Preliminary Study on Using Acoustic Reflection Coefficient to Characterize Seafloor Features

 

Chih-Wei LIN; Shu-Kun HSU; Ching-Hui TSAI; Yet-Chung CHANG

 

A wide-band, frequency-modulated, subbottom profiling system (the chirp sonar) can produce artifact-free sediment profiles in real time . The function of a sediment profiler is to record echoes from the interfaces between sedimentary layers ; these layers correspond to breaks in acoustic impedance , generating reflections of the acoustic signal . The image is generated by calculating the envelope of each cross-correlationed acoustic return , mapping the amplitude of each sample in the envelope to a shade of gray and then stacking the arrays of shaded values side-by-side on a display .

Underwater acoustic transducers are made with piezoelectric crystals . An electric field applied to these materials causes a deformation related to electrical excitation. These mechanical deformations in turn create acoustic waves. Since the transmitted Full Spectrum pulse is highly repeatable and its peak amplitude is precisely known, the sediment reflectivity values can be estimated from the peak pulse amplitude measurements of the bottom returns.

The reflection coefficient anomaly is due to the acoustic impedance of sediments and the surface roughness . This study area is between the Kaoping Canyon and Fangliao Canyon. Mud volcanoes and gassy sediments have been identified from this marine geophysical survey, created by lateral tectonic compression with gas/fluid upward migration and rich dissociation of gas hydrate. Most of surface sediments are mud in this area, and the reflection coefficient is about -15(dB). Some very low reflection coefficient implies that the scattered field is predominant caused by the rough surface. The gas seeps distribute in water depth from five meters. When the deep-towed fish pass through seepage, the reflection coefficient is significantly decreased .