Seafloor Imagery - Sidescan and Backscatter

Seafloor Imagery 

Sidescan and Backscatter 

An introduction of sidescan 

operation, data collection and 

Processing 

Harold Orlinsky 

Harold@hypack.com

Topics to cover 

• Introduction to the Sidescan 

• Sidescan Theory 

• Sidescan Positioning 

• Resolution of systems (object detection) 

• Backscatter 

• Data collection 

• Sidescan Mosaicking ( processing ) 

• Data Interpretation 

• Sidescan data used for bottom classification 

That should keep us busy for the session.

Sidescan systems – why use them 

• A sidescan sonar can be used 

for a wide variety of survey 

operations. 

• Search and recovery 

• Geological Identification 

• Pre / Post dredge surveys 

• Target verification and 

location 

• It is also a useful tool for correlating 

• and verifying bathymetric data 

Shipwreck with shadow (in white) 

Sidescan data showing sediment change

Sidescan data for survey work 

A side scan: 

Will not give you depth information. Use a single beam 

or multibeam sonar for that. 

A side scan: 

Will give me information about targets on the seafloor, 

including their position and height above the bottom. 

Will give you real time information about the height of 

the fish above the bottom. 

Can be used for seafloor classification. 

Has a swath that is not depth dependent, like a 

multibeam.

Sidescan types 

Sidescan frequency can vary from 100kHz to over 

1Mhz. The higher the frequency, the better your 

resolution, but the shorter your range scale. 

Example: 100 Khz might be able to detect objects at 300meters, 

while a 900Khz will work to 45 meters 

Sidescan can be analog or digital. With an Analog 

system, an A/D is needed. See the Side Scan 

Hardware presentation for details. 

Some sidescan systems are multi-ping, and can be 

used while travelling at a high speed ( 10 to 12 kts ) 

Single ping systems require you to survey at slow speeds to insure 

100% coverage.

What you find with a Sidescan Record 

Shadow 

Water Column 

How the sonar sees the object: 

• Water Column - Provides information about towfish height. 

• Target - features off the seafloor will produce a shadow. 

• Bottom sediment - Signal will differ based upon return angle on incident 

and seabed absorption.

What to do with Side Scan data 

Mosaics and Targeting 

Mosaic: 

Survey lines can be merged together 

to provide a 2D representation of the 

seafloor and saved as a GeoTif file. 

Targeting: Capture and measure 

objects on seafloor. Targets can be 

saved to a file for reports 

Bottom Classification: GEOCODER 

Sample Mosaic of 5 survey lines 

Target report in HYSCAN

Sidescan Theory

Sonar system components 

• The sonar system (towfish) has a transmit and receive component 

taking the signal (digital) and sending to the acquisition computer for 

display and storage. 

• The echo distance is determined by time, similar to other sonar 

systems 

Display 

Transmitter 

/ receiver 

Sidescan 

Signal 

Distance = time (round trip) x Sound Velocity 

2 

0 Time of pulse ------- 

Bottom 

echo 

Note the use of 

Sound Velocity. It is 

needed to compute 

the distance. The 

default of 1500 m/s 

will give an error

Sidescan operations - Positioning 

A sidescan, can be towed or fixed to a vessel.In either case, a swath of 

data is collected - port and starboard across the seafloor. 

During towed operations, the vessel is beyond the object when the sonar 

detects the echo. It is important to know the position of the sidescan, 

and use that for processing and targeting

Sidescan positioning 

Getting the position of the towfish is critical. Seeing 

an object but not knowing where it is, will not be very 

useful. 

•Cable out (layback) 

•Acoustic positioning system (USBL) 

•ROV (inertial systems on board) 

•Ship mounted (uses on board GPS with offsets) 

Which one is best Dependent of survey operations 

and water depth.

