John L. Spiesberger

Tomographic Maps of the Ocean Mesoscale. Part 1: Pure Acoustics

Abstract A field test of ocean acoustic tomography was conducted in 1981 for a two month period in a 300 km square at 26°N, 70°W in the North Atlantic (just south of the MODE region). Nine acoustic deep-sea moorings with sea floor transponders for automated position keeping and with provisions for precise time keeping were set and recovered. From the measured travel times between moorings, various displays of the three-dimensional field of sound speed (closely related to temperature) have been obtained by inversion procedures. These procedures use historical ocean data as a reference, but all information from the in situ surveys has been withheld; the “pure” tomographic results were then compared to direct in situ observations. The tomographically derived spatial mean profile compares favorably to an equivalent profile from the in situ observations; both differ significantly from the historical average. Maps constructed at three day intervals for a two month period show a pattern of eddy structure in agre...

New full‐wave approximation for ocean acoustic travel time predictions

A new full‐wave parabolic approximation is introduced that is valid for a wide range of grazing angles. By Fourier synthesis it yields travel times of ocean acoustic multipaths that are insensitive to a reference speed of sound. After depths and sound speeds are transformed to new coordinates, the highly efficient ‘‘split‐step Fourier’’ algorithm is used to solve the new approximate wave equation for forward propagation. Accuracy of the new approximation has been tested by comparison to a broadband normal mode model in a range‐independent environment. At 1000 km range and with a pulse of resolution 20 ms at center frequency 75 Hz, computed travel times of 24 multipaths agreed with maximum difference 3.4 ms, mean difference 0.9 ms, and rms difference 1.5 ms. This approximation may prove to be an efficient method for accurate travel time predictions of multipaths over a wide range of acoustic frequencies and for basin scale distances.

Stability and identification of ocean acoustic multipaths

A phase‐coded signal with 64‐ms resolution was transmitted at 10‐min intervals for a 48‐day period between an acoustic source moored at 2000‐m depth and a bottom mounted receiver at ∠3000‐m depth and at ∠900‐km range. About 16 multipaths were resolved. They were stable in the presence of ocean fluctuations and could be identified (with some exceptions) from ray theory. The precision to which daily travel‐time fluctuations along multipaths could be measured was better than 10 ms. The resolution, stability, identification, and precision is adequate for acoustic monitoring of mesoscale ocean variability by measuring travel‐time variations along ray paths.

Ocean acoustic tomography - Travel time biases

The travel times of acoustic rays traced through a climatological sound‐speed profile are compared with travel times computed through the same profile containing an eddy field. The exact travel time difference is compared with its constituent terms, one of which is linearly related to the deviation of the sound speed (referenced to the climatological profile) and the others which are nonlinearly related to the deviation. At ranges that are much greater than the eddy scale and for eddy fields whose range‐averaged temperature anomaly is small, the numerical results are: (1) the values of the nonlinear terms are insensitive to changes in the positions of the eddies and (2) the nonlinear terms are approximately proportional to the range between the source and receiver and to the square of the eddy anomaly. At a 1084‐km range in the east Atlantic at 24u2009°N (36u2009°N) the nonlinear terms can account for 17% (90%) of the exact travel time change where the temperature anomalies associated with the eddies are typicall...

Ocean Acoustical Ray-Tracing Software RAY

Funding was provided by the Office of Naval Research under contract N00014-86-C-0358 and the nOffice of Naval Technology under contract N00014-90-C-0098.

Probability density functions for hyperbolic and isodiachronic locations

Animal locations are sometimes estimated with hyperbolic techniques by estimating the difference in distances of their sounds between pairs of receivers. Each pair specifies the animals location to a hyperboloid because the speed of sound is assumed to be spatially homogeneous. Sufficient numbers of intersecting hyperboloids specify the location. A nonlinear method is developed for computing probability density functions for location. The method incorporates a priori probability density functions for the receiver locations, the speed of sound, winds, and the errors in the differences in travel time. The traditional linear approximation method overestimates bounds for probability density functions by one or two orders of magnitude compared with the more accurate nonlinear method. The nonlinear method incorporates a generalization of hyperbolic methods because the average speed of sound is allowed to vary between different receivers and the source. The resulting isodiachronic surface is the locus of points on which the difference in travel time is constant. Isodiachronic locations yield correct location errors in situations where hyperbolic methods yield incorrect results, particularly when the speed of propagation varies significantly between a source and different receivers.

