A POST-PROCESSING TECHNIQUE TO REMOVE BACKGROUND-NOISE FROM ECHO INTEGRATION DATA

Echo integration of biological organisms with a low target strength ca n be difficult because of the problem of setting suitable pre-integrat ion thresholds, and this is particularly acute with higher frequencies , such as 120 or 200 kHz, which are often used in studies of euphausii ds. One solution is to integrate data without any threshold and then r emove background noise during post-processing. Unthresholded, integrat ed data of micronekton and Euphausia superba collected with a SIMRAD E K500 at frequencies of 38 and 120 kHz are presented. The underlying ba ckground noise level follows a 20 log R + 2 alpha R relationship, whic h can be scaled to the minimum volume backscatter (Sv) in each layer d uring a transect and then subtracted from the entire data set to remov e background noise. The utility of this procedure is demonstrated by m aking comparisons of Sv at each frequency and investigating the effect of noise removal on the identification of targets based on the dB dif ference (120 kHz Sv-38 kHz Sv). (C) 1996 International Council for the Exploration of the Sea