I don't mean to be a smart ass here, but failure to see a planar from one side is by no means a sign that one is not present. Angle of incidence is very important to picking up a planar flaw. If you look at this video I've uploaded
http://www.youtube.com/watch?v=oZR846TSQds You can see an example of the importance of angle of incidence using an inspection mirror, a flash light, and video camera. The camera is approximately 90 degrees minus a few degrees. It starts with the mirror turned flat, in line with the flashlight both laid down on a counter top in line with each other. As this example begins, only ambient light can be seen at this point, as the mirror is turned up and the camera lense/mirror incident angle gets to within 30 degrees of 45 out to each other, you will note an increase in the light intensity. As it gets within 20 degrees a definite increase of intensity is noted, and at about 45 you can see the direct reflection from bulb of the flash light. At that point from the cameras point of view, the most light is being reflected.
Going on past that point angling in towards the flash light, you will note that when the mirror gets within 20-25 degrees of the plane of the flash light, a reflection from the mirror can be seen on top of the flashlight rim. When it gets to the same plane you can see a direct reflection.
An ultrasonic wave will behave the same way. You have to be impinging on the planar at an angle that reflects enough energy back to the transducer to register on the scope. If the planar flaw is outside approximately 20 degrees from the angle of energy incidence, it is not likely to be seen and more likely to redirect the sound downwards via reflection and refraction. this means that from the opposite side of what is considered nearest to perpendicular to the planar, it could barely be seen if it all.
Even in that, the sound could also be turned down by a haz interaction, a weld fusion zone (transmission/reflection coefficient) interaction. and many other modes.
I have to disagree that a planar is eliminated (even almost).
A secondary creep wave could have also been set up at the cladding layer, or the back side of the weld. This could mode convert the sound to an L wave, and assuming initial exam in shear, would throw off where the actual flaw is if there is one. This is because the differences in velocities. If your machine thinks the reflection coming back is in shear wave mode and the machine is calibrated in shear, then the time of flight will be off because of the conversion to L wave which is roughly twice as fast as the T wave.
One thing that is absolutely true, everything is conjecture without the exact details of the exam. Everything is academic until exact details are known.
Respectfully,
Gerald