Principles of Side Scan Sonar
PRESENTED BY
NIKA BHINGARDE
SABNA THILAKAN
(SECOND YEAR ME –FOUND ENGG, GEC,PONDA) ON 23.09.2014
Introduction to Sidescan sonar
Sidescan sonar was developed during World War II to
detect submerged enemy submarines.
It is an acoustical instrument that is normally towed behind
a vessel and emits acoustical signals to both sides.
The acoustical pulses are reflected from features on the sea
floor and returned to the recorder to show even small-scale
positive and negative relief of the sea floor (such as rock
outcrops, sand shoals, channels, and wrecks).
It has also found useful in identifying sea floor areas of
different sediment composition.
Some terminologies:-
Footprint: Seafloor area sampled by a single
acoustic signal pulse.
Fish: An instrument towed behind a ship -- e.g.
sidescan sonar and its casing.
Hull: The main body or structure of a ship.
Schematic of Side Scan sonar
SIDE VIEW FRONT VIEW
Working of sidescan sonar
In side-scan sonar systems, acoustical energy is projected laterally from a pair of
transducers mounted in a towed cylindrical body or "towfish.“
Electrical energy, supplied through the electromechanical tow cable, is applied to
the piezoelectric transducers in the towfish. This energy causes them to vibrate,
creating sound waves which travel through the water. Sound is reflected from the
seabed or structure, received by the same transducers, transmitted back up the tow
cable to the recorder, and printed on continuous chart paper
Side-scan sonar transducers typically vibrate at preselected frequencies from 50 to
500 kHz. Most of the popular commercial units operate at frequencies between 100
and 500 kHz. The 100-kHz frequency provides greater range, up to 400 m on each
channel and is most often used for sea bottom mapping and locating objects. A
frequency of 500 kHz gives a shorter range, up to 100 m per beam (also known as a
channel), but provides greater resolution and is recommended for detailed
inspection of coastal structures undermost circumstances
Side Scan sonar in operation
TOWFISH AND GRAPHIC SIDE SCAN SONAR RECORD
RECORDER
TYPICAL SIDE SCAN SONAR
COMPONENTS
•The echo received along
time will represent the
bottom reflectivity along the
swath* , and particularly the
presence of irregularities or
small obstacles.
•The signal is recorded
laterally and hence the name
‘Side Scan sonar’.
* a broad strip or area of
something
Sonograph Characteristics
The continuous paper image of the bottom or
structure produced by the recorder is remotely
similar to low level oblique aerial photographs.
A reasonable description of the side-scan sonar
beam is arrived at by comparison to the light areas
and shadows formed by an obliquely held flashlight
in a darkened room.
High frequency (short wavelength) sound attenuates
quickly but can resolve small things; conversely,
lower frequency (longer wavelength) sound travels
farther but has less resolving power.
Side-scan sonar geometry
Sample side-scan sonar record (sonograph)
A sonograph usually contains two channels of sonar information representing
the bottom to the right and left of the towfish.
Two dark parallel lines, representing the initial acoustical pulse, run just right
and left of the center of the sonograph
The track of the boat and towfish are along these center lines (line A).
The surface return (line B) is often the next line closest to the center line (line A).
Line C, the initial bottom return, is recognizable as the start of the darker tone.
Total water depth can be calculated by adding the distances on the sonograph of
the output pulse to the surface (A to B) and the output pulse to the bottom (A to
C).
The dark line perpendicular to the line of travel (line E) is an event mark created
by the operator for later reference.
Event marks are usually used for highlighting an interesting feature or for
referencing position.
The distance of a
target
perpendicular to
the line of travel
can be calculated
once the height of
the towfish above
the bottom is
known (as shown in
the figure) by
simple
trigonometry
Calculation of slant range
The height of a
target can be
calculated using
similar triangles
Ht Hf
Ls Rs Ls
Which can be
written as:
H f Ls Calculation of target height
Ht
Rs Ls
The sonograph image is of varying shades with
each shade a function of the intensity of the
returning acoustical pulse.
The stronger the returning pulse the darker the
image.
Factors that affect the intensity of the returning
signal are
• acoustic reflectivity of the target,
• slope of the target face,
• contrast between the target and surrounding
material, and
• the number of reflecting surfaces.
Target material and orientation influences
Acoustic reflectivity
The acoustic reflectivity of the target is a function of
the acoustic impedance of the material.
Acoustic impedance = ρ x c
where ρ = material density
and c = speed of sound through
the material
The coarser the sediment, the higher the reflectivity.
Therefore, gravel reflects more acoustic energy than
sand, which reflects more than silt or clay.
Target material and orientation influences
Slope of the target face
As the slope of the target face becomes more
perpendicular to the incoming sound wave, the
strength of the reflected signal increases.
The acoustic shadow zone where no signal is
reflected, shows up as a white area on the sonograph.
The sound is not reflected by the hole but reflects off
the far side of the hole. Consequently, the shadow is
closer to the trackline, and the object's reflection is
farther from the trackline
Vessel speed effects
Distortion parallel to the trackline of the towfish
occurs due to varying boat speeds.
Distortions perpendicular to the line of travel is a
function of the height of the fish and the distance of
the object from the fish and oscillations in these
positions.
A Sample Side Scan Sonar Image Interpretation
On this photograph of a sidescan sonar record, the position of the towfish is shown by the vertical black
line in the center. The white strips to either side of the towfish position show the water column, and the
port and starboard-looking images of the sea floor appear as the grey shaded areas that extend to the
margins of the photo. The fairly uniform, light-grey, background color of the images indicates that mud
covers most of the sea floor in this area. Further evidence of the muddy consistency of the sea floor is
provided by the round, crater-like features which surround the rectangular outline of a sunken barge on
the left-hand side of the record. These pockmarks were imprinted in the soft sea bed by spilling cargo that
rained down from the sinking barge.. Where prominent structural components of the barge stood as
barriers to sound propagation, they cast the "acoustic shadows" (white areas of low backscatter) that
appear in and adjacent to the wreck. Photo by Ralph Lewis
Limitatons of Side Scan Sonar
Wave effects:
Periods for successful survey is limited to seasons of low
wave energy i.e summer months.
Current conditions:
Major problem occurs when the current is perpendicular
to the path of survey vessel.
Lack of contrast or too much contrast between target and the
surrounding bottom can be a potential problem.
Other site limitations:
Maintaining constant speed and towfish elevation
required for good results.
Presence of other vessels can make it difficult to keep the
vessel on track
