Submeter bathymetric mapping of volcanic and hydrothermal features on the East Pacific Rise crest at 9°50′N

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q01006, doi:10.1029/2006GC001333.Recent advances in underwater vehicle navigation and sonar technology now permit detailed mapping of complex seafloor bathymetry found at mid-ocean ridge crests. Imagenex 881 (675 kHz) scanning sonar data collected during low-altitude (~5 m) surveys conducted with DSV Alvin were used to produce submeter resolution bathymetric maps of five hydrothermal vent areas at the East Pacific Rise (EPR) Ridge2000 Integrated Study Site (9°50′N, “bull's-eye”). Data were collected during 29 dives in 2004 and 2005 and were merged through a grid rectification technique to create high-resolution (0.5 m grid) composite maps. These are the first submeter bathymetric maps generated with a scanning sonar mounted on Alvin. The composite maps can be used to quantify the dimensions of meter-scale volcanic and hydrothermal features within the EPR axial summit trough (AST) including hydrothermal vent structures, lava pillars, collapse areas, the trough walls, and primary volcanic fissures. Existing Autonomous Benthic Explorer (ABE) bathymetry data (675 kHz scanning sonar) collected at this site provide the broader geologic context necessary to interpret the meter-scale features resolved in the composite maps. The grid rectification technique we employed can be used to optimize vehicle time by permitting the creation of high-resolution bathymetry maps from data collected during multiple, coordinated, short-duration surveys after primary dive objectives are met. This method can also be used to colocate future near-bottom sonar data sets within the high-resolution composite maps, enabling quantification of bathymetric changes associated with active volcanic, hydrothermal and tectonic processes.This work was supported by an NSF Ridge2000 fellowship to V.L.F. and a Woods Hole Oceanographic Institution fellowship supported by the W. Alan Clark Senior Scientist Chair (D.J.F.). Funding was also provided by the Censsis Engineering Research Center of the National Science Foundation under grant EEC-9986821. Support for field and laboratory studies was provided by the National Science Foundation under grants OCE-9819261 (D.J.F. and M.T.), OCE-0096468 (D.J.F. and T.S.), OCE-0328117 (SMC), OCE-0525863 (D.J.F. and S.A.S.), OCE-0112737 ATM-0427220 (L.L.W.), and OCE- 0327261 and OCE-0328117 (T.S.). Additional support was provided by The Edwin Link Foundation (J.C.K.)

Urban agriculture, dietary diversity and child health in a sample of Tanzanian town folk

Undernutrition and micronutrient deficiency continue to be two of the major health burdens in less developed economies. In this study, we explore the link between urban agriculture, dietary diversity and child health, using weight-for-age and height-for-age Z-scores. The study makes use of two rounds of observational data for urban Tanzania and employs an instrumental variables estimation approach. We show that practising urban agriculture leads to the consumption of a greater variety of food items and the health status of urban children living in households practising urban agriculture significantly improves in the short and, more importantly, long term

Bathymetry and Seafloor Acoustic Backscatter Imagery with a Volume Search Sonar

Volume search sonars designed for mine hunting applications could be used for environmental sensing, particularly seafloor relief and texture. This capability is explored with a system that transmits a stepped FM pulse over a 243 deg vertical fan beam centered on nadir. It receives with 27 pairs of beams, symmetrically steered about nadir in the fore‐aft direction and spaced at 7.16‐deg intervals across track. The receive beam pair geometry allows simultaneous views of the seafloor in forward, vertical, and aft profiles. Pulse compression, monopulse processing techniques, and temporal and spatial filtering are used to estimate bathymetry and seafloor acoustic backscatter imagery of a sandy bottom and a muddy bottom. Three monopulse techniques have been investigated: conjugate product, difference over sum, and reduced beamwidth which is the most promising for this application. Results are presented for data collected while surveying at roughly 25 knots, showing the combined effects of acoustic geometry and survey speed on the resolution of the bathymetry and acoustic backscatterimagery and on bottom coverage. [Work supported by NRL Grant No. N00173‐00‐1‐G912.

Comparison of Extended Kalman Filtering with Split-aperture Processing for Angle of Arrival Estimation in Multibeam Echo-sounding

Echo angle of arrival estimates may be obtained from the FFT beamformed output of a multibeam sonar system at each successive time sample. Whether tracking disparate targets in the water column or tracking a mostly continuous surface such as the seafloor, the sequence of angles of arrival versus time is inherently nonlinear and nonstationary, and lends itself to processing with an extended Kalman filter. This filter provides both an approximation of the optimum (in a least squares sense) angles of arrival estimation and a measure of the associated errors. This technique is applied to quadrature‐sampled field data collected with the 26‐elementhydrophone array of a 160‐kHz multibeam sonar system. Results are compared to arrival angle estimates obtained with a conventional split‐aperture processor. [Work supported by NOAA Grant 111833.

3-D Spatial Sampling with a Cylindrical Multibeam Sonar ,

Various beam pair combinations can be formed with cylindrical multibeam sonar arrays to obtain a 3D spatial sampling of a patch of seafloor for each ping. This capability is explored with a 286 deg sector cylindrical array transmitting a stepped FM pulse over a 243 deg vertical fan beam centered on nadir and receiving with twenty‐seven beam pairs, symmetrically steered about nadir in the fore‐aft direction and spaced at 7.16 deg intervals across track. Conventional conjugate product techniques yield two across‐track profiles of 26 soundings each (52 phasors total) per ping. However, by combining one or two phasors along‐track and one, two or three phasors across‐track, soundings can be obtained at the spatial bisectors of the angles between the 52 phasors. This yields three profiles consisting of 51 soundings each, for a total of 153 soundings. This effectively creates a 3D patch of detected seafloor for each ping, which may overlap with the patches from adjacent pings depending on the sonars attitude changes and altitude above the seafloor

Diagnostic sur le maraîchage de contre-saison au Lac Alaotra

Aucune étude sur le marîchage n'avait été entreprise jusqu'alors au Lac Alaotra, zone de production prioritaire rizicole. Pourtant l'augmentation de la pression foncière et l'accroissement de la population obligent les agriculteurs à rechercher de nouvelles sources de revenus et on assiste à un important développement des cultures maraîchères de contre-saison. Afin de mieux cerner les potentialités de celle-ci et les possibilités d'intervention de la recherche et du développement, un diagnostic a été réalisé sur l'ensemble de la région. Outre un zonage géographique des productions, cette étude propose une typologie des systèmes d'exploitation maraîchers et dégage les principaux problèmes techniques ou d'approvisionnement rencontrés sur cultures légumières. Le suivi des marchés permet de décrire les différentes filières de commercialisation des légumes et les stratégies de chacun des acteurs, et de déterminer les productions maraîchères à développer et les actions à entreprendr