Thicknesses of mud drapes over rough surfaces from acoustic attenuation in sonar backscatter

What is it about?

Backscattering data from sidescan and other sonars has mostly been used in a qualitative manner (interpreting spatial patterns in the images of these data or variations in texture). Their use as true remote-sensing data, i.e., detecting physical properties of the seabed, has been more limited, partly perhaps because formally the backscattered intensity is a function of several factors (seabed roughness, incidence angle, the acoustic impedance ("hardness") of the seabed and other properties of the near-surface) making inversion of these data seemingly impossible. In this article, variations in signal intensity are shown to be interpretable in situations where we suspect the backscattered signal is returned by a rough hard surface underlying a thin mud drape, using the attenuation through the drape. For the reconnassance sonar used, the low 6.5 kHz signal allows penetration to perhaps 10s of m in extreme cases; for higher frequency sonars the penatration will be smaller (the article outlines how penetration depth can be estimated for given geometry and frequency, based on published attenuation measurements on different muds). Attenuation alone produces a simple logarithmic decrease in signal intensity (i.e., the signal looses intensity by a constant amount of decibels per metre travelled through mud). Hence, using measurements of attenuation rate in decibels per metre and the geometry of the seabed imaging, we can easily evaluate the amount of signal loss where the path length is known or, if signal loss is known, work out the path length. A lava flow imaged near Hawaii is covered by a 1-2 m layer of mud. Plotting the data of the lava flow in a modified form of the range from the sonar, the decrease in backscatter was shown to be compatible with attenuation rates in the mud layer, thus confirming a simple approach in this case. For lava flows of differing backscatter intensity, the varied thicknesses of their mud drapes could in principle be worked out, providing an indirect dating tool based on sediment thickness. Backscatter intensity variations across a mid-ocean ridge were studied, where we anticipate the sediment thickness generally increases with age of the oceanic crust. Here, the decrease in signal intensity with age allowed a rough estimate of the sedimentation rate at the ridge (6 mm/yr). Interestingly, the diversity of backscatter allowed the diversity of sediment thickness also to be worked out - sediment does not just thicken on average but disperses to some extent into local depressions, leading to an increasing range of sediment thickness.

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Why is it important?

The article shows how thicknesses of mud drapes can be evaluated from acoustic backscatter variations where the seabed comprises a rough, hard layer with overlying mud. Using backscatter strength variations rather than absolute values overcomes the lack of calibration of these systems. The simple model allows the penetration depths of sonars to be evaluated for interpretation. Backscatter variability provides one of the few ways we have to assess how pelagic sediments on mid-ocean ridges become redistributed (alternatively, more direct assessment is possible with deep-tow sediment profilers, though such datasets are expensive to collect and of more limited spatial extent).