Sven Nordsiek

A new approach to fitting induced-polarization spectra

Best fitting of induced-polarization (IP) spectra by different models of Cole-Cole type evidences discrepancies in the resulting model parameters. The time constant determined from the same data could vary in magnitude over several decades. This effect, which makes an evaluation of the results of different models nearly impossible, is demonstrated by induced polarization measurements in the frequency range between 1.4 mHz and 12 kHz on thirteen mixtures of quartz sand and slag grains. The samples differ in size and the amount of the slag grains. Parameters describing the IP spectra are derived by fitting models of the Cole-Cole type to the measured data. The fitting quality of the generalized Cole-Cole model, the standard Cole-Cole model, and the Cole-Davidson model is investigated. The parameters derived from these models are compared and correlated with mass percentage and grain size of the slag particles. An alternative fittingapproach is introduced, using the decomposition of observed IP spectra into ...

Estimating permeability of sandstone samples by nuclear magnetic resonance and spectral-induced polarization

Two techniques to estimate permeability are compared in this paper: nuclear magnetic resonance (NMR) and spectral-induced polarization (SIP). Both methods are based on relaxation processes. NMR records the relaxation of hydrogen nuclei after excitation in an external magnetic field. The phenomenon of induced polarization can be characterized by a relaxation of ions after excitation by an electric field. Hydrogen nuclei are concentrated in the pore water, the current flow is restricted to the pore space for most reservoir rocks, and permeability is related to the pore space geometry. Based on the similarity between fluid movement and current flow in the pore space, different relations have been published linking parameters derived from NMRand SIP data to predict permeability. NMR, SIP and permeability data have been acquired on 53 sandstone samples of the cretaceous Bahariya Formation (Western Desert, Egypt) including 27 samples showing a lamination that causes anisotropy. We compare the applicability of k...

Spectral Induced Polarisation – a Geophysical Method for Archaeological Prospection in Peatlands

Abstract Compared to conventional electrical conductivity surveys the spectral induced polarisation (SIP) method measures the frequency dependence of conductivity amplitude and the phase shift between injected current and the resulting voltage signal. The phase shift is related to polarisation effects of materials. Laboratory investigations of peat, wood and soil samples show a considerable variation in the spectra of conductivity amplitude and phase shift. From the results of laboratory investigations it could be concluded that characteristic features of the phase spectra can be used to detect wooden relics of trackways or pile dwellings in the wetlands subsurface. Field surveys in the Federsee bog in southern Germany and the Campemoor bog in northern Germany prove that the method of spectral induced polarisation can be considered as a prospecting tool for archaeological surveying in wetlands. A shallow trackway in the Campemoor bog could be clearly verified by higher phase values measured at a frequency of 3Hz. The weak phase signal of a somewhat deeper plankway in the Federsee bog became only visible after data processing.

ESTIMATION OF VAN GENUCHTEN-MUALEM PARAMETERS AND THE SATURATED HYDRAULIC CONDUCTIVITY FROM SIP MEASUREMENTS

We performed laboratory measurements on samples of unconsolidated sediments with the method of spectral induced polarisation (SIP) and with hydrological experiments. We determined integrating parameters, e. g. the normalized chargeability , from the SIP spectra by application of the Debye decomposition (Nordsiek and Weller, 2008). The normalized chargeability, that is the ratio of the total chargeability to the DC resistivity, is a measure of the strength of the polarisation effect. In this study is the key parameter for the development of two empirical power laws to estimate parameters of the common hydrological model by Mualem (1976) and van Genuchten (1980).