Coastal Systems Station Research Articles

Model/data comparisons of subcritical grazing angle detection of targets buried under a rippled sand interface

Subcritical grazing angle target detection results for a sphere buried in Coastal System Station’s Facility 383 pond are compared to predictions from two different models. Both models use perturbation theory to calculate the transmission across the rough interface due to ripple scattering. One is a simple sonar equation model that uses a first-order expression [Eq. (16), Jackson et al., IEEE J. Ocean. Eng. 27, 346–361 (2002)] to calculate the penetration, taking into account the measured ripple profile and the target strength estimate for the buried sphere. The other model uses the penetrating field to calculate target backscattering via a T-matrix formalism adapted to account for the specified sinusoidal roughness. These models have been combined with a reverberation model to predict detection SNR. The reverberation model uses a power law fit to the sand/water interface roughness spectrum determined from measured roughness profiles. [Work supported by ONR.]

Exploiting sediment acoustics properties for subcritical buried target detection using a synthetic aperture sonar aboard the AUV Reliant

The Coastal Systems Station High Frequency/ Low Frequency Synthetic Aperture Sonar (HF/LFSAS) was recently integrated into a Bluefin Robotics, Inc. Odyssey III Family autonomous underwater vehicle (AUV). This vehicle, designated Reliant, has been used in several at-sea experiments to determine the feasibility of using bottom roughness to enhance detection of buried targets at subcritical grazing angles. Previously, the HF/LFSAS was deployed on a towed 21-in. vehicle and made some remarkable buried target detections at subcritical angles. However, due to the complexities associated with using a towed platform, only a limited amount of data was obtained during these exercises. This sparse data set made it difficult to quantify the enhancing effects of bottom roughness on buried target detection. An extensive run matrix for buried target detection was completed this year using the Reliant AUV. The primary variables in the experiments were ripple orientation, standoff range, and grazing angle. Results from these sea tests are presented and compared to theoretical models.

Buried target detection with a synthetic aperture sonar

Synthetic aperture sonar (SAS) systems can provide the high resolution and high signal-to-noise ratios that are important in detecting buried targets. Previous experiments with the Coastal Systems Station SAS have shown good results for above critical grazing angle targets and mixed results for below critical angle targets. Interest in characterizing the environmental conditions, which can allow detection at sub-critical angles, has prompted additional SAS testing. This paper describes a buried target experiment that included two 1.5-meter by 0.5-meter cylindrical targets, one 35-cm silicon-fluid-filled sphere, and two 58-cm air-filled spheres. These targets were buried under sandy sediment in 17 meters of water approximately 1 mile offshore from Panama City Beach, Florida. Above critical angle and below critical angle runs were made during November 2001 and June 2002. Environmental conditions and the seafloor ripple structure were reported by divers and, during the 2002 experiment, measured by the Applied Physics Laboratory/University of Washington with their second-generation in situ measurement of porosity (IMP2) instrumentation.

The measurement of acoustic dispersion in loosely consolidated, saturated sediments using a water-filled impedance tube

At frequencies of several kilohertz and below, the measurement of sound propagation in marine sediments is difficult due to the larger wavelengths. Laboratory experiments can be limited by the size of the facility required for propagation studies but are amenable to material property measurements. Impedance tube techniques can be used to measure the complex interfacial properties of small samples over a broad and continuous range of frequencies. From this, frequency-dependent sound speed and attenuation is obtained. Results from compressional wave speed and attenuation measurements made with a laboratory impedance tube using artificial and natural water-saturated sediments will be presented and compared to existing propagation models. Variation of attenuation with frequency will be discussed. [Work supported by the U.S. Navy Office of Naval Research and the Coastal Systems Station.]

Multibeam volume acoustic backscatter imagery and reverberation measurements in the northeastern Gulf of Mexico.

Multibeam volume acoustic backscatter imagery and reverberation measurements are derived from data collected in 200-m-deep waters in the northeastern Gulf of Mexico, with the Toroidal Volume Search Sonar (TVSS), a 68-kHz cylindrical sonar operated by the U.S. Navy's Coastal System Station. The TVSS's 360-degree vertical imaging plane allows simultaneous identification of multiple volume scattering sources and their discrimination from backscatter at the sea surface or the seafloor. This imaging capability is used to construct a three-dimensional representation of a pelagic fish school near the bottom. Scattering layers imaged in the mixed layer and upper thermocline are attributed to assemblages of epipelagic zooplankton. The fine scale patchiness of these scatterers is assessed with the two-dimensional variance spectra of vertical volume scattering strength images in the upper and middle water column. Mean volume reverberation levels exhibit a vertical directionality which is attributed to the volume scattering layers. Boundary echo sidelobe interference and reverberation is shown to be the major limitation in obtaining bioacoustic data with the TVSS. Because net tow and trawl samples were not collected with the acoustic data, the analysis presented is based upon comparison to previous biologic surveys in the northeastern Gulf of Mexico and reference to the bioacoustic literature.

