Odor plume

2 Surface Source or Hidden Source Sometimes source munitions are hidden, either intentionally or unintentionally, with neither soil nor water above them. In this case, we may expect that chemical signatures will be found in the vapor state. It is possible that sampling of surfaces could lead to detection, but normally a search will be in air. Many animals routinely face this same task. Whether looking for a meal or a mate, a common approach is to follow an odor plume to its source. 

A classic form of plume, familiar to everyone, is the smoke from a just extinguished candle. Close observation of this plume in still air reveals that it is tightly constrained and very smooth for some distance above the wick. A few inches above the wick the character of the plume changes. It begins 

13The term seafloor is used generically to refer to the bottom of any body of water, even a field flooded for irrigation. 

1 Plumes in Water Several fine researchers have approached the study of underwater plumes with different objectives. While all this work is undoubtedly instructive, a series of articles [28-32] produced by Webster and Weissberg and their colleagues may apply most directly. They have examined the structure [28] of plumes in controlled experiments and produced photographs of dye plumes to study their development. They also took the point of view of a hungry crab [32], In its attempt to find the food source indicated by the plume, the crab manipulates its sensors within the plume. The structure of the plume makes it necessary. Chapter 5 is devoted to a description of these plumes. 

2 Plumes in Air It is clear that, if the supply of molecules is adequate, plumes can form in air as well. While there may not be enough concentration in the air above buried sources to form plumes, in the case of unburied explosives, such as in IEDs or UXO covered loosely with rubble, there may be exploitable plumes. When such plumes develop, there will often be some level of urgency associated with locating the source. Sometimes, when logs seem to be following 

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The results demonstrate (i) that in turbulent odor plumes, purely chemical spatial gradients can be calculated when measuring with sensors scaled to lobster olfactory organs, (ii) that rapid odor access to the lobster s olfactory organs (under low ambient flow conditions) is accom-... 

A third important characteristic of a female moth s sex-pheromone plume is its nonuniformity. Simulation of odor plumes using ionized air has shown clearly that a plume is not a simple concentration gradient but instead is distinctly filamentous and discontinuous (47,48). Furthermore,... 

Webster and Weissburg (2001) visualized instantaneous versus time-averaged odor plumes by laser-induced fluorescence. The spatially varying plume at any particular point in time matters more to an animal than an average plume shape. The mean direction and speed of airflow may be relatively constant, but the animal may extract information from concentration differences on very small temporal and spatial scales. 

Clark, L. and Shah, P. S. (1992). Information content of prey odor plumes what do foraging Leach s storm petrels know In Chemical Signals in Verteirates vol. 6, ed. R. L. Doty and D. Miiller-Schwarze, pp. 421-427. New York Plenum. 

Vickers, N. J. (2000). Mechanisms of animal navigation in odor plumes. Biological Bulletin of the Marine BiologicalLaboratory 198,203-212. 

Webster, D. R. and Weissburg, M. J. (2001). Chemosensory guidance cues in a turbulent chemical odor plume. Limnology and Oceanography 46,1034-1047. 

Search strategies that exploit the natural tendency of the molecules to remain adsorbed on surface particles and to become trapped in the boundary layers near the surface are likely to be most productive. If odor plumes from buried sources do develop, they are likely to remain close to the surface until they dissipate from turbulence. 

A much-simplified classification, based on these tables, is shown in Table 4.6. The scale of interest is very different for trace chemical or odor plumes from that for which the original classification system was developed. Therefore, it is proposed that the simplified classification scheme of Table 4.6 will be sufficiently applicable to provide an indication of the more and the less favorable conditions for following plumes. In this presentation the lighter shading denotes those conditions that are generally more favorable darker shading indicates more instability and hence less well defined plumes. 

Much research has been applied toward understanding the ways that various animals use odor plumes. It is not within the scope of this book to exhaustively survey the resulting literature. One aspect of this research has direct bearing on the use of plumes to locate sources of chemical molecules such as explosives. As they are studying their animal subjects, these researchers construct two kinds of mathematical models to assist them. 

Zimmer-Faust, R. K., C. M. Finelli, N. D. Pentcheff, and D. S. Wethey. Odor plumes and animal navigation in turbulent water flow A field study. Biol. Bull. 188, 111-116 (1995). 

Farrell, J. A., J. Murlis, X. Long, W. Li, and R. T. Card6. Filament-based atmospheric dispersion model to achieve short time scale structure of odor plumes. Environ. Fluid Mech. 2, 143-169 (2002). 

Webster, D. R., S. Rahman, and L. P. Dasi. On the usefulness of bilateral comparison to tracking turbulent chemical odor plumes. Limnol. Oceanogr. 46, 1048-1053... 

Weissburg, M. J. and R. K. Zimmer-Faust. Odor plumes and how blue crabs use them in finding prey. J. Exp. Biol. 197, 349-375 (1994). 

Moore, P. A., N. Scholz, and J. Atema. Chemical orientation of lobsters, Homarus americanus, in turbulent odor plumes. 7. Chem. Ecol. 17, 1293-1307 (1991). 

Editor The other question brought to mind by the vulture reference concerns odor plumes. An entire, Chapter 5, of the book is devoted to the structure of plumes in air and water. Most animals, not just vultures, have an instinctive sense of how to follow a plume to its source. As you have used animals in the field, have you observed any behaviors that seem to indicate plume following How far have you seen animals effectively follow an explosive plume ... 

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