Aerodynamic sampling for landmine trace detection - CiteSeerX

SPIE Aerosense, Vol. 4394, paper 108, April 2001

Aerodynamic sampling for landmine trace detection

Gary S. Settles*, Douglas A. Kester** Gas Dynamics Lab, Mechanical and Nuclear Engineering Dept., Penn State University ABSTRACT Electronic noses and similar sensors show promise for detecting buried landmines through the explosive trace signals they emit. A key step in this detection is the sampler or sniffer, which acquires the airborne trace signal and presents it to the detector. Practicality demands no physical contact with the ground. Further, both airborne particulates and molecular traces must be sampled. Given a complicated minefield terrain and microclimate, this becomes a daunting chore. Our prior research on canine olfactory aerodynamics revealed several ways that evolution has dealt with such problems: 1) proximity of the sniffer to the scent source is important, 2) avoid exhaling back into the scent source, 3) use an aerodynamic collar on the sniffer inlet, 4) use auxiliary airjets to stir up surface particles, and 5) manage the “impedance mismatch” between sniffer and sensor airflows carefully. Unfortunately, even basic data on aerodynamic sniffer performance as a function of inlet-tube and scent-source diameters, standoff distance, etc., have not been previously obtained. A laboratory-prototype sniffer was thus developed to provide guidance for landmine trace detectors. Initial experiments with this device are the subject of this paper. For example, a spike in the trace signal is observed upon starting the sniffer airflow, apparently due to rapid depletion of the available signalladen air. Further, shielding the sniffer from disruptive ambient airflows arises as a key issue in sampling efficiency. Keywords: mines, explosive detection, chemical trace detection, aerodynamic sampling, sniffers, flow visualization, aerosols.

1. BACKGROUND This paper is an outgrowth of the 1997-2000 DARPA Unexploded Ordnance/Dog’s Nose research program, an effort to apply technology to the solution of a serious global problem1. Insofar as trained dogs are known to be the most sensitive and versatile landmine detectors, part of this effort was aimed at a better understanding of how dogs are able to accomplish this feat2,3. The major thrust of the effort, however, was to develop sensors capable of mimicking the extraordinary sensitivity of the dog’s nose for landmine detection4,5. The role of the Penn State Gas Dynamics Laboratory in this effort was twofold: 1) we applied flow visualization and optical diagnostics to explore the aerodynamics of canine olfaction3, and 2) we conducted a basic experiment on sniffer aerodynamics in order to provide guidance for the design if sniffers for landmine trace detectors. This second task is the main subject of the present paper. However, it is first necessary to provide some additional background. 1.1 Explosives fate and transport A key advancement during the DARPA UXO/Dog’s Nose Program was the characterization, by an Explosives Fate and Transport Team, of the actual trace signal level one can expect to find above a buried mine in a minefield6-8. Previous information had been sketchy and inaccurate. It is now known that explosives and related compounds from the mine enter the surrounding soil, collect below the mine, and percolate up to the surface in a “halo” pattern about the mine location. Moisture and daily variations in soil temperature aid this process. For the ubiquitous TNT mine, the prevalent trace signal comes from 2,4-DNT: more volatile than TNT and always present in the military-grade explosive. The “halo” size on the ground above the mine depends on the size of the mine, but is typically expected to be on the order of 60 cm in diameter. This determines the order of magnitude of the pertinent length scale for surface trace searches for buried landmines: 1 m or less. Anecdotal evidence from handlers of explosive-detection dogs has earlier indicated that the animals could detect a mine within a 5 m radius. If this is true, then airborne trace transport (i.e. chemical plume tracing) must also play a role, discussed below in terms of

*

[email protected], phone (814) 863-1504, fax 865-0118, http://www.me.psu.edu/psgdl, Gas Dynamics Laboratory, Mechanical and Nuclear Engrg. Dept., 301D Reber Bldg., University Park, PA 16802 USA. ** now with York International, 1419 Monroe St., 1st Floor, York, PA 17404 USA, [email protected], phone (717) 771-6993.

