VDL Log (Variable Density) Tools & Interpretation
The Variable Density - VDL Log in essence is a further refinement of the conventional CBL (Cement Bond Log) by including a full waveform display, presented on the log as 'a variable intensity' in shades between black and white, representing maximum and minimum in amplitudes. It is used mainly in the interpretation of the cement-to-formation bond (Cementing In Drilling). You might be interested in cement evaluation & cement evaluation tool.
https://www.drillingmanual.com/vdl-log-variable-density-tools-interpretation/