Sidescan positioning 

Where is the sonar located: 

At the vessel or towed system 

When at (on) the vessel, the position is set, but there are other problems 

When being towed, we lose confidence of the position of the targets 

Benefits 

Problems 

Towed Close to bottom Position (not 

Minimal motion 

accurate) 

Deep water work 

Hull Accurate position Shallow water work 

Mounted Won’t hit bottom (see next slide) 

Vessel motion will show

Operating a fixed mount (on survey vessel) 

RANGE SCALE 

Optimum Fish Height 

50 meter 4 – 10 meters 



A rule of thumb to get the best results: 

Use 8% to 20% of range scale for fish height 

•Why 8 – 20% or range scale This 

altitude will provide the best angle of 

incidence (reflection of an object) for a 

sonar return. 

Too shallow, high angle 

•When using a fixed mounted sidescan 

the image may degrade in water depths 

greater then 40 meters. The angle is too 

steep 

Too high, high angle

Towing a sidescan system 

Towing a sidescan allows for deep water operations. 

Some things to consider: 

1. Use of an A-frame or another method to deploy 

and recover the towfish 

2. Cable and winch system 

3. Position system – cable counter or Acoustic 

position 

4. Ship maneuverability 

Towfish deployment

Sidescan Resolution

Across track range resolution 

Why sonars work better at the outer ranges 

Distinguishing two independent objects can be referred to as 

the sonar resolution. 

If two objects are too close, they will appear as one on the sidescan 

record. Getting these objects further apart will show them as 

independent objects. How close can they be Half the pulse length. 

Example: A 500khz system has a pulse length of 1 cm. 

In practice, that won’t be seen, and cannot distinguish an object that 

close. We can approach this value further away from nadir. 

The footprint of sonar (arriving energy) is longer near nadir due to the 

angle of incident. Conversely, the further away, the more the bottom 

footprint approaches the pulse length. 

At the very far end of the signal, the best resolution would be obtained 

First 

return

Along track resolution 

why objects in the far field cannot be distinguished 

The resolution in the along track direction is dependent on the sonar 

Horizontal beam width. 

The image below represents what happens when targets in the far field 

are inside the angular resolution of the sonar. They become 

indistinguishable and look as a single object 

At the near field, these objects can be distinguished. 

Horizontal 

beam width 

First 

return

A combination effect – where object detection is ideal 

Looking at both the near field and far field constraints, as well as 

maximizing the best seen area, the sonar will work best in the region 

of the Optimal Zone of Operation (OZO) 

Targets is these areas 

might be missed or 

cannot be measured 

accurately 

Ideal location 

for targets

How to do a self check of object detection 

During survey operations, set the range scale of the sonar 

to your operating range. 

Run past a know object. A buoy block or sand waves will 

work 

Ensure that you can see the entire swath. If you can’t, 

shorten the range scale until you do. 

If you don’t see a known object, 

chances are you won’t find the 

unknown object.

Backscatter

Backscatter 

How we see images on the seafloor. 

The sonar sends out energy, it gets deflected in all directions. The 

return signal can be from the surface and water column are 

reverberations, though typically low energy. The reverberations 

that come from the bottom are the ones we use to distinguish 

sediment types 

Surface 

Air bubbles 

fish 

Bottom features

Backscatter 

Not all sound returning from the sonar is from direct echoes. On a flat 

bottom, the sound wave will reflect off the bottom and keep going. 

When there is some roughness on the bottom (and there usually is), 

the sound waves scatters on the return. 

The sound that comes back is detected, and can be used to 

distinguish different bottom types.

Sidescan data collection

Collecting sonar data is different than echosounder data 

Bold statement, but it is true. 

Survey operations will change when running a sidescan sonar 

• Vessel speed (slow is best – 5kts is typical) 

• Vessel handling (you can’t stop when towing a sidescan) 

• No turns during collection 

• line spacing is fixed (unlike multibeam collection) 

• Nadir area is obscured. 

• Visual display of waterfall gives immediate results 

• (sometimes that is good enough for folks) 

• Sidescan files are large, more storage needed 

But…. Even with all that, sidescan by itself or part of a survey operation 

provides very good results.

