Arre Verweerd

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.

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.