Environmental Engineering Reference
In-Depth Information
Figure 10.
SES96 Sediment echosounder profile 1: cross section of the mud belt at 24
◦
50S
showing stratified, gas-free mud over westward prograding sandy layers. Towards the coast, at
a depth of about 70 m, the muddy surface sediment changes to coarser (silt, sand) material.
The near-shore region (water depths of less then 150 m) is characterized by
sedimentation of organic matter forming a NNW-SSE striking (coast parallel)
diatom mud layer, which grows to more then 10 m thickness towards the
coast (Fig. 11). At water depths between 80 m and 130 m, acoustic anomalies,
so-called blankings, occur that indicate free gas accumulations at about 6 m
depth in the mud layer (Fig. 11). The gas bubbles are concentrated under a less
permeable layer inside the mud. Comprehensive acoustic investigations on gas-
charged mud in Eckernforde Bay, Baltic Sea, used similar acoustic frequencies
to our study [41] and found that between 2% (mean) and 8% of the pore space
are occupied by bubbles.
In direction to the coast, the isolated acoustic gas blankings change into a
permanent gas-charged layer at about 3 to
<
1 m sub-bottom depth. This gas-
charged layer intersects the sea-bottom at water depths of about 40 m (Fig. 11).
Near the coast, between 22
◦
50'S and 23
◦
10'S, east of 14
◦
15'E the flat sea
bottom changes to morphological features like pockmarks (Fig. 11) and very
rough sea bottom surface (Fig. 11). These features indicate recent eruptions of
free gas and/or gas-charged mud blocks. Upward-travelling gas bubbles in the
water column from one of the pockmarks were observed in the high-frequency
channel of the SES96 echosounder during station work. The area affected by
