Acoustic Backscattering From a Sand and a Sand/Mud Environment: Experiments and Data/Model Comparisons

Abstract

The results from two bottom backscattering experiments are described in this paper. These experiments occurred within about 1 km of each other but were separated by approximately five years (1999 and 2004). The experimental methods used in the second experiment were changed based on lessons learned in the first experiment. These changes and the motivation for them are discussed. The sediment at each experiment site would generally be classified as the same (as a well-sorted medium sand sediment) before the weather events (Hurricane Ivan and Tropical Storm Matthew) that occurred in late September and early October 2004. As a result of these weather events, the sediment present during the October 18, 2004 experiments was much more complicated than that in 1999 and in many places had a mud/sand surface layer. The environmental measurements in both experiments were sufficient to separate physical mechanisms responsible for scattering. For shallow grazing angles (less than 45 ), backscattering at frequencies between 20 and 150 kHz was attributable to sediment interface roughness in 1999, whereas volume scattering dominated in 2004. Furthermore, in 2004, volume heterogeneity within the mud/sand surface layer is a probable mechanism for the scattering feature seen in the data in the 20 -30 region. Above 200 kHz, the frequency dependence of both the 1999 data and the 2004 data indicates that a new scattering mechanism is coming into play. Other results within this issue [Ivakin, IEEE J. Ocean. Eng., vol. 34, no. 4, Oct. 2009] indicate that scattering from shells is a viable candidate for explaining the data above 200 kHz.

A combined boundary integral and Lambert's Law method for modelling multibeam backscatter data from the seafloor

“…Detailed modelling taking into account all these factors is also impractical because multibeam surveys are not normally accompanied by geophysical and geological surveys to determine poro-and viscoelastic rock properties necessary to model adequately the data or interpret the results. It is also unclear whether scattering observed over a wide range of grazing angles and frequencies is dominated by the roughness of the sediment-water interface alone or whether scattering within the subsurface (volume scattering) is also important Williams et al, 2009).…”

Model-based seafloor characterization employing multi-beam angular backscatter data—A comparative study with dual-frequency single beam

“…11 Williams et al 30 revealed that the physics behind the backscattering mechanism appeared to be changing in the critical frequency region of 150-300 kHz. Recently, Williams et al 31 concluded that at 200 kHz or higher frequencies, a new scattering mechanism comes into play for the backscatter data obtained from both the SAX99 and SAX04 experiments. At high frequencies (150 kHz to 2 MHz), a significant difference in scattering strength from the surrounding medium and the embedded coarse material in controlled laboratory experiments is also observed.…”

Towards a fully calibrated multibeam echosounder

“…(6) and it is given by SL + 10 · log 10 (r 4 0 /H 4 ) + 10 · log 10 (e −4k w H ) + BS + 10 · log 10 (A I /r 2 0 ) = 160dB rel. 1μ Pa (13) where the insonfied area is A I = 0.15 m 2 , and where r 0 = 1 m. For a grazing angle equal to 25 • the range is about R = 28 m, BS≈ -23 dB [7], and A I = 0.0035 m 2 . Consequently, the pressure level will be 111 dB rel.…”

“…The estimated BS values at normal incidence have been compared with results obtained by another processing method [6]; both results are based on the same data set, and therefore, the comparison also serves to validate the code developed. In addition, BS values have been compared with BS values obtained at SAX04' [7] -at a site similar to that of GulfEx11.…”