Investigation of geotechnical and geochemical relationships by parameter cross‐correlation methods,oslofjorden and dramsfjorden,Norway

Abstract

A cross‐correlation matrix was computer developed to investigate the statistical dependence of geotechnical properties on other properties as an aid to learning why such dependence exists. Linear correlation coefficients, based on 10,000 data values from borings in four major basins along the axis of the Oslofjorden, and one boring at the south end of the Dramsfjorden, are presented for 17 measured or calculated geotechnical and geochemical parameters. Confidence levels of 99 percent and 99.99 percent were calculated for all parameters, and causative and fortuitous relationships or linkages were investigated using regrouped matrices showing correlations significant at the 99.99 percent confidence level. A table summarizes parameters correlating in both fjords, only in the Oslofjorden, only in the Dramsfjorden, and in neither fjord.

Generally, relationships were as expected. Examples of unexpected absence of correlations follow: In the Oslofjorden, the clay‐size fraction and the plastic limit correlated only with the activity and the liquid limit, respectively. The lack of correlation with other parameters may be related to the character of the clay‐size fraction. In the Dramsfjor‐den, the sensitivity did not correlate with any other parameter, which may be related to the anoxic soils found in this fjord.     

San Diego trough geotechnical test area

Abstract

The San Diego Trough Geotechnical Test Area, located about 24 km southwest of San Diego in a water depth of about 1.2 km, lies near the base of the Coronado Escarpment directly north of the Coronado Fan. A new bathymetric map delineates a shallow basin in the soft, highly plastic, clayey silts flooring the Test Area. Measurements of shear strength by vane and static cone pene‐trometer, and bulk density by nuclear densitometer, were made in place from the submersible Deep Quest. Sixteen short (< 1.6 m) gravity cores were collected from ships.

The geotechnical properties show little areal variation and generally change uniformly with depth within the 55 km² Test Area. Silt is the predominant grain size, averaging about 62%. In‐place bulk density shows little change with increasing depth, values range from 1.23 to 1.26 Mg/m³; laboratory density values increase with depth, ranging from 1.30 to 1.52 Mg/m³ between the surface and a depth of about 1.1 m. The difference between the in place and laboratory values may indicate sampling densification of the cored sediment. Water content in the cores decreases uniformly within the range of 249 to 43% dry weight. Shear strength increases linearly with depth. The laboratory shear strength values are lower than the in place values, which range from 4 kPa at the surface to about 29 kPa at a depth of 3.27 m. Predictor equations relate Atterberg limits, bulk density, water content, and laboratory and in place shear strength to depth. Sedimentation‐compression e log p curves have an equivalent compression index of 1.5 to nearly 2. Excluding rurbidite layers and sampling disturbance effects, all cores indicate a uniform depositional environment in the surface to 1.6 m of sediment sampled. The geotechnical properties indicate that the sediments in the west central and southwest parts of the Test Area exhibit vertical heterogeneity due to thin silt‐sand layers, presumably of turbidity current origin, that originated from the Coronado Canyon.     

In situ pore‐pressure measurement in Mississippi delta front sediments

Abstract

A differential piezometer was used to monitor excess pore pressure in the soft clayey seafloor sediments of Block 28, South Pass, Mississippi delta, from September 1975 to March 1976. An ambient excess pore pressure of about 32 kPa was measured at a depth of 6.4 m below the mudline in a water depth of 19 m. Storm‐wave‐generated cyclic fluctuations of ± 4 kPa about the ambient were measured during Hurricane Eloise. Irregular, long‐period, small‐amplitude fluctuations in excess pore pressures persisted for 4 days following the storm. An effective stress analysis was made by using excess pore pressures; in situ field vane‐shear strength, t _fv, measurements; and laboratory wet unit weights measured by Lehigh and NOAA. The effective stress of the SEA‐SWAB site soil was calculated to be zero to a depth of about 6 m, below which it increased to 3.5 kPa at a depth of 15 m. Values of c´ = 4.6 kPa, ?´ = 56°, and T _FV/σ_vo(c/p) =0.1–0.2 were calculated, and it was concluded that these data do not represent the in situ condition of the soil because of the probability that the measured soil properties were affected by the presence of gas. However, it is clear that the soil is significantly underconsolidated.     

Impact of Sea-Level Rise on Sea Water Intrusion in Coastal Aquifers

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.     

Numerical simulations for the non-linear Molodensky problem

We present a boundary element method to compute numerical approximations to the non-linear Molodensky problem, which reconstructs the surface of the Earth from the gravitational potential and the gravity vector. Our solution procedure solves a sequence of exterior oblique Robin problems and is based on a Nash-Hörmander iteration. We apply smoothing with the heat equation to overcome a loss of derivatives in the surface update. Numerical results show the error between the approximation and the exact solution in a model problem.     

