On the validity of the “thin” and “thick” double-layer assumptions when calculating streaming currents in porous media

We find that the thin double layer assumption, in which the thickness of the electrical diffuse layer is assumed small compared to the radius of curvature of a pore or throat, is valid in a capillary tubes model so long as the capillary radius is >200 times the double layer thickness, while the thick double layer assumption, in which the diffuse layer is assumed to extend across the entire pore or throat, is valid so long as the capillary radius is >6 times smaller than the double layer thickness. At low surface charge density (0.5 M) the validity criteria are less stringent. Our results suggest that the thin double layer assumption is valid in sandstones at low specific surface charge (<10 mC· m -2), but may not be valid in sandstones of moderate- to small pore-throat size at higher surface charge if the brine concentration is low (<0.001 M). The thick double layer assumption is likely to be valid in mudstones at low brine concentration (<0.1 M) and surface charge (<10 mC· m -2), but at higher surface charge, it is likely to be valid only at low brine concentration (<0.003 M). Consequently, neither assumption may be valid in mudstones saturated with natural brines. Copyright © 2012 Matthew D. Jackson and Eli Leinov

Investigation of guided wave propagation and attenuation in pipe buried in sand

Long-range guided wave testing is a well-established method for detection of corrosion defects in pipelines. The method is currently used routinely for above ground pipelines in a variety of industries, e.g. petrochemical and energy. When the method is applied to pipes buried in soil, test ranges tend to be significantly compromised and unpredictable due to attenuation of the guided wave resulting from energy leakage into the embedding soil. The attenuation characteristics of guided wave propagation in an 8 in. pipe buried in sand are investigated using a laboratory full-scale experimental rig and model predictions. We report measurements of attenuation of the T(0,1) and L(0,2) guided wave modes over a range of sand conditions, including loose, compacted, mechanically compacted, water saturated and drained. Attenuation values are found to be in the range of 1.65–5.5 dB/m and 0.98–3.2 dB/m for the torsional and longitudinal modes, respectively, over the frequency of 11–34 kHz. The application of overburden pressure modifies the compaction of the sand and increases the attenuation. Mechanical compaction of the sand yields similar attenuation values to those obtained with applied overburden pressure. The attenuation decreases in the fully water-saturated sand, and increases in drained sand to values comparable with those obtained for compacted sand. Attenuation measurements are compared with Disperse software model predictions and confirm that the attenuation phenomenon in buried pipes is essentially governed by the bulk shear velocity in the sand. The attenuation behaviour of the torsional guided wave mode is found not to be captured by a uniform soil model; comparison with predictions obtained with the Disperse software suggest that this is likely to be due to a layer of sand adhering to the surface of the pipe

Guided Wave Attenuation in Coated Pipes Buried in Sand

Long-range guided wave testing (GWT) is routinely used for the monitoring and detection of corrosion defects in above ground pipelines in various industries. The GWT test range in buried, coated pipelines is greatly reduced compared to aboveground pipelines due to energy leakage into the embedding soil. In this study, we aim to increase test ranges for buried pipelines. The effect of pipe coatings on the T(0,1) and L(0,2) guided wave attenuation is investigated using a full-scale experimental apparatus and model predictions. Tests are performed on a fusion-bonded epoxy (FBE)-coated 8” pipe, buried in loose and compacted sand over a frequency range of 10-35 kHz. The application of a low impedance coating is shown to effectively decouple the influence of the sand on the ultrasound leakage from the buried pipe. We demonstrate ultrasonic isolation of a buried pipe by coating the pipe with a Polyethylene (PE)-foam layer that has a smaller impedance than both pipe and sand and the ability to withstand the overb..

