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).
Mitchell, N. C., "A model for attenuation of backscatter due to sediment accumulations
and its application to determine sediment thickness with GLORIA sidescan sonar", J.
Geophys. Res., 98, 22477-22493, 1993.