Detection of mines in acoustic images using higher order spectral features

A new pattern-recognition algorithm detects approximately 90% of the mines hidden in the Coastal Systems Station Sonar0, 1, and 3 databases of cluttered acoustic images, with about 10% false alarms. Similar to other approaches, the algorithm presented here includes processing the images with an adaptive Wiener filter (the degree of smoothing depends on the signal strength in a local neighborhood) to remove noise without destroying the structural information in the mine shapes, followed by a two-dimensional FIR filter designed to suppress noise and clutter, while enhancing the target signature. A double peak pattern is produced as the FIR filter passes over mine highlight and shadow regions. Although the location, size, and orientation of this pattern within a region of the image can vary, features derived from higher order spectra (HOS) are invariant to translation, rotation, and scaling, while capturing the spatial correlations of mine-like objects. Classification accuracy is improved by combining features based on geometrical properties of the filter output with features based on HOS. The highest accuracy is obtained by fusing classification based on bispectral features with classification based on trispectral features.

Development of a systems engineering training plan at the U.S. Navy's coastal systems station

AbstractDoD, through acquisition, reform has pushed its R&D laboratories to change their approach to defense system development. Systems engineering methods and practices have become critical to the laboratories' near‐term success and long‐term survival. This study reports on the efforts of one such U.S. Navy R&D laboratory, the Coastal Systems Station (CSS), to improve its systems engineering capability by developing a comprehensive systems engineering training plan. The plan was developed by first performed an assessment of its systems engineering processes and knowledge. Then, CSS developed a systems engineering training plan to address the near‐term needs of its practicing and upcoming systems engineers. A multi‐criteria decision analysis tool, the Brown‐Gibson model, was used in the training selection process. Recommended training included an overview course and other in‐depth, topic‐specific courses as needed. © 2002 Wiley Periodicals, Inc. Syst Eng 5: 156–163, 2002

Buried target detection using synthetic aperture sonar

During the Sediment Acoustics Experiment in 1999, the Coastal Systems Station fielded a synthetic aperture sonar (SAS) operating at 20 kHz. The SAS was able to detect and image 1.5 × 0.5 m cylinders at burial depths up to 50 cm and grazing angles well below the critical angle. In order to clarify the effect of the rough surface (sand ripple field) on sound propagation into the sediment, an experiment was recently conducted using five 10-in. fluid-filled target spheres that were buried at depths ranging from 5 to 50 cm below the top of the spheres. The interior fluid was chosen to maximize the target strength of the spheres (approximately −11 dB) when buried in sediment with a sound speed of 1750 m/s. Using aspect-independent targets enables the determination of the sound level transmitted into the sediment as a function of the angle between the transmitted acoustic beam and the sand ripple direction. Complete 360 deg azimuthal patterns of target echo levels are presented, illustrating the variation due to transmit/receive azimuthal angle with respect to the ripple structure. These azimuthally dependent echo level patterns are presented for several incident grazing angles, all of which are subcritical. [Work supported by ONR Code 321TS.]

Navy Omni-Directional Vehicle (ODV) Development: Where the Rubber Meets the Deck

The Office of Naval Research sponsored the omnidirectional vehicle (ODV) development program of the Coastal Systems Station (CSS). CSS has investigated the application of ODV technology to Navy shipboard materials and ordnance handling. Under the Navy program, ODV technology was developed and a series of vehicles were built and tested. ODT technology was demonstrated to be applicable to the shipboard environment and shown to be able to overcome conditions of confined spaces, reduced traction, ship motion, decks heeled at high angles, and on-deck obstacles. This paper focuses on the Navy's demonstration of the capability of the ODV to operate under demanding environmental conditions, ODV mechanical simplicity, and adaptability of the technology for a wide range of applications. Potential commercial applications were identified in manufacturing and warehousing, and remotely controlled or autonomous platforms employed in nuclear facilities, hazardous waste cleanup, and other operations that require the movement and precise positioning of large, heavy objects. The Navy has implemented two cooperative research and development agreements (CRADA) and others are pending for further development and transfer of ODV technology to the private sector.

