Acoustic Travel-time Measurements Research Articles

Coherence analysis of multimegameter range ocean acoustic signals

The quality of acoustic travel time measurements depends on the coherence of the signal as well as the bandwidth. Ocean acoustic signals exhibit coherence properties in four ways: in time, in bandwidth, and in vertical and horizontal extent. (These are conjugate to Doppler broadening, pulse time spread, and rms vertical and horizontal arrival angle.) These properties have recently been measured and can be compared for several different ranges (80–5000 km), center frequencies (28–250 Hz), and geometries ( source on-bottom versus off-bottom). For signals of less than perfect coherence, techniques from communications theory can be used to improve the quality of travel time measurements by relaxing the resolution requirement. For instance, if the coherence bandwidth is less than the signal bandwidth, it is possible to consider subbands of the signal as separate measurements. How to best combine these measurements then depends crucially on the SNR. While acoustic thermometry has relied on electronically controlled sources, by allowing for the coherence structure of stochastic sources (such as T-phases, explosives, or marine mammals) an improved signal processing strategy may result. a)The ATOC Consortium: A. B. Baggeroer (MIT), T. G. Birdsall (U. Michigan), C. Clark (Cornell), J. A. Colosi (WHOI), B. D. Cornuelle (SIO), D. Costa (UCSC), B. D. Dushaw (U. Washington), M. Dzieciuch (SIO), A. M. G. Forbes (CSIRO, Hobart), C. Hill (MIT), B. M. Howe (U. Washington), J. Marshall (MIT), D. Menemenlis (MIT), J. A. Mercer (U. Washington), K. Metzger (U. Michigan), W. Munk (SIO), R. C. Spindel (U. Washington), D. Stammer (MIT), P. F. Worcester (SIO), and C. Wunsch (MIT).

Optimal array element localization

Measuring ocean acoustic fields at an array of sensors allows the application of advanced signal processing methods such as beamforming and matched-field processing. However, these methods require accurate knowledge of the positions of individual sensors. Typically, an acoustic survey is required to localize the sensors, a procedure referred to as array element localization (AEL). AEL generally consists of measuring the traveltimes of acoustic signals transmitted from a series of known positions to the sensors to be localized. These traveltime data can then be inverted for estimates of the sensor locations using a linearized inversion algorithm. An important issue in AEL is designing the configuration of acoustic source positions: a well-designed source configuration can produce substantially better AEL results than a poor configuration. In this paper, a procedure is developed to determine optimal AEL source locations for arrays of any configuration. This procedure minimizes the condition number of the Jacobian matrix of partial derivatives on which the linearized inversion is based. Minimizing the condition number in effect minimizes the magnification of data (traveltime) errors into model (sensor position) errors in the inversion. Monte Carlo simulations verify that optimal AEL source configurations can lead to significant improvements in sensor localization.

Physiological roles of ryanodine receptor subtypes

Continuous acoustic travel-time measurements from a two-dimensional array of pressure-gauge-equipped inverted echo sounders spanning the entire Ulleung Basin of the southwestern Japan/East Sea between June 1999 and July 2001 are used to examine the upper temperature and current patterns. A new method, referred to as the Residual GEM Technique, interprets the travel-time data into a three-dimensional (x,y,p) time-series of daily, synoptically mapped current and temperature fields. During the two-year measurement period, at least five non-repeating persistent flow patterns are found. The patterns during the first year coincide with changes in the total volume transport through the Korea/Tsushima Strait, while the patterns of the second year do not. The mean temperature of the basin displays strong interannual variability and is correlated with the total Korea/Tsushima Strait transport, with a higher mean temperature in the first year when total volume transport was higher. In addition, a new framework for describing the flow patterns is presented. A newly described cold-core eddy, referred to as the Dok Cold Eddy, is about 60 km in diameter and typically forms southwest of Dok Island when the Subpolar Front loops southward between Ulleung and Dok (Takeshima) Islands and sheds an eddy. The Dok Cold Eddy is highly variable in space and time, and it tends to propagate westward towards the coast of Korea, where it merges with cold waters from the north. Three such propagation events preceed the disappearance of the East Korean Warm Current, which then remains absent between June and November 2000. The Offshore Branch forms by branching in the Korea/Tsushima Strait and is present during much of our two-year observation period.

