Page 26 - Acquisition and Processing of Marine Seismic Data

1.2 MARINE ACOUSTIC METHODS 17

of these different sediment compositions scatters as short-, medium- and long-range sonars. As a
a different amount of energy back to the tow-fish general rule for the sonar systems, maximum
unit and hence they appear in different gray range decreases as the operating frequency
shades in the sonographs. increases. The long-range systems are towed at
Each signal emission is termed a ping. Sono- shallower depths close to the sea surface,
graphs consist of several successive pings along whereas short- and mid-range systems are of
the route of the tow-fish, and seafloor reflectivity higher resolution and must be towed at small
is demonstrated as the maps of gray shades, altitudes close to the seafloor. General proper-
which are proportional to the amplitude of the ties of these systems are as follows:
returned signal. Generally 8-bit grayscale map-
• Short-range side-scan sonar systems employ
ping is used, which allows the use of 256 differ-
beams with a relatively high frequency range
ent gray tones between black and white. In
between 250 and 1000 kHz and are generally
general, a high-amplitude return (i.e., high back-
used to map an area of approximately
scatter) is shown as black, or vice versa. Modern
maximum 250 m per side. They are generally
acquisition and processing software offers the
operated at shallow waters in continental
use of different color patterns to display the
shelves and provide very high-resolution
sonographs for a better analysis of the small-
seabed images, generally delineating the
scale targets. The targets with a positive relief
small natural structures and man-made
on the seafloor prevent the signal from penetrat-
small-scale targets.
ing back of the targets, constituting an
• Medium-range systems operate at a
amplitude-free shadow zone, which enables us
50–250 kHz frequency band and generally
to discriminate targets as well as their heights
have a maximum range of approximately
from the seafloor. In practice, sonar data is col-
lected along several parallel lines with a certain 1 km per side. These systems are used to map
amount of overlap (e.g., 10% of the sonar range). continental slopes and relatively deep
water areas.
At the end of the survey, these parallel lines are • Long-range sonar systems use relatively
merged to produce one large reflectivity map of low-frequency signals (generally around
the seafloor, termed the sonar mosaic. 10 kHz) and provide morphologic data up to
As is the case in multibeam echosounders, the
horizontal resolution of the side-scan sonar sys- 20 km range per side. They are used in
reconnaissance surveys to quickly map
tem is defined in along- and across-track direc-
relatively large areas in considerably lower
tions. Along-track resolution is the minimum
resolution.
distance in which two parallel targets on the sea-
floor lying along the survey line can be distin-
guished as two separate objects. Similarly, 1.2.3 Subbottom Profiler
across-track resolution is defined as the mini-
mum distance in which two parallel targets on Subbottom profilers are basically single chan-
the seafloor lying perpendicular to the survey nel seismic systems operating at 1–10 kHz fre-
line can be detected as two separate objects. quency band (generally 3.5 kHz), which
Across-track resolution is a function of beam provides a high-resolution stratigraphic display
width, signal frequency and pulse length, while of the uppermost sediments. Depending on the
along-track resolution depends on ping rate and system frequency, the vertical resolution of
survey speed. many subbottom profiler systems is better than
We can classify side-scan sonar systems into 50 cm. Although their penetration depth reaches
three categories based on their maximum ranges 200 m below the seabed depending on the