1,721,087 research outputs found
The role of intimin and flagella in the persistence of Escherichia coli O157:H7 in poultry
No full textEscherichia coli O157:H7 has been implicated in many major and sporadic outbreaks of food-borne related disease in humans. Bovine and bovine derived meat products are cited as the main source of infection and deliberate inoculation studies of ruminants show that the surface-arrayed outer membrane protein intimin facilitates persistent colonisation. The prevalence of E. coli O157:H7 infection in birds is low, but several deliberate inoculation studies show that poultry are readily and persistently infected by this organism indicating a possible threat to public health. The mechanisms of E. coli O157:H7 colonisation of poultry are not understood, but flagella are important for the colonisation of poultry by avian pathogenic E. coli. Whether intimin plays a role in persistent colonisation of poultry has not been determined. To investigate the role of intimin and flagella, defined knockout single and double intimin and aflagellate mutants were constructed in a well characterised non-toxigenic E. coli O157:H7 isolate (NCTC 12900) and tested in adherence assays with an avian epithelial cell line (Div-1) and used to inoculate 1-day-old SPF chicks. In vitro, NCTC 12900 intimin contributed significantly to adherence, but not invasion, whereas NCTC 12900 flagella only contributed to invasion. NCTC 12900 intimin, but not flagella was required for micro-colony and AE lesion formation. In vivo studies revealed that the wild-type could form micro-colonies on the caecal mucosa of SPF chicks and could persistently colonise birds for up to 169 days, ceasing 9 days after the birds came into lay and 6% of eggs were contaminated on the eggshell. NCTC 12900 flagella, but not intimin contributed to persistent colonisation in the chick.</p
Effects of aligned fractures on the response of velocity and attenuation ratios to water saturation variation: a laboratory study using synthetic sandstones
Full text linkP-wave-to-S-wave ratios are important seismic characterization attributes. Velocity ratios are sensitive to the petrophysical properties of rocks and to the presence of gas. Attenuation ratios have also been shown to be sensitive to the presence of partial liquid/gas saturation. The relationship between liquid/gas saturation and P-wave and S-wave ratios has been used to distinguish gas-saturated rocks from liquid-saturated rocks. Aligned fractures are common in the Earth's crust and cause seismic anisotropy and shear wave splitting. However, most existing relationships between partial gas/liquid saturation and P-wave and S-wave ratios are for non-fractured rocks. We present experimental results comparing the effects of changing water saturation on Qs/Qp versus Vp/Vs ratios between a non-fractured rock and one containing fractures aligned parallel to wave propagation direction. We also study the effects of aligned fractures on the response of Vp/Vs to changing water saturation using synthetic fractured sandstones with fractures aligned at 45o and parallel to the wave propagation direction. The results suggest that aligned fractures could have significant effects on the observed trends, some of which may not be obvious. Fractures aligned parallel to wave propagation could change the response of Qs/Qp versus Vp/Vs ratios to water saturation from previously reported trends. Shear wave splitting due to the presence of aligned fractures results in two velocity ratios (Vp/Vs1 and Vp/Vs2). The fluid independence of shear wave splitting for fractures aligned parallel to wave propagation direction means the difference between Vp/Vs1 and Vp/Vs2 is independent of water saturation. For fractures aligned at oblique angles, shear wave splitting can be sensitive to water saturation and consequently be frequency dependent, which can lead to fluid and frequency-dependent differences between Vp/Vs1 and Vp/Vs2. The effect of aligned fractures on Vp/Vs ratios not only depends on the fracture effects on both P-wave and S-wave velocities but also on the effects of water saturation distribution on the rock and fracture stiffness, and hence on the P-wave and S-wave velocities. As such, these effects can be frequency dependent due to wave-induced fluid flow. A simple modelling study combining a frequency-dependent fractured rock model, and a frequency-dependent partial saturation model was used to gain valuable interpretations of our experimental observations and possible implications, which would be useful for field seismic data interpretation
Experimental rig to improve the geophysical and geomechanical understanding of CO2 reservoirs