Tomographic reconstruction of shallow-water bubble fields observed in the Scripps Pier bubble experiment

An experiment was performed in the Spring of 1997 off the pier at the Scripps Institute of Oceanography. The objective was to measure the near-shore properties of bubble fields. A major component of the experiment was the Delta frame designed at NRL-SSC. The triangular frame, 9.4 m on each side, supported two sources and eight hydrophones. Measurements of acoustic travel time and attenuation were made at eight frequencies between 39 and 244 kHz. Details of the measurement apparatus will be presented elsewhere at this meeting [J. W. Caruthers et al., ‘‘Observations of attenuation during the Scripps Pier bubble experiment’’]. In this paper, a tomography algorithm [D. Rouseff and F. S. Henyey, J. Acoust. Soc. Am. 101, 3032 (1997)] is applied to a sampling of the data to produce cross-sectional mappings of attenuation. The bubble concentrations are shown to be inhomogeneous and carried across the frame by rip currents. Possible improvements to the tomography algorithm for imaging strongly attenuating structures are discussed. [Work supported by ONR.]

Further evidence that the sound-speed algorithm of Del Grosso is more accurate than that of Chen and Millero

Estimates of the bottom depth of the ocean at 11 sites were determined by combining round-trip acoustic travel time measurements made by inverted echo sounders on the ocean bottom with sound-speed profiles determined from simultaneously measured temperature and salinity profiles. These depths were converted into pressures and were compared to independently measured bottom pressures to determine the accuracy of the algorithms used to calculate the sound-speed profile. The sound-speed algorithms tested were those derived by Del Grosso [J. Acoust. Soc. Am. 56, 1084–1091 (1974)] and by Chen and Millero [J. Acoust. Soc. Am. 62, 1129–1135 (1977)], as well as the corrected version of Chen and Millero’s algorithm recently published by Millero and Li [J. Acoust. Soc. Am. 95, 2757–2759 (1994)]. The results of this study agree with the results from recent acoustic tomography experiments which indicate that the algorithm of Del Grosso for the speed of sound in seawater is more accurate than the currently accepted standard algorithm of Chen and Millero. Del Grosso’s algorithm also produces more accurate results than those from the Millero and Li correction to the Chen and Millero algorithm.

Acoustic array navigation in shallow water

The inversion of acoustic field data for estimation of unknown environmental or geometric parameters is receiving considerable attention. The environmental parameters usually consist of bathymetry, sound speed in the water, and bottom properties such as sound speed, attenuation, and density. The geometric parameters consist of source and array sensor positions. In many situations it is assumed that the array sensor positions are known and in these cases it is important to the inversion process that the array sensor positions are known precisely. In this paper an acoustic navigation method for precisely determining sensor position is presented. Acoustic travel time measurements were used to navigate the sensors of a 94-m vertical array in shallow water (130 m). Array navigation performance was evaluated during a vertical array deployment in the Mediterranean from the R/V Alliance. A network of four bottom-moored acoustic transponders were interrogated from the Alliance and their replies were received by the vertical array sensors and telemetered to the Alliance and their replies were received by the vertical array sensors and telemetered to the Alliance for navigation processing. In October, over a 2-day period, current versus depth and array shape was monitored. During this period the local currents were small; the array shape was estimated to be almost straight and nearly vertical.

Use of acoustic wave travel-time measurements to probe the near-surface layers of the Sun

The variation of solar p-mode travel times with cyclic frequency nu is shown to provide information on both the radial variation of the acoustic potential and the depth of the effective source of the oscillations. Observed travel-time data for waves with frequency lower than the acoustic cutoff frequency for the solar atmosphere (approximately equals 5.5 mHz) are inverted to yield the local acoustic cutoff frequency nu(sub c) as a function of depth in the outer convection zone and lower atmosphere of the Sun. The data for waves with nu greater than 5.5 mHz are used to show that the source of the p-mode oscillations lies approximately 100 km beneath the base of the photosphere. This depth is deeper than that determined using a standard mixing-length calculation.

