Computation of Synthetic Seismograms in 2-D and 3-D Media Using the Gaussian beam Method

The Gaussian beam method is applied to vertical seismic profiling in 2-D and 3-D models. A simple approach to the computation of Gaussian beam seismograms in the vicinity of structural interfaces is proposed. The effects of (a) the radiation pattern of a point source, (b) non-causal attenuation, (c) transverse inhomogeneities in synthetic seismograms are studied on numerical examples.     

Effects and removal of systematic errors in crosshole georadar attenuation tomography

We have developed an algorithm that allows crosshole georadar amplitude data contaminated with systematic errors to be tomographically inverted. The effects of the errors, which may due to variable antenna-borehole coupling, the groundwater table, and 3-D heterogeneities in the vicinity of one or more boreholes, are included in a series of transmitter and receiver amplitude-correction factors. Tests with synthetic georadar sections demonstrate that the new approach is capable of producing reliable attenuation information, even when large systematic errors are present in the amplitude data. Standard inversions of crosshole georadar data acquired within a highly complex alpine rock glacier yield distorted tomograms. In contrast the new approach results in geologically useful images.     

A piecewise matched-field tracking algorithm

Matched-field tracking (MFT) algorithms have been successfully applied to both simulated and measured data to determine the most likely positions of a sound source that is localized ambiguously by a matched-field processing (MFP) system. They have been used to track sources moving linearly or on a circular path at constant speed and heading. The input to the trackers is a set of ambiguity surfaces, contiguous in time, generated by MFP. These algorithms assume that the track start and end times are known a priori; this restriction is removed in the piecewise MFT algorithm (PTA). The PTA was applied to narrow-band measured data collected during the PACIFIC SHELF 93 trial to successfully identify the significant source track segments     

A hybrid simplex genetic algorithm for estimating geoacousticparameters using matched-field inversion

Matched-fieId inversion (MFI) undertakes to estimate the geometric and geoacoustic parameters in an ocean acoustic scenario by matching acoustic field data recorded at hydrophone array with numerical calculations of the field. The model which provides the best fit to the data is the estimate of the actual experimental scenario. MFI provides a comparatively inexpensive method for estimating ocean bottom parameters over an extensive area. The basic components of the inversion process are a sound propagation model and matching (minimization) algorithm. Since a typical MFI problem requires a large number of computationally intensive sound propagation calculations, both of these components have to be efficient. In this study, a hybrid inversion algorithm which uses a parabolic equation propagation model and combines the downhill simplex algorithm with genetic algorithms is introduced. The algorithm is demonstrated on synthetic range-dependent shallow-water data generated using the parabolic equation propagation model. The performance for estimating the model parameters is compared for realistic signal-to-noise ratios in the synthetic data     

Gravettian environmental changes in a N — S transect of central Europe

The area defined as the N — S transect of Central Europe encompasses Slovenia, Croatia, Hungary, southern Slovakia, northern Austria, Moravia and southern Poland. Physical and biological environmental analysis have been undertaken for the Gravettian period of the last glacial, between 31 and 22 ka cal BP. Mammal faunas have been recorded from different provinces, which have climatic differences. These climatic differences are reflected in the sediments, the plant cover and the fauna. Over the course of several warm and cold events from 31–22 ka the biodiversity of the individual provinces has remained roughly the same, but important quantitative changes have occurred in the individual faunas.     

Internal structure of an alpine rock glacier based on crosshole georadar traveltimes and amplitudes

Rapid melting of permafrost in many alpine areas has increased the probability of catastrophic rock slides. In an attempt to provide critical structural information needed for the design and implementation of suitable mitigation procedures, we have acquired low frequency (22 MHz) cross‐hole radar data from within a fast‐moving rock glacier, an important form of alpine permafrost. Since the ice, rock and pockets of water and air found in the underground of high alpine areas have very different dielectric permittivities and electrical conductivities, the radar method was well‐suited for investigating the structure and state of the rock glacier. Our interpretation of the radar velocities and attenuations was constrained by geomorphological observations, borehole lithological logs and the results of a surface seismic survey. The radar data revealed the existence of a discontinuous 7–11 m thick ice‐rich zone distinguished by high velocities (0.14–0.17 m/ns) and low attenuations (0.04–0.09 m^?1) and a thin underlying ice‐free zone characterized by moderate velocities (0.11–0.12 m/ns) and low attenuations (0.04–0.09 m^?1). Beneath these two zones, we observed a prominent band of high velocities (0.14–0.17 m/ns) and moderately high attenuations (0.10–0.20 m^?1) associated with unconsolidated glacial sediments and numerous large air‐filled voids, which in the past were probably filled with ice. At greater depths, the variably dry to water‐saturated sediments were represented by generally lower velocities (0.08–0.10 m/ns) and higher attenuations (0.16–0.24 m^?1). The bedrock surface was represented by an abrupt ～0.03 m/ns velocity increase. We speculate that the disappearance of ice, both laterally and with depth, occurred during the past one to two decades.     

Computation of synthetic seismograms in 2-D and 3-D media using the Gaussian beam method

Summary The Gaussian beam method is applied to vertical seismic profiling in 2-D and 3-D models. A simple approach to the computation of Gaussian beam seismograms in the vicinity of structural interfaces is proposed. The effects of (a) the radiation pattern of a point source, (b) noncausal attenuation, (c) transverse inhomogeneities in synthetic seismograms are studied on numerical examples.

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Effects of climate warming on the distributions of invasive Eurasian annual grasses: a South African perspective

Threats posed by Eurasian annual grasses to ecosystem function have received little attention. Therefore, protocols for prioritising these alien annual species and likely future dimensions of their spread are urgently required. Here we modelled these grasses potential distribution and shifts in distribution ranges in South Africa under current and future climate scenarios. We applied a modelling framework (BIOMOD), which integrated a variety of parametric statistical and non-parametric rule based models to point distribution records of 29 invasive grass species. Correspondence between modelled and recorded distributions was calculated using the model accuracy criteria called the AUC (Area under the Curve). Based on this criteria 12 C₃ species were excellently modelled (AUC = 0.9–1), 11 C₃ species had good model accuracy (AUC = 0.7–0.8) and four C₃ and four C₄ species fell into the fair (AUC = 0.6–0.7) model accuracy class. Mean temperature of the coldest month was the strongest environmental parameter, for most of the alien grass distributions. Modelled distributions of the alien annual grasses projected into the future indicated range contractions in all C₃ species, except Briza minor, which were accompanied by shifts in species distribution ranges into higher altitudes. All C₄ species displayed habitat loss of relatively similar magnitude with climate warming and shifts in their distribution ranges also into higher elevations. These findings conclude that climate change will hinder the spread of European annual grasses in southern Africa. However, shifts in their distributions into pristine areas at higher elevations could pose a threat to the natural vegetation by altering fire regimes.