Export 140 results:
Sort by: Author Title Type [ Year  (Desc)]
Cornuelle, B.  1990.  Some practical aspects of ocean acoustic tomography. Oceanographic and geophysical tomography: Les Houches, session L, 1988. ( Desaubies Y, Tarantola A, Zinn-Justin J, Eds.)., Amsterdam: North-Holland Abstract
Greenland Sea Project Group.  1990.  Greenland Sea Project: a venture toward improved understanding of the oceans' role in climate. EOS Trans. AGU. 71:750-751and754-755. AbstractWebsite

The Greenland Sea is one of the few major areas where convective renewal of intermediate and deep waters contribute to world-ocean ventilation. Basin-scale cyclonic circulation, boundary currents advecting of Atlantic and Polar origin, mixing across the fronts related to the boundary currents, wintertime heat loss to the atmosphere, ice formulation and related brine release and sequences of penetrative plumes control the renewal. The field work of the Greenland Ice Project began in the summer of 1987. This paper presents and explains the objectives of the Greenland Sea Project and summarizes preliminary results from the first intense field phase which covered the seasonal cycle from summer 1988 to summer 1989. The paper discusses the following: large-scale circulation; convection; circulation and convection variability; exchange across fronts; sea ice; sea ice interactions and copepod life cycles in the European Arctic seas.

Agnon, Y, Malanotterizzoli P, Cornuelle BD, Spiesberger JL, Spindel RL.  1989.  The 1984 bottom-mounted Gulf Stream tomographic experiment. Journal of the Acoustical Society of America. 85:1958-1966.   10.1121/1.397849   AbstractWebsite

In this paper, data from a Gulf Stream tomographic experiment carried out in October 1984 are analyzed. The experiment used acoustic sources and receivers bottom mounted beneath the stream to measure Gulf Stream dynamics. However, due to an unfortunate electronic malfunction of the source, only 2 days of acoustically measured travel time data are available. Nevertheless, some new and positive results are obtained. Bottom reflected acoustic rays having up to two bottom bounces are unambiguously identified by solving the direct problem of tracing rays both in a reference climatological profile and in actual range‐dependent sound‐speed sections from a hydrographic survey carried out during the experiment. It is also shown that these rays do not appear to be affected by important nonlinearities so that they can be used to provide consistent results in inverse solutions.

Cornuelle, B, Munk W, Worcester P.  1989.  Ocean Acoustic Tomography from Moving Ships. Journal of Geophysical Research-Oceans. 94:6232-6250.   10.1029/JC094iC05p06232   AbstractWebsite

Mesoscale mapping of the ocean sound speed field in a 1000×1000 km area by means of ocean acoustic tomography is greatly enhanced by augmenting a few acoustic moorings with a movable ship-based receiver. Computer simulations based on realistic noise levels in the measured acoustic travel times give 5% (1%) residual variance in ΔC(x;y,z) for four (six) acoustic source moorings in an ocean perturbed in the gravest baroclinic mode. For comparison, objective mapping based on traditional vertical profiles requires 3 times the steaming distance to yield equivalent residual error. Detailed results depend on many parameters: the assumed mesoscale spectrum and vertical mode structure, the number of observed multipaths, the mooring configuration, the number of ship stations and the travel time signal level (due to mesoscale eddies) and noise level (due to internal waves and position-keeping errors). These parameters have critical values, below which there is distinct deterioration and beyond which there is little gain. We believe that the critical values can be attained in practice so the ultimate limit on mesoscale mapping is imposed by the internal wave-induced travel time error. This assumes that position keeping of the submerged acoustic sources and receiver by a combination of satellite navigation and high-frequency acoustics can be achieved with ±10-m accuracy. The present study assumes a stationary ocean; a second paper will deal with reciprocal transmissions yielding currents and hence the barotropic mode. This is required in a dynamic ocean model for estimating ΔC(x,y,z;t). All this is preparatory to a tomography experiment in the Greenland Sea in 1988–1989.

Hildebrand, JA, Dorman LM, Hammer PTC, Schreiner AE, Cornuelle BD.  1989.  Seismic tomography of jasper seamount. Geophysical Research Letters. 16:1355-1358.   10.1029/GL016i012p01355   AbstractWebsite

A vertical section of the interior structure of Jasper Seamount was modeled using a spectral tomographic inversion of P wave travel times. An array of ocean bottom seismographs (OBSs) deployed over the seamount detected the arrivals from a series of ocean bottom shots. A reference velocity model reveals that average compressional velocities within the seamount are similar to those found within Kilauea and are consistently slower than velocities at equivalent depths in typical oceanic crust. This suggests Jasper Seamount has a high average porosity. Perturbations from the reference model were imaged by tomographic inversion. A high velocity zone within the northwest flank of the seamount may result from dikes associated with a radial rift or from a shallow solidified magma reservoir. A low velocity summit may result from shallow, explosive eruptions. The tomographic model is consistent with the results of gravity, magnetic and dredging analyses.

