ESC Publications - Cambridge Univesity
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Isobaric specific heat capacity of natural gas as a function of specific gravity, pressure and temperature
No full textThis study was undertaken to support a remediation technique that will be applied to contaminated groundwater in the vicinity of Ghonan landfill, eastern Saudi Arabia. Preliminary field investigations involved hydrogeological characterization of the entire area. The improved understanding of the underlying geology and groundwater movement gained from the preliminary studies helped in determining a test site within the proximity of the landfill. One remediation well and five monitoring wells were constructed at the test site. Optimum pumping rate for the remediation well was determined to range from 2.642 through 7.926 gallons per minute (gpm) (0.01 m3/min through 0.03 m3/min) based on site-specific hydrogeological investigations and mathematical simulation. A concentration of 0.05 mg/L of methyl tertiary-butyl ether (MTBE) contaminated the aquifer in the test site. The simulated concentration of MTBE at the point of exposure after a period of 2 years was found to be higher than the maximum contaminant level of 0.005 mg/L set by the United States Environmental Protection Agency (U.S. EPA). The results of risk assessment conducted revealed that domestic use of groundwater in the study area through any of the exposure pathways (ingestion, dermal contact, and inhalation in the shower) may lead to development of health risks to human receptors. The landfill, which is being operated as a hazardous landfill and a dump site, may become a source of groundwater pollution in its vicinity in the near future. As a potential health risk, it should be controlled properly by remediating the aquifer and implementing environmental measures to the landfill users. © 2014 Saudi Society for Geosciences
Imaging topographic growth by long-lived postseismic afterslip at Sefidabeh, east Iran
No full textThis paper describes observations and models of the postseismic deformation following the 1994 Sefidabeh earthquake sequence in east Iran, which shed light on the nature of the earthquake cycle and the mechanisms of topographic growth in the region. Interferometric synthetic aperture radar observations show creeping fault motion (“postseismic afterslip”) on an array of faults. Some of these faults probably represent the extensions of those that ruptured in the blind thrust-faulting earthquakes in 1994, and cut through the entire seismogenic layer, while others are shallow and break up the hanging walls of the coseismic faults. The postseismic slip accommodates at least part of the vertical displacement gradient resulting from the buried coseismic slip, which was concentrated at depths of greater than ∼5 km. The postseismic afterslip is visible for over 16 years following the earthquakes. Agreement between the areas of postseismic uplift and indications of long-term motion preserved in the geomorphology suggest that shallow fault slip during seismic cycles similar to the one we have observed governs the development of the landscape in the region. Slip on an array of shallow faults provides a mechanism for the development of short-wavelength topography and geological structures above active thrust faults and has important implications for the interpretation of shallow geological features produced in regions experiencing similar seismic cycles to that at Sefidabeh
The origin of celestine–quartz–calcite geodes associated with a basaltic dyke, Makhtesh Ramon, Israel
No full textSpectacular celestine geodes occur in a Jurassic peri-evaporitic sequence (Ardon Formation) exposed in Makhtesh Ramon, southern Israel. The geodes are found only in one specific location: adjacent to an intrusive contact with a Lower Cretaceous basaltic dyke. Celestine, well known in sedimentary associations worldwide and considered as a low temperature mineral, may therefore be associated with magmatic-induced hydrothermal activity. Abundant fluid inclusions in celestine provide valuable information on its origin: gas-rich inclusions in celestine interiors homogenized at T>=200°C whereas smaller liquid-rich inclusions record the growth of celestine rims at T<=200°C. Near 0°C melting temperatures of some fluid inclusions and the occurrence of hydrous Ca-sulphate solid crystals in other inclusions indicate that celestine precipitated from variably concentrated Ca-sulphate aqueous solutions of meteoric origin. Celestine crystallized from meteoric water heated by the cooling basaltic dyke at shallow levels (c. 160 m) during a Lower Cretaceous thermal perturbation recorded by regional uplift and magmatism. The 87Sr/86Sr ratio of geode celestine, 0.7074, is similar to that measured in the dolostones of the host Jurassic sequence, but differs markedly from the non-radiogenic ratio of the dyke. Strontium in celestine was derived from dolostones preserving the 87Sr/86Sr of Lower Jurassic seawater, while sulphur (δ34S = 19.9‰) was provided by in situ dissolution of precursor marine gypsum (δ34S = 16.8‰) indicated by relict anhydrite inclusions in celestine. Low-temperature meteoric fluid flow during the Campanian caused alteration of the dyke into secondary clays and alteration of geodal celestine into quartz, calcite and iron oxides
