Egon Zimmermann

A high-accuracy impedance spectrometer for measuring sediments with low polarizability

Spectral impedance measurements are receiving increased attention with regard to the characterization of soils, sediments and rocks, particularly in terms of the internal rock structure, the mineralogical composition and the chemistry of fluids contained in porous rocks. In fluid-saturated, porous sedimentary rocks, which are of particular relevance for many hydrological and environmental problems, the polarization processes that give rise to an observed phase shift between input current and output voltage signals are caused by the interaction of the electrolyte in the pores of the rock with electrically charged mineral surfaces. However, this phase response is relatively weak, typically smaller than 10 mrad and sometimes even of the order of only 1 mrad. In order to reliably measure such phase responses in the relevant frequency range, a high-accuracy impedance spectrometer is required. This system must allow phase measurements between 1 mHz and 1 kHz with a phase accuracy better than 0.1 mrad. In this paper, we present a new impedance spectrometer which meets these requirements. It is based on the four-point measurement method and offers a measurement range from 1 mHz to 45 kHz. Furthermore, we present design information for the sample holder and the electrodes, and methods for performing numerical corrections to reduce measurement errors. The overall accuracy of the setup was validated using water and sand with well-defined polarizable objects.

An overview of the spectral induced polarization method for near-surface applications

Over the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IP data for imaging purposes and the increasing application of the method in near-surface investigations. We summarize here the current state of the science of the SIP method for near-surface applications and describe which aspects still represent open issues and should be the focus of future research efforts. Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing data base of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has also been made in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IP data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3D, time-lapse and multi-frequency IP data, which defines a 5D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.

EIT measurement system with high phase accuracy for the imaging of spectral induced polarization properties of soils and sediments

A powerful method for the non-invasive structural characterization of material is electrical impedance tomography (EIT) combined with the capabilities of impedance spectroscopy. This method determines the complex resistivity magnitude and phase images at a set of different measurement frequencies. We are particularly interested in the application of such an advanced approach for the improved characterization of soils and sediments, which only show a weak polarizability. Here, typical phase values lie between 1 and 20 mrad only, requiring instrumentation with relatively high phase resolution and accuracy. In this paper, we present a new spectral EIT data acquisition system for laboratory applications, which operates in the frequency range from 1 mHz to 45 kHz and which was developed to meet these requirements. In this context, we also present a new measurement method based on current injection swapping, which leads to significantly improved phase images, particularly for higher measurement frequencies. The system and the new measurement method are tested on a water-filled tank and column containing different 2D and 3D targets (metallic and biological objects). The tests prove a phase accuracy of 1 mrad for frequencies of up to 1 kHz and higher, resulting in a clear discrimination of the objects on the basis of the reconstructed phase images.

Spectral induced polarization for the characterization of free-phase hydrocarbon contamination of sediments with low clay content

