Aerial Photography for Survey Purposes with a High Resolution, Small Format, Digital Camera

This paper discusses the work carried out to obtain stereoscopic "photography" from airborne platforms with a Kodak DCS200 digital camera, as part of an investigation into the suitability of current digital camera technology for use in air survey. Details of the work, from the initial test flight in a Cessna light aircraft through to tests utilizing a Thruster microlight platform, are documented. The paper concludes with a summary of the present shortcomings in obtaining aerial photography for survey purposes with still video technology and discusses the future prospects.     

Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments

A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth's magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.     

A design study of an energy-dissipating cladding system

A design study has been conducted to explore the use of structural cladding panels with energy-dissipating cladding-to-frame connections for seismic-resistant design. The study identifies several issues involved in the modelling and analysis of frames with energy-dissipating cladding-to-frame connections, establishes concepts for design, and provides a preliminary assessment of the force and deformation demands that are likely to be placed on panels and connections. Non-linear dynamic analyses indicate that the clad frames perform well, based on observations about maximum interstorey drifts, maximum plastic hinge rotations in the frames, and maximum ductility demands on the cladding-to-frame connections.     

Seismic Hazard and Risk Assessment for Tulbagh,South Africa: Part I – Assessment of Seismic Hazard

In this part of our study the probabilistic seismic hazard analysis (PSHA) for Tulbagh was performed. The applied procedure is parametric and consists essentially of two steps. The first step is applicable to the area in the vicinity of Tulbagh and requires an estimation of the area-specific parameters, which, in this case, is the mean seismic activity rate, , the Gutenberg-Richter parameter, b, and the maximum regional magnitude, m_max. The second step is applicable to the Tulbagh site, and consists of parameters of distribution of amplitude of the selected ground motion parameter. The current application of the procedure provides an assessment of the PSHA in terms of peak ground acceleration (PGA) and spectral acceleration (SA). The procedure permits the combination of both historical and instrumental data. The historical part of the catalogue only contains the strongest events, whereas the complete part can be divided into several subcatalogues, each assumed complete above a specified threshold of magnitude. In the analysis, the uncertainty in the determination of the earthquake was taken into account by incorporation of the concept of `apparent magnitude'. The PSHA technique has been developed specifically for the estimation of seismic hazard at individual sites without the subjective judgement involved in the definition of seismic source zones, when the specific active faults have not been mapped or identified, and where the causes of seismicity are not well understood. The results of the hazard assessment are expressed as probabilities that specified values of PGA will be exceeded during the chosen time intervals, and similarly for the spectral accelerations. A worst case scenario sketches the possibility of a maximum PGA of 0.30g. The results of the hazard assessment can be used as input to a seismic risk assessment.     

Multiple phases of North African humidity recorded in lacustrine sediments from the Fazzan Basin, Libyan Sahara

The Fazzan Basin of south-west Libya is at present arid with less than 20 mm of rainfall per annum. However, regionally extensive limestones, lacustrine sands and coquina (fossiliferous carbonate rock) deposits show that the Fazzan Basin previously contained a large palaeolake, indicating that the climate in the past was more humid. Optically stimulated luminescence (OSL) dating techniques have been applied to key lacustrine deposits within the basin in an attempt to provide an internally consistent chronology for this humidity record. Results indicate that palaeolake sediments within the Fazzan Basin record a very long history of palaeohydrological change, ranging from present day arid conditions to humidity capable of sustaining a lake with an approximate area of 76,250 km². The existence of humid periods in mid oxygen isotope stage 5 and the early Holocene is confirmed. An older lacustrine event, tentatively correlated to oxygen isotope stage 11, is also recognized. In addition, evidence is presented for at least two humid phases beyond the age range over which the conventional OSL dating technique is applicable. This study demonstrates that OSL dating of palaeolake sediments within the Fazzan Basin offers the potential to provide a detailed record of North African humidity spanning several glacial–interglacial cycles.     

Accurate Determination of Electron Densities in Active and Quiescent Prominences: the Mid-Infrared Advantage

We have analyzed all lines in the MIR (8 to 20 micron) spectra of a quiescent and two time-frames of an active prominence. In the quiescent prominence, in addition to those lines found by Zirker (1985), we have identified a higher excitation hydrogen line and two helium recombination lines. Accounting for instrumental broadening, we can further separate out the Doppler and the Stark contributions to the line width. The former yields maximum temperatures of 6200 K, 34000 K and 12000 K and the latter electric field strengths of 7, 17, and 10 V cm^-1 for the above prominences, respectively. We show that these electric fields when divided by 2.2 are equal to the normal electric field in Holtsmark's quasi-static Stark broadening theory. Hence, we obtain electron densities of N₃=2.4(0.3), 9.1(1.2), and 5.5(0.6) in units of 10¹⁰ cm^-3 respectively. Using the same assumptions as made by Zirker, namely, (1) the strongest line (7-6) is optically thin, (2) the population of the lower level (n=6) is determined by direct radiative recombination and photo-ionization, (3) the equality of proton and electron densities, and (4) the thickness of the prominence is at least 10⁸ cm, we derive a new inequality, N_e 1.83 × 10⁸ T^0.75 e^-2195/T. Substituting our maximum temperatures into the right-hand side, we find upper bound N_e values of 9, 43, and 30 in the same units as above. These upper bound values are comfortably higher than our measurement, unlike those of Zirker's derived from the same set of assumptions. We have also observed the helium recombination spectrum which has been postulated by Tandberg-Hanssen as one of three possible ways of equilibrating the triplet/singlet ratio. Surprisingly, it is present in the quiescent as well as in the active prominence. We show that no meaningful values can be found for the turbulent velocities by combining the helium with the hydrogen line widths.