Coalescence Microseismic Mapping
No full textEarthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations. If mis-picks are made, these will contribute to the location, therefore causing potential bias. For data recorded by a dense seismic array, direct imaging methods can be applied instead. We describe the ‘coalescence microseismic mapping’ method, which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array. By continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio-temporal coordinates of the origins of seismic events. Noisy data are migrated away from the correct origin, so do not contribute to errors in location. The method is rooted in a Bayesian formulation of event location traveltime inversion, allows imaging of source locations and has the capacity to handle errors in modelled traveltimes. It has the advantage of working with any 3-D velocity model, which therefore may include anisotropy. It also automatically incorporates both P- and S-wave data. A multiresolution grid search leads to an efficient implementation, with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality. We discuss the theory and implementation of this method and illustrate it with real data from microseismic events in Iceland caused by melt intrusion in the crust
Normal mode splitting due to inner core attenuation anisotropy
No full textThe Earth's inner core displays transverse velocity anisotropy with cylindrical symmetry, which causes the anomalous zonal splitting of inner core sensitive normal modes. In this paper, we extend existing theory for calculating normal mode splitting from models of cylindrical velocity anisotropy to include models of anelastic attenuation anisotropy. Furthermore, we derive the equations that can be used to rewrite the attenuation anisotropy parameters natural to normal mode considerations in terms of body wave attenuation anisotropy for compressional and shear waves
Sulfur degassing due to contact metamorphism during flood basalt eruptions
No full textAbstract We present a study aimed at quantifying the potential for generating sulfur-rich gas emissions from the devolatilization of sediments accompanying sill emplacement during flood basalt eruptions. The potential contribution of sulfur-rich gases from sediments might augment substantially the magma-derived sulfur gases and hence impact regional and global climate. We demonstrate, from a detailed outcrop-scale study, that sulfur and total organic carbon have been devolatilized from shales immediately surrounding a 3-m thick dolerite sill on the Isle of Skye, Scotland. Localized partial melting occurred within a few centimetres of the contact in the shale, generating melt-filled cracks. Pyrite decomposed on heating within 80 cm of the contact, generating sulfur-rich gases (a mixture of H2S and SO2) and pyrrhotite. The pyrrhotite shows 32S enrichment, due to loss of 34S-enriched SO2. Further decomposition and oxidation of pyrrhotite resulted in hematite and/or magnetite within a few cm of the contact. Iron sulfates were produced during retrogressive cooling and oxidation within 20 cm of the contact. Decarbonation of the sediments due to heating is also observed, particularly along the upper contact of the sill, where increasing δ13C is consistent with loss of methane gas. The geochemical and mineralogical features observed in the shales are consistent with a short-lived intrusion, emplaced in <5 h. The dolerite magma contains pervasive pyrite and localized sulfur concentrations greater than the sulfur concentration at sulfide liquid saturation, consistent with addition of sulfur (perhaps from sediments) at a late stage. Our study provides evidence for desulfurization, as well as decarbonation, of shales adjacent to an igneous intrusion. The liberated fluids, rich in sulfur and carbon, are likely to be focused along regions of low pore fluid pressure along the margins of the sill. The sulfur gases liberated from the sediments would have augmented the sulfur dioxide (and hydrogen sulfide) yield of the eruption substantially, had they reached the surface. This enhancement of the magmatic sulfur budget has important implications for the climate impact of large flood basalt eruptions that erupt through thick, volatile-rich sedimentary sequences
