Angus I. Best

A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand

Remote seismic methods, which measure the compressional wave (P wave) velocity (Vp) and shear wave (S wave) velocity (Vs), can be used to assess the distribution and concentration of marine gas hydrates in situ. However, interpreting seismic data requires an understanding of the seismic properties of hydrate-bearing sediments, which has proved problematic because of difficulties in recovering intact hydrate-bearing sediment samples and in performing valid laboratory tests. Therefore a dedicated gas hydrate resonant column (GHRC) was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both Vp and Vs made at frequencies and strains relevant to marine seismic investigations. Thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested. The results show a bipartite relationship between velocities and hydrate pore saturation, with a marked transition between 3 and 5% hydrate pore saturation for both Vp and Vs. This suggests that methane hydrate initially cements sand grain contacts then infills the pore space. These results show in detail for the first time, using a resonant column, how hydrate cementation affects elastic wave properties in quartz sand. This information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation.

Role for Flagella but Not Intimin in the Persistent Infection of the Gastrointestinal Tissues of Specific-Pathogen-Free Chicks by Shiga Toxin-Negative Escherichia coli O157:H7

ABSTRACT Shiga toxin (Stx)-positive Escherichia coli O157:H7 readily colonize and persist in specific-pathogen-free (SPF) chicks, and we have shown that an Stx-negative E. coli O157:H7 isolate (NCTC12900) readily colonizes SPF chicks for up to 169 days after oral inoculation at 1 day of age. However, the role of intimin in the persistent colonization of poultry remains unclear. Thus, to investigate the role of intimin and flagella, which is a known factor in the persistence of non-O157 E. coli in poultry, isogenic single- and double-intimin and aflagellar mutants were constructed in E. coli O157:H7 isolate NCTC12900. These mutants were used to inoculate (105 CFU) 1-day-old SPF chicks. In general, significant attenuation of the aflagellate and intimin-aflagellate mutants, but not the intimin mutant, was noted at similar time points between 22 and 92 days after inoculation. The intimin-deficient mutant was still being shed at the end of the experiment, which was 211 days after inoculation, 84 days more than the wild type. Shedding of the aflagellar and intimin-aflagellar mutants ceased 99 and 113 days after inoculation, respectively. Histological analysis of gastrointestinal tissues from inoculated birds gave no evidence for true microcolony formation by NCTC12900 or intimin and aflagellar mutants to epithelial cells. However, NCTC12900 mutant derivatives associated with the mucosa were observed as individual cells and/or as large aggregates. Association with luminal contents was also noted. These data suggest that O157 organisms do not require intimin for the persistent colonization of chickens, whereas flagella do play a role in this process.

Role of NleH, a type III secreted effector from attaching and effacing pathogens, in colonization of the bovine, ovine, and murine gut

ABSTRACT The human pathogen enterohemorrhagic Escherichia coli (EHEC) O157:H7 colonizes human and animal gut via formation of attaching and effacing lesions. EHEC strains use a type III secretion system to translocate a battery of effector proteins into the mammalian host cell, which subvert diverse signal transduction pathways implicated in actin dynamics, phagocytosis, and innate immunity. The genomes of sequenced EHEC O157:H7 strains contain two copies of the effector protein gene nleH, which share 49% sequence similarity with the gene for the Shigella effector OspG, recently implicated in inhibition of migration of the transcriptional regulator NF-κB to the nucleus. In this study we investigated the role of NleH during EHEC O157:H7 infection of calves and lambs. We found that while EHEC ΔnleH colonized the bovine gut more efficiently than the wild-type strain, in lambs the wild-type strain exhibited a competitive advantage over the mutant during mixed infection. Using the mouse pathogen Citrobacter rodentium, which shares many virulence factors with EHEC O157:H7, including NleH, we observed that the wild-type strain exhibited a competitive advantage over the mutant during mixed infection. We found no measurable differences in T-cell infiltration or hyperplasia in colons of mice inoculated with the wild-type or the nleH mutant strain. Using NF-κB reporter mice carrying a transgene containing a luciferase reporter driven by three NF-κB response elements, we found that NleH causes an increase in NF-κB activity in the colonic mucosa. Consistent with this, we found that the nleH mutant triggered a significantly lower tumor necrosis factor alpha response than the wild-type strain.

Comparison of in situ and laboratory acoustic measurements on Lough Hyne marine sediments

Compressional wave velocity and attenuation were measured at frequencies of 200–1500 Hz on seafloor sediments at Lough Hyne, Ireland, using a mini-boomer source and hydrophone array. Velocity and attenuation were also measured in the laboratory at 200–800 kHz on a 1 m long sediment core taken from the site. The in situ results indicate an average sediment phase velocity of about 1600 m/s and sediment quality factor of 10–20. The laboratory core measurements give an average phase velocity of 1793±26 m/s and quality factor of 16±5. The poorly sorted, Lough Hyne sediments are highly attenuating and highly dispersive when compared to values published in the literature for well-sorted, marine sediments such as clean sands and marine clays. The results are consistent with the few published data for poorly sorted sediments, and indicate that intrinsic attenuation is highest when the mass ratio of mud (clay + silt) to sand grade particles is close to unity. It is proposed that compliance heterogeneities are most abundant when mud and sand grade particles are present in roughly equal proportions, and that the observations support local viscous fluid flow as the most likely loss mechanism.

Tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments

Remote prediction of gassy marine sediment properties is important for geohazard assessment. Gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity-frequency and attenuation-frequency relationships that depend on gas bubble size, gas content, and sediment elastic properties. An acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at Dibden Bay, Southampton, United Kingdom. A vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed. A miniboomer in the seabed above the array was used to generate broadband (600 Hz to 3000 Hz) acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m (subaerial exposure) at low tide and 2.35 m at high tide. The calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum (over 200 dB/m) that shifts in frequency from 1050 Hz at low tide to 1250 Hz at high tide, thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments. Modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum, supported by X-ray computed tomography scan observations on a pressure core (which also indicate that bubble shape depends on sediment type). Modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum, implying that gas diffusion and nonspherical bubbles are significant.

Shallow seabed methane gas could pose coastal hazard

Abnormally high levels of methane gas in seafloor sediments could pose a major hazard to coastal populations within the next 100 years through their impact on climate change and sea level rise. Marine scientists have known for many years that biogenic methane (CH4) is generated in shallow seabed sediments on continental margins, especially in rapidly deposited muddy sediments with high organic matter content (see Methane Flux Control in Ocean Margin Sediments (METROL) project in Mienert et al., [2004]). Gassy sediments are found in river deltas, estuaries, and harbors, but also in deeper waters on continental shelves and slopes. Human activities can accelerate natural seafloor gas generation by increasing the supply of sediments and organic matter from rivers through deforestation and intensive farming, and also by the disposal of human waste at sea. When this extra organic matter becomes buried to about one meter beneath the seabed, biogeochemical processes start to convert it to CH4 [Floodgate and Judd, 1992]. The impact of this extra CH4 could be felt within the next 100 years, assuming a one-centimeter-per-year sediment accumulation rate.

Effects of fluids and dual-pore systems on pressure-dependent velocities and attenuations in carbonates

TheeffectsoffluidsubstitutiononP-andS-wavevelocitiesin carbonates of complex texture are still not understood fully. The often-used Gassmann equation gives ambiguous results when compared with ultrasonic velocity data. We present theoretical modelingofvelocityandattenuationmeasurementsobtainedata frequency of 750 kHz for six carbonate samples composed of calcite and saturated with air, brine, and kerosene.Although porosities 2%‐14% and permeabilities 0‐74 mD are relatively low, velocity variations are large. Differences between the highest and lowest P- and S-wave velocities are about 18% and 27% for brine-saturated samples at 60 and 10 MPa effective pressure, respectively. S-wave velocities are measured for two orthogonal polarizations;forfourofsixsamples,anisotropyisrevealed.The Gassmann model underpredicts fluid-substitution effects by 2% for three samples and by as much as 5% for the rest of the six samples. Moreover, when dried, they also show decreasing attenuationwithincreasingconfiningpressure.Tomodelthisbehavior, we examine a pore model made of two pore systems: one constitutesthemainanddrainableporosity,andtheotherismade ofundrainedcracklikeporesthatcanbeassociatedwithgrain-tograincontacts.Inaddition,thesedriedrocksamplesaremodeled to contain a fluid-filled-pore system of grain-to-grain contacts, potentiallycausinglocalfluidflowandattenuation.Forthetheoretical model, we use an inclusion model based on the T-matrix approach,whichalsoconsiderseffectsofporetextureandgeometry, and porefluid, global- and local-fluidflow. By using a dualpore system, we establish a realistic physical model consistently describingthemeasureddata.

The frequency dependence of compressional wave velocity and attenuation coefficient of intertidal marine sediments

To advance the present understanding of the frequency dependence of compressional wave velocity and attenuation in marine sediments a series of well-constrained in situ acoustic transmission experiments (16 to 100kHz) were performed on intertidal sediments. The processing techniques incorporated in situ spreading losses, sediment to transducer coupling and thorough error analyses. Significant variations in velocity and attenuation were observed over scales of tens of meters within the same sediment type. Velocity was generally nondispersive in sands, while highly variable silt velocities prevented any meaningful dispersion estimates from being determined. The attenuation coefficient was proportional to frequency for 75% of the experimental sites. The measured compressional wave properties were compared to predictions from the Grain-Shearing model. For the sandy sites, the phase velocities predicted by the Grain Shearing model exceed those measured, while predicted phase velocities agreed with measured gro...

An anisotropic model for the electrical resistivity of two-phase geologic materials

Electrical and electromagnetic surveys of the seafloor provide valuable information about the macro and microscopic properties of subseafloor sediments. Sediment resistivity is highly variable and governed by a wide range of properties including pore-fluid salinity, pore-fluid saturation, porosity, pore geometry, and temperature. A new anisotropic, twophase, effective medium model describes the electrical resistivity of porous rocks and sediments. The only input parameters required are the resistivities of the solid and fluid components, their volume fractions and grain shape. The approach makes use of the increase in path length taken by an electrical current through an idealized granular medium comprising of aligned ellipsoidal grains. The model permits both solid and fluid phases to have a finite conductivity useful for dealing with surface charge conduction effects associated with clay minerals and gives results independent of grain size hence, valid for a wide range of sediment types. Furthermore, the model can be used to investigate the effects of grain aspect ratio and alignment on electrical resistivity anisotropy. Good agreement was found between the model predictions and laboratory measurements of resistivity and porosity on artificial sediments with known physical properties.

The role of free gas in the activation of submarine slides in Finneidfjord

Free gas is evident on high resolution seismic profiles near the 1996 Finneidfjord submarine landslide, Norway. A bright reflector at about 6 m sub-seabed depth on intact terrain adjacent to the slide coincides with the initial failure surface. We hypothesise that the bright reflector comprises free gas collecting in relatively sandy layers, and that the free gas could have contributed to the generation of excess pore pressures and the initiation of the submarine landslide. Preliminary measurement of the bright layer reflection coefficient from seismic sections, and interpretation of the available geotechnical data, support this hypothesis.