ASA 126th Meeting Denver 1993 October 4-8

5aUW13. Some observations of acoustic variability and array performance at short range near Vestfjorden.

Donald R. Del Balzo

SACLANT Undersea Res. Ctr., Viale San Bartolomeo, 400, 19138 La Spezia, Italy

An experiment was conducted in October 1989 in shallow water (250-320 m) near Vestfjorden in support of sonar system research at SACLANTCEN. This talk describes some observations of acoustic variability and array performance at short range for one-way pulse propagation during a 46-min period. The signals were 0.5-s LFM pulses in the frequency band of 340--345 Hz. They were received at a range of 13 km on 59 elements of a towed array. Large variability in the received signal level across the array, by as much as 10--15 dB, was observed on some individual pulses. These acoustic variations appear to be related to depth variability along the array due to a small array tilt. Normal mode calculations in this environment indicate the existence of a deep acoustic null in the vicinity of the measured data. Average array signal gain degradation for all data with SNR>20 dB is only 0.6 dB, so the array beamforming performance is close to the theoretical maximum on this 27-wavelength aperture in shallow water.