A model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with GLORIA sidescan sonar

Abstract

Where the seafloor consists of a highly backscattering surface covered with a drape of fine-grained sediment, and where the sediment does not contribute significantly to the backscattered signal, the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer. A first-order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments. Data collected with Geological Long-Range Inclined Azdic (GLORIA) are used to develop and test this model. At an acoustic frequency of 6.5 kHz, the range of sediment thickness that the sonar can potentially recover is 0-20 m, a range that is often below the resolution of vertical profilers. In the first example, the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation, which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer. The surface is a lava flow north of Hawaii covered by 1-2 m of fine-grained sediments (Clague et al., 1990), and GLORIA data analyzed with the model suggest an attenuation coefficient of 0.2-0.4 dB/m, a value consistent with results from laboratory and field measurements of sediment attenuation. In the second example, a rocky surface is buried by varying amounts of sediment, which is analyzed at almost constant grazing angle. The surface is the volcanic basement of the Southeast Indian Ridge, and GLORIA data reveal the thickening of the sediment cover with distance away from the spreading center. This is used to derive an average sedimentation rate of 6 mm/ka, which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions. Furthermore, the model is used to calculate, from image amplitude distributions, the sediment thickness distribution, which represents the accumulation of sediments in ponds and the exposure of abyssal hills. These sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age. I explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes (the first-order model assumes a uniform thickness at the subfootprint scale).When combined with the sedimentation rate, estimates of the thickness of draping sediment give the age of the underlying rock. (GLORIA), a long-range sidescan sonar [Somers et al., 1978]. The data show variations in backscatter amplitude with range from the sonar and with decreasing grazing angle that are caused by an increasing path length in the attenuating sediment cover. Such variations may potentially be used to recover the sediment thickness. The second application is to midocean ridges, where the relatively steady rain of pelagic material produces an almost linear decay in the average backscatter strength away from the spreading ce...

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238U, 230Th, and 231Pa excesses

“…6a) demonstrate that no single, simple mechanism, such as subsurface assimilation of hydrothermally altered rocks, can explain both the Cl/K 2 O ratios and the 238 U- 230 Th isotopic relationships we observe. Due to the complex speciation and solubility of U under various redox conditions (Mitchell, 1993), it is difficult to precisely predict the behavior of U in the crust. It follows from the discussion above, however, that Cl could be mainly added by seawater brines (Michael and Cornell, 1998) (Fig.…”

On the Benefits of Using Both Dual Frequency Side Scan Sonar and Optical Signatures for the Discrimination of Coral Reef Benthic Communities

“…The 500 kHz sonogram shows a better definition of the coral mounds and higher reflectivity over sand. This is due to the fact that higher frequency sound experiences less attenuation in penetrating sediment than lower frequency sound, resulting in overall less penetration and a higher backscatter (Mitchell, 1993). Syringodium generates bright but fuzzy backscatter indicative of a strong echo return while the fuzziness indicates that the return is not from a distinct hard object (Figure 2b).…”

Effects of using inclined parametric echosounding on sub-bottom acoustic imaging and advances in buried object detection

“…To overcome this limitation, Wunderlich et al (2005) used a non-vertical (30°) beam incidence within a study of archaeological settlement remnants. Other studies employed non-vertical incidence acoustic setups to map sediment coverage (Mitchell 1993) and shallow gas (Schneider von Deimling et al 2013). Szender and Kosalos (1997) and Leighton and Evans (2008) presented bistatic (spatially separated acoustic source and receiver) approaches to track underwater cables.…”