Nicholas C. Makris

Critical population density triggers rapid formation of vast oceanic fish shoals.

Similarities in the behavior of diverse animal species that form large groups have motivated attempts to establish general principles governing animal group behavior. It has been difficult, however, to make quantitative measurements of the temporal and spatial behavior of extensive animal groups in the wild, such as bird flocks, fish shoals, and locust swarms. By quantifying the formation processes of vast oceanic fish shoals during spawning, we show that (i) a rapid transition from disordered to highly synchronized behavior occurs as population density reaches a critical value; (ii) organized group migration occurs after this transition; and (iii) small sets of leaders significantly influence the actions of much larger groups. Each of these findings confirms general theoretical predictions believed to apply in nature irrespective of animal species.

Fast object recognition in noisy images using simulated annealing

A fast simulated annealing algorithm is developed for automatic object recognition. The object recognition problem is addressed as the problem of best describing a match between a hypothesized object and an image. The normalized correlation coefficient is used as a measure of the match. Templates are generated on-line during the search by transforming model images. Simulated annealing reduces the search time by orders of magnitude with respect to an exhaustive search. The algorithm is applied to the problem of how landmarks, e.g., traffic signs, can be recognized by a navigating robot. We illustrate the performance of our algorithm with real-world images of complicated scenes with traffic signs. False positive matches occur only for templates with very small information content. To avoid false positive matches, we propose a method to select model images for robust object recognition by measuring the information content of the model images. The algorithm works well in noisy images for model images with high information content.<<ETX>>

THERMAL IMAGING REVEALS SIGNIFICANTLY SMALLER BRAZILIAN FREE-TAILED BAT COLONIES THAN PREVIOUSLY ESTIMATED

Abstract Using data collected with thermal imaging technology, we found a major reduction in population estimates of colony size in the Brazilian free-tailed bat (Tadarida brasiliensis) from 54 million, obtained in 1957 without this technology, to 4 million in 6 major cave colonies in the southwestern United States. The 1957 census was based on human visual observations of cave emergence flights that were subject to potentially high errors. The recent census was produced using an accurate, reproducible counting method and based on complete temporal records of colony emergences. Analysis of emergence flights from dusk through darkness also revealed patterns in group behavior that would be difficult to capture without thermal infrared technology. Flow patterns of bats during emergence flights exhibited characteristic single, double, or triple episodes, with the peak flow during the 1st episode. A consistent rhythmic pattern of flow episodes and pauses was revealed across colonies and was independent of emergence tempo.

Tracking Large Variable Numbers of Objects in Clutter

We propose statistical data association techniques/or visual tracking of enormously large numbers of objects. We do not assume any prior knowledge about the numbers involved, and the objects may appear or disappear anywhere in the image frame and at any time in the sequence. Our approach combines the techniques of multitarget track initiation, recursive Bayesian tracking, clutter modeling, event analysis, and multiple hypothesis filtering. The original multiple hypothesis filter addresses an NP-hard problem and is thus not practical. We propose two cluster-based data association approaches that are linear in the number of detections and tracked objects. We applied the method to track wildlife in infrared video. We have successfully tracked hundreds of thousands of bats which were flying at high speeds and in dense formations.

The effect of saturated transmission scintillation on ocean acoustic intensity measurements

Coherence theory is used to analyze the statistical properties of ocean‐acoustic intensity fluctuations measured after saturated multipath propagation. Previous analyses in this area have been implicitly limited to certain special cases for which the time‐bandwidth product of the field received from a given source is unity. In this paper, the statistical description is extended and generalized to be a function of measurement time and temporal coherence. As a result, the well known 5.6‐dB transmission loss (TL) standard deviation of Dyer is found to be a special case of a more general TL standard deviation that approximates 4.34 sqrt(1/μ) dB when the time‐bandwidth product μ is large. Therefore, the TL standard deviation approaches zero for increasing μ, as it must in the deterministic limit of an arbitrarily large sample size. A similar generalization is obtained for the TL mean, from which it is found that the sonar equation must be corrected for a μ‐dependent bias that vanishes in the deterministic limi...

