Stephen J. Stanic

Measurements of high-frequency shallow-water acoustic phase fluctuations

A shallow-water high-frequency (HF) acoustic propagation experiment was conducted just off shore in Panama City, FL. Several broad-band high-resolution sources and receivers were mounted on stable platforms and deployed in water depths of 8-10 m. Signals covering the frequency range from 20 to 200 kHz were transmitted from the sources to two spatially separated receivers. The data were analyzed to provide estimates of the signal phase variances as a function of frequency and source-to-receiver range. These phase variabilities are correlated with small-scale water column thermal variabilities and ocean swell conditions.

Observation of high-frequency sound propagation in shallow water with bubbles due to storm and surf

An experiment was performed off the shore of Panama City, FL, to measure the spatial and temporal coherence of high-frequency signals that were transmitted between fixed towers. Transmission was along paths at mid depths in about 10 m of water. During the time of the experiment, there were two stormy days with breaking waves and nearby high surf. It was observed that pulse-to-pulse variations (over seconds) in travel times, over a range of frequencies, increased dramatically from those observed on quiet days. Average travel times also increased by about 3%. Dispersion was also observed. The distance between the source and the receiver towers was approximately 60 m. By assuming that bubbles were either generated by breaking waves and advected down ward and/or generated by surf and advected outward, these results are explained. Estimates of the average bubble density and bubble-density variations are made.

Vertical profiles and horizontal scales of bubble clouds in the surf zone as measured with a distributed array of upward‐looking sonars

A collaborative, multi‐institute experiment was recently performed in the vicinity of the Scripps Pier, whose goal was the study of bubble‐field generation, transport, and distribution, as influenced by surf zone conditions. The collaboration produced an ensemble of instrumentation at the site for measuring bubbles, ambient noise, temperature and salinity, currents, and surface waves. This paper emphasizes data gathered by APL‐UW using a set of four upward‐looking transducers (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles from four locations. The primary locus of measurement activity was defined by NRL‐SSC’s triangular frame (Delta Frame) and the four transducer locations for the APL‐UW system bracketed this region. The transport of bubbles via rip currents emerged as a key feature surf zone bubble environment. Images of volumetric backscattering strength versus time and depth reveal episodic events (by way of increased scattering level) whi...

Effects of bubbles on high‐frequency sound propagation in very shallow water

The results of an experiment performed off the research pier at the Scripps Institute of Oceanography that measured the acoustic effects of small bubbles in very shallow water (6 m) are discussed. The results presented are in coordination with researchers who conducted other measurements simultaneously. During the experiment, rip currents passed through a field of measurement instruments 300 m offshore. These rip currents were laden with bubbles created in the surf between the instruments and the shore. Pulse signals, between 39 and 244 kHz, were propagated to 10 m through the bubble clouds. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, bubble populations are calculated by an iterative procedure based on the well‐known resonant bubble approximation. [Work supported by the Office of Naval Research.]

The Scripps Pier Bubble Experiment of 1997 and overview of rip events and their effects on acoustics measurements

An experiment to determine the dynamics, distributions, and acoustic effects of bubbles in shallow water just offshore from active surf was performed in the Spring of 1997. The region of interest was just North of the pier at the Scripps Institution of Oceanography. An area from a few tens of meters to a thousand meters from the beach was instrumented by several researchers. An effort was made to acquire a comprehensive dataset of coastal dynamics, bubble distributions, and acoustic propagation. A major emphasis was placed on a smaller region approximately 400 m2 in area and about 300 m offshore, where most of the instruments were clustered. The purpose of this paper is to provide an overview of the experiment, a summary of the runs made over the several days of experiments, and a context for the observed rip events based on an acoustic device known as the ‘‘Delta‐Frame.’’ Two rip events (Run 5, event starting about 14:48 PST 7Mar97 and Run 7, event starting about 15:38 PST 8Mar97) selected for coordinate...

