Nancy R. Bedford

Determination of source depth from the spectra of small explosions observed at long ranges

The acoustic pressure versus time signature of an underwater explosive source is a sensitive function of the charge weight and the depth of the explosion. If those parameters are known, then the amplitude of the primary shock pulse and the amplitudes and spacings of the following bubble pulses may be accurately predicted. The power spectrum of the signal is characterized by a scalloped form, which also is determined by the charge weight and depth. Even though the signal observed at long range is distorted due to propagation effects, the scalloped spectral structure is preserved. By using power cepstrum techniques, it is possible to determine the characteristic period of the spectrum. From the relation between the spectral period and the bubble pulse period, the exact detonation depth may be determined from a received shot signal if the charge weight is known. An example of such a measurement is presented in this paper. A series of signals from 1.8 lb Mk 61 SUS charges detonated at nominal depths of 60 ft ...

Multiple-source localization using GPS technology and received arrival time structure analysis in an air-deployed system

An efficient and robust method has been developed to locate multiple impulsive sources in an ocean environment. Global position system (GPS) receivers were installed on sonobuoys to obtain their locations within a few meters of accuracy. A sonobuoy field was deployed in a ring-type pattern. Charges were then set off at arbitrary locations within the ring, High-resolution plots were used to obtain direct path and/or first bottom bounce arrivals on each buoy. A model grid of arrival times was constructed, corresponding to the dimensions of the buoy field. A ray model previously developed here at the Applied Research Laboratories at the University of Texas at Austin (ARL:UT) was used to obtain model travel times. The minimum value of the least-square-type error between the real arrival times and the modeled travel times resulted in an unambiguous location of the source, within the limits of the grid spacing chosen. This value was calculated by picking one receiver as the reference and then summing the timing errors of the remaining receivers relative to the reference. Successive iterations with finer grid spacings result in source localization within the accuracy of the buoy locations. The localization routine was extended by allowing permutations of the pulse arrivals on each buoy to account for multiple sources closely separated in time and/or space. An automated correlation technique is presented as an alternative to the leading edge-detection method used here for obtaining relative arrival times. Two proof-of-concept experiments were performed and some results of data obtained at Lake Travis and the Gulf of Mexico are presented.

Development of an arctic ray model.

An Arctic ray model ICERAY, which is based on a ray model previously developed for deep ocean environments, is described. An acoustic propagation description for Arctic regions is achieved by introducing top layering above the ocean environment and allowing receivers to be located in either the ocean volume or the top (ice) layers. Compressional and shear ray components are included. An evanescent field, in a ray form, is shown to account for the dominant component of the field for long‐range propagation to receivers in the ice. Comparisons to an existing full wave model and corresponding analytic expressions demonstrate the accuracy of this ray approach if the evanescent field is included.

Long‐range sensing of explosive source depths using cepstrum

The acoustic signature of an underwater explosive source is a sensitive function of the source weight and depth of explosion; its power spectrum is characterized by a scalloped form. Even though the signal observed at long range is distorted due to propagation effects, the scalloped spectral structure is preserved; a characteristic period of the receiver signal, which is approximately the bubble‐pulse period of the source, may be determined and used to measure the explosion depth. Examples of such measurements are presented in this paper. Signals from 1.8‐lb charged detonated at nominal depths of 60 and 300 ft were examined. The bubble‐pulse period of each source was determined onboard the source ship. The shots were received at ranges of 250–300 nm at one location and 650–700 nm at another. The depths as estimated at the two ranges, respectively, were distributed about the onboard measurements with standard deviations of 0.7 and 1.3 ft for the 60‐ft sources and approximately 2.0 and 5.0 ft for the 300‐ft...

A study of the influence of an off‐shore rise on low‐frequency modal propagation with Arctic surface loss.

Simple up‐slope propagation of underwater acoustic energy can usually be modeled by an adiabatic normal mode approximation, provided the slope is not too severe. Bottom interacting paths in such an environment are usually sufficiently attenuated to make consideration of mode coupling unnecessary. An environment further complicated by an off‐shore rise can cause additional acoustic energy to be introduced into lower‐order modes due to bottom interaction. These low‐order modes will then propagate in deep water until encountering the continental shelf. Experimental results from the Arctic Ocean between 25 and 45 Hz suggest such propagation conditions. These results, and their interpretation in the context of a coupled mode study, will be presented. Environmental parameters and source depth will be varied to explore under what conditions mode coupling can be ignored when considering detection and localization problems. Arctic surface loss will be applied to the modeling results to show how mode varying attenu...