Asymmetric Hydrodynamic Impact and Dynamic Response of Vessels

Theoretical simulations are compared with drop test experiments of vessel free-fall impact on calm water, with zero and nonzero initial heel angles. The impact force and moment are predicted by using a theory described in Xu et al. (1998). The resulting motions of vessels at asymmetric impact include vertical water entry and transverse rolling. The downward deceleration equation combines the hydrostatic and hydrodynamic forces and the weight of hull, while the rolling equation takes into account the restoring moments due to asymmetry. Another simplified approach of a water entry dynamics and injury model (Gollwitzer and Peterson, 1996) is also applied to evaluate the impact acceleration for symmetric cases. The experiments were conducted at the Coastal Systems Station, Panama City, Florida, using a 2 ft × 8 ft (0.61 m × 2.44 m) prismatic hull model with 20 deg deadrise. The comparisons between the asymmetric impact theory and experiment shows varying degrees of qualitative and quantitative agreement.

Optimal amplitude shading for arrays of irregularly spaced or non-coplanar elements

The majority of optimal amplitude shading methods for arrays of irregularly spaced or non-coplanar elements rely on numerical optimizations and iterative techniques to compute the desired weighting function because analytic solutions generally do not exist. Optimality is meant here in the Dolph–Chebyshev sense to provide the narrowest mainlobe width for a given sidelobe level. A simpler and more efficient technique to compute the shading weights for arbitrary line array shapes or element spacings is presented and it is shown that it is sufficient to sample the optimal Dolph–Chebyshev window, computed for a uniform line array of equivalent aperture length, at the element position of the nonuniform array. Examples are given for narrow-band plane-wave beamforming with curved arrays in which phase compensation is achieved by projecting the elements on a line tangent to the array. For the same mainlobe width, the resulting peak sidelobe levels are within 3 dB of a 30-dB Dolph–Chebyshev weighted uniform line array of equal aperture length and number of elements. Results are presented for computer simulations and for data collected at sea with the Toroidal Volume Search Sonar by the Coastal Systems Station, Panama City, Florida. [Work sponsored by ONR-NRL (Contract No. N00014-96-1-G913).]

Motion compensated high-frequency synthetic aperture sonar

The Coastal Systems Station has recently completed a set of sea tests using a high-frequency synthetic aperture sonar (HFSAS) mounted on a towed underwater vehicle. The processed results from these tests demonstrate that 2.5-cm resolution can be achieved in shallow water from a small physical array. This system had a Doppler unit to control the ping rate, but had no inertial navigation system and depended entirely upon acoustic backscatter from the seabed to correct for vehicle deviations in the line of sight. In these sea tests the dual overlapped phase center technique [Sheriff, Symposium on Autonomous Underwater Vehicle Technology, pp. 236–245, 1992] was used as the motion compensation algorithm. The tests were conducted over a target field in the Gulf of Mexico that contained a variety of targets, both small sphere and minelike targets. This paper will present results from the HFSAS in the form of a point response function and an acoustic image. The HFSAS achieved between 2.5- and 3-cm resolution on the smallest sphere and produced excellent images of the remaining targets out to a range of approximately 70 m. [Work supported by ONR.]

FFT beamforming with cylindrical arrays: Application to the Toroidal Volume Search Sonar

FFT beamforming is typically applied to arrays of coplanar and equally spaced elements. For curved arrays, conventional delay and sum beamforming should be the norm because the elements are not coplanar and the projection of their position on a tangent plane yields unevenly spaced elements. However, with appropriate phase correction coefficients, FFT beamforming can be applied to the elements contained in a 90° sector of a cylindrical array if beams formed within ±35° about broadside are kept. Within this sector results are essentially identical to those obtained via conventional beamforming but the FFT technique is more efficient computationally and better suited to real-time applications. The error bounds associated with FFT beamforming on cylindrical arrays are discussed and we present applications of the technique to data collected with the Toroidal Volume Search Sonar by the Coastal System Station, Panama City, FL. [Work sponsored by ONR-NRL (Contract No. N00014-96-1-G913).]

A novel approach for obtaining bubble dissolution measurements at sea

A hybrid system for measuring the dissolution of gas bubbles in an oceanic environment was developed utilizing the Coastal Systems Station’s Light Scattering Bubble Counter (LSBC). The LSBC independently measures flow velocity and bubble size, and is capable of distinguishing bubbles from particulate matter. In the hybrid system, the dynamic range of the LSBC measurements is 50 μm to about 300 μm in radius. The system is filled at depth (30 feet maximum without internal pressurization) and initially purged of all free gas. Pneumatically controlled valves are used to establish a closed recirculating system in which a flow can be generated in either direction using a large-volume peristaltic pump driven at low-volume velocity. Clear air is forced through a ceramic fritted disk to create the initial bubble population. This bubbly mixture is recirculated through the system at a nominal flow rate of 22 l/min, which corresponds to a velocity of 30 cm/s through the LSBC aperture. The bubble radii are continuously measured using the LSBC, resulting in time-dependent bubble size distributions. These distributions are used to estimate bubble dissolution rates for the size range indicated. [Work supported by ONR.]

