A sensor node having two types of sensors: sound and seismic units was used for signal collection in a test with
different moving light vehicles on a gravel road in a quiet area. An analysis of signals from the node at low frequencies
(less than 100 Hz) shows the possibility of tested vehicles detection at long distance. The sound signals for the vehicle
motion were detected above the lowest frequencies of 15-20 Hz only while the seismic signals had the maxima in that
frequency band. Another test was conducted on the ground to find the common vibrations of a light vehicle and the
ground due to vehicle passby in frequencies below 100 Hz. For this signal collection the same sensor node was used. An
additional 3-x accelerometer was installed in the vehicle cabin above the transmission. For start time synchronization of
recorded signals from the node on the ground and 3-x accelerometer in the vehicle cabin a radio channel was used.
Results for this test revealed the vehicle vibrations due to motion were detected on the ground with all three components
of the 3-axes geophone for the test track entire distance.
The National Center for Physical Acoustics (NCPA) at the University of Mississippi is working on the application of
ultrasonic Doppler sonars in air for personnel motion detection. Two traditional Doppler sonar configurations, a
monostatic and a bistatic, are being studied. In the monostatic configuration, the distance between the transmitter and
the receiver is small. The proximity of the source to the receiver places a limitation on the system associated with the
overloading of the receivers' input due to acoustic energy leakage from the transmitters' output. The maximum range
of detection is therefore limited by the dynamic range of the acquisition system. In a bistatic Doppler ultrasonic sonar,
the source and receiver are spaced apart and the acoustic energy along the direct path does not constrain the maximum
acoustic power level output of the transmitter. In a monostatic configuration the acoustic signal suffers from beam
spreading and natural absorption during propagation from the transmitter to the target and from the target back to the
receiver. In a bistatic configuration the acoustic propagation is in one direction only and theoretically the detection
distance can be twice the monostatic distance. For comparison the experiments of a human walking in a building
hallway using the bistatic and monostaic Doppler sonars in air were conducted. The experimental results for human
signatures from these Doppler sonars are presented and discussed.
The focus of this paper is a review of methods and algorithms for human motion detection in the presence of nonstationary environmental background noise. Human footstep forces on the ground/floor generate periodic broadband seismic and sound signals envelopes with two characteristic times, T<sub>1</sub> (the footstep repetition time, which is equal to the time of the whole body periodic vibrations) and T<sub>2</sub> (the footstep duration time, which is equal to the time interval for a single footstep from "heel strike" to "toe slap and weight transfer"). Human body motions due to walking are periodic
movements of a multiple-degrees-of-freedom mechanical system with a specific cadence frequency equal to 1/T<sub>1</sub>. For a
walking human, the cadence frequencies for the appendages are the same and lie below 3 Hz. Simultaneously collecting footstep seismic, ultrasonic, and Doppler signals of human motion enhance the capability to detect humans in quiet and noisy environments. The common denominator of in the use of these orthogonal sensors (seismic, ultrasonic, Doppler) is a signal-processing algorithm package that allows detection of human-specific time-frequency signatures and discriminates them using a distinct cadence frequency from signals produced by other moving and stationary
objects (e.g. vehicular and animal signatures). It has been experimentally shown that human cadence frequencies for
seismic, passive ultrasonic, and Doppler motion signatures are equivalent and temporally stable.
Detection and identification of vehicles obscured by forest canopy is a particularly challenging military problem.
Imaging techniques, e.g. laser radar imaging a target through gaps in foliage, require extensive data, making this
approach processing-intensive and time-consuming. A new method for standoff detection of a vehicle obscured under
forest canopy by remotely sensing the vibration of foliage with a laser Doppler vibrometer (LDV) has been proposed.
The method uses the effect of the vehicle engine creating sound waves, which then travel through the air and then couple
into tree leaves, causing them to vibrate. The presence of a vehicle can be determined by the spectrum of the leaves'
vibrations. Experimental study has shown that vibration velocity of leaves excited by sound from a vehicle is high
enough to be reliably detected with a LDV. The vibrations of leaves excited with simulated vehicle acoustic stimuli and
a real vehicle were successfully measured with a LDV in the laboratory and in an outdoor environment. The effect of
wind on measurements have been studied and discussed in the current work.
