Cable Strumming Research Articles

The ONR three octave research array at Penn State

A new, large diameter towed research array has been built to support the experimental efforts of the Office of Naval Research (ONR) Ocean Acoustics Program. Array development was a collaborative effort between the Centre for Maritime Research and Experimentation (CMRE) in La Spezia, Italy, and the Penn State Applied Research Laboratory (PSU). PSU chose the array specifications based on discussions within the underwater acoustics community, prior measurement experience, cost efficiency, and flexibility in scientific measurements, while CMRE designed and assembled the array. The design includes three, forward-nested acoustic apertures cut for 1, 2, and 4 kHz, and hence, has been dubbed the Three Octave Research Array (THORA) in deference to its predecessor, the Five Octave Research Array (FORA). However, this system has a modular design to allow addition of acoustic apertures if scientific interest and sponsor support warrant. The array currently consists of a 50 m acoustic module and a 25 m vibration isolation module to help isolate the acoustic module from cable strum. This ship towed system has nearly 1 km of tow cable and a maximum measurement depth of 500 m. The array’s design, its use in the ONR NESMA23 Pilot experiment, and data quality will be discussed.

Concept Tests for a New Wire Flying Vehicle Designed to Achieve High Horizontal Resolution Profiling in Deep Water

Abstract Efficiently profiling the water column to achieve both high vertical and horizontal resolution from a moving vessel in deep water is difficult. Current solutions, such as CTD tow-yos, moving vessel profilers, and undulating tow bodies, are limited by ship speed or water depth. As a consequence, it is difficult to obtain oceanographic sections with sufficient resolution to identify many relevant scales over the deeper sections of the water column. This paper presents a new concept for a profiling vehicle that slides up and down a towed wire in a controlled manner using the lift created by wing foils. The wings provide a novel low-power method of propulsion along the cable by using the free stream velocity of the wire moving through the water in similar fashion to a sailboat sailing up wind. Scale model tests show a wide range of achievable profiling glide slopes for tow cable angles between vertical and 45°, and effective isolation of cable strum vibration from the towed vehicle body. The concept is not depth limited and will offer two-dimensional resolution that meets or exceeds current undulating tow bodies over the full water column. Additionally, this system could be used simultaneously with many other deep towed instrument packages to produce complementary datasets.

자동기록식 수중청음기를 이용한 저주파 해양잡음의 측정

Ocean noise may be used for monitoring wind speed and rainfall rate on the sea surface, as well as for tracking whales' migration routes. In particular, low-frequency ocean noise has recently been of concern with relation to the behavior of marine mammals. Low-frequency ocean noise has been increasing over the past few decades due to increase of ship traffic and offshore oil industry activities. Mechanical noise such as flow noise and cable strumming noise may be induced if low-frequency ocean noise is measured by cabled traditional hydrophone in high current areas. To successfully measure low-frequency ocean noise in a shallow water environment with strong current, we developed a self-recording hydrophone. This paper describes the main configurations of the self-recording hydrophone and presents some results on measured data.

Application of the empirical mode decomposition and Hilbert-Huang transform to seismic reflection data

Advancements in signal processing may allow for improved imaging and analysis of complex geologic targets found in seismic reflection data. A recent contribution to signal processing is the empirical mode decomposition (EMD) which combines with the Hilbert transform as the Hilbert-Huang transform (HHT). The EMD empirically reduces a time series to several subsignals, each of which is input to the same time-frequency environment via the Hilbert transform. The HHT allows for signals describing stochastic or astochastic processes to be analyzed using instantaneous attributes in the time-frequency domain. The HHT is applied herein to seismic reflection data to: (1) assess the ability of the EMD and HHT to quantify meaningful geologic information in the time and time-frequency domains, and (2) use instantaneous attributes to develop superior filters for improving the signal-to-noise ratio. The objective of this work is to determine whether the HHT allows for empirically-derived characteristics to be used in filter design and application, resulting in better filter performance and enhanced signal-to-noise ratio. Two data sets are used to show successful application of the EMD and HHT to seismic reflection data processing. Nonlinear cable strum is removed from one data set while the other is used to show how the HHT compares to and outperforms Fourier-based processing under certain conditions.

