Bottom reverberation

Bottom reverberation measurements were performed with ELAC LAZ-4700 echosounder at 30 and 50 kHz. The echosounder through an A/D converter to the signal processing computer system. The sounding pulse durations were 0.3, 0.6 and 1 ms, and the interval between consecutive pulses was about 0.5 s. The bottom reverberation signals were recorded on disk in blocks of 64 or 128 pulses with set of echosounder technical data, physical parameters of emitted signals and A/D converter sampling frequency. Due to the coupling of the acoustic measuring system with the vessel satellite navigation system (GPS) the position and time were also registered for each pulse. The echo signal envelopes were read by 12-bit resolution A/D converters with a sampling frequency from 3 to 9 kHz.

The measurement of the reflected acoustic wave parameters are refer to the normal incidence of the beam on the bottom and covered practically the entire Polish economic zone of the Baltic Sea along transectes more than 4000 km in length. Just the data set of echo profiles measured at 30 kHz contains about 5800 echo-integrated profiles, which corresponds to about 500000 acoustic pulses put in for analysis. The vessel routes during cruises 1991-1994, along which the echo signals were recorded at both 30 and 50 kHz are mapped.

The majority of measurements were performed while the vessel was sailing. To obtain maximum separation from the influence of the ship's rolling, the acoustic transducers were mounted on the towed V-fin body, which was more stable than the vessel itself. However, during extremely harsh weather conditions, the acoustic transducers often appeared to rock through even a few degrees, which influenced the form and the level of echo signal envelopes. To eliminate unwanted altered echo pulses five procedures were proposed and applied during off line signal processing of selected pings.

  1. Echo signals in the block whose flight time was shorter than some threshold value were selected (this corresponded to the vertical incidence of the acoustic beam).
  2. Echo signals with local maxima of the envelope's echo gravity centre (its highest position).
  3. The selection of locally strongest echo signals.
  4. Conjunction of 2 and 3 method.
  5. Alternative of 2 or 3 method.

Each of this methods qualified about 25% of signals for further processing. However, it must be said that because various pulse-selection criteria were taken for averaging, differences appeared in the values obtained for the backscattering strength. These differences did not usually exceed 2 dB and they depended both on the character of the bottom and on the weather conditions.

To calculate the backscattering strength from the bottom materials, the formalism of volume reverberation was applied. This emerged from the fact that besides the zone of sands, the layer of sediments taking part in backscattering had a thickness equal to tens of spatial lengths of the sounding pulse and had to be treated as volume scattering medium.

Parameters of bottom reverberation signals:
  1. The integral backscattering strength.
  2. The reflection coefficient at the water-sediment interface.
  3. The reverberation time, determined as the time, in which 50 or 90% of acoustic energy of bottom reverberation returned.
  4. The attenuation coefficient describing the attenuation of acoustic waves in bottom materials.
  5. The statistical properties of bottom reverberation signals and the shapes of envelopes represented by such parameters as the statistical moments of signal envelope, fluctuation and similarity measures of the signals and envelope skewness.

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Jaroslaw Tegowski<tegowski@iopan.gda.pl>, 12.03.1996