Gideon Gouws

Spectral Induced Polarization Measurements on New Zealand Sands - Dependence on Fluid Conductivity

Spectral induced polarization (SIP) measurements explore the variation of the complex conductivity (σ*) of a material with frequency. Much of this variation results from polarization effects associated with the electric double layer on the surfaces of pore spaces. The consequent dependence of the SIP signature on pore structure thus has the potential to provide a link to the hydraulic properties of the material, which have a similar dependence. We report here on the variation of the SIP signature of unconsolidated sands, typical of those found in coastal aquifers in New Zealand, with the conductivity of the pore fluid. The SIP parameters of the measurements are modelled in terms of a Cole-Cole model and demonstrate the independence of relaxation time on fluid conductivity. The contribution of surface conductivity to the overall conductivity is calculated and the variation of the imaginary part of the surface conductivity with fluid conductivity is tested against two models for the origin of surface conductivity. The measured hydraulic conductivity is also compared with estimates provided by three proposed equations relating hydraulic properties to structural and electric properties.

Anisotropic microstructured poly(vinyl alcohol) tissue-mimicking phantoms [Letters]

Novel microstructured PVA phantoms mimicking fibrous tissues have been developed using a simple freeze-casting process. Scanning electron micrographs reveal highly anisotropic microstructure with dimensions of the order of 5 to 100 μm. Characterization of an example phantom revealed acoustic properties consistent with those found in fibrous tissues. At 20 MHz, the velocity measured parallel to the microstructure orientation of 1555 ms-1 was significantly greater than that perpendicular to the microstructure of 1537 ms-1. The attenuation coefficient was measured to be 5 dB·mm-1 and proportional to the 1.6 power of frequency, which is in good agreement with that for normal human myocardium.

The low frequency electrical properties of sea ice

Impedance measurements were performed across the frequency range 40 Hz-1MHz on laboratory grown sea ice using a custom four-electrode capacitive cell. The measurements were performed in-situ, throughout the ice growth in horizontal and vertical orientations. Two-electrode measurements were susceptible to electrode polarization and a four-electrode measurement was required to measure the true electrical properties of the ice. A broadband mathematical model is presented which distinguishes polarization and conduction in the bulk material from space charge polarization at interfaces. The fit of this model to the impedance data gives insight into the relative contribution of these phenomena. The bulk material undergoes a dielectric relaxation and we have examined the temperature dependence of the relaxation time and susceptibility. Aspects of the susceptibility and DC conductivity can be reconciled with a conductive dielectric model in which brine is confined to regular ellipsoids.

A 3D Faraday Shield for Interdigitated Dielectrometry Sensors and Its Effect on Capacitance

Interdigitated dielectrometry sensors (IDS) are capacitive sensors investigated to precisely measure the relative permittivity (ϵr) of insulating liquids. Such liquids used in the power industry exhibit a change in ϵr as they degrade. The IDS ability to measure ϵr in-situ can potentially reduce maintenance, increase grid stability and improve safety. Noise from external electric field sources is a prominent issue with IDS. This paper investigates the novelty of applying a Faraday cage onto an IDS as a 3D shield to reduce this noise. This alters the spatially distributed electric field of an IDS affecting its sensing properties. Therefore, dependency of the sensor’s signal with the distance to a shield above the IDS electrodes has been investigated experimentally and theoretically via a Green’s function calculation and FEM. A criteria of the shield’s distance s = s0 has been defined as the distance which gives a capacitance for the IDS equal to 1 − e−2=86.5% of its unshielded value. Theoretical calculations using a simplified geometry gave a constant value for s0/λ = 1.65, where λ is the IDS wavelength. In the experiment, values for s0 were found to be lower than predicted as from theory and the ratio s0/λ variable. This was analyzed in detail and it was found to be resulting from the specific spatial structure of the IDS. A subsequent measurement of a common insulating liquid with a nearby noise source demonstrates a considerable reduction in the standard deviation of the relative permittivity from σunshielded=±9.5% to σshielded=±0.6%. The presented findings enhance our understanding of IDS in respect to the influence of a Faraday shield on the capacitance, parasitic capacitances of the IDS and external noise impact on the measurement of ϵr.

Characterisation of anisotropic poly(vinyl-alcohol) gels prepared using a two-zone controlled directional freezing process

PVA phantoms prepared using an improved two-zone controlled directional freezing process exhibit tailorable, repeatable, anisotropic properties representative of real tissues. The directional freeze process induces a preferred microstructural orientation with significantly increased Youngs Modulus along the direction of freezing. The uniformity of the microstructure is consistent throughout the sample as confirmed through velocity measurements.

Low frequency permittivity measurements of sea ice

A portable impedance measuring system and capacitive sensor array was developed to perform in-situ measurements on sea ice. This equipment was installed close to McMurdo Sound in the Antarctic and measurements are currently being made during the Southern winter and spring. These measurements are compared to measurements made on laboratory grown ice using a commercial impedance analyser. From these measurements information on the temporal and structural evolution of the ice can be obtained.

Mid-IR absorbance and its relation to static permittivity as a robust in-field tool tracking oil deterioration

A study of the link between the infra-red (IR) absorbance and the relative static permittivity Ɛr of liquid hydrocarbons is of special interest for developing in-situ oxidation monitoring tools. In particular, where IR measurements are difficult to implement but cost efficient and durable capacitive probes can be used. This paper will explore this link by exposing a paraffinic hydrocarbon to oxidation in an accelerated degradation process, while measuring the IR absorption and Ɛr values during this process. It is shown to what extent the IR response of the hydrocarbon liquid changes in the 500 to 4000 cm-1 window, and how this can be translated into a measured increase in Ɛr during oxidation time. The correlation coefficient between IR absorbance at around 1720 cm-1 and Ɛr increase with oxidation time was 99.7%. This remarkably good agreement shows that capacitive probes have the potential to be used as a substitutional in-field tool for in-situ degradation monitoring of hydrocarbon liquids.

Properties of PVA tissue phantoms produced by directional solidification.

Polyvinyl alcohol (PVA) cryogel samples prepared by repeated directional freezing and thawing steps are shown to display an anisotropic structure due to the effect of ice templating during the freezing process. The mechanical stability of the material is improved by the application of a number of directional freeze steps followed by thawing. The anisotropy of the material is confirmed by measurements of acoustic impedance and a simple model is proposed to explain the variation of impedance with the microstructure.

Preparation and characterization of Poly(Vinyl-Alcohol) phantoms mimicking fibrous tissue

Microstructured PVA phantoms mimicking fibrous tissue have been made using a simple freeze casting process. SEM images reveal highly anisotropic microstructure with dimensions of the order of 5–20 um. Characterization of a sample from 20–50 MHz reveals a constant velocity of 1560 ms−1 and an attenuation coefficient slope of approximately 0.25 dB/(mm-MHz).

Acoustic impedance matching with porous aluminium

Highly regular microstructured porous aluminium samples show excellent properties for use in transducers. Three dimensional modeling reveals by varying sample porosity from 0 to 75 %, acoustic impedance in the range 3–15 MRayl can be realized, which can be used to improve impedance matching between the transducer and sample material.