ATOC/Pioneer Seamount cable after 8 years on the seafloor: Observations, environmental impact

A study was conducted on the impacts of the presence of the Acoustic Thermometry of Ocean Climate (ATOC)/Pioneer Seamount cable on the benthos from nearshore waters adjacent to its origin at Pillar Point Air Force Station in Half Moon Bay, California to its terminus 95 km along its length on Pioneer Seamount. The coaxial Type SD cable was installed, unburied on the seafloor in 1995. Thirteen sites along the cable route were surveyed using the Monterey Bay Aquarium Research Institute (MBARI) ROVs Ventana and Tiburon equipped with cable-tracking tools. Quantitative comparisons of biological communities and seafloor features between cable and control sites were performed at nine stations. Forty-two hours of video footage and 138 push cores were collected over 15.1 km of seafloor. Approximately 12.1 km of the cable was observed (13% of the cable route). This study documents the appearance and condition of the cable and the underlying seafloor, and the effects of the cable on biological communities along its route. Limited self-burial of the cable has occurred during the 8-year deployment, particularly over the continental shelf and upper slope. Cable strumming by nearshore wave action has incised rocky siltstone outcrops. Several observations of kinks and snags in the cable on the upper slope (∼240 m depth) suggest contact with trawling gear. Few changes in the abundance or distribution of benthic fauna were detectable from video observations (epifaunal) and sediment core samples (infauna). Of 17 megafaunal groups and 19 infaunal taxa, no tests evaluating the overall effect of the cable were statistically significant. While these results indicate that the biological impacts of the cable are minor at most, three megafaunal groups exhibited cable-related changes at one or more stations. Actiniarians (sea anemones) colonized the cable when it was exposed on the seafloor, and were therefore generally more abundant on the cable than in surrounding, sediment-dominated seafloor habitats. Some fishes were also more abundant near the cable, apparently due to the higher habitat complexity provided by the cable. The study also documents general changes in the benthos across the Central California continental margin.

Robust adaptive beamforming for hydrodynamic self-noise rejection

White noise gain (WNG) is a metric widely used in the radar community for limiting the degree of adaptivity of an adaptive beamformer (ABF). Constraining adaptation of an ABF algorithm has two key effects: (1) protection against self-nulling associated with steering vector mismatch, and (2) limitation of white noise gain associated with squinting a beampattern to place a null on a mainlobe interferer. Hydrodynamic self-noise, or cable strum, commonly manifests itself as a source of mainlobe interference for passive acoustic towed horizontal line arrays. Cable strum is the result of vibrations excited in the array body by vortex shedding in the presence of flow. Strum is particularly detrimental on beams near endfire and typically exhibits very high dynamic range. As such, it requires an aggressive adaptation strategy for its effective removal. In this work, a white noise gain constraint for the rejection of cable strum is derived based on the scaled orthogonal projection approach of Cox et al. [IEEE Trans. ASSP 35 (1987)]. Tradeoffs involved in balancing strum rejection performance against mismatch-induced signal self-nulling are examined. [Sponsored by the Dept. of the Navy, under Air Force Contract No. F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and not necessarily endorsed by the U.S. Air Force.]

Cable strum self-noise cancellation for sonar towed arrays

Nonacoustic self-noise observed on marine seismic streamers and towed sonar arrays represents a serious problem for acoustic source detection at low frequency. Towed array self-noise, also known as cable strum, consists of mechanical vibrations induced by vortex shedding. Transverse vibrations in the array body subject each hydrophone pressure head to local accelerations. The resultant acoustic response can be several orders of magnitude stronger than the water-borne acoustic signals of interest. In this paper, a beamspace, time-domain adaptive signal processing architecture for the coherent rejection of broadband nonacoustic self-noise is presented. The approach is based on the recognition that most vibrational modes of a towed array propagate at phase speeds substantially less than those of acoustic signals in the water column. This property supports the formation of a signal-free, strum reference using the same sensor that samples the acoustic data. The approach removes the need for additional measurement channels, such as accelerometers or strain gauges, to independently sense the undesirable distortions introduced by cable strumming. The phenomenology underlying flow-induced self-noise for towed arrays is discussed and characterized using k-w analysis. An overview of the method is presented and cancellation performance is demonstrated using snapshots of passive sonar towed array data. [Work sponsored by the Department of the Navy, under Air Force Contract F19628-95-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the U.S. Air Force.]

Automatic classification of low-frequency Arctic ambient noise

A technique is described which can be used to automatically classify Arctic ambient noise data collected using an array of hydrophones suspended below the ice. The technique is currently designed for analyzing low-frequency (5–60 Hz) noise collected from the central Arctic pack ice. It is capable of distinguishing between nearby or distant active pressure-ridging events, thermal ice-cracking events, biological noise, cable strum, flow noise, and microseisms. Different noise types are distinguished by the power spectra, cross spectral matrices, noise directivity, and modal decomposition of the received sound. The technique is designed to be used in a real-time system which can automatically classify the dominant noise contained in the data over a specified time interval. This technique is applied to data obtained in the Lincoln Sea over 5 days in April 1988 to show how the ambient noise characteristics change with time during this period. Application of the algorithms produced a quantitative classification of the noise type that was clearly recognized in displays of the distinguishing measures.

