Spectral Induced Polarisation is an extension of conventional electrical resistivity measurements. The method is used in geophysical research to investigate and chracterise soils and rocks. Targets are material type, porosity, permeability and salt content. In division 8.2 SIP is applied on building materials (Division 7.1) and wood (Division 4.1) to characterise pore structures and to delineate moisture and salt distribution.
Principle of resistivity measurement with a four electrode array
The electrical resistivity is measured in SIP by applying sine wave currents in the frequency range between 1 mHz und 10 kHz. Impedance Z and phase φ are determined by dividing measured voltage U by applied current I. To determine complex resistivity (or its reciprocal, complex conductivity) a configuration factor has to be applied, which depends on electrode configuration and sample geometry.
Principle of complex resistivity measurements
To investigate samples of granular or solid materials we have developed a set of measurement cells, which are now used in several institutions (Germany, France, USA). For lab measurements we are using a two channel device built by Forschungszentrum Jülich (SIP-ZEL). Samples and cells are placed in climate controlled chambers (temperature, humidity, CO2). Interpretation is done by fitting mathematical models to the data, mostly the generalised Cole-Cole model. The parameters determined herewith, e. g. the time constant τ, are correlated with material properties (e. g. pore size).
SIP-measurement cell. Sample is located between inner electrodes (potential measurement). Coupling to outer (current) electrodes by water or agar-agar gel.
SIP-measurement cells for different sample sizes and geometries. Right: Brick sample with medical adhesive electrodes.
SIP laboratory working place (SIP-ZEL) with climate chamber
For on site measurements (soil, masonry, trees) we are using a special version of the SIP 256C (Radic Research), which allows simultaneous potential measurements on all electrode pairs as well as multifrequency and double source measurements to accelerate the field work. Coupling is done by metal stakes (soil), nails (trees) or adhesive medical electrodes (masonry).
SIP measurements at masonry (SIP on building materials).
SIP tomography measurements at trees (SIP on wood).
Measurement on (large) inhomogenenious objects require an additional processing step. Large datasets acquired with several electrode distances and arrays have to be inverted by 2D or 3D schemes to reveal the approximate structure (electrical resistivity tomography, ERT). We are using the commercial software RES2DINV as well as custom programs of Dr. Thomas Günther, LIAG: Wallbert (masonry) and DC2dTree (trees).
Principle of ERT (electrical resistivity tomography) by numerous measurements at different positions with different depths of penetration. The final result is achieved by 2D or 3D inversion schemes.
ERT result across a river embankment after 2d inversion (DEISTRUKT). High resistivities (dark) point to sandy, dry material, low values (bright) to clay, loam or topsoil.
Research topics
Completed Research Projects
Research cooperation
BAM scientists have co-founded the working group on IP in the German Geophysical Society (DGG). BAM has developed and maintains a reference material for round robin tests. The working group organizes yearly national and international workshops.
Service
Division 8.2
Dr. rer. nat.