Human motion can be characterized as a periodic, temporal process of a mechanical system and can be detected by
active and passive ultrasonic methods. The active method utilizes Doppler ultrasound to characterize the motion of
individual body parts (torso, legs, arms, etc.). The friction forces of a footstep produce broadband sound signals that can
be measured by passive ultrasonic sensors. Comparison of Doppler motion and the footstep signals reveals a strong
correlation of features between the footstep friction and the maximum Doppler shift. This article presents test results
from measurements of human motion and evaluates the detection range for the passive ultrasonic method.
Seismic methods for footstep detection exploit low frequency vibration waves, typically below 100 Hz. There are two
limiting factors for detection of human footsteps at these frequencies: walking styles and the background noise floor.
The walking style changes the dynamic footstep force on the ground and, therefore, limits the maximum distance at
which walkers may be detected. For seismic frequencies, the background vibration noise floor is higher in urban areas
than in quiet areas. This article presents and discusses test results of human footstep measurements as a function of
distance using the seismic method in quiet and urban areas.
Methods of human detection utilizing low-frequency seismic signals (typically below a few hundred Hertz) from
footsteps are well known in the literature and in a practice. This frequency band is used for seismic detectors. Different
walking styles (regular, soft, and stealthy) result in different vibration signatures in the low-frequency range that limit
the maximum ranges for this method of footstep detection. For example, the stealthy walking style was undetectable
even a few meters from a seismic detector. Human footsteps generate broadband frequency vibrations in the
ground/floor and sound in the air from a few Hertz up to ultrasonic frequencies. The dynamic forces from footsteps that
are normal to the ground/floor are the primary cause of the low-frequency component in these signals. Striking and
sliding contacts between a foot and the ground/floor produce the high-frequency responses. The physical mechanisms
involved in the generation of high frequency signals and the possibility of their application for human footstep detection
were investigated by the authors [A. Ekimov, and J. M. Sabatier "Vibration and sound signatures of human footsteps in
buildings," J. Acoust. Soc. Am., 120, 762-768 (2006)]. The present paper introduces an approach for human footstep
detection using a passive ultrasonic method. The passive method employs an ultrasonic sensor that is sensitive to the
sound from sliding contacts. Test results for the detection of a walking person indoors and outdoors are presented and discussed.
Proc. SPIE. 6204, Photonics for Port and Harbor Security II
KEYWORDS: Acoustics, Signal to noise ratio, Signal detection, Interference (communication), Electronic filtering, Target detection, Environmental sensing, Sensors, Linear filtering, Detection and tracking algorithms
Stevens Institute of Technology is performing research aimed at determining the acoustical parameters that are necessary for detecting and classifying underwater threats. This paper specifically addresses the problems of passive acoustic detection of small targets in noisy urban river and harbor environments. We describe experiments to determine the acoustic signatures of these threats and the background acoustic noise. Based on these measurements, we present an algorithm for robustly discriminating threat presence from severe acoustic background noise. Measurements of the target's acoustic radiation signal were conducted in the Hudson River. The acoustic noise in the Hudson River was also recorded for various environmental conditions. A useful discriminating feature can be extracted from the acoustic signal of the threat, calculated by detecting packets of multi-spectral high frequency sound which occur repetitively at low frequency intervals. We use experimental data to show how the feature varies with range between the sensor and the detected underwater threat. We also estimate the effective detection range by evaluating this feature for hydrophone signals, recorded in the river both with and without threat presence.
The human footstep is one of several signatures that can serve as a useful parameter for human detection. In early research, the force of footsteps was measured on load cells and the input energy from multiple footsteps was detected in the frequency range of 1-4 Hz. Cress investigated the seismic velocity response of outdoor ground sites to individuals that were crawling, walking, and running. In his work, the seismic velocity response was shown to be site-specific and the characteristic frequency range was 20-90 Hz. The current paper will present vibration and sound pressure responses of human footsteps in a broad frequency range. The vibration and sound in the low-frequency band are well known in the literature and generated by the force component normal to the ground/floor. This force is a function of person's weight and a manner of motion (e.g. walking, running, etc). Forces tangential to the ground/floor from a footstep and the ground reaction generate the high frequency responses. The interactions of foot and the ground/floor produce sliding contacts and the result is a friction signal. The parameters of this friction signal, such as frequency band and vibration and sound magnitudes as functions of human walking styles, were studied. The results of tests are presented and discussed.
