-grained fractions of soil samples from eastern Spain in the laboratory. We use the parallel-plate method in combination with an impedance analyzer and focus on the frequency of 200 MHz . The measurements are compared to physical properties such as volumetric water content, dry density, clay fraction, and carbonate content. The results show the well-known increase in dielectric permittivity with increasing water content, as presented in the literature; however, our values are systematically higher. This deviation may be caused by the exceptionally high carbonate content of the samples. We establish a basic relationship between dielectric permittivity and water content that is characteristic for soils in the research area. In addition to the dominating influence of water on permittivity, we find a correlation with dry density...

A New Telluric KCl Probe Using Filloux’s AgAgCl Electrode

The measurement of the telluric fields long-time variations requires stable instruments in the period range above 1 day. Obviously, most problems arise from drifting voltages between the telluric probes. Good results have been achieved using a three chamber Hempfling KCl probe together with Fillouxs AgAgCl electrode. However, a one chamber probe of 0.6 m length and 0.06 m diameter filled with saturated KCl solution may be sufficient for some applications and additionally allows permanent control of the electrolytes salt concentration. In a field test the telluric field at a single site was simultaneously observed using one and three chamber probes separated by 25 and 55 m resp. For periods shorter than 1 hour the noise level of the electrodes was found to be less than 1 (mV)2/Hz whereas in the period range between 1 hour and 1 day it increases to 100 (mV)2/Hz. Consequently, surveys investigating long periods of the telluric field can be carried out using small electrode separations of a few tens of meters. Furthermore, the stability of this probe negates the need to interrupt the time series for servicing of the probe. However, the one chamber probe is somewhat inferior to the three chamber probe with regard to temperature dependence at long periods.

Magnetotelluric Transfer Functions: Phase Tensor and Tipper Vector above a Simple Anisotropic Three-Dimensional Conductivity Anomaly and Implications for 3D Isotropic Inversion

The influence of anisotropic conductivity structures on magnetotelluric transfer functions is not easy to analyse in its entire complexity. In this study, we investigate the spatial and frequency-dependent behaviour of phase tensors and tipper vectors above a 3D anisotropic conductivity anomaly. The anomaly consists of a simple cubic block embedded in a homogeneous half space. Using a 3D FD code, we compare an isotropic, 2 anisotropic models with an anisotropy factor of 10 and one anisotropic model with the anisotropy factor of 100. The results show characteristic differences between the isotropic and anisotropic cases. For the anisotropic anomalies, the tipper vectors are parallel over the entire area despite the 3D geometry of the anomalous body. The size of the tipper vectors depends on the position of the site relative to the anomaly’s boundaries and the direction of the anisotropic strike. Above the anomalous anisotropic body, the main diagonal elements of the phase tensor show the well-known split. Outside the anomaly, the phase tensor principal axis rotates according to the site position in contrast to the constant tipper direction. The 3D inversion of the forward data using an isotropic 3D code (ModEM) yields a very good fit for all cases. Whereas the inversion result matches the isotropic model, wave-like structures with high conductivity contrast occur for the anisotropic models. These structures extend far beyond the extension of the original anomalous body. Thus, the study reveals important indications of the existence of anisotropic conductivity structures for observed magnetotelluric transfer functions.

Influence of pipe filling, geometry and antenna polarisation on GPR measurements

The number of buried cables and pipes in urban areas has increased tremendously over the last decades. As the precise documentation of their layout is often missing, damage occurs frequently during construction work. Consequently there is urgent need for non-destructive methods providing reliable and fast prediction of their location. The Applied Geophysics Section of Frankfurt University takes part in the DETECTINO Project(Hildesheim, Germany) which aims at the development of an all-inclusive utility locating tool. The appraisal of GPR reflection patterns requires profound knowledge of the underlying physical processes. This is best achieved by deploying the locating tool on well defined objects, such as the Frankfurt utility detection test site providing a large variety of pipes and cables in different geotechnical settings. The exact knowledge of the geotechnical data allows the comparison of theoretical and observed data by adequate modelling. In this contribution we present modelled and measured data for pipes with varying depth, diameter and filling, for parallel and perpendicular source polarisation.

Ulrich Schmucker (1930–2008)

Ulrich Schmucker passed away in a Beijing, China, hospital in the early morning of 27 October 2008. Ulrich had been in China to attend the 19th International Workshop on Electromagnetic Induction in the Earth, a biannual workshop he had helped found and had attended for more than 30 years without fail.

Chapter 14 On the determination of the electrical 3-D conductivity distribution beneath iceland with long-period magnetotellurics

Abstract The cause for Icelands existence is assumed to be the upwelling of a hot mantle plume. An important result of more than 200 magnetotelluric (MT) measurements over the last decades is a layer of increased conductivity at an average depth of 10 km within the Icelandic crust. Three-dimensional (3-D) model studies show the effects of various conductivity structures on the magnetotelluric transfer functions for periods between 10 and 100,000 s. In particular the resolution of the plume structure and the influence of the highly conductive seawater is discussed. The assumed plume can be detected for a plume-host conductivity contrast > ∼3. Using these values, it is possible to differentiate between the highly conducting crust layer and the plume head as they show a characteristic pattern at clearly separated period ranges in the magnetotelluric transfer functions. Thus the influence of the crustal conductor does not mask the effects of the assumed plume structure. Also the influence of the seawater does not shield the effects of the mentioned conductive zones in crust and mantle; however, the coast effect can be detected up to 50 km inside the island. An assumed plume stem cannot be detected in the model results.