Gaillard, F, Cornuelle B.  1987.  Improvement of Tomographic Maps by Using Surface-Reflected Rays. Journal of Physical Oceanography. 17:1458-1467.   10.1175/1520-0485(1987)017<1458:iotmbu>;2   AbstractWebsite

The results of the experiment conducted in the northwest Atlantic in 1981 have demonstrated the possibilities of acoustic tomography. The first maps, based only on purely refracted rays, showed the evolution of a cold eddy, confirmed by direct measurement of temperature and salinity. A more complete use of the 1981 dataset, with incorporation of surface-reflected rays, is proposed here. The addition of new data reduces the statistical error on the estimation of the sound speed field. Resolution at levels already well estimated in the earlier computations is improved, and individual maps exhibit a better continuity. Information is now available about the average properties of the upper layers of the ocean, which could not be monitored with purely refracted rays.

Cornuelle, B, Howe BM.  1987.  High Spatial-Resolution in Vertical Slice Ocean Acoustic Tomography. Journal of Geophysical Research-Oceans. 92:11680-11692.   10.1029/JC092iC11p11680   AbstractWebsite

Most studies of ocean acoustic tomography have assumed that little horizontal information is available from the many acoustic multipath travel times observed in a single vertical plane (slice) between source and receiver moorings. There is in fact significant small-scale information present in such data sets. We examine single vertical slice tomography in spectral terms, and show that the acoustic measurements resemble a high-pass filter, which is more sensitive to small scales (shorter than 100 km) than to longer scales, with the exception of the mean, which is well measured. The sensitivity extends to scales smaller than 10 km, in theory, although the level of the ocean energy spectrum is so low at these scales that even small data errors limit the measurement. We use analytical calculations supplemented by numerical simulations with realistic data sets to show that accurate reconstructions of the high wave number features are possible out to the limits of the parameterization (9.2-km wavelength) when the power spectrum of the ocean features is white or red, the total measurement error is 1 ms, and multiple receivers are used. The ultimate limit of spatial resolution may be smaller still, depending on array configuration, measurement errors, and the shape of the power spectrum.

Roemmich, D, Cornuelle B.  1987.  Digitization and Calibration of the Expendable Bathythermograph. Deep-Sea Research Part a-Oceanographic Research Papers. 34:299-307.   10.1016/0198-0149(87)90088-4   AbstractWebsite

A study was undertaken of signal digitization and temperature calibration in expendable bathythermographs (XBT's) to learn how to minimize temperature errors in that system. An XBT digitizer was built into a PC-type microcomputer and used to calibrate 24 XBT probes at 5 temperature points, and later, to calibrate 72 probes at a single temperature. Twenty of the first set of probes were fastened rigidly in pairs and dropped in the ocean as a field test of the calibrations. Calibration of individual probes reduced the standard deviation of temperature calibration errors from around 0.05°C to <0.01°C. The calibration procedure is simple and nondestructive, so the probes can be used normally after calibration. Errors in the temperature digitizer can be held to about 0.01°C by periodic adjustment. An advantage of the PC-based digitizer is the ease with which the calibrations are accomplished and applied to the ocean tracers. Two substantial sources of transient systematic error in XBT temperatures are mentioned: an electronic transient lasting about 0.1 s occurs on entry of the probe into seawater, and a longer transient is due to the thermal mass of the XBT nose.

Cornuelle, BD, Malanotterizzoli P.  1986.  A Maximum-Gradient Inverse for the Gulf-Stream System. Journal of Geophysical Research-Oceans. 91:566-580.   10.1029/JC091iC09p10566   AbstractWebsite

Acoustic tomography uses integrating measurements which require inverse methods to resolve the averages into estimates of spatial structure. Statistical inverse methods have been extensively used to solve the reconstruction problem over different tomographic ranges and configurations. These inverses become very difficult to apply in frontal regions like the Gulf Stream (GS) system, where the statistics are acutely inhomogeneous and anisotropic and the mean is not a likely representation of the GS front at any time. In this paper we propose an alternative inverse which asks for the solution which gives a front instead of asking for the smoothest solution. The inverse solution minimizes the errors in the fit to the data while simultaneously maximizing the sum of the squares of the gradients observed in the reconstructed section and minimizing the absolute value norm for stability. The inverse is aimed at detecting changes in the GS front, thus the data are used to estimate the perturbations to a previous estimate of the frontal structure, instead of reconstructing the entire front as a perturbation from some average state. This approach is intended to merge well with eventual dynamic updating schemes and can be used with various types of data, given a proper model. Several examples have been run intercomparing the traditional linear least squares (LLSI) with the maximum gradient inverse (MGI), from very idealized cases to a real Gulf Stream section reconstructed from hydrographic data. Different transceiver configurations were also compared and mid-depth instruments were found to be superior to bottom mounted instruments. The simplest cases show a significant improvement in the estimate of the Gulf Stream front by the MGI compared to the weighted least squares inverse (LLSI). As the cases became more complicated (and more realistic), the differences between inverse methods become less pronounced, although the strength and location of the perturbation maxima were always determined more accurately by the MGI. The decline is at least partially due to the numerical algorithm which lumps data misfits and external constraints (the maximum gradient) into a single penalty criterion which is minimized. The most immediate way to overcome this limitation is to break up the problem into a two-step procedure, first a least squares inverse to fit the data and second an iterative, nonlinear optimization maximizing the gradient and minimizing the absolute value norm.