Full text linkWe intend to perform experiments that simulate real Carbon Capture and Storage (CCS) conditions in the laboratory, and hence provide the necessary knowledge to interpret field seismic surveys. Primarily, our research is focused on determining seismic rock properties (i.e., wave velocities and attenuation) of real and artificial 50 mm diameter brine-CO2-bearing sandstone and sand samples that are representative host rocks of real CCS scenarios. Accordingly, we have integrated into a new triaxial cell system both an ultrasonic pulse-echo method for accurate velocity (± 0.3%) and attenuation (± 0.1 dB cm-1) measurements, and an electrical resistivity tomography (ERT) method to monitor homogeneity of pore fluid distribution within the samples. The use of ERT provides calibration data for field scale techniques (such as marine controlled source electromagnetic surveying) but also allows measurements of bulk resistivity, fluid diffusion monitoring, flow pathway characterization, and determination of the relative permeability for different brine/brine-CO2 ratios. By simultaneously measuring ultrasonic P- and S-wave velocities and electrical resistivity, we also provide data for joint inversion of seismic and electric field data. Furthermore, the stress-strain behaviour of the sample is continuously monitored with the aid of electrical gauges, so that we deal consistently and simultaneously with the geophysical and geomechanical response of the reservoir when submitted to CO2 injections
Integrated geophysical and hydromechanical assessment for CO2 storage: shallow low permeable reservoir sandstones
Full text linkGeological reservoirs can be structurally complex and can respond to CO2 injection both geochemically and geomechanically. Hence, predicting reservoir formation behaviour in response to CO2 injection and assessing the resulting hazards are important prerequisites for safe geological CO2 storage. This requires a detailed study of thermal-hydro-mechanical-chemical coupled phenomena that can be triggered in the reservoir formation, most readily achieved through laboratory simulations of CO2 injection into typical reservoir formations. Here, we present the first results from a new experimental apparatus of a steady-state drainage flooding test conducted through a synthetic sandstone sample, simulating real CO2 storage reservoir conditions in a shallow (?1 km), low permeability ?1mD, 26% porosity sandstone formation. The injected pore fluid comprised brine with CO2 saturation increasing in steps of 20% brine/CO2 partial flow rates up to 100% CO2 flow. At each pore fluid stage, an unload/loading cycle of effective pressure was imposed to study the response of the rock under different geomechanical scenarios. The monitoring included axial strains and relative permeability in a continuous mode (hydromechanical assessment), and related geophysical signatures (ultrasonic P-wave and S-wave velocities Vp and Vs, and attenuations Qp?1 and Qs?1, respectively, and electrical resistivity). On average, the results showed Vp and Vs dropped ?7% and ?4% respectively during the test, whereas Qp?1 increased ?55% and Qs?1 decreased by ?25%. From the electrical resistivity data, we estimated a maximum CO2 saturation of ?0.5, whereas relative permeability curves were adjusted for both fluids. Comparing the experimental results to theoretical predictions, we found that Gassmann's equations explain Vp at high and very low CO2 saturations, whereas bulk modulus yields results consistent with White and Dutta–Odé model predictions. This is interpreted as a heterogeneous distribution of the two pore fluid phases, corroborated by electrical resistivity tomography images. The integration of laboratory geophysical and hydromechanical observations on representative shallow low-permeable sandstone reservoir allowed us to distinguish between pure geomechanical responses and those associated with the pore fluid distribution. This is a key aspect in understanding CO2 injection effects in deep geological reservoirs associated with carbon capture and storage practices
Characterization of buried inundated peat on seismic (Chirp) data, inferred from core information
No full textPeat horizons provide a wide range of critical environmental and direct archaeological information for both archaeologists and Quaternary geologists. At present, such data are typically obtained from terrestrial exposures or cores, and occasional offshore cores. These data can provide invaluable and detailed site-specific environmental information but require a relatively high spatial sampling strategy to provide more regional-scale information. Through a comparison of laboratory, in situ acoustic and sedimentary analyses, this paper presents evidence to suggest that peat buried in fine to medium grained, marine, siliciclastic sediments has an easily identifiable acoustic signature. The very low densities recorded by buried peats result in a distinct negative peak in the reflectivity series. Comparison of synthetic seismograms with in situ seismic data confirms that this negative peak can be easily identified from seismic profiles. Reanalysis of a decade of Chirp (sub-bottom) data, acquired from the Solent Estuary, indicates that possible extensive peat deposits, dating from the Late-glacial to early Holocene, can be traced at depth in this estuary using geophysical methods. The results of this study could be significant for future research into submerged landscape reconstructions
Laboratory determination of the full electrical resistivity tensor of heterogeneous carbonate rocks at elevated pressures
