Ingo H. Stender
Forschungsanstalt der Bundeswehr fur Wasserschall-und Geophys., D-24148 Kiel, Germany
A conventional side scan sonar gives only a qualitative image of seafloor backscatter distribution. It does not allow quantitative measurements of the bottom backscattering strength. Because of the advantages of the side scan sonar (imaging and profiling capacity and easy handling), a conventional 100-kHz side scan sonar was redesigned. Signal generation and transducers remained unchanged. The received raw signal is amplified, digitized at 20 kHz, and stored on a 486 personal computer. Data are displayed as color coded images on screen and hard copy. Attenuation with distance and angle dependency (using Lambert's law) are compensated. This digital attachment provides relative backscatter data. On the muddy seafloor of the CBBL/JOBEX experimental site in Eckernforde Bay the digital side scan sonar shows a rather uniform bottom with low backscatter values. The spatial variation is less than 8 dB, mainly caused by trawl marks on the bottom. But inside one pockmark the backscatter strength varies as much as 28 dB. This high variation cannot be attributed to sediment changes; all sediment samples inside the pockmark were mud. The strong backscatter is presumably caused by the gassy layer inside the pockmark which nearly coincides with the water--sediment interface.