Rocket-borne and groundbased observations of coincident field-aligned currents,electron beams,and plasma density enhancements in the afternoon auroral oval

Results are reported from a rocket experiment conducted at Søndre Strømfjord, Greenland, on 22 August 1976, at 16.00 M.L.T. A series of plasma, particles, and fields and wave experiments were carried on board the payload, and the venture was supported by data from the AE-C satellite and by groundbased ionosondes and magnetometers at the launch site and at Godhavn. Two regions of field-aligned electron precipitation, electron density and temperature enhancements, and field-aligned upflowing current sheets were intercepted by the rocket. The density enhancements were also observed by groundbased ionosondes. Significant discrepancies were found between the currents carried by the streaming electrons in the 0.15–10 keV range and the upflowing currents seen by the on board magnetometer, suggesting that the upflowing current could not be the primary driver of the electron acceleration mechanism. The E-region was unstable to the combined Gradient-Drift and Farley-Buneman instability, and plasma turbulence was observed in situ, but the absolute density fluctuations were too small to return detectable HF-radar power to the ground.     

Incorporating vegetation into visual exposure modelling in urban environments

Visual exposure modelling establishes the extent to which a nominated feature may be seen from a specified location. The advent of high-resolution light detection and ranging (LiDAR)-sourced elevation models has enabled visual exposure modelling to be applied in urban regions, for example, to calculate the field of view occupied by a landmark building when observed from a nearby street. Currently, visual exposure models access a single surface elevation model to establish the lines of sight (LoSs) between the observer and the landmark feature. This is a cause for concern in vegetated areas where trees are represented as solid protrusions in the surface model totally blocking the LoSs. Additionally, the observer's elevation, as read from the surface model, would be incorrectly set to the tree top height in those regions. The research presented here overcomes these issues by introducing a new visual exposure model, which accesses a bare earth terrain model, to establish the observer's true elevation even when passing through vegetated regions, a surface model for the city profile and an additional vegetation map. Where there is a difference between terrain and surface elevations, the vegetation map is consulted. In vegetated areas the LoS is permitted to continue its journey, either passing under the canopy with clear views or partially through it depending on foliage density, otherwise the LoS is terminated. This approach enables landmark visual exposure to be modelled more realistically, with consideration given to urban trees. The model's improvements are demonstrated through a number of real-world trials and compared to current visual exposure methods.     

Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis

A vertical profile of maximum horizontal principal stress, SHmax, orientation to 5 km depth was obtained beneath the Swiss city of Basel from observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 m exploration well (OT2) imaged from 2550 m to 2753 m across the granitic basement-sediment interface at 2649 m; and a 5 km deep borehole (BS1) imaged entirely within the granite from 2569 m to 4992 m. Stress-related wellbore failure in the form of breakouts or drilling-induced tension fractures (DITFs) occurs throughout the depth range of the logs with breakouts predominant. Within the granite, DITFs are intermittently present, and breakouts more or less continuously present over all but the uppermost 100 m where they are sparse. The mean SHmax orientations from DITFs is 151 ± 13° whereas breakouts yield 143 ± 14°, the combined value weighted for frequency of occurrence being N144°E ± 14°. No marked depth dependence in mean SHmax orientation averaged over several hundred meters depth intervals is evident. This mean SHmax orientation for the granite is consistent with the results of the inversion of populations of focal mechanism solutions of earthquakes occurring between depths of 10–15 km within regions immediately to the north and south of Basel, and with the T-axis of events occurring within the reservoir (Deichmann and Ernst, this volume). DITFs and breakouts identified in OT2 above and below the sediment-basement interface suggest that a change in SHmax orientation to N115°E ± 12° within the Rotliegendes sandstone occurs near its interface with the basement. The origin of the 20–30° change is uncertain, as is its lateral extent. The logs do not extend higher than 80 m above the interface, and so the data do not define whether a further change in stress orientation occurs at the evaporites. Near-surface measurements taken within 50 km of Basel suggest a mean orientation of N–S, albeit with large variability, as do the orientation of hydrofractures at depths up to 850 m within and above the evaporite layers and an active salt diapir, also within 50 km of Basel. Thus, the available evidence supports the notion that the orientation of SHmax above the evaporites is on average more N–S oriented and thus differs from the NW–SE inferred for the basement from the BS1/OS2 wellbore failure data and the earthquake data. Changes in stress orientation with depth can have significant practical consequences for the development of an EGS reservoir, and serve to emphasise the importance of obtaining estimates from within the target rock mass.     

