Seismic velocity structure of oceanic crust by inversion using genetic algorithms     

Guy G. Drijkoningen  Robert S. White 《Geophysical Journal International》1995,123(3):653-664

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Tracing groundwater flow in the Borden aquifer using krypton-85   总被引：3，自引：0，他引：3  

William M. Smethie Jr.  D.Kip Solomon  Sherry L. Schiff  Guy G. Mathieu 《Journal of Hydrology》1992,130(1-4):279-297

Krypton-85 was measured in air, soil gas, and ground water at the Borden aquifer in Ontario in October 1989. The measured specific activities in air and soil gas were 52.0 ± 2.0 and 53.6 ± 1.8 disintegrations per min (dpm) cm⁻³ krypton. These measurements are in excellent agreement with the global atmospheric trend and demonstrate that krypton-85 enters the water table at the Borden site without a lag in the soil gas reservoir. The krypton-85 specific activity in five groundwater samples ranged from 44.9 to 9.5 dpm cm⁻³ corresponding to groundwater ages of 2–17 years with a monotonic decrease in specific activity (increase in age) along the groundwater flow path. Travel times calculated from a two-dimensional steady-state model of groundwater flow agree well with the krypton-85 ages in the main recharge region of the aquifer where flow is predominantly vertical, but were 30–40% older than the krypton-85 age downstream of the main recharge area where the flow is mainly horizontal. The effect of dispersion on the distribution of krypton-85 was determined by modelling the transport of krypton-85 in the Borden aquifer with a two-dimensional time-dependent advection dispersion model using the steady-state flow field. Agreement between model specific activity and observed specific activity was excellent for samples in the main recharge region, but the model specific activities were 30–50% lower than observed specific activities in the region of horizontal flow. The differences in travel times and krypton-85 ages and in model krypton-85 and observed krypton-85 specific activities are considered to be small given the heterogeneities that exist in the hydraulic conductivity and aquifer geometry and hence in the groundwater flow field. The model simulated krypton-85 distribution was not sensitive to changes in longitudinal dispersivity and was only weakly sensitive to changes in transverse dispersivity. The geochemical inertness, well-defined source function, and insensitivity to dispersion of krypton-85 allow estimates of groundwater age to be made in a straightforward manner and measurement of krypton-85 can significantly enhance the characterization of groundwater flow in many shallow subsurface systems.  相似文献   

Observations of currents and density structure across a buoyant plume front     

Guy Gelfenbaum  Richard P. Stumpf 《Estuaries and Coasts》1993,16(1):40-52

Observations of the Mobile Bay, Alabama, plume during a flood event in April 1991 reveal significant differences in the current field on either side of a front associated with the buoyant plume. During a strong southeasterly wind, turbid, low salinity water from Mobile Bay was pushed through an opening in the west side of the ebb-tidal delta and moved parallel to the coast. A stable front developed between the low salinity water of the buoyant plume (11‰) and the high salinity coastal water (>23‰) that was being forced landward by the prevailing winds. Despite the shallow water depth of 6 m, measurements of currents, temperature, and salinity show large shears and density gradients in both the vertical and the horizontal directions. At a station outside of the buoyant plume, currents at 0.5 m and 1.5 m below the surface were in the same direction as the wind. Inside the plume, however, currents at 0.5 m below the surface were parallel to the coast, 45°, off the direction of the wind and the magnitude was 45% larger than the magnitude of the surface currents outside the plume. Beneath the level of the plume, the currents were identical to the wind-driven currents in the ambient water south of the front. Our observations suggest that the wind-driven surface currents of the ambient water converged with the buoyant plume at the front and were subducted beneath the plume. The motion of the ambient coastal surface water was in the direction of the local wind stress, however, the motion of the plume had no northerly component of motion. The plume also did not show any flow toward the front, suggesting a balance between the northerly component of wind stress and the southerly component of buoyant spreading. In addition, the motion of the plume did not appear to affect the motion of the underlying ambient water, suggesting a lack of mixing between the two waters.  相似文献   

Geochemistry of basalts from the Dumisseau Formation, southern Haiti: implications for the origin of the Caribbean Sea crust     

Gautam Sen  Rosemary Hickey-Vargas  David Guy Waggoner  Florentin Maurrasse 《Earth and Planetary Science Letters》1988,87(4)

Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO₂ contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm.La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO₂ (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO₂ (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts.⁸⁷Sr⁸⁶Sr ratios vary from 0.70280 to 0.70316,¹⁴³Nd¹⁴⁴Nd from 0.512929 to 0.513121, and²⁰⁶Pb²⁰⁴Pb from 19.00 to 19.27. LREE-depleted lavas have high¹⁴³Nd¹⁴⁴Nd (0.51309–0.51310) typical of MORBs, whereas¹⁴³Nd¹⁴⁴Nd in the LREE-enriched lavas varies widely (0.512929–0.513121).Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.  相似文献   

Strength Behavior of Marine Sands at Elevated Confining Stresses     

Rebecca Desrosiers  Armand J. Silva 《Marine Georesources & Geotechnology》2002,20(1):1-19