Data collection in HYPACK 

Two programs are running: HYPACK Survey (for GPS 

and positioning) and SIDE SCAN SURVEY (to handle 

the sidescan system) 

Your computer 

HYPACK 

SURVEY 

SHARED 

MEMORY 

POSITION, HEADING, 

TIDE,, DEPTH 

SIDE SCAN 

SURVEY 

GPS 

Side Scan Sonar

Setting up a Sidescan Device 

• In HYPACK Hardware, add 

the driver to position your 

main vessel 

• GPS.DLL in this example. 

• Create a 2 nd mobile for your 

towfish position. 

• Named ‘Towed Array’ in 

this example. 

• Select a driver to position the 

towfish. 

• Towfish.DLL in this 

example. 

• Add the HYSWEEP.DLL to 

your towfish mobile. 

Setup menu for 

Towfish.DLL 

HYPACK hardware 

Device set-up has 

3 items in 2 mobile

Setting up a Sidescan Device 

In Sidescan Hardware: 

• Select position device: 

• HYPACK Navigation for 

vessel mount. 

• HYPACK Mobile 

for towfish mount. 

• Select Sidescan from the 

list of sensors. 

• Check Specific Sonar 

Identification (for 

processing data in 

Geocoder)

Connecting the sonar 

Sonar connections: 

There is serial (Imanegex) and USB (Tritech ,C-Max 

and the NI A/D board), but all the others are network 

based. 

Just know the port address and you’re set

Survey planning: 100% and 200% coverage 

100% coverage: At 100 meter range scale, line spacing would be set 

for 160 meters. This provides a 20 meter off-track error while 

surveying. 

200% coverage: This survey plan ensures that the bottom is covered 

twice by the Sidescan, covering the nadir region with the second 

line pass 

Line spacing 

160 meters 

Swath width 

200m 

Line spacing 

80 meters 

Note the overall swath 

width is twice the range 

scale

Data collection 

Sidescan survey 

Waterfall, Signal window, 

coverage map (real time 

mosaic) are all the tools you 

see during data collection

Sidescan TOOLS 

Many sidescan systems can be 

controlled inside SIDE SCAN 

SURVEY. 

In the Tools menu, select 

Sidescan Device Control. 

Settings for Range, Scale, 

Pinging, and certain gain controls 

can be sent to the sonar. 

During data collection, minimizing 

any gain changes or adjustments 

is best. 

A gain change will make the 

image look “darker”, but can be 

interpreted as a different bottom 

type. 

Know the survey area – and fix the 

range scale and gains before your 

start. 

Sample control boxes 

for different sonars

But… you still want to make sure the gains are set properly 

Display in waterfall with no adjustments. Notice the 

darken area near nadir, and the lighten area on the 

outer edge 

Raw Data: Gain = 0 

Auto TVG – Sigma = 7 

Use the gain controls to enhance the image, and to balance the signal

Maintaining a Proper Bottom Track is critical 

Why removing the bottom 

track isn’t a good idea: 

The bottom track will provides 

a visual display of how close 

the towfish is to the bottom. 

If you see something like 

this…Speed up or pull the 

cable in fast!

Capture and Measuring a contact: 

• To mark a target, double click in the waterfall. 

• To measure a target, select the up-arrow icon (4 th one in), 

and then double click. A target window will open 

Height of Contact (H) = L * A / R 

Three points along the 

image are needed 

altitude (A) , shadow length 

(L), total distance (R)

The target height may not be exact by two operators, due to the 

subjectiveness of shadow measurements 

Target Height Measurement 

Using the target window 

• Set Bar 1 to end of water column. 

• Set Bar 2 to target/shadow edge 

• Set Bar 3 to far end of shadow. 

• Use the signal display to assist in finding the target and shadow (look 

for low or weak signal following the target) 

Height above bottom 

Mark and save target. 

Target will be displayed in 

HYPACK survey

Sidescan mosaicking and 

targeting

Sidescan Mosaic Processes 

Stage 1: Raw View 

Load raw sidescan data – HSX, HS2, XTF or LOG files 

Adjust bottom track, smooth heading and track line data.


