Using Sidescan Sonar for Underwater Survey

This is an introduction to using sidescan sonar imagery for archaeological work - using real data from our Swan River Project. Blackwall Reach is stretch of limestone cliffs over one of the deepest areas of the Swan River, best known today as a recreational cliff jumping spot. There are several known archaeological features in the area. It's often used as a training area for scuba divers - leading to many wild and wonderful rumours.

A focus of MAAWA's Swan River project is to systematically and rigorously record the cultural heritage hidden beneath Blackwall Reach, and the first step of this process was a sidescan sonar survey of the area. The visibility for divers underwater ranges from 1-5m, which makes any work slow, and large diving surveys impractical. With the help of the Western Australian Museum, the survey was carried out in under an hour.

The sidescan ‘fish’ is towed behind the boat, sending sound waves out at an angle towards the seabed. It records the sound waves that reflect back, and displays them on a screen. The harder the object, the stronger the reflection and the brighter the point. Objects that stick up off the seabed cast shadows. The GPS location is always recorded by a receiver mounted on the boat. Once corrected for the length of the towing line (layback), this allows the imagery to be loaded into GIS software such as QGIS or ArcGIS. If the layback is not taken into account, the georeferencing will be offset incorrectly.

The map

The imagery collected is displayed below – along with GIS data from the WA Museum’s Shipwreck Database, David Jackson’s divingwawrecks.com and the ‘Swan Divers’ website (link now broken: swandivers.org). Like GIS software, the map has various tools for managing layers and measuring features. Combining geographical information like this is useful for investigating maritime cultural landscapes.

Test your skills

1. Verify the WA Museum’s information on the Blackwall Reach Barge.

   • The map starts centred on the Blackwall Reach study area. Look in the map layers menu to find which symbol represents Museum database entries. They already display on the map, so you should not need to change anything yet.

   • Locate the entry for Blackwall Reach Barge by clicking on the map points (towards the south)

   • Zoom in until the structure becomes clear. Click outside the information bubble if it gets in the way.

   • Measure the length of the wreck using the measurement tool, and compare it to the database entry.

2. Verify Swan Diver’s reports of a large wreck and a nearby tinny, due east of Point Childey and near the “Porthole Cruiser”.

   • Hint: you will have to load different imagery for this area

   • How does the sidescan imagery match up with the information on GPS points?

3. There appears to be a consistent error in the GPS points – why might this be?

4. Bonus question (really tough): Can you locate the car wreckages shown below?