Energy spectrum of HeII in a strong magnetic field and bound-bound transition probabilities

The ground state energy of the Heii (singly ionized helium) atom is determined in magnetic fields up to 10¹² G. The 13 lowest excited states and bound-bound transition probabilities are calculated in magnetic fields from 10⁷ to 10⁹ G.     

Organic matter in the Soultz HDR granitic thermal exchanger (France): natural tracer of fluid circulations between the basement and its sedimentary cover

Organic matter has been observed in cores of the Soultz-sous-Forêts granite (Alsace, France) at depths between 2158 and 2160 m, in a highly fractured and altered zone. The granite is overlain by a 1400-m-thick sedimentary cover containing petroleum (Pechelbronn oil field). The Soultz area is devoted to Hot Dry Rock geothermics thanks to a high geothermal gradient (up to 100 °C/km). During drilling operations, an artesian source produced oil in a fractured zone of the sedimentary cover (Buntsandstein). Its gas-chromatography and mass-spectrometry (GC-MS) analysis revealed the presence of normal alkanes (n-alkanes, i.e. linear hydrocarbons) centered on C₁₇, branched alkanes with a major C₁₉ peak, and few unsaturated hydrocarbons. The aromatic fraction is present in small amounts. In the Soultz granite, where it is altered, organic matter is exclusively associated with tosudite (interstratified clay mineral) which crystallized in plagioclase sites during a hydrothermal alteration episode. Organic matter has been later displaced and concentrated along veinlets in which illite and carbonates have crystallized during another hydrothermal alteration stage. The soluble organic matter analyzed by GC-MS is composed of aliphatic acids, n-alkanes with a bimodal C₁₈ and C₂₄-C₂₅ centered distribution, alkylbenzenes and aromatic acids. Organic compounds in the granite would either originate from a single source (immature sediments) or from two sources (immature sediments and migration of the Pechelbronn oil). No real evidence was found to prove which hypothesis is the best one. The presence of organic matter in the granite shows the importance of fluid flows between the sedimentary cover and the granitic basement through major fractures. In addition, the impregnation of plagioclase pseudomorphs with organic matter is made possible due to their high interconnection degree and to the intergranular microfracturation of the granite. The succession of several hydrothermal events with different physico-chemical characteris- tics may also be inferred from the occurrence of organic matter found in association with neoformed clay minerals in the granite.     

System-wide submerged aquatic vegetation model for Chesapeake Bay

A predictive model of submerged aquatic vegetation (SAV) biomass is coupled to a eutrophication model of Chesapeake Bay. Domain of the model includes the mainstem of the bay as well as tidal portions of major embayments and tributaries. Three SAV communities are modeled: ZOSTERA, RUPPIA, and FRESHWATER. The model successfully computes the spatial distribution and abundance of SAV for the period 1985–1994. Spatial distribution is primarily determined by computed light attenuation. Sensivitity analysis to reductions in nutrient and solids loads indicates nutrient controls will enhance abundance primarily in areas that presently support SAV. Restoration of SAV to areas in which it does not presently exist requires solids controls, alone or in combination with nutrient controls. For regions in which SAV populations exist at the refuge level or greater, improvements in SAV abundance are expected within 2 to 10 years of load reductions. For regions in which no refuge population exists, recovery time is unpredictable and will depend on propagule supply.     

Fresh-water lenses and practical limitations of their three-dimensional simulation

Fresh-water lenses are the major sources of water supply in many atoll islands in the Pacific and Indian Oceans, particularly in dry seasons. Several two- and three-dimensional models are currently available for the simulation of atoll-island aquifers; however, 2D models cannot include 3D spatial variability of material properties, they must simplify the boundary conditions, and they cannot correctly simulate pumping wells. In an attempt to overcome these difficulties, a 3D model, SALTFLOW, was adopted for the simulation of Home Island in the Indian Ocean. This exercise required a discretisation on the order of a few metres and time steps of a few hours requiring significantly high CPU times. High CPU demand proved to be a difficult challenge but cannot be considered a serious practical limitation with today's advanced computers. The exhaustive data demands of the model (e.g., 3D distributions of hydraulic conductivity, porosity, dispersivities, and spatial and temporal variations of recharge and extraction rates) proved to be more problematical. Although the Home Island data set is unusually comprehensive by any standards, nonetheless the quality and quantity of the available data proved inadequate to meet the calibration needs of a highly karstic aquifer system. The Home Island modeling demonstrates the practical limitations of 3D models. It raises the concern that our ability to develop computer codes capable of simulating complex systems now exceeds our ability to supply the input data necessary for reliable calibration. Finally, the paper demonstrates the importance of the transient calibration in reliable simulation of various management options and emphasises that transient calibration should be considered as an integral part of any similar 2D or 3D modeling. Electronic Publication