Cornuelle, BD.  1985.  Simulations of Acoustic Tomography Array Performance with Untracked or Drifting Sources and Receivers. Journal of Geophysical Research-Oceans. 90:9079-9088.   10.1029/JC090iC05p09079   AbstractWebsite

Ocean tomography as originally proposed required all sources and recievers to be tautly moored and acoustically tracked to separate travel time perturbations due to mooring motion from those due to ocean features. It is possible to process the tomographic travel times to estimate both ocean sound speed perturbations and mooring offsets, effecting a separation without external tracking. A side effect of this processing is a check on the ray identification, since the varying instrument positions can be used as a synthetic array for estimating ray angle. Simulations and examples with actual data were used to contrast mapping performance with and without mooring tracking for a variety of ray data sets. In general, the ocean maps degrade when the tracking data are withheld. However, when many high-precision ray travel time measurements are available, the degradation is small; in these cases it would be possible to deploy free-drifting instruments as part of a monitoring experiment.

Cornuelle, B, Wunsch C, Behringer D, Birdsall T, Brown M, Heinmiller R, Knox R, Metzger K, Spiesberger J, Spindel R, Webb D, Worcester P.  1985.  Tomographic maps of the ocean mesoscale. Part 1: Pure acoustics. Journal of Physical Oceanography. 15:133-152.   10.1175/1520-0485(1985)015<0133:TMOTOM>2.0.CO;2   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 agreement with the direct observations within computed mapping errors, but these mapping errors are too large for many oceanographic purposes. The mapping errors are the result of an unexpectedly large noise variance in travel time. (A 1983 experiment, using sources with larger bandwidth, reduced this variance to acceptable limits.) The 1981 tomographic results strongly suggest that the ocean sometimes undergoes transitions too rapid to be mapped over such large areas by shipboard observations.

Worcester, P, Cornuelle B.  1982.  Ocean acoustic tomography: Currents. Current Measurement, Proceedings of the 1982 IEEE Second Working Conference on. 2:131-135., Hilton Head Island, South CArolina   10.1109/ccm.1982.1158437   Abstract

Synoptic maps of the geostrophic current structure of the mesoscale field can be constructed from the three-dimensional density field provided by ocean acoustic tomography with unidirectional acoustic transmissions. Reciprocal acoustic transmissions can extend the technique by permitting one to directly measure the current field, including the barotropic component. A preliminary reciprocal acoustic transmission experiment at long range (300 km) and low frequency (400 Hz) is planned for autumn 1982.

Cornuelle, BD.  1982.  Acoustic Tomography. IEEE Transactions on Geoscience and Remote Sensing. 20:326-332.   10.1109/tgrs.1982.350450   AbstractWebsite

High-resolution measurements of the density field in the ocean are prohibitively expensive if traditional ship-borne instruments are used. Tomography uses acoustic remote sensing to infer ocean structure, and avoids many of the limitations of direct measurements. Sound pulses follow distinct trajectories through the water from source to receiver, and the travel time for a given pulse is a known functional of the sound speed field. This functional can be inverted to recover an estimate of the sound speed field through which it passed. The inversion is accomplished with either detenninistic linear matrix inversion or stochastic optimal estimation, and the sound speed field estimate returned can be converted to an estimate of density. A numerical simulation of the pilot tomography experiment is presented to demonstrate that tomography can be effective in reproducing significant ocean features.

Behringer, D, Birdsall T, Brown M, Cornuelle B, Heinmiller R, Knox R, Metzger K, Munk W, Spiesberger J, Spindel R, Webb D, Worcester P, Wunsch C.  1982.  A demonstration of ocean acoustic tomography. Nature. 299:121-125.   10.1038/299121a0   AbstractWebsite

Over the past decade oceanographers have become increasingly aware of an intense and compact ocean ‘mesoscale’ eddy structure (the ocean weather) that is superimposed on a generally sluggish large-scale circulation (the ocean climate). Traditional ship-based observing systems are not adequate for monitoring the ocean at mesoscale resolution. A 1981 experiment mapped the waters within a 300 × 300 km square south-west of Bermuda, using a peripheral array of moored midwater acoustic sources and receivers. The variable acoustic travel times between all source–receiver pairs were used to construct the three-dimensional (time-variable) eddy fields, using inverse theory. Preliminary results from inversions are consistent with the shipborne and airborne surveys.

Malanotte-Rizzoli, P, Cornuelle B, Haidvogel D.  1982.  Gulf Stream acoustic tomography: modelling simulations. Ocean Modelling. 46:10-15. Abstract