Resonance behavior of many types of landmines was first experimentally discovered in 2000 (Donskoy <i>et al</i>. in <i>Proceedings of SPIE </i>Vol. 4394, pp. 575-582, 2001). Laboratory studies and field tests have shown that mine’s resonance response is a complex phenomenon dependent upon interaction between soil and mines and their respective properties. Although the resonance effect was successfully used by various research teams for detection of landmines, there were no thorough studies on various factors influencing buried mine's resonance response. This paper presents results of theoretical and experimental investigation of this problem including multi-modal structure of mine's vibration response, effect of burial depth and soil condition. In the modeling efforts we considered multiple modes of vibration of mine casing and represented them as oscillators with effective parameters. This approach allowed for simplification of analysis and expanding existing lump-element model to account for multiple vibration modes. The experimental tests were focused on studying the effects of burial depth and soil moisture content on resonance behavior of soil-mine system. The tests have shown that a resonance frequency initially decreases with burial depth, as expected. However, an anomalous resonance frequency increase was observed at greater depths; soil moisture even further increases the resonance frequency.
In recent years, innovative vibro-modulation technique has been introduced for detection of contact-type interfaces such as cracks, debondings, and delaminations. The technique utilizes the effect of nonlinear interaction of ultrasound and vibrations at the interface of the defect. Vibration varies on the contact area of the interface modulating passing through ultrasonic wave. The modulation manifests itself as additional side-band spectral components with the combination frequencies in the spectrum of the received signal. The presence of these components allows for detection and differentiation of the contact-type defects from other structural and material inhomogeneities. Vibro-modulation technique has been implemented in N-SCAN damage detection system. The system consists of a digital synthesizer, high and low frequency amplifiers, a magnetostrictive shaker, ultrasonic transducers and a PC-based data acquisition/processing station with N-SCAN software. The ability of the system to detect contact-type defects was experimentally verified using specimens of simple and complex geometries made of steel, aluminum, composites and other structural materials. N-SCAN proved to be very effective for nondestructive testing of full-scale structures ranging from 24 foot-long gun barrels to stainless steel pipes used in nuclear power plants. Among advantages of the system are applicability for the wide range of structural materials and for structures with complex geometries, real time data processing, convenient interface for system operation, simplicity of interpretation of results, no need for sensor scanning along structure, onsite inspection of large structures at a fraction of time as compared with conventional techniques. This paper describes the basic principles of nonlinear vibro-modulation NDE technique, some theoretical background for nonlinear interaction and justification of signal processing algorithm. It is also presents examples of practical implementation and application of the technique.
The paper presents results of the field test of the nonlinear seismo-acoustic technique for detection and discrimination of land mines. The tests were conducted in summer 2001 at the U.S. Army's outdoor testing facilities. Plastic antitank mines (M19, VS1.6, VS2.2) and plastic antipersonnel mines (M14, VS50, TS50) were confidently detected at their maximum burial depths in both gravel and dirt lanes. Mine M14 is one of the smallest mines and is very difficult to detect by other techniques. The test proved that the nonlinear seismo-acoustic detection algorithm is very sensitive to AT and AP mines, while completely insensitive to false targets, such as rocks, chunks of metal or wood, thus promising to deliver high probability of detection with low false alarm rate. The results of the tests are in good agreement with the developed physical model of the seismo-acoustic detection.
Seismo-acoustic detection has demonstrated a high potential for the detection of land mines with a low probability of false alarms. A key element in the implementation and optimization of this new detection approach is the physical model of the mine-soil system. The validated model of the mine-soil system employs a mass-spring approach, which characterizes the dynamic response of the system using very few parameters derived from the dynamic mechanical impedances of the soil and the mines. This presentation describes the model and the results of the impedance measurements of live antitank and antipersonnel mines. The paper also deals with the optimization of the detection algorithm and its performance based on mine types, burial depth, and soil condition.