Probing Europa's interior with natural sound sources

Europas interior structure may be determined by relatively simple and robust seismo-acoustic echo sounding techniques. The strategy is to use ice cracking events or impacts that are hypothesized to occur regularly on Europas surface as sources of opportunity. A single passive geophone on Europas surface may then be used to estimate the thickness of its ice shell and the depth of its ocean by measuring the travel time of seismo-acoustic reflections from the corresponding internal strata. Quantitative analysis is presented with full-field seismo-acoustic modeling of the Europan environment. This includes models for Europan ambient noise and conditions on signal-to-noise ratio necessary for the proposed technique to be feasible. The possibility of determining Europas ice layer thickness by surface wave and modal analysis with a single geophone is also investigated.

Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.

Deterministic reverberation from ocean ridges

The deterministic relationship between low‐frequency reverberation and detailed geomorphology is documented for wide‐area insonifications of the western Mid‐Atlantic Ridge. Charted reverberation registers precisely with extended ridges and has a high correlation with negative transmission loss, confirming previous analysis with lower resolution data [J. Acoust. Soc. Am. 95, 1865–1881 (1993)]. For a given ridge, prominent returns come from steep escarpments and cliffs that face the bistatic source and receiving arrays. This was hypothesized in the above reference, but could not be proven without the higher resolution waveforms and more sophisticated charting procedures of the present analysis. Monostatic and bistatic returns from the same lineated ridge at 1/2, 11/2, and 21/2 convergence‐zone ranges are used to thoroughly document the registration with steep scarps. A decrease in resolution of the scattering site is also documented for increasing measurement range. Ambiguity from the horizontal receiving array is resolved by two independent methods that show close agreement for prominent returns. The first is an environmental symmetry breaking (ESB) technique that requires detailed knowledge of bathymetry to be incorporated into a range‐dependent propagation model. The second is a global inversion of reverberation charts obtained from differing receiving‐array locations and orientations. The global inversion requires no a priori environmental information and has not been previously applied to field data.

The array invariant

A method is derived for instantaneous source-range estimation in a horizontally stratified ocean waveguide from passive beam-time intensity data obtained after conventional plane-wave beamforming of acoustic array measurements. The method has advantages over existing source localization methods, such as matched field processing or the waveguide invariant. First, no knowledge of the environment is required except that the received field should not be dominated by purely waterborne propagation. Second, range can be estimated in real time with little computational effort beyond plane-wave beamforming. Third, array gain is fully exploited. The method is applied to data from the Main Acoustic Clutter Experiment of 2003 for source ranges between 1 to 8 km, where it is shown that simple, accurate, and computationally efficient source range estimates can be made.

A unified model for reverberation and submerged object scattering in a stratified ocean waveguide

A unified model for reverberation and submerged target scattering in a stratified medium is developed from wave theory. The advantage of the unified approach is that it enables quantitative predictions to be made of the target-echo-to-reverberation ratio in an ocean waveguide. Analytic expressions are derived for both deterministic and stochastic scattering from the seafloor and subseafloor. Asymptotic techniques are used to derive expressions for the scattering of broadband waveforms from distant objects or surfaces. Expressions are then obtained for the scattered field after beamforming with a horizontal line array. The model is applied to problems of active detection in shallow water. Sample calculations for narrow-band signals indicate that the detection of submerged target echoes above diffuse seafloor reverberation is highly dependent upon water column and sediment stratification as well as array aperture, source, receiver, and target locations, in addition to the scattering properties of the target and seafloor. The model is also applied to determine the conditions necessary for echo returns from discrete geomorphologic features of the seafloor and subseafloor to stand prominently above diffuse seafloor reverberation. This has great relevance to the geologic clutter problem encountered by active sonar systems operating in shallow water, as well as to the remote sensing of underwater geomorphology.