Initial Analysis Of The Data From The Vertical DIFAR Array

The vertical DIFAR array constructed by the Marine Physical Laboratory is composed of 16 elements with an inter-element spacing of 15 m. Each element measures both the three orthogonal components of acoustic pmicle velocity as well as acoustic pressure. The simultaneous measurement of acoustic pressure and particle velocity permits the calculation of quantities of fundamental physical importance, e.g., the acoustic energy density and vector acoustic intensity. In addition, a spatial decomposition of the acoustic field can performed with the 4component data recorded by a single element. This decomposition can be accomplished using not only a conventional DFAR "cardioid-steering" approach, but also high resolution beamforming techniques. Data collected from the first sea test of the array are used to estimate the potential and kinetic energy density spectra, the horizontal active intensity spectrum, and to perform single-element and whole-amy beamforming using both the conventional and minimum variance methods. Comparison of the energy density spectra indicates that the sensor-motion-sensitive geophone data are useable down to around 10 Hz, and the horizontal active intensity generated by a 225-Hz source indicates the net flux of energy away from the source, as expected. Results from beamforming on 225-Hz source show that the minimum variance approach provides a significantly greater azimuthal resolution than the conventional method for this high S N R signal. I. ~ ~ ~ r o ~ u ~ $ i o ~ analysis of the first set of data collected by the Marine Physical Laboratory's vertical DIFAR array. The main features of the array's hardware are discussed in the companion paper [1] and are briefly summarized in Section 11. A description of the engineering sea test conducted in summer, 1991, then is given in Section se of this paper is to present an 111. In Section IV, the theoretical framework used for the analysis of these data is presented, followed by a discussion of some of the results in Section V. Finally, a summary of h e conclusions are given in Section VI. The main p i n t of e presentation is that the vertical DFAFt array is capa le of rnaking high quality, ocean acoustic measurements with surprisingly good spatial resolution in z h u t h (when high resolution beamforming techniques are used) as well as good resolution in the vertical direction. The companion paper [ 11 discusses in detail the hardware features of the veaical DFAR array. Here, only a brief summary is given. The may is composed of 16 elements, with an inter-element spacing of 15 m. Each of the elements contains three orthogonally-oriented geophones lo measure the three components of acoustic particle velocity, and a hydrophone to measure acoustic pressure. Note that standard DEAR sonobuoys measme only the two horizontal components of particle motion as well as pressure. A recently-coined term for new-design sonobuoys containing all four component sensors is "TRIFAR" sonobuoys. ( ally stands for DIrectio LOW Frequency Andy elements of the array dso con pass for measurhg the orientation of the horizontal geophones with respect to the earth's magnetic field and a high frequency ~ r o ~ ~ m a b l e from 9 to 12 IrHz) data acquisition system provide acoustic element localization capability. In the engineering sea test, the data sampling frequency was set at 600 Hz, which provided an upper useable frequency limit of 270 Mz. However, during future experiments, the sampling frequency can be changed, to either 150 Hz, 300 Hz, 1280 Hi9 or 2400 Hi, from the recording platform. ( issuingcommands s change is accom-