Stage 2: Scan View 

Full line can be viewed using the scroll bar. 

Adjust color, gain, TVG and display options (water column removal, 

display range of sonar image). 

Capture and measure targets (GeoTif and JPG images).


Stage 3: Mosaic 

Create mosaics, export GeoTiff images. Apply image filters. Ability to use 

a Border file for mosaic creation in specific area.

Tiling of Mosaics 

Based on the 

resolution and the 

extents of your 

mosaic, HYSCAN 

may decide to subdivide 

your mosaic 

into several GeoTIF 

files. 

This allows you to 

mosaic a big area at 

a high resolution. 

It takes a little 

longer, but you’ll get 

the detail you need.

Sidescan Controls Adjustments 

Right-click on a side-scan image to get access to the 

Sidescan Controls window. ( short cut Shift+F9 ). Sidescan 

controls and a sample of the data set appear in a single 

window. 

Select Color palate. 

Gray Scale, Gold, Copper, Rust, Pastel, Intensity, Custom. Invert Color scale 

Set Gains 

Manual or Auto TVG 

Remove Water Column 

Set Display Range 

Show Range lines 

RESET button sets 

gain thresholds to 

best fit value


Colors Tab 

• Select color palate 

• Adjust overall brightness and contrast 

• Invert colors option


Gains Tab 

• Set Gain and TVG 

• Port = Stbd. 

• Port and Stbd. Independent 

• Advanced Tab: 

• Apply Auto TVG and TVG curve (P1, P2 and P3) 

Show TVG Tab

Targeting 

Select Target (double 

left-click on feature) 

Target 

window 

opens 

Target screen displays contact. Use to take 

measurements, capture image and classify (new) 

In Stage 2, double click on any feature. 

Use sliders (top) to determine the 3 points along the target. Use the signal 

display to help. 

Click and drag to measure contact, capture JPG or TIF and enter any notes 

Save to a TGT file.

Targeting: Example

Measuring a contact: 

Three points along the image are needed 

altitude (A) , shadow length (L), total distance (R) 

Height of Contact (H) = L * A / R 

Each target will have a shadow; use the tools in Sidescan Survey or 

HYSCAN to measure contact

Target Viewer and Classification 

Use the target viewer to examine all contacts. 

Arrow keys to scroll through the list of targets. 

Data fields in the target viewer are editable for any changes or updates. 

Report can be saved to a RTF file 

TargetViewer program can run without a HYPACK key

Target Classification Database 

Select Tools – Target Classification 

Database to open/revise database 

Creating the Database 

Target database can be made 

of any target image. User 

can add a confidence level 

and notes to each target 

Using the Database in Target Viewer 

In Target Viewer, select from classification drop 

down the targetID. Use the To open the 

comparison window 

Target classification 

comparison 

Classification 

database 

Target Viewer

Resolution Calculator 

Determine the optimal mosaic 

resolution based on your 

sonar's characteristics 

• Don't waste time and space 

on heavily-interpolated 

imagery 

• Calculate Along-track and 

across-track resolution 

• Load & Save sonar profiles 

across HYPACK projects 

• When closed, the best 

resolution is applied to the 

mosaic setup

Mosaic Filter options 

AVERAGE: 

Smoothes the mosaic 

by setting each pixel 

to the average of the 

3x3 pixel 

neighborhood 

SHARPEN: 

Sharpens the mosaic 

by enhancing pixel 

contrast relative to a 

3x3 pixel 

neighborhood 

MEDIAN: 

Smoothes the mosaic 

while preserving 

edges by setting each 

pixel to the 3x3 media 

value.

Using Coverage Map to manage targets 

Blue bounding box = previous 

line Red bounding box = 

current line 

Note the difference in 

position (approx 6 meters) 

when viewed in Scan View. 

Possible cause could be 

improper layback setting. 

• When a target is taken, a mark is generated on the coverage 

map. Use this feature to verify location or to confirm target

Target Viewer 

• Used to view side scan targets marked in HYSCAN. 

