MULTIPLE REFLECTIONS AND HEAD WAVES IN THE GULF OF SUEZ*

Inferior reflection quality in the Gulf of Suez at the target depth interval is attributable in part to surficial multiple reflections. An excellent example of the latter is observed on a typical seismic line in the northern portion of the Gulf. An increase in prominence of the multiple reflections appears associated with decreasing depth to a dipping highvelocity layer. Inversion of a second-order polynomial time-distance function, fitted to the observed refraction onset time-distance values, gives the velocity-depth function for sediments between the water bottom and a high-velocity layer. Velocities thus determined increase non-linearly with depth from a value near water velocity at the water bottom. Depths to the high-velocity layer are obtained from the associated head-wave linear time-distance function and by ray tracing in the overlying sediments. As the high-velocity layer approaches the water bottom from sub-water depths exceeding 0.6 km to a depth of 56 m, intensity of the multiple reflections increases to the extent of completely dominating individual records to a time of at least 3 s. The estimated plane-wave normal-incident reflection coefficient at the top of the high-velocity layer increases with decreasing depth to this layer, approaching 0.5 at the shallowest depth. This strong reflection coefficient further substantiates the existence of multiple reflections between the high-velocity layer and water layer. However, existence of water-layer multiples cannot be ruled out. The estimated water-bottom reflection coefficient is approximately 0.3, a substantial value. Multiple reflections of considerably less intensity are apparent where the high-velocity layer is deepest, and it is likely that such are waterlayer multiple reflections. Unfortunately, water-layer multiple reflections and multiple reflections between the water surface and high-velocity layer cannot be separated by their coincidence with time-distance (normal moveout) curves, the configuration of each visibly matching the curves equally well.     

Application of numerical modelling to a case of compaction-driven dolomitization: a Jurassic palaeohigh in the Po Plain, Italy

Dolomitization of a carbonate platform can occur at different times and in different diagenetic environments, from synsedimentary to deep burial settings. Numerical simulations are valuable tools to test and select the model that, among different hypotheses compatible with field and geochemical data, best honour mass balance, kinetic and thermodynamic constraints. Moreover, the simulation can predict the distribution of the dolomitized bodies in the subsurface and evaluate porosity changes; valuable information for the oil industry. This study is the first attempt to reproduce and investigate the compaction dolomitization model. The diagenetic study of the Jurassic carbonate basin and palaeohigh system of the Po Plain indicates that the carbonates of the palaeohighs were dolomitized by basin compaction fluids. The main goal of the simulations is to evaluate the origin and evolution of the dolomitizing fluids and to provide insights regarding the distribution of the potential reservoir‐dolomitized bodies in the Po Plain. The modelling process is subdivided into two steps: basin modelling and reactive transport modelling. The SEBE3 basin simulator (Eni proprietary) was used to create a three‐dimensional model of the compacting system. The results include compaction fluid flow rate from the basin to the palaeohigh, compaction duration and a determination of the total amount of fluid introduced into the palaeohigh. These data are then used to perform reactive transport modelling with the TOUGHREACT code. Sensitivities on dolomite kinetic parameters suggest that dolomitization was an efficient process even at low temperatures, with differences mainly related to the dynamics of the process. Fluid composition is one of the main constraints, the sea water derived compaction fluid is proven to be efficient for dolomitization due to its relatively high Mg content. Simulations also confirmed that permeability is the most important factor influencing fluid flow and, consequently, the dolomite distribution in the formation. Permeable fractured zones have a strong influence, diverting the dolomitizing fluids from their normal path towards overlying or lateral zones. Moreover, the simulations showed that, after dolomite replacement is complete, the dolomitizing fluids can precipitate dolomite cement, causing over‐dolomitization, with related localized plugging effects in the zone of influx. Mass balance calculations indicate that in the dolomitization compaction model, the amount of compaction water fluxed from the basin to the carbonate is the main constraint on dolomitization efficiency. This observation implies that the ratio between the volume of the basin undergoing compaction and the volume of the palaeohigh is a limiting factor on the final size of the dolomitized bodies. An isolated palaeohigh could be an ideal site for pervasive replacement dolomitization due to the large volume of compaction fluids available compared with the carbonate rock volume. In the case of large platforms, the more permeable margin lithofacies are the most likely sites for compaction model dolomitization. The combined use of a basin simulator and reactive transport modelling has proved to be a successful method to verify model reliability and it provides insights into the volumetric distribution of diagenetic products.     

A recent build-up of atmospheric CO2 over Europe. Part 1: observed signals and possible explanations

We analysed interannual and decadal changes in the atmospheric CO₂ concentration gradient (ΔCO₂) between Europe and the Atlantic Ocean over the period 1995–2007. Fourteen measurement stations are used, with Mace-Head being used to define background conditions. The variability of ΔCO₂ reflects fossil fuel emissions and natural sinks activity over Europe, as well as atmospheric transport variability. The mean ΔCO₂ increased by 1–2 ppm at Eastern European stations (∼30% growth), between 1990–1995 and 2000–2005. This built up of CO₂ over the continent is predominantly a winter signal. If the observed increase of ΔCO₂ is explained by changes in ecosystem fluxes, a loss of about 0.46 Pg C per year would be required during 2000–2005. Even if severe droughts have impacted Western Europe in 2003 and 2005, a sustained CO₂ loss of that magnitude is unlikely to be true. We sought alternative explanations for the observed CO₂ build-up into transport changes and into regional redistribution of fossil fuel CO₂ emissions. Boundary layer heights becoming shallower can only explain 32% of the variance of the signal. Regional changes of emissions may explain up to 27% of the build-up. More insights are given in the Aulagnier et al. companion paper.     

