P R E S E N T E D B Y

N I K A B H I N G A R D E

S A B N A T H I L A K A N

( S E C O N D Y E A R M E –F O U N D E N G G , G E C , P O N D A ) O N 2 3 . 0 9 . 2 0 1 4

Principles of Side Scan Sonar

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 RECORDER

SIDE SCAN SONAR RECORD

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.

Calculation of slant range

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 target height

The height of a target can be calculated using similar triangles

Which can be written as:

ss

f

s

t

LR

H

L

H

ss

sf

tLR

LHH

The sonograph image is of varying shades witheach shade a function of the intensity of thereturning acoustical pulse.The stronger the returning pulse the darker theimage.Factors that affect the intensity of the returningsignal 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 towfishoccurs 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