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micrometeorology. This also raises the topic of the transfer rate of explosives traces from soil to air, which is absolutely crucial to any attempt to detect mines by airborne sampling (“sniffing”). Briefly, the Explosives Fate and Transport Team6-8 was able to quantify the mass of explosive-related compounds in contaminated soil above a buried mine: on the order of tens of µg/kg of soil7. 2,4-DNT was prevalent in the boundary layer of air directly above a buried mine, with a concentration on the order of 100 picograms/liter at equilibrium. However, less than one liter of air may be available for sampling, depending upon weather conditions. This highlights the need to consider sampling airborne particulates disturbed from the ground as well, since they carry a thousand-fold higher concentration of explosive-related signal. Finally, the team also found that soil moisture helps to release explosive vapors from buried mines, explaining why detection dogs seem to work better in wet conditions. Some mines, however, release much more trace signal than others, and metalencased mines are expected to be very difficult to detect by chemical trace sensing. 1.2 The role of micrometeorology The surface temperature of soil is documented to vary some 30° C over 24 hours, depending on location, soil type, and weather conditions. Such a temperature variation can and does desorb trace explosives from the surface, creating a detectable airborne vapor signal. What becomes of this signal, however, depends heavily on micrometeorological conditions near the soil surface. Once again, although the handlers of trained detection dogs are mindful of wind conditions and time of day, a proper scientific consideration of the micrometeorological problem with respect to mine detection has never been done. The following discussion provides some general background relevant to the issue of airborne sampling of chemical signals emanating from the ground, especially where favorable vs. adverse meteorological conditions are concerned. On sunny days a strong temperature gradient develops above the soil from morning through sunset. The soil can be 40-65° C while the air above it is in the 25° range, for example. In deserts, where many landmines are buried worldwide, the largest gradient of all occurs. This produces unstable thermal convection from the soil to the air, causing desorbed trace explosives to be transported vertically (on windless days) by thermals, see Fig. 1. Measurements9 have characterized typical thermals as arising from surface areas of 1 or more m2, rising at speeds of about ¼ m/sec, and occurring at frequencies in the range of 4/minute. Wide variability is expected, but these numbers give at least an impression of the scale of the phenomenon in space and time. Visualizations of thermals10 show typical mushroom-shaped convection cells that transport explosive traces skyward above a land mine. Under these adverse micrometeorological conditions no layer of concentrated trace signal can be expected to hug the ground above a buried landmine.

Fig. 1 - Micrometeorological regimes above a minefield, in order of complexity The situation is different, however, from sunset until the next morning. A stable temperature-inversion pattern emerges, with soil temperatures perhaps in the 10° C range and warmer air temperatures above the soil during the summer. Stable stratification then develops with a layer of colder air lying directly over the soil during windless conditions. Through the

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evening and night, a concentration of vapor traces from buried mines can accumulate in this stable, ground-hugging layer, facilitating the task of mine detection. Thus mines will be most detectable by way of their airborne trace signals during the evening, night, and early morning hours in calm weather when the ground is moist. If there is a prevailing wind, however, the aerodynamics of trace transport from a minefield is enormously complicated by mixed free- and forced convection, usually unsteady as well and often fully turbulent. In sunny weather, thermals will be transported horizontally as well as vertically while mixing with surrounding air due to inherent turbulence in the planetary boundary layer. For example, assuming thermals rise at ¼ m/sec, a horizontal breeze of the same speed (only ½ mph) is enough to tilt the thermals to a 45° angle with respect to the vertical. Clearly, above a certain mean wind speed, forced convection dominates free convection and all trace signals emanating from a minefield are vigorously mixed in the logarithmically-varying velocity field of the surface layer. This airflow exhibits gustiness characteristic of the prevalent turbulent scales, a vertical density gradient due to heating by the soil, and a “roughness offset” due to vegetation and ground irregularities. Despite having been studied since World War I, the instantaneous behavior of airborne chemical plumes, such as the explosive trace plume from a land mine, is still not well understood in terms of micrometeorology and fluid dynamics. Flow visualization, a necessary early step in understanding, has been applied only sporadically. In fact, there appears to be a disconnect between the animal behavior research community, where most of the chemical plume research has been done, and the fluids and meteorology communities, where key knowledge resides with which to advance the topic. To summarize this knowledge briefly, the downwind behavior of a chemical trace plume is strongly dependent upon the stability of the atmospheric boundary layer. Under unstable conditions (i.e., typical daytime conditions over a warm or evaporating surface) the plume will soon be torn apart by the strain rates of the buoyancy-induced thermals and the surface-layer coherent structures caused by the interaction of the wind shear and buoyancy. By contrast, under stable conditions (i.e., typical early evening and night conditions under clear skies with a temperature inversion) the stable stratification eliminates the largest turbulent eddies and, for the same overall conditions, produces a lower-speed flow of lower turbulence level (Fig. 2).

