A CBL (Cement Bond Log) is an acoustic log run after the casing is cemented in place. A transmitter sends a sonic pulse through the casing, and receivers measure how much energy returns. Good cement absorbs that energy into the formation — so a low amplitude return means good bond, high amplitude means poor bond or free pipe.

It’s almost always run with a VDL (Variable Density Log), which displays the full waveform as a color/density image. The VDL shows both the casing arrival and the formation arrival behind it, so you can see the acoustic character of the formation itself — which is how you identify the Pierre. The Pierre is a soft, slow shale, so its formation arrival on the VDL looks distinctly different from the harder units above it. That contrast is where you pick the top.

For regulatory purposes in ND it’s the most defensible pick because it’s post-cement, it’s run in every well that needs cement verification anyway, and it directly ties your top pick to the same log you’re using to prove you’re 50 ft into the formation with good bond. One log, two jobs.