Propagation characteristics from the trans‐Arctic propagation source as measured at a receiver in the Lincoln Sea

The trans‐Arctic propagation (TAP) source was a Russian device deployed north of Svalbard transmitting at about 20 Hz. Transmissions received in the Lincoln Sea north of Ellesmere Island were recorded on a 20‐element vertical array located on the continental shelf. The propagation path between the source and receiver is over the Arctic Mid Ocean Ridge, then skirting the Morris Jesup Plateau, and finally propagating up the continental slope. The ice cover along this path of propagation is highly varied, ranging from central Arctic roughness levels (usually about 1–2.5 m standard deviation) to the higher roughness (about 4 m s.d.) of the Canadian Archipelago. Amplitude and phase fluctuations of signal and environmental noise over varied time periods, and the results for the statistical character of the water column will be presented. Comparisons will be made with modeled results to help determine the effects from this complex environment and to predict the impact of variation in ice conditions along the pro...

The extraction of acoustic propagation parameters in shallow water using synthetic aperture processing

Synthetic aperture processing (SAP) is a method of extracting horizontal wave numbers from recordings on one, or on a very few hydrophones. The authors have reported results from a range‐dependent experiment using an array in deep water [J. Acoust. Soc. Am. 93, 2374(A) (1993)]. In the experiment reported in this presentation SAP have been combined with the robustness of a 48‐element vertical array covering most of the water column in 400 m of water. A determination was made of detailed shallow water propagation parameters using a moving cw sound source projecting 25 and 45 Hz. Estimations can be made for mode depth functions, mode attenuation, and mode eigenvalues as a function of source range. At 25 Hz almost all propagating modes are bottom interacting. So a determination of the effects of bottom properties on acoustic propagation can be made without direct measurement of bottom parameters. The environment is slightly range dependent, and experimental results will be compared with parameters obtained fr...

A study of the influence of an offshore rise on low‐frequency modal propagation

Simple up slope propagation of underwater acoustic energy can usually be modeled by an adiabatic normal‐mode approximation, provided the slope is not too severe. Bottom interacting paths in such an environment are usually sufficiently attenuated to make consideration of mode coupling unnecessary. An environment further complicated by an offshore rise can cause additional acoustic energy to be introduced into lower‐order modes due to bottom interaction. These low‐order modes will then propagate in deep water until encountering the continental shelf. Experimental results from the Arctic Ocean between 25 and 45 Hz suggest such propagation conditions. These results, and their interpretation in the context of a coupled mode study will be presented. Emphasis will be placed on the conditions under which mode coupling will occur, and when it must be considered in prediction and localization problems. The coupled mode model, COUPLE [R. B. Evans, J. Acoust. Soc. Am. 74, 188–195 (1983)], is used as the vehicle for t...

Environmental correlate to underwater acoustic noise in the Arctic Ocean

In an effort to find a physical parameter to use as a predictor of acoustic noise in the Arctic Ocean, a modified version of the AIDJEX plastic ice model was used to study the mechanical energy balance of the sea ice cover in the vicinity of long‐term acoustic measurements taken in the Lincoln Sea in 1989–90. It was assumed that pa−pw (the rate at which the air works on the ice minus the rate at which the ice works on the water) was the power available for generating noise and that a fixed fraction of this power was converted locally into acoustic noise intensity in the water. The dominant environmental variable in this term was the geostrophic wind velocity, derived from barometric pressure measurements from The Coordinated Arctic Buoy Program [Colony and Rigor, APL/Washington]. A relative noise intensity was determined by calculating the quantity pa−pw on a grid over the Arctic Ocean, weighting by transmission loss from the grid points to the measurement site, and summing. Monthly time series correlatio...

VLF cw signal analysis

Very‐low‐frequency (VLF) sound waves transmitted at 7 and 10 Hz from a source towed at a depth of 122 m were sampled over the bottom half of the water column by an 18‐element vertical hydrophone array anchored on the bottom (3170 m deep). Analysis included the comparison of measured beam levels with corresponding eigenray and adiabatic normal mode models at short (0.7–20 km) and long (35–60 km) ranges. At short ranges, the VLF measurements and both models were consistent in indicating easily distinguishable direct, bottom, bottom–surface, and bottom–surface–bottom interacting arrivals. At longer range, the arrivals typically consisted of three from the surface and three from the bottom, corresponding to two, three, and four bottom interactions. The continuous range over which a particular arrival order dominated was 5 km or less. An anomalous bottom loss observed at 30°–34° at 7 Hz appears to be the result of a slow hydrate layer in the sediment. Except for these angles, VLF bottom loss was low compared t...