Ultrasonic isolation of buried pipes

Long-range guided wave testing (GWT) is used routinely for the monitoring and detection of corrosion defects in above ground pipelines. The GWT test range in buried, coated pipelines is greatly reduced compared to above ground configurations due to energy leakage into the embedding soil. In this paper, the effect of pipe coatings on the guided wave attenuation is investigated with the aim of increasing test ranges for buried pipelines. The attenuation of the T(0,1) and L(0,2) guided wave modes is measured using a full-scale experimental apparatus in a fusion-bonded epoxy (FBE)-coated 8 in. pipe, buried in loose and compacted sand. Tests are performed over a frequency range typically used in GWT of 10–35 kHz and compared with model predictions. It is shown that the application of a low impedance coating between the FBE layer and the sand effectively decouples the influence of the sand on the ultrasound leakage from the buried pipe. Ultrasonic isolation of a buried pipe is demonstrated by coating the pipe with a Polyethylene (PE)-foam layer that has a smaller impedance than both the pipe and sand, and has the ability to withstand the overburden load from the sand. The measured attenuation in the buried PE-foam-FBE-coated pipe is found to be substantially reduced, in the range of 0.3–1.2 dB m⁻¹ for loose and compacted sand conditions, compared to measured attenuation of 1.7–4.7 dB m⁻¹ in the buried FBE-coated pipe without the PE-foam. The acoustic properties of the PE-foam are measured independently using ultrasonic interferometry and incorporated into model predictions of guided wave propagation in buried coated pipe. Good agreement is found between the experimental measurements and model predictions. The attenuation exhibits periodic peaks in the frequency domain corresponding to the through-thickness resonance frequencies of the coating layer. The large reduction in guided wave attenuation for PE-coated pipes would lead to greatly increased GWT test ranges; such coatings would be attractive for new pipeline installations

A guided wave inspection technique for wedge features

Numerous engineering components feature prismatic wedge-like structures that require Non-destructive Evaluation (NDE) in order to ensure functionality or safety. This paper focuses on the inspection of the wedge-like seal fins of a jet engine drum, though the capabilities presented will be generic. It is proposed that anti-symmetric flexural edge modes, feature guided waves localised to the wedge tips, may be used for defect detection. Although analytical solutions exist that characterise the ultrasonic behaviour of ideal wedges, in practise real wedges will be irregular (containing for example truncated tips, are built onto an associated structure or have non straight edges) and therefore generic methodologies are required to characterise wave behaviour in non-ideal wedges. This paper uses a semi-analytical finite element (SAFE) methodology to characterise guided waves in wedge-like features with irregular cross-sections to assess their suitability for NDE inspection and compare them to edge modes in ideal wedges. The science and methodologies required in this paper are necessary to select an appropriate operating frequency for the particular application at hand. Additionally, this paper addresses the practical challenge of excitation and detection of flexural edge modes by presenting a piezoelectric based dry-coupled transducer system suitable for pulse-echo operation. The paper therefore presents the scientific basis required for industrial exploitation, together with the practical tools that facilitate use. The study concludes with the experimental demonstration of the edge wave based inspection of a seal fin, achieving a signal-to-noise ratio of 28 dB from a 0.75 mm radial tip defect

Supporting materials for manuscript entitled 'Ultrasonic Isolation of Buried Pipes'

This set contains data in support of some of the figures appearing in an open access manuscript entitled Ultrasonic Isolation of Buried Pipes that is published at the Journal of Sound of Vibration (DOI:10.1016/j.jsv.2015.10.018). The data set contains ultrasonic guided wave attenuation measurements performed on a full-scale experimental rig, and model simulations output obtained using the software Disperse. The data set contains three files. Details on the specific data is provided within an extra file (readme file).This set contains data in support of some of the figures appearing in an open access manuscript entitled 'Ultrasonic Isolation of Buried Pipes' that is published at the Journal of Sound of Vibration (DOI:10.1016/j.jsv.2015.10.018). The data set contains ultrasonic guided wave attenuation measurements performed on a full-scale experimental rig, and model simulations output obtained using the software Disperse. The data set contains three files. Details on the specific data is provided within an extra file ('readme' file)