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Vera, MD, Heaney KD, Grp N.  2005.  The effect of bottom interaction on transmissions from the North Pacific Acoustic Laboratory Kauai source. Journal of the Acoustical Society of America. 117:1624-1634.   10.1121/1.1854491   AbstractWebsite

Acoustic signals transmitted from a 75-Hz broadband source near Kauai as part of the North Pacific Acoustic Laboratory (NPAL), experiment were recorded on an array of receivers near California at a range of 3890 km, and on a vertical line array at a range of 3336 km in the Gulf of Alaska. Because the source is approximately 2 m above the seafloor, and the bottom depth at the receivers near California is approximately 1800 m, acoustic interaction with the bathymetry complicates the identification of the recorded arrivals with those present in numerical simulations of the experiment. Ray methods were used to categorize acoustic energy according to interactions with the sea bottom and surface and to examine the significance of seafloor geometry. A modal decomposition was also used to examine the role of range-dependent bathymetry and to associate the effects on the acoustic field with seafloor features at specific ranges. Parabolic-equation simulations were performed in order to investigate the sensitivity of the received signal to geoacoustic parameters; shear excitations within the seafloor were modeled using a complex-density, equivalent-fluid technique. Incorporation of bottom interaction into models of the propagation enables an identification between experimental and simulated arrivals. (c) 2005 Acoustical Society of America.

Worcester, PF, Cornuelle BD, Hildebrand JA, Hodgkiss WS, Duda TF, Boyd J, Howe BM, Mercer JA, Spindel RC.  1994.  A Comparison of Measured and Predicted Broad-Band Acoustic Arrival Patterns in Travel Time-Depth Coordinates at 1000-Km Range. Journal of the Acoustical Society of America. 95:3118-3128.   10.1121/1.409977   AbstractWebsite

Broadband acoustic signals were transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the eastern North Pacific Ocean during July 1989. The sound-speed field along the great circle path connecting the source and receiver was measured directly by nearly 300 expendable bathythermograph (XBT), conductivity-temperature-depth (CTD), and air-launched expendable bathythermograph (AXBT) casts while the transmissions were in progress. This experiment is unique in combining a vertical receiving array that extends over much of the water column, extensive concurrent environmental measurements, and broadband signals designed to measure acoustic travel times with 1-ms precision. The time-mean travel times of the early raylike arrivals, which are evident as wave fronts sweeping across the receiving array, and the time-mean of the times at which the acoustic reception ends (the final cutoffs) for hydrophones near the sound channel axis, are consistent with ray predictions based on the direct measurements of temperature and salinity, within measurement uncertainty. The comparisons show that subinertial oceanic variability with horizontal wavelengths shorter than 50 km, which is not resolved by the direct measurements, significantly (25 ms peak-to-peak) affects the time-mean ray travel times. The final cutoffs occur significantly later than predicted using ray theory for hydrophones more than 100-200 m off the sound channel axis. Nongeometric effects, such as diffraction at caustics, partially account for this observation.

Cornuelle, BD, Worcester PF, Hildebrand JA, Hodgkiss WS, Duda TF, Boyd J, Howe BM, Mercer JA, Spindel RC.  1993.  Ocean Acoustic Tomography at 1000-Km Range Using Wave-Fronts Measured with a Large-Aperture Vertical Array. Journal of Geophysical Research-Oceans. 98:16365-16377.   10.1029/93jc01246   AbstractWebsite

Broadband acoustic signals transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of information available from tomographic techniques used in the vertical plane connecting a source-receiver pair. A range-independent, pure acoustic inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological data without using any information from concurrent environmental measurements. Range-dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, although the limited spatial resolution of concurrent direct measurements does not provide a strong cross-validation, since the typical cast spacing of 20-25 km gives a Nyquist wavelength of 40-50 km. The small travel time signals associated with high-wavenumber ocean variability place stringent but achievable requirements on travel time measurement precision. The forward problem for the high-wavenumber components of the model is found to be subject to relatively large linearization errors, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two-dimensional tomographic array. The high-ocean-wavenumber resolution that is in principle available from tomographic measurements is therefore achievable only under restricted conditions.

Dushaw, BD, Worcester PF, Cornuelle BD, Howe BM.  1993.  Variability of Heat-Content in the Central North Pacific in Summer 1987 Determined from Long-Range Acoustic Transmissions. Journal of Physical Oceanography. 23:2650-2666.   10.1175/1520-0485(1993)023<2650:vohcit>;2   AbstractWebsite

The evolution of the heat content in the central North Pacific Ocean during summer 1987 has been measured using acoustic transmissions between transceivers deployed in a triangle approximately 1000 km on a side. The acoustically determined heat contents of the source-receiver sections agree with heat contents computed from CTD and XBT data obtained during May and September 1987. The accuracy of acoustical measurements of range-averaged heat content is comparable to estimates from CTD and XBT data. Transmissions at four-day intervals allow the continuous observation of heat content and show that it varies on time scales of weeks or less. The magnitude of these variations is of the same order as that observed from XBT sections, which are only occasionally available. Ocean-atmosphere heat exchange from bulk formulas accounts for only about half of the observed heat content increase from May through September 1987, indicating that advective effects are important in the region. The excess heat change is calculated to be of order 50-150 W m(-2). The advective component of the near-surface heat budget is roughly in phase with the surface flux component.

Worcester, PF, Cornuelle BD, Spindel RC.  1991.  A Review of Ocean Acoustic Tomography - 1987-1990. Reviews of Geophysics. 29:557-570. AbstractWebsite

Research in ocean acoustic tomography during the last quadrennium has resulted in substantial progress in understanding the capabilities and limitations of the technique. Theoretical studies and numerical simulations have led to greater understanding of the oceanographic information available in a vertical slice from acoustic transmissions between a single pair of instruments, of the horizontal geometries required to map the ocean mesoscale field with specified precision, and of the properties of tomographic reconstructions of the two-dimensional vector field of current. Simultaneously, the instrumentation used in tomographic experiments has been significantly improved, increasing the precision of the measurements and making gyre and basin scale experiments feasible between moored instruments. Experimental efforts to test the accuracy with which the ocean temperature and current fields can be measured acoustically have now demonstrated that tomographic techniques provide measurements with oceanographically useful precision up to ranges of about 1000 km. Such demonstrations are difficult due to the incompatibility between point measurements and the spatial averages provided by tomographic techniques. The experiments have also yielded significant information on the character of acoustic propagation at long range in the ocean. Experimental precision is now adequate to distinguish between competing algorithms for sound speed as a function of temperature, salinity, and depth. Finally, and most importantly, emphasis has shifted to use of the technique for studying the ocean, rather than on understanding the capabilities and limitations of the technique. Two major experiments, the Greenland Sea Tomography Experiment and the Gulf Stream Extension Tomography Experiment, both conducted during 1988-89, were devoted to improving our understanding of ocean dynamics, although results are not yet available. There is increased emphasis on exploiting the integrating nature of acoustic transmissions to study gyre and global scale temperature variability, phenomena difficult to study in any other way.