SIDE SCAN OVERVIEW

Sidescan Survey Overview 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 operations A side scan: A side scan: Will not give you depth information. Use a single beam or multibeam sonar for that. Will not give you depth information. Use a single beam or multibeam sonar for that. A side scan: A side scan: Will give me information about targets on the seafloor, including their position and height above the bottom. 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. Will give you real time information about the height of the fish above the bottom. Can be used for seafloor classification. Can be used for seafloor classification. Has a swath that is not depth dependent (unlike a multibeam system) Has a swath that is not depth dependent (unlike a multibeam system)

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.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 metersExample: 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 card is needed. See the Side Scan Hardware presentation for details.Sidescan can be analog or digital. With an Analog system, an A/D card is needed. See the Side Scan Hardware presentation for details. Sidescan systems can be multi-ping, and can be used while travelling at a high speed ( 10 to 12 kts )Sidescan systems can be multi-ping, and can be used while travelling at a high speed ( 10 to 12 kts ) Single ping systems require you to survey at slower speeds to insure 100% coverage.Single ping systems require you to survey at slower speeds to insure 100% coverage. Sidescan systems are phase differing (interferometric), providing sidescan and simultaneous bathymetrySidescan systems are phase differing (interferometric), providing sidescan and simultaneous bathymetry Refer to the HYSWEEP notes for logging of this type of systemRefer to the HYSWEEP notes for logging of this type of system

What you find with a Sidescan Record 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. How the sonar sees the object: Water Column Shadow

Tow Fish Height Above the Bottom - use the 8% to 20% rule during survey Why 8 – 20% or range scale? This altitude will provide the best angle of incidence (reflection of an object) for a sonar return.Why 8 – 20% or range scale? This altitude will provide the best angle of incidence (reflection of an object) for a sonar return. When using a fixed mounted sidescan the image may degrade in water depths greater then 40 meters. The angle is too steepWhen using a fixed mounted sidescan the image may degrade in water depths greater then 40 meters. The angle is too steep RANGE SCALEOptimum Fish Height 50 meter4 – 10 meters 75 meter6 – 15 meters 100 meter8 – 20 meters

Image quality vs. location  The sidescan will have no return at the nadir area.  The sidescan image typically degrades at the far end of the swath When running survey operations, the best image will be in the center of the swath Targets is these areas might be missed or cannot be measured accurately Ideal location for targets

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

Sidescan Deployment  Sidescan sonar transducers are typically mounted on a towfish. For shallow water operations they can be hull mounted. Towfish deployment

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 Operations Towed verses Hull (fixed) mounted systems. BenefitsProblems TowedSonar is close to bottomPosition of towfish Minimal motionmay not be known Ideal for Deep water Hull Accurate positionNot Ideal for deep water MountedWon’t hit bottom Vessel motion will show in sidescan imagery Two options for running the system: towed or hull mounted. The list shows the benefits and problems with each method.

What to do with Side Scan data Mosaics and Targeting Mosaic: HYSCAN & GEOCODER Survey lines can be merged together to provide a 2D representation of the seafloor and saved as a GeoTif file. Targeting: HYSCAN Targeting: HYSCAN Capture and measure objects on seafloor. Targets can be saved to a file for reports Bottom Classification: GEOCODER Target report in HYSCAN Mosaic of 5 lines