Comparisons of high-frequency acoustic fluctuations with ocean-temperature and sea-surface fluctuations in shallow water

Dynamic ocean processes produce small thermal variations that induce spatial and temporal variability in the oceans index of refraction and in the spatial scale along an acoustic propagation path. This paper reports measurements and analysis of thermal microstructure effects on ping-to-ping amplitude and phase variability of shallow-water direct-path acoustic propagation in the 20-200 kHz frequency range. These measurements were conducted during a joint experiment conducted by the Naval Research Laboratory and the North Atlantic Treaty Organization Supreme Allied Commander Atlantic (SACLANT) Undersea Research Centre, La Spezia, Italy, in 8 m of water off American Beach, located between Pisa and Livorno, Italy. Experimental observations are compared with predictions for isotropic and anisotropic turbulence, as well as for sea-surface swell. Measured phase and log-amplitude variances coincide with predictions and are relatively insensitive to weak water-column stability. The sea-surface swell dominates phase variances for this data and turbulence dominates log-amplitude variances. These results provide a reasonable lower limit on high-frequency ping-to-ping amplitude and on phase variability produced by benign shallow-water thermal fluctuations.

Surface backscattering statistics for observations conducted near Kiel, Germany

In May 1993, an experiment was performed off the coast of Kiel, Germany to study the temporal variability of backscattering from the ocean boundaries. In the sea‐surface backscattering component of the experiment, two dual‐mode acoustic transducers were used to ensonify the surface with 1 or 3 ms pings at 20–90 kHz and incident angles of 65.1°−80.9° from normal. The transducers also recorded the backscattered sound. Analysis of the overall returns shows that they vary significantly from ping to ping, such that the distribution of backscattered energy appears nearly Gaussian. However, close comparison of all the signals from a given run indicates the presence of strong salient patterns in some cases, which suggest scattering from individual features (presumed to be wave crests) which move through the beam pattern from ping to ping. The statistics of the backscattered signal from these features, and their relation to the overall backscattered energy statistics, is the topic of this paper. [This work was fun...

The measurements of the effect of storm‐produced bubbles on high‐frequency sound propagation

A high‐frequency acoustics experiment to measure the spatial and temporal coherence of direct path signals was performed in shallow water, off the coast of Panama City, Florida. Transmission was along acoustic paths between fixed towers at middepth in 10 m of water. During the experiment there were two stormy days with strong winds, breaking waves and, nearby, high surf. It was observed that: (a) average time of arrivals increased significantly over those measured on calm days; (b) pulse to pulse variations in arrival times (over seconds) increased, showing an increase of variance of about a factor of 6, and (c) dispersion was observed. The range between the source and receivers was approximately 60 m. The only known cause for these variations is the presence of bubbles. Although no direct bubble density or distribution was measured, it is possible to construct a plausible bubble model to estimate the total average void fraction along the acoustic path and the variation necessary to cause the increase in ...

Observations of attenuation during the Scripps Pier bubble experiment

An experiment that measured the effects of bubbles just offshore from active surf was performed in the spring of 1997 off the pier at the Scripps Institution of Oceanography. A region from a few tens of meters to 1000 m from the beach was instrumented by several researchers. An emphasis was placed on a region approximately 400 m2 in area and about 300‐m offshore where most of the instruments were clustered. Located in this area was a triangular‐shaped frame called the Delta Frame, which had sources at two vertices and eight hydrophones along the perimeter (each side was 9.4 m long). Eight frequencies, from 39 to 244 kHz, were propagated sequentially from each source and received on each hydrophone. All 16 transmissions occurred within 12 ms and were repeated at a 1‐s interval during each of ten 88‐min data runs. Bubbles produced in the surf zone were carried out to the experimental region by rip currents and had dramatic effects on all the instruments. This paper discusses their effects on attenuation as ...

The Naval Research Laboratory’s high‐frequency multimode towed vehicle

To obtain high‐resolution measurements of acoustic spatial variability in shallow water (SW) and very shallow water (VSW) over a wide range of environmental conditions, the Naval Research Laboratory has developed a highly stable multimode underwater vehicle that can be towed and its depth and attitude controlled via commands from the towing vessel. The vehicle was configured with manual and semiautonomous control systems, data acquisition and communications systems, and an acoustic sensor system for measuring acoustic variability in water depths from 20 to 100 ft. Two high‐frequency (20‐ to 600‐kHz) arrays were mounted flush on the sides of the tow body and operated in either broad‐beam mode or ‘‘phased‐beam’’ mode using the latest ‘‘Doily’’ technology developed at ARL Penn State. In ‘‘phased‐beam’’ mode the arrays form sine and cosine shape functions, with the beam steer angle dependent on frequency of operation. Through selection of tow speed and pulse repetition rate, the scattering characteristics of ...