2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

Distribution, status and conservation of Indian gray wolf (Canis lupus pallipes) in Karnataka, India

Abstract The Indian gray wolf Canis lupus pallipes is the major large carnivore in the plains of Karnataka, India. We carried out a study on its distribution and status from November 2001 to July 2004. We estimated 555 wolves occupying about 123 330 km2 of the state. In the past 40 years, wolves have disappeared from the southern plateau from an area of about 31 801 km2. Their distribution is now largely restricted to the north-eastern dry plains. The wolf has also disappeared in recent years from some ‘protected areas’ such as Melkote Temple Wildlife Sanctuary, and their present population is largely found in ‘non-protected’ areas. Blackbucks are the only natural prey of wolves in Karnataka, but their density in most parts of the state is extremely low. The major prey species is domestic livestock, especially sheep. The available ‘remote area’ (forests or rocky terrains) in the wolf-occupied regions determined the status of the wolf. Killing of adult wolves and pups was common throughout the range of the wolf. However, such killings were made largely by local sheepherders with small sheep holdings and not by nomadic shepherds who maintained large sheep herds. The forests in the north-eastern parts of the state exist in small patches every few kilometers. Because each wolf pack ranges over large distances and is by and large a commensal species, we propose that the management of these small forest patches, considering them as components of a larger landscape, is the only effective conservation practice for the wolf. Although existing locally in low densities, because of a large ranging area of a single pack, the seemingly isolated wolf packs can become parts of a large metapopulation, providing a sustainable population