A testing program was initiated to determine the stress-strain and strength behavior of two very different marine sands (a calcareous sediment from South Australia and a siliceous sediment from the United Kingdom) at elevated confining pressures. The testing matrix consisted of a series of isotropically consolidated, undrained (CIU) and drained (CID), triaxial compression tests on samples of naturally deposited calcareous and siliceous sediment and remolded calcareous sediment. It was found that the calcareous samples displayed little cemented behavior during shear. For tests conducted at pressures up to 1.5 MPa, a significant amount of particle crushing occurred in the calcareous samples but not in the siliceous samples. Particle degradation and reorientation facilitates transitions from dilative to contractive behavior with increases in confining stress. The calcareous sediment exhibited contractive behavior at confining pressures above approximately 500 kPa and the siliceous sediment remained dilative at stresses up to 1 MPa during undrained loading. Comparison with data collected by the University of Sydney (CID tests with confining pressures up to 60 MPa) showed that most of the variations in strength behavior occurred within the low stress range (up to 2 MPa) tests conducted at URI. This was evident in the friction angle data and in the reloading Young's modulus data.  相似文献   

Making management procedures operational innovations implemented in South Africa   总被引：1，自引：0，他引：1  

Plaganyi  Eva E.; Rademeyer  Rebecca A.; Butterworth  Doug S.; Cunningham  Carryn L.; Johnston  Susan J. 《ICES Journal of Marine Science》2007,64(4):626-632

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Lithological controls on biological activity and groundwater chemistry in Quaternary sediments     

Rebecca Bartlett  Simon H. Bottrell  Karen Sinclair  Steve Thornton  Ian D. Fielding  Dave Hatfield 《水文研究》2010,24(6):726-735

Depth profiles of solute chemistry and sulfate isotopic compositions are presented for groundwater and pore water in a sequence of Quaternary glacial outwash sediments. Sand units show evidence for hydraulic connection to the surface and thus modern sources of solutes. Finer‐grained sediments show a general pattern of increasing solute concentrations with depth, with sulfate derived from ancient rainwater and pyrite oxidation in the soil/drift. In these sediments sulfate has undergone bacterial sulfate reduction (BSR) to produce biogenic sulfide. In clay sediments, with d₁₀ ≤ 1·6 µm, high concentrations of sulfate and acetate now co‐exist, implying that BSR is inhibited. The correlation with smaller sediment grain size indicates that this is due to pore size exclusion of the sulfate reducing bacteria. Mechanical restriction of microbial function thus provides a fundamental limitation on microbial respiration in buried clay‐rich sediments, which acts as a control on the chemical evolution of their pore waters. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Association between Messinian drainage network formation and major tectonic activity in the Marche Apennines (Italy)     

Sonia Scarselli  Guy D. H. Simpson  Philip A. Allen  Giorgio Minelli  Lorenzo Gaudenzi 《地学学报》2007,19(1):74-81

The Marche Apennines (Italy) offer an excellent opportunity to constrain the temporal and spatial relationships between drainage network formation and tectonic activity. Using a combination of field data, seismic lines and boreholes we show that the main deformation phase took place during the Messinian when the area, affected by the Messinian sea level drop, emerged and evolved from marine to continental conditions. The results highlight that during the Messinian emersion a drainage network developed contemporaneously with an increase in tectonic activity that could be related to sea level fall and river erosion. The present‐day river system, which is dominated by transverse rivers that cut straight across the tectonic grain, is located in older Messinian palaeovalleys, even though the region was subsequently covered by water until the late Pliocene–early Pleistocene.  相似文献   

Geochemistry of four tropical montane watersheds,Central Panama     

Russell S. Harmon  W. Berry Lyons  David T. Long  Fred L. Ogden  Helena Mitasova  Christopher B. Gardner  Kathleen A. Welch  Rebecca A. Witherow 《Applied Geochemistry》2009

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Characterizing the vadose zone and a perched aquifer near the Vosges ridge at the La Soutte experimental site,Obernai, France     

Pascal Sailhac  Maksim Bano  Mickael Behaegel  Jean-François Girard  Ester Falgàs Para  Juanjo Ledo  Guy Marquis  Pierre-Daniel Matthey  José Ortega-Ramírez 《Comptes Rendus Geoscience》2009,341(10-11):818-830

We consider a series of hydrogeophysical techniques that provide a multiscale investigation of the water content in the vadose zone and of the perched aquifer at the experimental site of “La Soutte” in the Vosges Mountains (France). It is located in a catchment area where several springs and streams occur along fractured volcanic and weathered plutonic rocks. The site is the object of a long-term study that uses both continuous and repeated measurements to monitor hydrogeological processes. The main results from AMT and DC resistivity techniques allow the determination of a high-resolution 3D resistivity model over a large range of depths (from 10⁰ to 10³ m). We discuss their use and propose a hydrogeological model (porosity, water conductivity and water content). We also use MRS and GPR for a detailed investigation of the shallow part of the catchment that consists of soil and weathered rocks of highly varying thickness (0 to 15 m). MRS is used to map the thickness and total water volume content by unit surface of the saturated weathered zone. It also yields estimates of the vadose zone thickness through the depth to the top of the saturated zone. Moreover, we show results from GPR CMP measurements that yield estimates of the water content and porosity in the shallowest layer (0–30 cm) by simple interpretation of the ground direct wave.  相似文献