Mass movement-induced fold-and-thrust belt structures in unconsolidated sediments in Lake Lucerne (Switzerland)

High-resolution seismic imaging and piston coring in Lake Lucerne, Switzerland, have revealed surprising deformation structures in flat-lying, unconsolidated sediment at the foot of subaqueous slopes. These deformation structures appear beneath wedges of massflow deposits and resemble fold-and-thrust belts with basal décollement surfaces. The deformation is interpreted as the result of gravity spreading induced by loading of the slope-adjacent lake floor during massflow deposition. This study investigated four earthquake-triggered lateral mass-movement deposits in Lake Lucerne affecting four sections of the lake floor with areas ranging from 0·25 to 6·5 km² in area. Up to 6 m thick sediment packages draping the subaqueous slopes slid along the acoustic basement. The resulting failure scars typically lie in water depths of >30 m on slopes characterized by downward steepening and inclinations of >10°. From the base-of-slope to several hundred metres out onto the flat plains, the wedges of massflow deposits overlie deeply (10–20 m) deformed basin-plain sediment characterized by soft sediment fold-and-thrust belts with arcuate strikes and pronounced frontal thrusts. The intensity of deformation decreases towards the more external parts of the massflow wedges. Beyond the frontal thrust, the overridden lake floor remains mostly undisturbed. Geometrical relationships between massflow deposits and the deformed basin-plain sediment indicate that deformation occurred mainly during massflow deposition. Gravity spreading induced by the successive collapse of the growing slope-adjacent massflow wedge is proposed as the driving mechanism for the deformation. The geometry of fjord-type lakes with sharp lower slope breaks favours the deposition of thick, basin-marginal massflow wedges, that effectively load and deform the underlying sediment. In the centre of the basins, the two largest massflow deposits described are directly overlain by thick contained (mega-)turbidites, interpreted as combined products of the suspension clouds set up by subaqueous mass movements and related tsunami and seiche waves.     

COMPARISON AMONG EMPIRICAL PROBABILITY DENSITY FUNCTIONS OF THE VERTICAL VELOCITY IN THE SURFACE LAYER BASED ON HIGHER ORDER CORRELATIONS

Vertical velocity fluctuations were measured in theatmospheric surface layer by means of an ultrasonic anemometer andhigher order correlations were calculated on two time series, recordedin unstable and neutral conditions, and selected for the wholemeasurement period on the basis of the inversion test (stationaritytest). Comparisons have been made between observed and predictedcorrelations by considering Gaussian joint-PDF and Gram-Charlierseries expansions truncated to the fourth and sixth order as doneearlier by Frenkiel and Klebanoff. A bi-Gaussian PDF, given by amixture of two Gaussian PDFs, has also been considered. This lasthas been constructed assuming that either the first three or the firstfour moments are given, and the relationships between correlationfunctions of different order are derived. The departure from Gaussianbehaviour in both stability conditions is derived. Though Gram-Charlier series expansions show a good correspondence toexperimental reality, their use as non-Gaussian probabilitydistributions cannot be suggested in theoretical approaches andshould be considered with care in practical applications, due topossible occurrences of small negative probabilities. The resultsshown in this paper support the applicability of the bi-Gaussian PDFcreated using up to the fourth moment.     

GEOMORPHOLOGICAL AND DENDROCHRONOLOGICAL ANALYSES OF A COMPLEX LANDSLIDE IN THE SOUTHERN APENNINES

Complex landslides, capable of reactivation, are typical slope movements in high relief areas. Due to their distribution, size and kinematics, these landforms represent a major hazard, posing a high risk to populations, settlements and infrastructures. This paper integrates geomorphological analyses, instrumental measurements and dendrochronological approaches in assessing a large, reactivated landslide system on the southern piedmont of Monte Sirino (southern Italy). The landslide system is associated with weak geological structures, earthquake activity, and rapid recent incision of the mid-Pleistocene Noce lake deposits. Potential reactivation triggers include a higher regional annual rainfall, one of the highest in southern Italy, and more frequent heavy snowfalls in recent decades. Reactivation of the Sirino landslide system has important implications for the motorway connecting Salerno and Reggio Calabria, which crosses it. The results of our study show that the slide is reactivated with an almost decadal frequency and that major reactivations are correlated to prolonged snowfall, which occurs with increasing frequency in the southern Apennines. The last observation suggests the need for similar studies on the behaviour of other landslide systems in the southern Apennines, performing integrated approaches such as geotechnical and dendrogeomorphological analysis.