An optimized chemiluminescence detector for tropospheric NO measurements

A detector for the chemiluminescent measurement of NO in background air is described. A large reduction of interferences is achieved by using a stabilized ozone generator which allows operation of the instrument at lower O₃ concentrations. Purification and humidification of the O₃ stream further reduces interferences and shortens the instrumental clean-up time, which is important for aircraft missions. From a series of laboratory tests and from measurements performed aboard an aircraft it is demonstrated that the remaining interferences are acceptable for measurements in the undisturbed troposphere. In particular, no remnant NO signal is observed in clean air at night. During flight, a detection limit (2) of 20 ppt is achieved for a 1 min integration time.     

Numerical simulation of turbulent convective flow over wavy terrain

By means of a large-eddy simulation, the convective boundary layer is investigated for flows over wavy terrain. The lower surface varies sinusoidally in the downstream direction while remaining constant in the other. Several cases are considered with amplitude up to 0.15H and wavelength ofH to 8H, whereH is the mean fluid-layer height. At the lower surface, the vertical heat flux is prescribed to be constant and the momentum flux is determined locally from the Monin-Obukhov relationship with a roughness lengthz _o=10^–4 H. The mean wind is varied between zero and 5w _*, wherew _* is the convective velocity scale. After rather long times, the flow structure shows horizontal scales up to 4H, with a pattern similar to that over flat surfaces at corresponding shear friction. Weak mean wind destroys regular spatial structures induced by the surface undulation at zero mean wind. The surface heating suppresses mean-flow recirculation-regions even for steep surface waves. Short surface waves cause strong drag due to hydrostatic and dynamic pressure forces in addition to frictional drag. The pressure drag increases slowly with the mean velocity, and strongly with /H. The turbulence variances increase mainly in the lower half of the mixed layer forU/w _*>2.     

The influence of induced polarization on long-offset transient electromagnetic data

The diffusion of electromagnetic fields is dependent not only on conductivity, but also on magnetic permeability, dielectric permittivity and polarizability, i.e. dispersive conductivity. The long‐offset transient electromagnetic (LOTEM) method is mainly used to determine the spatial distribution of conductivity in the subsurface. However, earlier work on loop‐loop TEM suggests that transient EM methods can also be affected by induced polarization (IP). Numerous 1D forward calculations were carried out to study the IP effect on LOTEM data, using the Cole‐Cole relaxation model to simulate the polarizability of the ground. Besides the polarizability of each layer, the IP effect depends on the LOTEM field set‐up and the spatial distribution of conductivity in the ground. In particular, near‐surface layers with high chargeabilities can significantly distort the late time transients of the electric field components in the vicinity of the transmitter. The influence of polarizable layers on the magnetic field components can be neglected under normal circumstances. In 1997 and 1999, LOTEM measurements were carried out at Mt. Vesuvius in Italy to explore the geological structure of the volcano. Sensitivity studies on the effect of polarizable layers suggest that high chargeabilities in connection with conductive layers at greater depths would result in a detectable distortion of the electric field transients. Although the simultaneous IP measurements revealed high chargeabilities in a near‐surface layer, no evidence of IP effects could be found in the measured LOTEM data. We conclude that the observed chargeabilities are local and that 3D effects are probably present in the data. Another aspect is the measurement of the system response, which is usually measured by placing a receiver very close to the transmitter. Therefore, large distortions can be expected if near‐surface polarizable layers exist. This was verified in practice by field measurements in an area with high chargeabilities in Longerich, Cologne.     

A novel method for automating the checking and correction of digital elevation models using orthophotographs

This article presents a complete mathematical model, which translates discrepancies between two orthophotographs created from different photographs, into precise corrections of the Digital Elevation Model (DEM). These corrections are the differences from the real surface and, if applied over the existing DEM, can produce a more accurate one. The mathematical model is straightforward, and is not approximate, and therefore there is no need for iterations.
Possible applications include checking of automatically created DEMs, refinement of existing DEMs using aerial photographs and update of orthophotographs based on the previous DEM and new imagery.     

Seismic reflectivity of detachment faults of the Iberian and Tethyan distal continental margins based on geological and petrophysical data

Low-angle detachment faults are key to our understanding of the tectonic evolution of magma-poor rifted continental margins. In seismic images of present-day rifted margins the identification and interpretation of such features is, however, notoriously difficult and ambiguous. We address this problem by studying the structure and seismic response of such faults through a synoptic interpretation of petrophysical data and geological evidence from the distal segments of the present-day West Iberian and the ancient Tethyan margins. On the basis of the geologically well-constrained remnants of the Tethyan margins, which are spectacularly preserved and exposed in the Alps of Eastern Switzerland, vertical profiles at four key geological settings of a typical magma-poor rifted margin are constructed and their synthetic seismic responses are compared to the observed seismic data from corresponding locations in the present-day Iberian margin. The seismic structure of these profiles is considered as the sum of deterministic large-scale and the stochastic small-scale components. Both components are analyzed for all pertinent lithologies. The large-scale structures are derived from laboratory measurements on samples from both, the West Iberian and Tethyan margins, whereas the small-scale fluctuations are constrained predominantly on the basis of well-log data from the Iberian margin. Different realizations of the simulated stochastic small-scale velocity fluctuations illustrate the potential variability of impedance contrasts and its impact on the seismic response from lithological interfaces and fault structures. Our results indicate that the nature of the seismic response from low-angle detachment faults is largely determined through the fracture-healing behavior of the surrounding rocks. Geological evidence from the exposed fragments of the Tethyan margins indicate that fracture-healing is generally well developed in crustal lithologies, but largely absent in mantle lithologies. It is for this reason that low-angle, intra-crustal detachment faults tend to be seismically undetectable. Conversely, crust–mantle detachments have a complex and variable seismic response, depending on the nature of the damaged zone and on the frequency content of the seismic data. These model-based inferences are consistent with the available evidence from the present-day Iberian passive margin and thus open new perspectives for the interpretation of the corresponding seismic images.     

Short Static GPS Sessions: Robust Estimation Results

Least-squares estimation (LS) yields results of low accuracy in the presentce of GPS phase-corrupting environmental conditions. We present a robust estimator that clearly identifies outlying observations caused by obstacles like diagonal cables, branches, or leaves. It performs significantly better than standard LS and signal-to-noise ratio dependent weighting if unfavorable signal distortion occurs, and is equal to LS otherwise. The estimator is realized by an iterated LS algorithm using an equivalent weight matrix. It is a generalization of the Danish Method to heterogeneous and correlated observations. The excellent peformance of the estimator for processing short static sessions is demonstrated using data obtained from an investigation of GPS signal obstruction. ? 2002 Wiley Periodicals, Inc.     

A new and simple method for qualitative sampling of meiobenthos-communities

The introduced collecting method is based on a common entomologist exhauster, which is specified for the fieldwork in aquatic systems with a sieve-inset and a bottom-lid. The living organisms are collected visually from the sampled debris and no further extraction is needed. The apparatus is designed for qualitative sampling of microcrustaceans (e.g. Cladocera, Copepoda, Ostracoda) and minimises disturbance at the sampling site.     

Holocene paleoenvironmental change inferred from two sediment cores collected in the Tibetan lake Taro Co

Journal of Paleolimnology - The Tibetan Plateau, also known as the “Water Tower of Asia” because of its function as a water storage and supply region, responds dramatically to modern...