Acoustic-optic hybrid wave filter development

Digital filters are developed for mitigation of wave-induced image degradation in laser vibrometer data taken from the water surface. The target is ensonified by a submerged acoustic projector operated near 10 kHz. The filters are evaluated for effectiveness in restoration of the image. Unbeamformed (raw) target data are acquired from a surface with waves of variable height. Images are formed with and without the filters to determine restoration performance. [Work sponsored by Coastal Systems Station under Internal Research funding.]

Acoustic‐optic hybrid imaging experiment.

A series of experiments to determine the ability of laser vibrometers to receive and process echoes from the water surface are described. Imaging of the bottom and objects lying on the bottom is attempted. The excitation is accomplished by a submerged acoustic source operating near 10 kHz. The degradation of the image is quantified as a function of wave height. Experiment results are reported. [Work supported by Coastal Systems Station under Internal Research funding.]

Superconducting magnetic sensors operating from a moving platform

Sensors incorporating Superconducting Quantum Interference Devices (SQUIDs) provide the greatest sensitivity for magnetic anomaly detection available with current technology, The Coastal Systems Station (CSS) has pioneered the use of superconducting magnetic sensors in the field operating onboard airborne and undersea moving platforms, where environmental and instrument noises present operational problems which are not encountered under laboratory conditions. Basic conceptual approaches for utilizing these sensors onboard moving platforms, including field deployable sensor configurations, are described. Environmental and instrumental technical issues are also addressed. A historical sketch of sensor development, current sensor capabilities, recent technology advances, and future trends is included. Naval applications and dual use opportunities for this technology are given based on experience developed at CSS. >

Simulation of synthetic aperture sonars

Coastal Systems Station is developing a high-fidelity simulation of low- and high-frequency imaging synthetic aperture sonars in coastal regions. This simulation includes a coherent bottom and surface reverberation model that treats reverberation as a Gaussian random process; 3-D coherent beam patterns for the projector and receiver arrays; a 3-D coherent target strength model of an arbitrarily shaped target that treats the target strength as the coherent sum of the inelastic scattering from a large number of facets, edges, and point scatterers; and the inclusion of multipaths in the calculation of the reverberation and target returns. This simulation generates synthetic images of proud and buried targets. A number of examples of images will be presented showing the effects of system parameters and operating conditions on synthetic aperture sonar imaging. [Work supported by ONR.]

Active synthetic aperture sonar: The past 25 Years

This paper presents a brief description of the active synthetic aperture concept, its advantages and disadvantages, followed by a historical overview from Coastal Systems Station viewpoint. The overview starts in the late 1960s with the initial concept studies, progresses to the simple single-channel systems developed in the mid to late 1980s, and concludes with the present high area search rate, multi-channel, multi-aspect, realtime, motion compensated systems. Images produced by each system will be presented. The paper concludes with a brief description of present SAS efforts at CSS and the most pressing research areas for future SAS development.

Underwater Inspection for Welding and Overhaul

ABSTRACTSignificant progress has been made in making underwater ultrasonic thickness gauging and magnetic particle inspections available to the fleet. Under sponsorship from the Naval Sea Systems Command, Director of Ocean Engineering, Underwater Ship Husbandry Division (NavSea 00C5), the Coastal Systems Station has developed complete hardware packages supporting these two nondestructive test methods, and has introduced them to military inspectors at a shore intermediate maintenance activity, a destroyer tender, and a naval shipyard. Performance trials conducted prior to taking the systems to the field have been accepted by NavSea, Ships' Concepts Group, Materials Subgroup, Metals Division (NavSea 5142) as evidence that these inspections can reliably be performed underwater. Avenues for certifying specially trained divers and inspectors are being developed; for the first time the Navy will have all of the elements in place for underwater inspections satisfying the requirements of Mil‐Std‐271 (Requirements for Nondestructive Testing Methods) and the Naval Ships' Technical Manual Chapter 074.Underwater ultrasonic thickness gauging has also been slated for use in the fleet, as data from laboratory and field trials have consistently shown that reliable results can be obtained by a team comprising a certified topside inspector and a diver. In tests performed at the Ship Repair Facility (SRF), Yokosuka, underwater readings were compared to those taken in dry dock by SRF inspectors, and independently by contract inspectors. On the basis of approximately 800 locations, differences between the data sets were found to be randomly distributed, with a standard deviation on the order of 0.02“. This level of accuracy is largely sufficient to distinguish plate which will need replacement during overhaul, or plate which is thick enough to weld on.