Perturbations in travel time and ray geometry due to mesoscale disturbances: A comparison of exact and approximate calculations

Exact calculations of the changes in eigenray travel time and geometry, due to range‐dependent mesoscale disturbances, are compared to three approximations. In the first approximation, the sound‐speed perturbation is integrated along the unperturbed path. In the second approximation, the range average of the sound‐speed perturbation is integrated along the unperturbed path. The third approximation uses the range‐average raytrace to estimate changes of travel time and geometry. Calculations made both for a measured section through a mesoscale eddy and for a simplified model of an eddy (with variable strength, position, and boundary width) suggest that: (i) while none of the approximations tested is uniformly the best, the first approximation is to be preferred ; (ii) the eddy position relative to the ray path is quite significant; and (iii) the exact travel‐time perturbations are not sensitive to whether the eddy boundaries are assumed sharp or gradual. For an eddy perturbation of ±9 m/s (±1.9u2009°C or Rossby...

Listening for climatic temperature change in the northeast Pacific: 1983–1989

Data are presented from an acoustic experiment designed to detect climatic trends of temperature in the ocean with basin‐scale resolution. These data are presented as an intriguing new way to recognize changes in spatially averaged temperature. In 1983, travel times of acoustic signals (133 Hz, 60‐ms resolution) were measured over 4000 km between a source and receiver mounted near Oahu and northern California, respectively. In 1987, measurement was begun on the travel times along six additional sections in the northeast Pacific, each at a distance of 3000 to 4000 km. Travel times changed by about ±0.2 s at each receiver at interannual periods. Changes in acoustic travel time exceeding about ±0.03 s are due to changes in the spatially averaged temperature along each section. A change of ±0.03 s is equivalent to a change in spatially averaged temperature of only about ∓0.02u2009°C in the upper kilometer of the ocean. The dynamical processes responsible for the temperature variability along the acoustic sections...

A new algorithm for sound speed in seawater

Travel times of acoustic pulses across a 3000-km section in the northeast Pacific are used to estimate an algorithm for the speed of sound in seawater. This algorithm, derived from tomographic techniques, is inconsistent both with the international standard algorithm derived by Chen and Millero [J. Acoust. Soc. Am. 62, 1129–1135 (1977)] and with the algorithm of Del Grosso [J. Acoust. Soc. Am. 56, 1084–1091 (1974)]. Both previous algorithms were derived from laboratory experiments. The additive correction, δcu2002(mu2009s−1), to Del Grosso’s sound speeds between 0- and 4-km depth is δc(p)=−0.104u2009189u2009813u20098×10−2p+0.210u2009143u2009906u20090×10−4p2−0.141u2009411u2009578u20098×10−6p3+0.317u2009812u2009236u20090×10−9p4−0.217u2009806u2009024u20093×10−12p5 with p being pressure-gauge units in kg cm−2. The rms error of δc is about 0.05u2009mu2009s−1 and 0.1u2009mu2009s−1 between the intervals of 0 to 2 km and 2 to 4 km, respectively. At about 3-km depth, sound speeds predicted by Chen and Millero and Del Grosso are about 0.7u2009mu2009s−1 and 0.2u2009mu2009s−1 too fast, respectively. An accurate alg...

Probability distributions for locations of calling animals, receivers, sound speeds, winds, and data from travel time differences

A new nonlinear sequential Monte Carlo technique is used to estimate posterior probability distributions for the location of a calling animal, the locations of acoustic receivers, sound speeds, winds, and the differences in sonic travel time between pairs of receivers from measurements of those differences, while adopting realistic prior distributions of the variables. Other algorithms in the literature appear to be too inefficient to yield distributions for this large number of variables (up to 41) without recourse to a linear approximation. The new technique overcomes the computational inefficiency of other algorithms because it does not sequentially propagate the joint probability distribution of the variables between adjacent data. Instead, the lower and upper bounds of the distributions are propagated. The technique is applied to commonly encountered problems that were previously intractable such as estimating how accurately sound speed and poorly known initial locations of receivers can be estimated from the differences in sonic travel time from calling animals, while explicitly modeling distributions of all the variables in the problem. In both cases, the new technique yields one or two orders of magnitude improvements compared with initial uncertainties. The technique is suitable for accurately estimating receiver locations from animal calls.