Ultrasonic Testing is performed so as to check for flaws at a given depth. (Thats why we hope the inspectors know their geometry.) The back side of the welded member will show up as a flaw (like a void) if the proper depth and angle of the shoe are not considered (or the shoe is worn). So, yes the backing bar will show up as a flaw if the inspector doesnt have the proper geometry input into the scope. But even if the depth were set greater than the thickness of the member, it would be located 1 1/4" deep in the member, which would indicate that it is in fact the backing bar, not the main member. (And would show up until the sound wave was hitting the root pass.)
I've been involved with both the design, as a Civil Engineer, and in the inspection, as the managing P.E. of a testing lab in California. The most common areas I've seen problems develop in these welds are: #1 - The fit-up (gap) is inconsistent due to fresh heading (grinding) of the driven pile, or mushrooming of the top of the pile causing a larger diameter on the driven portion; #2 - lack of proper welding procedure mandating a thorough cleaning of the root pass.
If done properly, these welds will be fine, but should be checked using UT.
Good luck,
Jim
