The Maritime Security Laboratory (MSL) at Stevens Institute of Technology supports research in a range of areas
relevant to harbor security, including passive acoustic detection of underwater threats. The difficulties in using passive
detection in an urban estuarine environment include intensive and highly irregular ambient noise and the complexity of
sound propagation in shallow water. MSL conducted a set of tests in the Hudson River near Manhattan in order to
measure the main parameters defining the detection distance of a threat: source level of a scuba diver, transmission loss
of acoustic signals, and ambient noise. The source level of the diver was measured by comparing the diver's sound with
a reference signal from a calibrated emitter placed on his path. Transmission loss was measured by comparing noise
levels of passing ships at various points along their routes, where their distance from the hydrophone was calculated with
the help of cameras and custom software. The ambient noise in the Hudson River was recorded under varying
environmental conditions and amounts of water traffic. The passive sonar equation was then applied to estimate the
range of detection. Estimations were done for a subset of the recorded noise levels, and we demonstrated how variations
in the noise level, attenuation, and the diver's source level influence the effective range of detection. Finally, we
provided analytic estimates of how an array improves upon the detection distance calculated by a single hydrophone.
Stevens Institute of Technology has established a research laboratory environment in support of the U.S. Navy in the
area of Anti-Terrorism and Force Protection. Called the Maritime Security Laboratory, or MSL, it provides the
capabilities of experimental research to enable development of novel methods of threat detection in the realistic
environment of the Hudson River Estuary. In MSL, this is done through a multi-modal interdisciplinary approach. In
this paper, underwater acoustic measurements and video surveillance are combined. Stevens' researchers have developed
a specialized prototype video system to identify, video-capture, and map surface ships in a sector of the estuary. The
combination of acoustic noise with video data for different kinds of ships in Hudson River enabled estimation of sound
attenuation in a wide frequency band. Also, it enabled the collection of a noise library of various ships that can be used
for ship classification by passive acoustic methods. Acoustics and video can be used to determine a ship's position. This
knowledge can be used for ship noise suppression in hydrophone arrays in underwater threat detection. Preliminary
experimental results of position determination are presented in the paper.
Stevens Institute of Technology has established a new Maritime Security Laboratory (MSL) to facilitate advances in
methods and technologies relevant to maritime security. MSL is designed to enable system-level experiments and data-driven
modeling in the complex environment of an urban tidal estuary. The initial focus of the laboratory is on the
threats posed by divers and small craft with hostile intent. The laboratory is, however, evolvable to future threats as yet
unidentified. Initially, the laboratory utilizes acoustic, environmental, and video sensors deployed in and around the
Hudson River estuary. Experimental data associated with boats and SCUBA divers are collected on a computer
deployed on board a boat specifically designed and equipped for these experiments and are remotely transferred to a
Visualization Center on campus. Early experiments utilizing this laboratory have gathered data to characterize the
relevant parameters of the estuary, acoustic signals produced by divers, and water and air traffic. Hydrophones were
deployed to collect data to enable the development of passive acoustic methodologies for maximizing SCUBA diver
detection distance. Initial results involving characteristics of the estuary, acoustic signatures of divers, ambient acoustic
noise in an urban estuary, and transmission loss of acoustic signals in a wide frequency band are presented. These
results can also be used for the characterization of abnormal traffic and improvement of underwater communication in a
shallow water estuary.
Time reversal acoustic (TRA) focusing allows concentration of elastic energy at a location in the soil being investigated to detect landmines. The TRA process is conducted by broadcasting a wide bandwidth signal and recording the surface vibration by a Laser Doppler Vibrometer (LDV). The system impulse response from speaker to the LDV output can then be computed by cross correlating the original and recorded signals for each channel. Each transducer re-radiates the time reversal impulse response. This provides efficient focusing of the seismic wave in both space and time, thus enhancing the nonlinear effects associated with soil and landmine vibrations. Using orthogonal initial signals the suggested TRA procedure can be implemented simultaneously with multiple transmitters to increase the scanning speed. The nonlinear effects were investigated using a phase inversion method where the TRA signal is broadcast a second time with an opposite sign and the two received signals are added in post processing. The summed signal contains mainly the results of nonlinear wave interaction and tends to cancel the linear response. Small scale land mine detection experiments were conducted using a laser Doppler vibrometer and an array of speakers in the frequency band 50-500Hz. They demonstrate that the TRA system provides high concentration of elastic wave energy in the tested area. The measurements of spectral density of the TRA focused signal reveal increased spectral density in the vicinity of mine resonance frequencies. The nonlinear TRA phase inversion method shows higher contrast between mine and no mine than the linear TRA method.
Proc. SPIE. 6204, Photonics for Port and Harbor Security II
KEYWORDS: Target detection, Signal to noise ratio, Detection and tracking algorithms, Sensors, Interference (communication), Linear filtering, Electronic filtering, Acoustics, Signal detection, Environmental sensing
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.
We present the general concept and results of a pilot study on land mine detection based on the application of Time Reverse Acoustics (TRA). Applying TRA is extremely effective at focusing seismic waves in time and space, significantly improving detection capabilities using both linear and nonlinear wave methods. The feasibility of the system was explored in the laboratory and in small scale field experiments. The system included a multi-channel TRA electronic unit developed at Artann, five speakers for seismic-wave excitation and noncontact (laser vibrometer) or contact (accelerometer) devices for measurements of the surface vibration. Experiments demonstrated the high focusing ability of the TRA system. We observed excitation of highly focused seismic waves in an area with dimensions of the order of one wavelength. In the presence of a buried mock mine, the method led to an increase in the surface vibration amplitude and to significant nonlinear distortion of the TRA focused signal. Localization via TRA depends on the frequency of excitation, the depth of the buried mine, and the form and size of a mine mock. The nonlinear acoustic effect-higher harmonic generation-provides higher contrast for the mock-mine signal-response than for the surrounding medium. We also successfully tested an inversion method of the nonlinear TRA measurements earlier developed for medical ultrasound applications.
Nonlinear acoustic technique has been recently introduced as a new tool for nondestructive inspection and evaluation of fatigued, defective, and fractured materials. Various defects such as cracks, debonding, fatigue, etc. lead to anomalous high level of nonlinearity as compared with flawless structures. One of the acoustic manifestations of such nonlinearity is the modulation of ultrasound by low frequency vibration. Two methods employing the nonlinear interaction of ultrasound and vibration were developed, namely vibro-modulation (VM) and impact-modulation (IM) methods. VM method employs forced harmonic vibration of a structure tested, while IM method uses impact excitation of structure natural modes of vibration. The feasibility tests were carried out for different objects and demonstrated high sensitivity of the methods for detection of cracks in steel pipes and pins, bonding quality in titanium and thermoplastic plates used for airspace applications, cracks in combustion engine, adhesion flaws in bonded composite structures, and cracks and corrosion in reinforced concrete. The model of the crack allowing to describe the modulation of sound by vibration is discussed. The developed nonlinear technique demonstrated certain advantages as compared with the conventional linear acoustic technique, specifically discrimination capabilities, sensitivity, and applicability to highly inhomogeneous structures.