Fig. 2 - The downwind shape of a chemical trace plume originating from a point source at the left-most extreme. Wind is from the left and is in the 1-5 m/sec (3-12 mph) range. The gray shades, from left to right, indicate contours of concentration × time. The values of these contours are 90, 70, 50, 30, 20, 10, and 1%, respectively (adapted from Ref. 11). The upshot of this for present purposes is that a proper minefield aerodynamic sampler must be designed as a compromise to deal with the wide variety of meteorological conditions that can be expected to occur. Under the best conditions of stable stratification and calm winds, it may only be necessary to sample close to the ground and scan the minefield area. On the other hand, a sunny, windy afternoon represents the worst meteorological conditions for trace detection in a minefield. Under such conditions, even trained dogs may have to give up and try again later. 1.3 The current state-of-the-art of aerodynamic sampling technology Research and technology development is required on the topic of airborne sampling of trace explosives in a minefield because the technology does not currently exist to accomplish the task. In fact, the state of aerodynamic sampling technology for traces of explosives, drugs, and other contraband is very rudimentary. Airborne samplers for particulates and air pollutants (impactors) are more highly-developed, following decades of EPA funding, but that is a fixed, static technology that is inappropriate for minefield use, or for the sort of sensors likely to be useful in mine detection12. The Penn State Gas Dynamics Lab has assumed

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the role of a center of expertise in the aerodynamic sampling of explosives, drugs, and other contraband, with projects and interests ranging from the current topic to aviation security13,14, medical detection, and chemical/biological agent detection15. If one searches the technical and patent literature for “sniffers,” one typically finds leak-detection equipment fitted with a highlength-to-diameter-ratio hose having a hand-held, pointed tip16. This hose is typically 2-7 mm in diameter by 1-2 m long and is intended for the detection of leaking gases such as helium. Such technology does not address the issues of standoff distance, required airflow rate, deposition loss in the transport line, and the need to cover large areas in limited time. It is thus entirely inappropriate for minefield use, so we must start at the beginning and invent a suitable airborne sampling technology. What is desired here is the development of something more practical, mobile, rugged and effective for field use. It is emphatically not advantageous to develop an airborne sampler with unnecessary complication. On the contrary, we hope to define clearly the requirements and parameters of sniffer function for this purpose, and thus to produce the simplest solution that meets these demands. For example, in the interest of ruggedness and robust reliability for extended use, an appropriate aerodynamic sampling system for minefields should have only one primary moving part (the air mover or fan). Little help can be found in the technical literature, but evolution is rather more obliging. 1.4 What was learned from dogs Lacking much original technology for mobile outdoor airborne trace sampling, we turned to the outstanding evolutionary example set by canines. By studying the external aerodynamics of canine scenting, we were able to learn principles that underlie the appropriate design of any mimicking device. Our results3 may be briefly summarized as follows: • For direct aerodynamic sampling of scent sources, “proximity is everything” • Air inhaled into a sniffer is a pure potential sink flow, not capable of being vectored (see Fig.3) • Samplers and sniffers should avoid exhaling back into the scent source, lest they disrupt the vapor-containing air layer that they attempt to sample • An aerodynamic “collar” should be used at the sampler inlet to avoid drawing in extraneous ambient air from behind • Auxiliary airjets may be used to stir up surface particles for particle detection of explosive-related chemical traces • Dogs sample a total airflow rate of roughly 60 ml/sec at very close proximity during normal scenting • Dogs always sniff at standoff distances less than 10 cm unless held back, and minimize this standoff distance to essentially zero whenever allowed.

a

b

Fig. 3 - Potential-flow streamlines about a dog’s nose during inhalation. a) in close proximity to a surface, streamlines sample the surface directly, b) at greater standoff distance, most of the inhaled air is extraneous to the surface sampling issue. Applied to a sniffer design for landmine detection, these results provide a basic framework of rules under which such a sniffer must function. First of all, it is clear that the sniffer inlet must get as close to the terrain as is feasible. This is not only because of the small (< 1 m) “footprint” of a typical landmine, but more especially due to the rapid dissipation of airborne trace signal by

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ambient micrometeorology just above the soil surface and the limited “reach” of an aerodynamic inlet17. Nevertheless, while sniffer proximity to the ground is crucial, actual ground contact should obviously be avoided. Thus a proper aerodynamic sniffer needs to skim the surface, take advantage of the ambient conditions where possible, and provide a certain level of aerodynamic isolation from disruptive air currents (to be discussed later).