Comparing quantitative precipitation forecast methods for prediction of sewer flows in a small urban area

Abstract

Due to the relatively small spatial scale, as well as rapid response, of urban drainage systems, the use of quantitative rainfall forecasts for providing quantitative flow and depth predictions is a challenging task. Such predictions are important when consideration is given to urban pluvial flooding and receiving water quality, and it is worthwhile to investigate the potential for improved forecasting. In this study, three quantitative precipitation forecast methods of increasing complexity were compared and used to create quantitative forecasts of sewer flows 0–3 h ahead in the centre of a small town in the north of England. The HyRaTrac radar nowcast model was employed, as well as two different versions of the more complex STEPS model. The STEPS model was used as a deterministic nowcasting system, and was also blended with the Numerical Weather Prediction (NWP) model MM5 to investigate the potential of increasing forecast lead-times (LTs) using high-resolution NWP. Predictive LTs between 15 and 90 min gave acceptable results, but were a function of the event type. It was concluded that higher resolution rainfall estimation as well as nowcasts are needed for prediction of both local pluvial flooding and combined sewer overflow spill events.

Editor D. Koutsoyiannis; Guest editor R.J. Moore     

Microstructure measures and the minimum size of a representative volume element: 2D numerical study

In the paper, a numerical study of the size of a representative volume element (RVE) for both the heat flow as well as the linear elasticity problems is presented. A particular two-phase random microstructure is studied and the method is applied to the digital image of reconstructed 2D realization of random media. The minimum size of RVE is determined by the investigation of the convergence of apparent properties as the size of RVE is increasing. Then, two estimates of the minimum RVE size are proposed and it is shown that the estimates are in a good agreement with the results determined by the investigation of the convergence of apparent properties. The minimum size of RVE can be successfully predicted based only on the microstructure morphology. The statistical measures used in this work are: the two-point probability and the linealpath functions.     

Petrographic,fluid inclusion and isotopic study of the Dikulushi Cu–Ag deposit,Katanga (D.R.C.): implications for exploration

The Dikulushi Cu–Ag vein-type deposit is located on the Kundelungu Plateau, in the southeastern part of the Democratic Republic of Congo (D.R.C.). The Kundelungu Plateau is situated to the north of the Lufilian Arc that hosts the world-class stratiform Cu–Co deposits of the Central African Copperbelt. A combined petrographic, fluid inclusion and stable isotope study revealed that the mineralisation at Dikulushi developed during two spatially and temporally distinct mineralising episodes. An early Cu–Pb–Zn–Fe mineralisation took place during the Lufilian Orogeny in a zone of crosscutting EW- and NE-oriented faults and consists of a sequence of sulphides that precipitated from moderate-temperature, saline H₂O–NaCl–CaCl₂-rich fluids. These fluids interacted extensively with the country rocks. Sulphur was probably derived from thermochemical reduction of Neoproterozoic seawater sulphate. Undeformed, post-orogenic Cu–Ag mineralisation remobilised the upper part of the Cu–Pb–Zn–Fe mineralisation in an oxidising environment along reactivated and newly formed NE-oriented faults in the eastern part of the deposit. This mineralisation is dominated by massive Ag-rich chalcocite that precipitated from low-temperature H₂O–NaCl–KCl fluids, generated by mixing of moderate- and low-saline fluids. The same evolution in mineralisation assemblages and types of mineralising fluids is observed in three other Cu deposits on the Kundelungu Plateau. Therefore, the recognition of two distinct types of (vein-type) mineralisation in the study area has a profound impact on the exploration in the Kundelungu Plateau region. The identification of a Cu–Ag type mineralisation at the surface could imply the presence of a Cu–Pb–Zn–Fe mineralisation at depth.     

Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO₂ or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between ?8.1‰ and ?6.8‰ for δ¹³C, +6.4‰ and +10.5‰ for δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO₂±CH₄±N₂) and aqueous-carbonic (H₂O-NaCl-CO₂±CH₄±N₂) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO₂-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.     

Paleomagnetic determination of emplacement temperatures of pyroclastic deposits: an under-utilized tool

Paleomagnetic data from lithic clasts collected from Mt. St. Helens, USA, Volcán Láscar, Chile, Volcán de Colima, Mexico and Vesuvius, Italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures. At Mt. St. Helens, the temperature of the deposits (T _dep ) at three sites from the June 12, 1980 eruption was found to be ≥532°C, ≥509°C, and 510–570°C, respectively. One site emplaced on July 22, 1980 was emplaced at ≥577°C. These new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates. Lithic clasts from pyroclastic deposits from the 1993 eruption of Láscar were fully remagnetized above the respective Curie temperatures, which yielded a minimum T _dep of 397°C. Samples were also collected from deposits thought to be pyroclastics from the 1913, 2004 and 2005 eruptions of Colima. At Colima, the sampled clasts were emplaced cold. This is consistent with the sampled clasts being from lahar deposits, which are common in the area, and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit. T _dep of the lower section of the lithic rich pyroclastic flow (LRPF) from the 472 A.D. deposits of Vesuvius was ~280–340°C. This is in agreement with other, recently published paleomagnetic measurements. In contrast, the upper section of the LRPF was emplaced at higher temperatures, with T _dep ~520°C. This temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections, with the upper section containing a greater proportion of vent-derived material that was initially hot. Our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation.