Many of these topics were discussed at an ONR-sponsored workshop held at Lake Arrowhead, California in March 1997. For a detailed description of scientific issues and research opportunities in long-range propagation, organized under the headings of Theoretical, Computer Simulations and Modeling, and Experimental, see the workshop report prepared by Peter Worcester (Scripps Institution of Oceanography Reference Series 98-8).
A good balance between field experiments, modeling, and theory is key to having a healthy program in long-range propagation. Field experiments are very demanding: they are the biggest cost item and require several months (and even years) of planning to be successful. Recently, ONR funded the "Dual-Frequency Experiment" to study the frequency and range dependence of acoustic fluctuations. In this experiment sound impulses having center-frequencies of 28 and 84 Hz were transmitted from a source off California to two vertical line arrays in the Pacific Ocean, at ranges of approximately three and five megameters. Analysisof the data collected for that experiment is still ongoing. This year ONR is executing, as a part of the North Paclfic Acoustic Laboratory (NPAL), a field experiment to study the 3-dimensional behavior of long-range propagation.
North Pacific Acoustic Laborator
The NPAL experiment is a collaborative effort involving investigators from several research institutions in the United States, including the Scripps Institution of Oceanography, the Applied Physics Laboratory at the University of Washington, and the Woods Hole Oceanographic Institution. As figure 1 illustrates, the configuration of the experiment consists of a 75-Hz (ATOC) source off Kauai transmitting acoustic pulses to a billboard array off California. The duration of the experiment is approximately one year, expected to end in August of 1999. In addition to the acoustic measurements there are several environmental measurements, including CTD and XBT surveys during the deployment and recovery phases of the at-sea instrumentation, as well as data collected from two "environmental" moorings stationed (apart) between the source and receiving array. These moorings consist of thermistors, ADCPs, and Seacats and are aimed at measuring the ocean (internal wave) fluctuations during the majority of the acoustics experiment.
The billboard array will provide both a horizontal and vertical aperture in the receiver. Figure 2 shows that the billboard array consists of 5 vertical line arrays, geometrically spaced, and a horizontal SOSUS array. This billboard array faces (approximately) normal to the sound propagation path emanating from Kauai. Navigation of the array elements will be possible so that fluctuations from array motions can be differentiated from those due to ocean processes. The data acquired by the billboard array,in conjunction with the environmental data, will be analyzed to determine the effect of oceanic variability on the vertical, horizontal, and temporal fluctuations of sound impulses propagated approximately 3900 kilometers. Analysis of the data will include an examination of the 3-D wavefront coherence, mode and ray scattering and diffusion, and ambient noise variability.