Fig. 4 - Conceptual design of an aerodynamic sniffer for landmine detection. Given a minerelated surface contamination zone (“scent source”) of diameter D, the sniffer inlet of diameter d is positioned at standoff distance h above the terrain. A fan draws airflow rate Q through the inlet, part of which is sampled by the sensor. After sampling, the airflow is discarded. Lateral separation between the sniffer and the scent source is indicated by distance ∆x. Not shown in this simplistic diagram are the particle separator, sensor resampling scheme, crosswind effect, etc. The design of the sniffer inlet plays a role in optimizing its performance as well. In order to skim the ground, a sniffer must be pointed downward. Its goal is to collect air in close contact with the ground, so it needs to be given as much directionality as possible in view of the fact that overt vectoring is incompatible with potential sink flow. The collar shown in Fig. 4 is a first step in this direction, limiting the inhaled airstream to a hemispherical “reach” zone beneath the inlet and not above or behind it. Such a collar is based on knowledge originally developed in the field of industrial ventilation17 and the aerodynamic principles of its design are well-known. Next comes the issue of exhaling the inhaled airstream after the detection step is finished. Dogs have evolved a complex variable-geometry nostril for this, but here biomimicry breaks down: No animal larger than a microbe has managed to evolve turbomachinery, but mankind has built small, light, quiet fans and blowers to move airstreams. Lacking these, the dog depends upon a bellows action that is unnecessary and unwise to mimic for the purpose of landmine detection. Instead, the airstream should be sampled near the ground and exhaled, after detection, elsewhere via a once-through system, as illustrated in Fig. 4. Finally comes the issue of particulates. It is very likely, from the DARPA Explosives Fate and Transport Team results7, that surface particulates can carry adsorbed explosive-related traces. Further, these particulates may be a thousand-fold more potent in the desired trace signal than the surrounding air. Whether or not a dog collects and desorbs these particles during sniffing is an issue of current debate, but our observations of canine sniffing3 definitely show particles disturbed and rendered airborne by the exhaled nostril airjets. This can certainly be mimicked, and particle collection/desorption is not to be ignored in a realistic chemical-trace mine-detection scenario.

2. A BASIC EXPERIMENT ON THE AERODYNAMICS OF SNIFFING Our work in the DARPA UXO/Dog’s Nose Program used flow visualization and other techniques to observe canine olfaction3. The results raised some basic questions about the aerodynamics of sniffing, e.g. what flow rate is required, as a function of distance from a chemical trace source, to acquire a detectable signal? Commercial sampler technology, described earlier, does not address such questions. We also discovered that basic data on sniffer performance, as a function of such variables as sniffertube diameter, scent source diameter, standoff distance, and lateral displacement, have apparently never been obtained. A basic experiment was thus designed to investigate the aerodynamic phenomena and performance of sniffing (Fig. 4). A stable thermal layer on a horizontal plane was used as a “scent” source according to the principle of Reynolds’ Analogy between heat and mass

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transfer. The detector was a thermocouple inside a simple sniffer tube. Flow patterns were observed by the schlieren optical method18, as shown for example in Fig. 6a.

Nondimensional Temp.

