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An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.

A new towedlvertical array system has been built to support the ONR ocean acoustic program's 6.1 experimental research efforts. The array consists of both a linear section for standard beamforming and a cardioid section giving it portistarboard discrimination capabilities. The linear section is comprised of 4 modules with halfwavelength hydrophone spacing corresponding to cutoff frequencies of 250, 500, 1000. and 2000 Hz. Each linear aperture is made up of 64 hydrophone channels with a total linear aperture length of 189 meters. The cardioid module consists of 78 hydrophone triplets arranged in an equilateral triangle with 38.5 mm spacing between the individual phones. The linear spacing between each triplet set is 0.2 meters for a cutoff frequency of 3750 Hz. In addition to the acoustic sensors, the array contains 3 non-acoustic sensor suites and and additional pressure sensor to provide realtime array heading, pitch, roil, and depth along with temperature at various positions along the array. Array control for setting of sampling rates, array gain, and monitoring of both acoustic and non-acoustic data is provided by a standard PC. The array supports sampling rates from 6.25-25 kHz for the acoustic data with 24 bit AID conversion. Array telemetry is ATMISONET with a data rate of 155 Mbls. The acquisition system acquires directly to SCSl ultra 320 disk and is based on a COTS Linux workstation. Since taking delivery in May 2002, the FORA has been deployed during three separate sea trials, including the recent Geoclutter experiment held April-May 2003 on the NJ shelf in the Atlantic Ocean. Some preliminary results and data are presented from the different trials illustrating array Capabilities along with an assessment of the array data quality for meeting current and future scientific objectives.

In September 2010 a long-range acoustic communication (LRAC10) experiment was carried out in deep water off the Southern California Coast. The experiment involved two mobile components: (1) a source towed slowly at a speed of 2–3 knots at ∼75-m depth and (2) a horizontal line array towed at 3.5 knots at a depth of ∼200 m. Phase-coherent communication sequences were transmitted in the frequency band of 200–300 Hz at various ranges (100–700 km). Initial analysis of the LRAC10 data demonstrates that an information rate of 50 bits/s can be achieved over ∼550-km range using quadrature-phase shift-keying (QPSK) modulation and error-correction coding combined with beamforming.

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