Fire and Brimstone: The Microbially Mediated Formation of Elemental Sulfur Nodules from an Isotope and Major Element Study in the Paleo-Dead Sea
No full textWe present coupled sulfur and oxygen isotope data from sulfur nodules and surrounding gypsum, as well as iron and manganese concentration data, from the Lisan Formation near the Dead Sea (Israel). The sulfur isotope composition in the nodules ranges between -9 and -11‰, 27 to 29‰ lighter than the surrounding gypsum, while the oxygen isotope composition of the gypsum is constant around 24‰. The constant sulfur isotope composition of the nodule is consistent with formation in an ‘open system’. Iron concentrations in the gypsum increase toward the nodule, while manganese concentrations decrease, suggesting a redox boundary at the nodule-gypsum interface during aqueous phase diagenesis. We propose that sulfur nodules in the Lisan Formation are generated through bacterial sulfate reduction, which terminates at elemental sulfur. We speculate that the sulfate-saturated pore fluids, coupled with the low availability of an electron donor, terminates the trithionate pathway before the final two-electron reduction, producing thionites, which then disproportionate to form abundant elemental sulfur
Triggering of microearthquakes in Iceland by volatiles released from a dyke intrusion
No full textWe suggest that carbon dioxide exsolved from a mid-crustal basaltic dyke intrusion in Iceland migrated upwards and triggered shallow seismicity by allowing failure on pre-existing fractures under the relatively low elastic stresses (100–200 kPa; 1–2 bar) generated by the dyke inflation. Intense swarms of microseismicity accompanied magmatic intrusion into a dyke at depths of 13–19 km in the crust of Iceland's Northern Volcanic Rift Zone during 2007–2008. Contemporaneously, a series of small normal earthquakes, probably triggered by elastic stresses imposed by the dyke intrusion, occurred in the uppermost 4 km of crust: fault plane solutions from these are consistent with failure along the extensional fabric and surface fissure directions mapped in the area, suggesting that the faults failed along existing rift zone fabric even though the mid-crustal dyke is highly oblique to it. Several months after the melt froze in the mid-crust and seismicity associated with the intrusion had ceased, an upsurge in shallow microseismicity began in the updip projection of the dyke near the brittle–ductile transition at 6–7 km depth below sea level. This seismicity is caused by failure on right-lateral strike-slip faults, with fault planes orientated 23 ± 3°, which are identical with the 24 ± 2° orientation in this area of surface fractures and fissures caused by plate spreading and extension of the volcanic rift zone. However, these earthquakes have T-axes approximately aligned with the opening direction of the dyke, and the right-lateral sense of failure is opposite that of regional strike-slip faults. We suggest that the fractures occurred along pre-existing weaknesses generated by the pervasive fabric of the rift zone, but that the dyke opening in the mid-crust beneath it caused right-lateral failure. The seismicity commenced after a temporal delay of several months and has persisted for over 3 yr. We propose that fluids exsolved from the magma in the dyke, primarily carbon dioxide, percolated updip and to shallower depths predominantly along pre-existing fractures. Increased pore pressure from the volatiles reduced the effective normal compressive stress on faults, increasing the likelihood of failure and allowing the modest stress changes generated by the intrusion to cause failure. Propagation of volatiles through the crust would also account for the observed time delay between the intrusion at depth and the shallow earthquake clusters. A further short-lived cluster of earthquakes at 2–4 km depth beneath the surface exhibits left-lateral strike-slip faulting with epicentres well orientated along a lineation which is identical with other subparallel strike-slip faults in the area that transfer motion between two adjacent spreading segments. These shallow earthquakes lie beyond lobes of significant positive Coulomb stress change caused by the intrusion, implying minimal modifications to the stress field in their vicinity; hence, they continue to respond to the regional stress field rather than the local stress field generated by the dyke intrusion
Seismic Imaging of Sequestered Carbon Dioxide
No full textTime-lapse, three-dimensional seismic surveys have imaged an accumulation of injected CO 2 at the Sleipner field, in the North Sea basin. The changing pattern of reflectivity suggests
that CO 2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone. Nine reflective horizons within the reservoir have been mapped
on six surveys acquired between 1999 and 2008