Design of periodic signals using FM sweeps and amplitude modulation for ocean acoustic travel-time measurements

A design procedure for an amplitude-modulated and nonlinear frequency-modulated (AM-NLFM) signal is introduced. The designed signal can drive a given transducer to its peak power to produce a sound pressure waveform into the water with a desired power spectrum and maximum possible energy. The signal can be formed either in the time domain or in the frequency domain. The frequency domain approach gives an output power spectrum precisely identical to a preferred shape. Therefore, the sidelobe levels after matched filtering are not raised by unwanted spectral magnitude ripples which exist when a time domain method is adopted. The absence of spectral ripples is desirable for applications requiring long range transmission and good multipath discrimination capability. An acceptable tradeoff between time resolution and sidelobe levels is achieved by properly choosing the desired power spectral shape. As the time resolution is usually the most critical specification for precision travel-time measurements, it is shown that by sacrificing some of the transducer's output power capability, a waveform with a considerably wider bandwidth can be transmitted, resulting in a significantly enhanced time resolution. A quasi-steady-state (QSS) approximation is used in the signal design, leading to a manageable and intuitive design procedure. >

Unique tomographic reconstruction of vector fields using boundary data

The problem of reconstructing a vector field v(r) from its line integrals (through some domain D) is generally undetermined since v(r) is defined by two component functions. When v(r) is decomposed into its irrotational and solenoidal components, it is shown that the solenoidal part is uniquely determined by the line integrals of v(r). This is demonstrated in a particularly simple manner in the Fourier domain using a vector analog of the well-known projection slice theorem. In addition, under the constraint that v (r) is divergenceless in D, a formula for the scalar potential phi(r) is given in terms of the normal component of v(r) on the boundary D. An important application of vector tomography, i.e., a fluid velocity field from reciprocal acoustic travel time measurements or Doppler backscattering measurements, is considered.

New estimates of sound speed in water

Measured travel times of acoustic pulses, propagated across a 3000-km section of the North Pacific, are inconsistent with travel times predicted with the internationally accepted algorithm for the speed of sound in water. The sound-speed algorithm predicts a speed that is too fast at oceanic pressures found below about 1-km depth. An accurate algorithm of sound speed is important for deriving temperature from measurements of acoustic travel time across oceans. Accurate estimates of the temperature field are probably important for better understanding the ocean’s role in determining weather, climate, and the distribution of marine organisms.

Basin-scale tomography: A new tool for studying weather and climate

Measurements of acoustic travel time across the northeast Pacific Ocean are used to detect large-scale (>500 km) thermal trends in the ocean interior. The trends are novel observations, and there is probably no method to verify them with other data sets. The results demonstrate the practicality of a new and powerful means to observe ocean temperature and to investigate its relation to weather, climate, and greenhouse warming.

New estimates of sound speed in water at high pressure

Measured travel times of acoustic pulses, propagated across a 3000-km section of the north Pacific Ocean, are inconsistent with travel times predicted with the internationally accepted algorithm for the speed of sound in water. The sound-speed algorithm predicts a speed that is too fast at oceanic pressures found below about 1-km depth. An accurate algorithm of sound speed is important for deriving large-scale temperatures from measurements of acoustic travel time across oceans. Accurate estimates of the large-scale thermal field are important for better understanding the ocean's role in determining weather, climate, and the distributions of marine organisms.

