Late Quaternary Climate Variations Reflected in Baltic Sea Sediments - The Baltic Sea Basin - page 106

5.4 Data
5.4.1 Seismoacoustic Survey
High-resolution sub-bottom profiling was performed using the parametric sediment
echosounder SES96 deployed during an expedition with R/V Poseidon in 2005
within the frame of the project GISEB (Harff 2005 ) . It has a high system bandwidth
and can therefore transmit short pulses without ringing (e.g. 1 period of 12 kHz).
Short pulses, narrow beams and the absence of side lobes result in less volume and
bottom surface reverberation compared to linear systems. This improves the signal-
to-noise ratio and therefore the usable depth range (penetration depth). The primary
transmitter frequency is about 100 kHz. During the profiling a secondary transmit-
ter frequency was selected in the range of 6-12 kHz (preferably 8 kHz) depending
on the water depth and the sediment type. All data are stored digitally on hard disk
including navigational data. A motion reference unit was used to correct for ship's
movement. A more detailed description of the SES96 sediment echosounder system
is available at www.innomar.com .
Coring sites and coring device parameters (load, core barrel length, steering of
the winch) were selected based on a first interpretation of the acoustic data. Profiling
lines and stations are plotted in the cruise track plot of Fig. 5.5 .
An echosounder record imaging the structure of the postglacial sediment
sequences of the Gotland Basin is depicted in Fig. 5.6 . The picture displays the
colour-coded acoustic echo strength, with red colours for strong reflections and blue
for weak echoes. The strength of the acoustic echoes (the reflectivity of a sediment
sequence) depends on the vertical gradient of the acoustic impedance (the product
of sound velocity times wet bulk density). A strong change in the vertical density
profile will therefore cause a strong reflection in the echosounder record. The range
of the density values of the Baltic Sea sediments extends from about 1,100 kg/m 3
(soft mud) up to 2,300 kg/m 3 (packed sand) whereas the sound velocity extends
from about 1,400 m/s (mud, soft clay) up to about 1,900 m/s (sand). Therefore,
most of the acoustic echoes reflect a change in density. In general, the echosounder
records are interpreted using core data. Selected echo bands are identified and traced
horizontally to map the thickness of Holocene and postglacial deposits as shown
below.
5.4.2 Sampling and Sediment Data
Most of the Baltic sediment data have been acquired within the frame of interna-
tional research projects. The first cores were taken in 1997 using R/V “Petr Kotsov”
(Project BASYS, Winterhalter 2001a ; Harff and Winterhalter 1996 , 1997 , Harff
et al. 2001a ) , but most of them have been sampled by R/V “Poseidon” (Project
GISEB, Harff 2005 ) . Within the map of Fig. 5.6 a master station is marked (57.28 ◦ N,
20.11 ◦ E). This position was selected (Winterhalter 2001b ) since the sediments here