You can use virtually any professional quality sidescan sonar with a magnetometer. You usually need to tow both devices independently. You usually tow the magnetometer behind the sidescan so the mag doesn't register the sidescan. Some sidescan units can interface to a mag and allow it to be towed behind the sidescan, but this isn't necessary.
The trick is the software. You want to overlay the magnetic data on the sidescan data, and make corrections for the relative displacement of the two. I know that the GeoAcoustics sidescan units do this, and their 'GeoPro' software allows for simultaneous real time sidescan and magnetic visualization. The GeoPro software runs on a Mac.
For most buried wrecks, it's very difficult to discriminate between benign sidescan contacts and magnetic contacts. However, when you have a sidescan contact that also registers on the mag, you have almost certainly found a man-made object.
Let me know if you have any questions. You can contact GeoAcoustics about the "GeoPro" software. They may or may not still carry it but I know for a fact that the people who wrote that software are still around and the software is up to date.
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