Axel Tillmann

An AMR sensor-based measurement system for magnetoelectrical resistivity tomography

A magnetoelectrical resistivity measurement system is proposed, which combines measurement of the electric potential and the magnetic field due to a current injection into a sample. Measurement of the electric potential, as well as the injected current, is similar to traditional electrical resistivity tomography (ERT) data acquisition. For the magnetic field measurements, 24 sensor modules have been developed using three component anisotropic magnetoresistive (AMR) sensors, mounted on a vertically moving scanning torus. The system is designed to operate in a typical laboratory magnetic noise environment without extensive shielding. To compensate for the effects of the Earths magnetic field, the AMR sensors are operated with a field feedback circuit. Optimal noise reduction is provided by the use of a lock-in frequency of 25 Hz, with sine wave modulation and measurement cycles of 10 s. The resolution of the system is better then 50 pT and the aimed accuracy is 0.1%. The system provides a data set of magnetic fields complimentary to traditional ERT to determine the internal conductivity distribution of cylindrical samples with the dimension of 0.1-m radius and 0.5-m height.

An Unsupervised Wavelet Transform Method for Simultaneous Inversion of Multimode Surface Waves

Dispersive surface waves are sensitive to the geometry of layered structures and the compressional and shear wave velocities within each layer. Therefore they can be used to estimate the structure and lithology of shallow soils. It is common practice to use the low frequency fundamental mode for inversion; thus, it is necessary to separate the single modes contained in the data interactively, which is time consuming and a source of error. The fast algorithm presented in this paper interprets all excited modes of a surface wave simultaneously and avoids the need to identify individual mode-numbers. The inversion is based on the group travel time of a Rayleigh wave, estimated from a single seismic trace by application of the Continuous Wavelet Transform (CWT). The resulting CWT distribution is fitted by a composition of wavelets determined from theoretical group-velocity dispersion relations of possible models, in connection with appropriate boundary conditions. A feasibility test on a synthetic dispersion ...

Hydrogeophysical characterization of subsurface solute transport at the Krauthausen test site: experiments and numerical modelling

We demonstrate the potential of geophysical methods, in particular Electrical Resistance Tomography (ERT) in conjuction with tracer experiments, to derive hydraulic variables (e.g. velocity) and solute transport parameters (e.g. dispersivity) in subsurface environments. For this purpose, real tracer experiments were monitored using crosshole time-lapse ERT at reference planes at the Krauthausen test site. Numerical inversion of an ERT dataset obtained from a numerical tracer experiment in a heterogeneous 3D flow domain reveals that the 2D distribution of the bulk electrical conductivity field at selected reference planes can be recovered. In 2000, a first small-scale tracer experiment at the Krauthausen field site combined with ERT was performed to monitor the change in bulk electrical conductivity at a reference plane.

Using CPT to Estimate an Aquifers Heterogeneity

The intensive growth of industrial and agricultural activity causes an increase of polluted areas. In order to predict the spreading of near-surface contamination it is essential to be familiar with the hydrogeological parameters of the investigated earth. The cone penetration technology is capable to resolve the structure of the subsurface in detail and logs various physical parameters during soil penetration. Cone penetration tests (CPT) were performed at the test site Krauthausen to investigate the aquifer’s heterogeneity in detail.