Acoustic navigation of a large-aperture array

Acoustic travel time measurements were used to navigate the elements of a large-aperture (900 m) acoustic array. Array navigation system performance was evaluated during a vertical deployment in the northeast Pacific from the Research Platform FLIP. A network of bottom-moored acoustic transponders were interrogated from FLIP and their 12-kHz replies were detected by receivers at 75-m intervals along the array. A nonlinear least-squares algorithm was used to estimate FLIP and array element positions from the travel time measurements. The FLIP positions derived from this procedure agreed with positions obtained from global positioning system (GPS) satellite navigation to within a 10-m rms error. Navigated positions for FLIP were internally consistent with a 0.5-m mean rms error and standard deviation of 1.1 m, and, for an array element, were consistent with a 2.8-m mean rms error and standard deviation of 0.8 m. The resulting time series of array and FLIP motions were analyzed with respect to wind, tidal, and internal wave forcing functions. Wind and tidal forcing had the greatest influence on FLIP motion, whereas array motion was governed by FLIP movement, tides, and higher frequency sources. Low-frequency array motion, with periods on the order of hours, was a result of FLIP towing the array over a horizontal range of 300 m in response to the wind and the semidiurnal tidal oscillations; the array remained within a 30-m horizontal range of FLIP’s position. Higher frequency array motion had apparent internal wave and surface-coupled components. The array shape was primarily straight and nearly vertical, to within approximately a 2° tilt, responding as a simple pendulum with small displacements.

Tomographic Reconstruction of 2-D Vector Fields: Application to Flow Imaging

Summary We examine the problem of reconstructing a 2-D vector field v(x,y) throughout a bounded region D from the line integrals of v(x, y) through D. This problem arises in the 2-D mapping of fluid-flow in a region D from acoustic travel-time measurements through D. For an arbitrary vector field, the reconstruction problem is in general underdetermined since v(x, y) has two independent components, vx(x, y) and vy(x, y). However, under the constraint that v is divergenceless (▿ v = 0) in D, we show that the vector reconstruction problem can be solved uniquely. For incompressible fluid flow, a divergenceless velocity field follows under the assumption of no sources or sinks in D. A vector central-slice theorem is derived, which is a generalization of the well-known ‘scalar’ central-slice theorem that plays a fundamental role in conventional tomography. the key to the solution to the vector tomography problem is the decomposition of the field v(x, y) into its irrotational and solenoidal components: v =▿φ+▿×ψ, where φ(x, y) and ψ(x, y) are scalar and vector potentials. We show that the solenoidal component ▿ x ψ can be uniquely reconstructed from the line integrals of v through D, whereas the irrotational component ▿φ cannot. However, when the field is divergenceless in D, the scalar potential φ solves Laplace's equation in D and can be determined by the values of v on the boundary of D. an explicit formula for φ from the boundary values of v is derived. Consequently, v(x, y) can be uniquely recovered throughout the region of reconstruction from the following information: line-integral measurements of v through this region and v measured on the boundary of this region.

Simulating ocean acoustic tomography measurements with Hamiltonian ray tracing

Tomographers map mesoscale ocean structure by inverting acoustic travel-time measurements through networks of underwater paths. To know where to deploy sensors and how to interpret their measurements, one must first understand the "forward problem," that is, how the sound channel and mesoscale features refract sound in three dimensions, and how such refraction alters the pulse-arrival sequence. We use a Hamiltonian ray-tracing program called HARPO to compute the refraction by continuous three-dimensional ocean models and to display the results in ways that add insight about refractive effects. We first simulate propagation in a simple range-independent sound channel, showing how pulse-arrival sequence depends on channel parameters and sensor placement. Next, we add linear range dependence and show that it is hard to extract range information from pulse measurements at one range. Finally, we add a simple model of a mesoscale eddy including its currents and show that deflection and splitting of the sound channel significantly alter the pulse-arrival sequence. Two diagrams that have not been widely used before are useful ways to display the arrival-time and ray-focusing perturbations caused by changes in ocean structure: they are plots of range versus launch angle and range versus travel time. Examples of azimuthal deflection, three-dimensional eigenrays, and reciprocal propagation through eddy currents are shown, and simplified methods for estimating the travel time of three-dimensional eigenrays are evaluated.