Induced polarization of soils and unconsolidated sediments

Summary Induced polarization has the potential to be used to image the water content and Cation Exchange Capacity in agriculture and soil sciences. In order to do so, we need a robust petrophysical model that can be used to interpret field data. Here we use two petrophysical models based on the classical Dynamic Stern Layer (DSL) polarization model. The first was developed using a volume averaging approach. A second DSL model was recently developed based on fractal theory. Furthermore, we include the presence of semi-conductors (pyrite or magnetite) in these two models. In order to test their predictions, the complex conductivity spectra of 400 samples of shallow sediments and soils were measured in the frequency range 1 Hz-45 kHz at 25°C and at three different NaCl solutions. The two models predict that the surface conductivity and the normalized chargeability are proportional to each other and depend on the CEC and water content. The experimental dataset favours the modified DSL model based on the fractal approach. The effects of the CEC and volume fraction of semi-conductors are investigated since framboidal pyrite is present in the investigated coastal soils and sediments from The Netherlands. Experiments show that the amount of semi-conductors can lead to an apparent cementation exponent below unity in agreement with the petrophysical models. This observation is further and qualitatively confirmed by additional measurements obtained by mixing a soil with different amounts of magnetite. The presence of the Organic Matter (OM) changes the CEC of the material acting therefore on both the surface conductivity and normalized chargeability. Our analysis shows that pure OM has a CEC of 160 ± 40 meq/100 g. An application of the volume-averaged-based DSL model to a field study in Guadeloupe is performed to showcase how we can map the water content and CEC of soils at a field scale using time-domain induced polarization measurements. The novelty of the analysis lies into the relationship between the normalized chargeability measured with the time-domain induced polarization method (injecting the current for a given period) and the normalized chargeability obtained in the frequency-domain and equal to the difference between the instantaneous conductivity and the DC conductivity. The application of the petrophysical models to field data implies that the amplification factor between the two normalized chargeability can be calibrated using experimental data or determined through numerical modelling.