The results of this experiment emphasize the importance of sniffer proximity to localize an airborne trace source. In other words, the “reach” of an aerodynamic sniffer inlet is strongly dependent upon the airflow rate through it. For example, in steady-flow operation our experimental inlet achieved a maximum sampled signal level at a 5 cm standoff distance h from the surface being sampled when the flow rate Q through the sampler was 1.5 liters/second (Fig. 5). However, when h was reduced to 2.5 cm the required flow rate Q dropped by a factor of 5 and a higher maximum signal level was obtained

1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.5

1.0

1.5

2.0

Q, liters/sec Fig. 5 - Data from the basic sniffer aerodynamics experiment. D= 25 cm, d = 2.5 cm, ∆x = 0, ! h= 2.5 cm, " h = 5 cm. In transient sniffer operation a surprising behavior was observed: the sampled signal rose quickly to a “spike” at the sniff onset (Fig. 6b), followed by signal decline due (apparently) to depletion of the available trace-saturated boundary layer on the surface. It was also observed that steady-flow sniffing shows extreme sensitivity to transient disruptive air currents, which destroy the symmetric flow pattern of Fig. 6a and literally “blow the signal away.”

a

b

Fig. 6 - a) Schlieren image of stable thermal boundary layer being drawn into sampler inlet during basic experiments on sniffer aerodynamics, b) Detected signal strength as a function of time from beginning of inhale airflow, demonstrating initial spike and signal depletion effect.

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These results help to understand evolved sniffing behavior in canines, and they suggest aerodynamic sampler design criteria for electronic-nose devices. Unfortunately the experiments had to be terminated before measurements could be made at large standoff distances and with unstably-stratified trace-bearing layers and their resulting thermal plumes, and before the effect of ambient wind could be quantified.

3. THE DESIGN OF AN AERODYNAMIC SAMPLER FOR LANDMINE TRACE DETECTION Given the above background, the remaining discussion concentrates on design issues of the aerodynamic sampling “front-end” to mate with a suitable detector for the purpose of landmine trace detection. 3.1 Sniffer/detector impedance matching Detector characteristics play an important role in sniffer design. For example, some detectors may allow or even take advantage of a relatively-high air volume flux Q. In this case no pre-concentration step is called for, and high Q also means an extended inlet “reach.” Most detectors, however, can accept only a very small airflow input. Mass spectrometers and ion-mobility spectrometers, for example, input only milliliters/minute or less from a total sampled airflow rate that needs to be at least 104 to 105 times that value for effective aerodynamic sampling (Fig. 5). This leads to what we have dubbed, by electrical analogy, the “impedance matching” problem between the high airflows that must be sampled in practice and the miniscule airflows that actually feed the trace detection step. If “impedance matching” is not done at all or is done poorly, most of the available signal is discarded. Since the signal level is a mere trace to begin with6-8, detection is likely to fail in such cases. One solution to the impedance-matching problem is to pre-concentrate the trace material by passing the sampled airflow across an appropriate pre-concentrator, then discarding the main flow and desorbing the pre-concentrated trace material with a separate clean-gas flow matching the input requirements of the detector. This works well for aviation security portals13-15,19 but may prove to be too slow for minefield work. Fast pre-concentration is thus a fruitful topic for further research. 3.2 Particulate-phase sampling The overwhelming emphasis in chemical trace detection of landmines has been on vapor-phase rather than particle-phase sampling. However, as demonstrated by Jenkins et al.7, much-greater trace signals can be accessed if particles are sampled as well. Dogs were observed to loft particles from surfaces by way of their nostril exhalations, and subsequently to inhale these particles3. The means to accomplish this artificially by way of auxiliary airjets attached to a sniffer inlet are relatively straightforward20. Nevertheless particle collection and desorption may come at a high cost in terms of the time required to disturb surface particles, inhale them, collect them, desorb them, and direct the desorbed vapors to the detector. An informal guideline stated during the DARPA UXO/Dog’s Nose program was that a 1-10 second time interval is allowable to detect a land mine. Given a mine “footprint” of less than 1 m2, this dictates an average mine-detector speed of 0.1-1 m/sec over the minefield terrain. This allows little or no time to pause for a desorption operation. Failure to pause while desorbing for several seconds means that the detection of trace explosives occurs after the mine has been passed over, not above it, confusing the process and adding to the danger involved. The separation of solid-phase particulates from an airstream is a known technology that nonetheless requires adaptation to the present problem. Depending upon airflow rate and other circumstances, it is done either by a cyclone separator or a particle impactor12,21. The first depends on centrifugal force to separate the heavy particles from the air, while the second uses the inertia of the particles to collect them at a sharp corner while allowing the air to make the turn. EPA-funded research21 has shown the capability to collect and remove all particles above 10 µm using one or a combination of the approaches described above. Thus the design challenge of integrating particle removal into a sniffer is within the current state-of-the-art. Even if such particles are not to be sensed for explosives, though, their removal may still be necessary to avoid clogging the inlet orifices of some detectors. Along the same lines, real minefield conditions impose some harsh realities on any explosive trace sampler/detector: insects, large solids (such as vegetation or trash), and the occasional collision of the sampling inlet with terrain are among the possible