Arctic ambient noise in the Beaufort Sea: Seasonal space and time scales

Data from the Beaufort Sea are used to quantify some of the seasonal characteristics of under-ice ambient noise. Noise at 10, 32, and 1000 Hz was studied and seasonal space and time scales were calculated. An interesting result is that summer 10-Hz noise apparently can often be contaminated by cable strum. The summer period is also the only time during which 1000-Hz noise is almost totally a product of ice motion. Within the central Beaufort Sea, lower frequency noise tends to have e-folding space scales of the order of 1000 km. This is true for 1000-Hz noise only during summer, with space scales for the rest of the year being ∼300 km. The time scale analysis also shows this trend. During summer, all frequencies have e-folding time scales of the order of 10–30 h. During fall and winter, the time scales for lower frequencies increase (up to 72 h or longer), but the 1000-Hz time scales drop to as low as 3 h.

Vortex-Induced Strumming Vibrations of Marine Cables With Attached Masses

Twenty full-scale test runs were conducted during the cable strumming experiments reported in this paper. These consisted of ten pairs of equivalent tests conducted in air and in water with a cable fitted with arrays of attached masses. The measured in-air natural frequencies are in good agreement with computed code predictions for the second and higher (up to n = 5) cable modes. The first mode frequency apparently was influenced by the sag of the cable. The measured mode shapes of the cable vibrations in water also are in agreement with the computed mode shapes. For the experiments in water the computed and measured natural frequencies are in good agreement. Drag coefficients in the range CD = 2.4 to 3.2 were commonly observed when the cable-attached mass system was strumming due to the water flow past it.

The FRAM IV vertical array—Design and performance

A vertical array was developed at NRL fur use in the eastern Arctic as part of the FRAM IV experiment. A primary objective was the measurement of the vertical sound field for long range coherent signals. The vertical directionality of ambient noise in the Arctic environment was also investigated. The array was 960 m long and had 28 working hydrophones. The design featured a fully balanced, current-mode signal transmission system, which provided maximum rejection of electrically induced interference. Provisions were also made for the reduction of flow noise and of cable strumming noise. The array was installed in late March 1982, at the site of the FRAM IV ice camp. It was operated successfully throughout a four week schedule of experiments. The recorded data exhibited a high signal to noise ratio.

Vortex-Excited Vibrations of Marine Cables

Recent experiments to study vortex-excited cable strumming in towing basins and at field sites are compared in this paper to small-scale laboratory experiments. The resonant cable strumming displacements and frequencies measured during the towing basin experiments with several 4.4-m long cables are in good agreement with laboratory experiments conducted with vibrating cylinders and cables. The field experiments were conducted with cables 23m long under spatially uniform flow conditions in a tidal current, for modes up to n = 7. The results from these field experiments also are in good agreement with basic laboratory-scale measurements of the strumming phenomenon. A strumming analysis of the SEACON II cable structure is briefly considered. Measured and calculated deflections of the cable structure are in excellent agreement when cable strumming effects are taken into account.

Active acceleration canceling for directional hydrophones

Directional transducers which are suspended from surface floats or moorings are subjects to suspension cable strumming and flow induced vibration. These inputs can mask the desired acoustical signals at low frequencies. A dynamic acceleration canceling system which senses case vibration and removes its response from the output of the acoustical sensor has been developed and tested. A reduction of vibration response by 14 dB has been measured in the laboratory with an additional acoustic sensitivity increase of 1 to 2 dB. The system comprises a simple piezoelectric accelerometer and associated electronics in addition to the usual hardware associated with the acoustic sensor. It can be used with any direct acting acoustic transducer scheme but not with geophone-type hydrophones.

Spurious Signals from Cable-Suspended Sonar Systems

Spurious sonar signals from ASW systems in the dynamic ocean environment, particularly those resulting from the hydromechanical flow-excitation phenomenon, plague motionsensitive equipment. Standing wave cable vibrations (cable strumming) and tone effects are characteristics of flexible cables in ocean currents. The magnitude of the water drag on a cable has been found to depend on cable strumming, increasing drag coefficients by as much as 35%. Results of recent studies show why the energy exchange to cylindrical shapes (i.e., flexible cables, hydrophones, and other components) is enhanced and limited by the transverse flow-excited vibrations. Engineering criteria based on these results provide effective designs to negate the causes of spurious signals from sonar systems.