The identification of organic pollutants in the soil and the subsurface is a goal of primary importance in the management of contaminated sites. However, only a few non-invasive techniques can be useful towards this goal. One such technique is spectral induced polarization. In this study, we investigate the spectral induced polarization effect of changing fluid saturation in a well-characterized porous medium, analysing the difference between air and hydrocarbons, at different degrees of water saturation. The experiments were conducted on fine eolic sand samples coming from an experimental site near Turin, Italy. Octanol and benzene were used as non-aqueous phase liquids. Samples were initially saturated with water having controlled electrical conductivity and subsequently de-saturated stepwise with injection of air at known pressure. The eolic sand samples were then re-saturated with the same water contaminated with hydrocarbons and then a non-aqueous phase liquid phase (either octanol or benzene) was injected in volumetric steps, in order to compare the effects of air and non-aqueous phase liquid invasion. At each saturation step, spectral induced polarization measurements were conducted in the 0.01 Hz to 1 kHz range using the ZEL-SIP04 impedance meter developed at the Forschungszentrum Juelich. The measurement setup guaranteed a 1 mrad phase precision for the entire frequency measurement range. Measurements were conducted under temperature controlled conditions at 20 (±0.5)° C. All spectral induced polarization curves show a peak in the range 0.01–1 Hz that changes in intensity and frequency with varying saturation and a high-frequency phase shift increase dominated by capacitive coupling effects of the measuring system. A multiple Cole-Cole model was fitted to the data. The effects of de-saturation on the low-frequency Cole-Cole parameters are that a) resistivity increases with decreasing water saturation but increases less with non-aqueous phase liquid than with the same volume of air; b) chargeability increases with decreasing water saturation but in presence of non-aqueous phase liquids its value is sometimes lower, sometimes higher and sometimes similar to the one observed in presence of air; c) the time constant τ increases with decreasing water saturation and is consistently larger with non-aqueous phase liquid than with air. These differences between air and non-aqueous phase liquid injection can be explained in terms of differences in non-aqueous phase distribution within the porous medium, as observed by X-ray micro-CT: while air is homogeneously distributed, non-aqueous phase liquids segregate under density effects. In summary, all spectral induced polarization effects of air and non-aqueous phase liquid injection in the considered porous medium are volumetric, i.e., are not due to interaction with grain surfaces or other electrical-chemical effects but are caused by pore obstruction by the electrically non-conductive phase.

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.

Broadband EIT borehole measurements with high phase accuracy using numerical corrections of electromagnetic coupling effects

Electrical impedance tomography (EIT) is gaining importance in the field of geophysics and there is increasing interest for accurate borehole EIT measurements in a broad frequency range (mHz to kHz) in order to study subsurface properties. To characterize weakly polarizable soils and sediments with EIT, high phase accuracy is required. Typically, long electrode cables are used for borehole measurements. However, this may lead to undesired electromagnetic coupling effects associated with the inductive coupling between the double wire pairs for current injection and potential measurement and the capacitive coupling between the electrically conductive shield of the cable and the electrically conductive environment surrounding the electrode cables. Depending on the electrical properties of the subsurface and the measured transfer impedances, both coupling effects can cause large phase errors that have typically limited the frequency bandwidth of field EIT measurements to the mHz to Hz range. The aim of this paper is to develop numerical corrections for these phase errors. To this end, the inductive coupling effect was modeled using electronic circuit models, and the capacitive coupling effect was modeled by integrating discrete capacitances in the electrical forward model describing the EIT measurement process. The correction methods were successfully verified with measurements under controlled conditions in a water-filled rain barrel, where a high phase accuracy of 0.8 mrad in the frequency range up to 10 kHz was achieved. The corrections were also applied to field EIT measurements made using a 25 m long EIT borehole chain with eight electrodes and an electrode separation of 1 m. The results of a 1D inversion of these measurements showed that the correction methods increased the measurement accuracy considerably. It was concluded that the proposed correction methods enlarge the bandwidth of the field EIT measurement system, and that accurate EIT measurements can now be made in the mHz to kHz frequency range. This increased accuracy in the kHz range will allow a more accurate field characterization of the complex electrical conductivity of soils and sediments, which may lead to the improved estimation of saturated hydraulic conductivity from electrical properties. Although the correction methods have been developed for a custom-made EIT system, they also have potential to improve the phase accuracy of EIT measurements made with commercial systems relying on multicore cables.

High temperature superconductor dc-SQUID microscope with a soft magnetic flux guide

A scanning SQUID microscope based on high-temperature superconductor (HTS) dc-SQUIDs was developed. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid-nitrogen-cooled SQUID sensor and back. The flux guide passes through the pick-up loop of the HTS SQUID, providing an improved coupling of magnetic flux of the object to the SQUID. The device measures the z component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in the x–y direction by a motorized computer-controlled scanning stage. A lateral resolution better than 10 µm, with a field resolution of about 0.6 nT Hz−1/2 was achieved for the determination of the position of the current carrying thin wires. The presence of the soft magnetic foil did not significantly increase the flux noise of the SQUID.

