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Sea bed plumesSea bedHang Hau FormationChannel plumeBlanketPillarLEGENDAcoustic turbidityFigure 21 - Morphology of Acoustic Turbidity on Seismic Profiles in the District (Not to Scale)The formation is relatively homogenous lithologically, consisting of very soft to soft, olive greyclayey silt Undrained shear strengths range from less than 3 to 20 kPa Disarticulated andarticulated bivalve shells are relatively common Comminuted shell debris, ranging in size fromless than 0 1 to 20 mm, is scattered throughout the matrix, but is sometimes concentrated in thinbeds and lenses Silt and fine sand lenses also occur regularly throughout the sequence and someblack mottling has been noted The sediments are commonly coarser towards, and at, the base ofthe formation.Acoustic turbidity (acoustic blanking or gas blanking) appears on seismic records as zones ofchaotic reflectors that obscure or obliterate any other reflectors within or beneath these zones. Itresults from gas within the sediments causing absorption and, or, scattering of the acoustic pulse(Judd, 1989). This creates significant attenuation of the return signal, and a relatively dramaticdrop m acoustic velocity (Gardner, 1987). Shallow, biogenic gas within sediments is produced byanaerobic bacteria feeding at depth on a source of carbon such as decaying organic matter.Extensive areas of acoustic turbidity have been identified, along the West Lamma Channel, andaround the Soko Islands and Chek Lap Kok (Figure 20) The major occurrences of acousticturbidity generally begin within the Hang Hau Formation and mask reflectors in the underlyingSham Wat and Chek Lap Kok formations and rock. However, west of the Soko Islands someacoustic turbidity occurs at depth in the Sham Wat Formation, and does not affect the overlyingHang Hau Formation The morphology of the turbidity on seismic records takes a number offorms (Figure 21), including wide extensive blankets, such as in the West Lamma Channel andaround the Soko Islands. This turbidity is also associated with infilled channels at the base of theHang Hau Formation, Channel plumes tend to lie across the channels, with their upper surfacegenerally at depths of 7 to 10 ni beneath the sea bed. The turbidity picks out the branching planof many channel systems but does not always extend along the entire length of all the channels inwhich it occurs. There is little evidence of columnar plumes reaching the sea bed, or of gasescape or gas pressure features at the sea bed Sea bed morphology associated with shallow gas,such as pockmarks (Hovland & Judd, 1988) have not been noted in Hong Kong, Localizeddoming of up to one metre of the sea bed, has been seen on some pinger seismic records in the137