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problems. In addition to fabricating the inlet from flexible and resilient material, a screen is necessary to trap insects and large solids before the particle removal and detector stages. Collision with terrain has the possibility to feed a large “slug” of dust into the inlet, which must usually be removed by a particle separator lest detector clogging result. 3.3 Integration of aerodynamic sampler and detector Additional integration issues include minimizing the length from the inlet to the detector input, managing fan size and power requirements, and related issues concerning a robust sampler/detector combination. Issues surrounding signal deposition on the walls of sampling tubes have been studied and are reported in the literature22. Heating of a (short) sampler tube is thought not to be either necessary or desirable. A heated probe might be used, for example, to resample the sampled airstream in order to provide direct input to a detector. Finally, there is the re-sampling issue: from a relatively-high sampled flow rate Q, a very-low re-sampled flow rate is fed to detectors like the ion-mobility spectrometer. If the originally-inhaled stream Q is not homogeneous, trace detection can be defeated at this step. Under such circumstances flow mixing vanes or “turbulators” are called for between the sampler inlet and the sensor, not unlike the ethmo-turbinates inside a dog’s nose. 3.4 Ambient wind isolation Finally, it is especially important to seek solutions to some of the practical issues of outdoor aerodynamic sampling under adverse conditions in a minefield. It was shown in our experiments with dogs3 and in the basic sniffer tests described earlier that an open, unshrouded inlet is extremely sensitive to crosswind disruption. This was seen as well in mine-detection field trials at Ft. Leonard Wood, MO, where adverse weather conditions (bright sunshine warming the ground and a stiff breeze) defeated most detection attempts. Styrofoam cups and cardboard boxes were used, with little success, in ad hoc attempts to create a “headspace” for sampling despite these difficult ambient meteorological conditions. A practical compromise is necessary between standoff distance to accommodate terrain and vegetation, suction airflow rate through the sniffer, and effectiveness in a crosswind situation. Preliminary experiments have shown some success in shrouding the sniffer inlet (Figs. 4 and 6a) by soft brush bristles or a fine-mesh screen around the collar periphery in order to reduce the deleterious effects of ambient crosswind on sampling efficiency. Such aerodynamic isolators work by producing a lateral pressure drop and thus a resistance to the crossflow, and they have the added advantage of providing a soft rather than a hard contact when inevitable sniffer/terrain collisions occur. They have little apparent effect, though, on the nature of the sampled airstream as illustrated in Fig. 6a. More work is definitely needed on this topic, however. In any case, the “headspace” concept that works so well for laboratory sampling has little or no relevance to outdoor sampling in a mine field. Placing an upturned canister in contact with the ground in order to build up an internal headspace vapor concentration for sampling is incompatible with mine detection on time, safety, and practicality grounds.

CONCLUSIONS Against a background of accumulated knowledge on canine olfaction, micrometeorology, and aerodynamics, we have attempted to state some guiding principles for sniffer design and practice in landmine detection. This work is preliminary, however, and much more needs to be done. The knowledge of the effects of large sniffer standoff distance, lateral separation between sniffer and trace source, and unstable thermal stratification, for example, is meager. Additional experiments along these lines are needed. We believe a proper aerodynamic sniffer needs to skim the surface, take advantage of the ambient conditions where possible, and provide a certain level of aerodynamic isolation from disruptive wind currents. Outdoor experiments in a realistic minefield scenario with flow visualization and measurements would be very useful in further building knowledge and experience in practical sniffer design.

ACKNOWLEDGMENT This work was funded by the DARPA Unexploded Ordnance/Dog’s Nose program (http://www.darpa.mil/ato/programs/uxo/), directed by Drs. R. E. Dugan and T. Altshuler. The assistance of J.D. Miller and L. J. Dodson-Dreibelbis and discussions with T. F. Jenkins, J. S. Kauer, V. George, J. M. Johnston, L. P. Waggoner and J. White are gratefully acknowledged.

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