Phase correction of electromagnetic coupling effects in cross-borehole EIT measurements

Borehole EIT measurements in a broad frequency range (mHz to kHz) are used to study subsurface geophysical properties. However, accurate measurements have long been difficult because the required long electric cables introduce undesired inductive and capacitive coupling effects. Recently, it has been shown that such effects can successfully be corrected in the case of single-borehole measurements. The aim of this paper is to extend the previously developed correction procedure for inductive coupling during EIT measurements in a single borehole to cross-borehole EIT measurements with multiple borehole electrode chains. In order to accelerate and simplify the previously developed correction procedure for inductive coupling, a pole–pole matrix of mutual inductances is defined. This consists of the inductances of each individual chain obtained from calibration measurements and the inductances between two chains calculated from the known cable positions using numerical modelling. The new correction procedure is successfully verified with measurements in a water-filled pool under controlled conditions where the errors introduced by capacitive coupling were well-defined and could be estimated by FEM forward modelling. In addition, EIT field measurements demonstrate that the correction methods increase the phase accuracy considerably. Overall, the phase accuracy of cross-hole EIT measurements after correction of inductive and capacitive coupling is improved to better than 1 mrad up to a frequency of 1 kHz, which substantially improves our ability to characterize the frequency-dependent complex electrical resistivity of weakly polarizable soils and sediments in situ.

Broadband Electrical Impedance Tomography for Subsurface Characterization Using Improved Corrections of Electromagnetic Coupling and Spectral Regularization

The low-frequency complex electrical conductivity in the mHz to kHz range has been shown to enable an improved textural, hydraulic, and biogeochemical characterization of the subsurface using electrical impedance spectroscopy (EIS) methods. Principally, these results can be transferred to the field using electrical impedance tomography (EIT). However, the required accuracy of 1 mrad in the phase measurements is difficult to achieve for a broad frequency bandwidth because of electromagnetic (EM) coupling effects at high frequencies and the lack of inversion schemes that consider the spectral nature of the complex electrical conductivity. Here, we overcome these deficiencies by (i) extending the standard spatial-smoothness constraint in EIT to the frequency dimension, thus enforcing smooth spectral signatures, and (ii) implementing an advanced EM coupling removal procedure using a newly formulated forward electrical model and calibration measurements. Both methodological advances are independently validated, and the improved imaging capability of the overall methodology with respect to spectral electrical properties is demonstrated using borehole EIT measurements in a heterogeneous aquifer. The developed procedures represent a significant step forward towards broadband EIT, allowing transferring the considerable diagnostic potential of EIS in the mHz to kHz range to geophysical imaging applications at the field scale for improved subsurface characterization.

Development and drift-analysis of a modular electromagnetic induction system for shallow ground conductivity measurements

Electromagnetic induction (EMI) is used for fast near surface mapping of the electrical conductivity (EC) for a wide range of geophysical applications. Recently, enhanced methods were developed to measure depth-dependent EC by inverting quantitative multi-configuration EMI data, which increases the demand for a suitable multi-channel EMI measurement system. We have designed a novel EMI system that enables the use of modular transmitter/receiver (TX/RX) units, which are connected to a central measurement system and are optimized for flexible setups with coil separations of up to 1.0 m. Each TX/RX-unit contains a coil, which is specifically adjusted for transmitting or receiving magnetic fields. All units enable impedance measurements at the coils, which are used to simulate its electrical circuit and analyze temperature-induced drift effects. A laboratory drift analysis at 8 kHz showed that 88% of the drift in the measured data is due to the change in the electrical transmitter coil resistance. The remaining 12% is due to changes in the transmitter coil inductance and capacitance, the receiver impedance and drifts in the amplification circuit. A measurement under field conditions proved that the new EMI system is able to detect a water-filled swimming pool with 50 mS m−1, using a coil separation of 0.3 m. In addition, the system allows in-field ambient noise spectra measurements in order to select optimal low-noise measurement frequencies.