No full textWe describe a measurement system capable of determining the full resistivity tensor of core samples at elevated, geologically representative, pressures using a galvanic method. It is suitable for heterogeneous rocks where it is difficult to measure tensorial resistivity without bias from sample selection and heterogeneity. We demonstrate the efficacy of the system using both synthetic data and measurements on carbonate rock core samples. The apparatus employs a computer controlled array of 16 electrodes to inject current into, and measure boundary voltages on, a 5 cm diameter cylindrical sample. A computationally efficient FE algorithm is used to retrieve the full resistivity tensor from the measured voltages. The algorithm uses isotropic Finite Element code to calculate anisotropic solutions for samples of arbitrary geometry. Initial results from Jurassic limestone and Triassic dolomite samples, reveal cm-scale heterogeneity and significant bulk anisotropy consistent with rock fabric observed in X-ray Computed Tomography scan images
Attenuation of seismic waves in methane gas hydrate-bearing sand
No full textCompressional wave (P wave) and shear wave (S wave) velocities (Vp and Vs, respectively) from remote seismic methods have been used to infer the distribution and volume of gas hydrate within marine sediments. Recent advances in seismic methods now allow compressional and shear wave attenuations (Q1p and Q1s, respectively) to be measured. However, the interpretation of these data is problematic due to our limited understanding of the effects of gas hydrate on physical properties. Therefore, a laboratory gas hydrate resonant column was developed to simulate pressure and temperature conditions suitable for methane gas hydrate formation in sand specimens and the subsequent measurement of both Q1p and Q1s at frequencies and strains relevant to marine seismic surveys. 13 dry (gas saturated) sand specimens were investigated with different amounts of methane gas hydrate evenly dispersed throughout each specimen. The results show that for these dry specimens both Q1p and Q1s are highly sensitive to hydrate saturation with unexpected peaks observed between 3 and 5 per cent hydrate saturation. It is thought that viscous squirt flow of absorbed water or free gas within the pore space is enhanced by hydrate cement at grain contacts and by the nanoporosity of the hydrate itself. These results show for the first time the dramatic effect methane gas hydrate can have on seismic wave attenuation in sand, and provide insight into wave propagation mechanisms. These results will aid the interpretation of elastic wave attenuation data obtained using marine seismic prospecting methods
Anomalous electrical resistivity anisotropy in clean reservoir sandstones
No full textWe report novel laboratory measurements of the full electrical resistivity tensor in reservoir analogue quartzose sandstones with clay contents less than 1.5%. We show that clean, homogeneous, visually uniform sandstone samples typically display between 15% and 25% resistivity anisotropy with minimum resistivity normal to the bedding plane. Thin-section petrography, analysis of fabric anisotropy, and comparison to finite-element simulations of grain pack compaction show that the observed anisotropy symmetries and magnitudes can be explained by syn-depositional and post-depositional compaction processes. Our findings suggest that: electrical resistivity anisotropy is likely to be present in most clastic rocks as a consequence of ballistic deposition and compaction; compaction may be deduced from measurements of electrical anisotropy; and the anisotropy observed at larger scales in well logging and controlled-source electromagnetic data, with maximum resistivity normal to bedding, is most likely the result of meso-scale (10-1 m – 101 m) periodic layering of electrically dissimilar lithologies
A new laboratory technique for determining the compressional wave properties of marine sediments at sonic frequencies and in situ pressures
Full text linkWe describe a new laboratory technique for measuring the compressional wave velocity and attenuation of jacketed samples of unconsolidated marine sediments within the acoustic (sonic) frequency range 1–10 kHz and at elevated differential (confining – pore) pressures up to 2.413 MPa (350 psi). The method is particularly well suited to attenuation studies because the large sample length (up to 0.6 m long, diameter 0.069 m) is equivalent to about one wavelength, thus giving representative bulk values for heterogeneous samples. Placing a sediment sample in a water-filled, thick-walled, stainless steel Pulse Tube causes the spectrum of a broadband acoustic pulse to be modified into a decaying series of maxima and minima, from which the Stoneley and compressional wave, velocity and attenuation of the sample can be determined. Experiments show that PVC and copper jackets have a negligible effect on the measured values of sediment velocity and attenuation, which are accurate to better than ± 1.5% for velocity and up to ± 5% for attenuation. Pulse Tube velocity and attenuation values for sand and silty-clay samples agree well with published data for similar sediments, adjusted for pressure, temperature, salinity and frequency using standard equations. Attenuation in sand decreases with pressure to small values below Q−1 = 0.01 (Q greater than 100) for differential pressures over 1.5 MPa, equivalent to sub-seafloor depths of about 150 m. By contrast, attenuation in silty clay shows little pressure dependence and intermediate Q−1 values between 0.0206–0.0235 (Q = 49–43). The attenuation results fill a notable gap in the grain size range of published data sets. Overall, we show that the Pulse Tube method gives reliable acoustic velocity and attenuation results for typical marine sediments
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