• Can be run as a stand alone program without a dongle. 

• Can save target info to RTF (Rich Text Format) that can be read into Microsoft Word.

Sidescan Interpretation

Take a look at this image. What is it

The Shadow Effect 

Imaging the seafloor from above will cast a shadow of the object. Sometimes 

the shadow can give you more information that just the data on the seafloor

Sidescan interpretation 

• Besides shadows, the feature 

itself can give some indication 

of the object. Colors ( light and 

dark ) can be inferred as 

different bottom types. In this 

example, we see an area of 

light and dark. 

• Without knowing anything else, 

we can say it is a different 

bottom type. 

• With ancillary information, 

depth data in this example, we 

can say it is a hard bottom 

(dark grey - deep), soft bottom 

(light grey-shallow).

Proving the chart is wrong 

On the chart, we 

see the outline 

for a dock. 

The sidescan is 

showing the 

piles 200 feet 

south. 

A position error 

Nope!

A floating dock

Sidescan for bottom 

classification 

(Geocoder)

GEOCODER 

• Developed by Dr. Luciano Fonseca, formerly of UNH-CCOM 

• Creates mosaics from: 

– Side Scan 

– Multibeam Average Backscatter 

– Multibeam Snippets 

• Advanced Backscatter 

Correction Algorithms 

• Bottom Classification using 

Angular Response Analysis 

60

GEOCODER Data Flow 

Side Scan Data 

Fix Bottom Detection 

SIDE SCAN 

SURVEY 

HSX 

SIDE SCAN 

MOSAIC 

HS2 

Multibeam Average Backscatter & Snippet Data 

GEOCODER 

HYSWEEP 

SURVEY 

HSX 

MBMAX 

HS2 

81X/7K/R2S 

GSF 

Bathymetric Model (optional) 

MTX or XYZ 

81X/7K/R2S 

TIF 

61

Input Data Files 

• Average Backscatter 

(HS2/GSF) 

• Snippets (HS2/GSF) 

• Side Scan (HSX/HS2) 

• Side Scan and DTM 

• Side Scan: (HSX/HS2) 

• DTM (XYZ-Gridded, MTX) 

Bringing in a DTM allows you to enhance your mosaic with angle varied 

gain algorithms. 

62

Remosaicing 

Users can remosaic entire lines or sections of lines. 

Normally used to change the overlap option.

Bathymetric Corrections 

Load a bathymetric model from 

MTX or XYZ to apply: 

• correction for actual angle of 

incidence for all data types. 

• improved area correction 

• improved slant-range 

correction for side scan data. 

• DTM must be a gridded XYZ or a 

MTX file.

Bottom Classification: Extracting the Beam Pattern 

• Each sonar can have its own particular 

faults that influence the acoustic beam 

pattern. 

• In GEOCODER, users can make 

adjustments based on the sonar’s beam 

pattern. 

1. Isolate a short swath over a flat, boring 

bottom. 

2. Use the ‘Extract Beam Pattern’ menu item to 

determine the adjustment for the sonar. 

3. Save the resulting beam pattern adjustment 

to a file. 

4. After extracting the beam pattern, or reading a 

previously extracted beam pattern file, your 

data will be automatically adjusted. 

AVI

Bottom Classification: Angular Response Analysis 

• Used to determine bottom classification across a swath. 

• Matches acoustic response to known bottom types. 

1. Create the mosaic. 

2. Calculate the ARA for the mosaic. 

3. Select a swath from the mosaic screen. 

4. Run the ARA – Patch ARA routine. 

AVI 1 

AVI 2 

Patch ARA Analysis Window 

Color-coded ARA 

Selecting a swath for ARA Analysis 

66

Output ARA results to DXF 

Export ARA results to a DXF contour map 

• You must enable the ‘Formal Inversion. 

• When completed, click the Save DXF menu item at the bottom.

A lot to see in an hour (or so). 

There is good reference material 

around, especially from the sonar 

manufactures 

- and most are here today 

Questions

Seafloor Imagery - Sidescan and Backscatter

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