Step 1: List what CBL actually measures:
A Cement Bond Log records the amplitude and travel time of an acoustic signal sent from a transmitter through the casing, cement and formation to a receiver. The amount of attenuation of this signal indicates whether the cement is well bonded to the casing and formation or whether channels of fluid or gas exist behind the pipe. This acoustic wave measurement is the sole physical principle behind cement bond investigation.
Step 2: Separate a correlation tool from an investigation tool:
A gamma ray curve is often recorded at the same time as the acoustic data, but only as a depth reference tool. It lets the engineer match the CBL depth scale to the open hole logs and to the casing collar locator so that any weak bond zone can be pinpointed at the correct depth. Being present in the same logging run does not make gamma ray part of the actual bond investigation, since it measures natural formation radioactivity, a property completely unrelated to cement bonding.
Step 3: Explain why neutron and electrical logs are excluded:
Neutron logs respond to hydrogen content and are designed to estimate porosity of the rock matrix in an open borehole. Electrical or resistivity logs measure the resistivity contrast of formation fluids to distinguish hydrocarbons from water, again in an open hole environment. Neither tool responds to the mechanical integrity of the cement sheath behind the casing, so neither is used in a cement bond investigation.
Step 4: Match to the given options:
Comparing with the four options, only Acoustic wave logging is genuinely used to investigate the cement bond, since it directly measures signal attenuation caused by the bonding condition. Gamma ray, Neutron and Electrical logging do not investigate the bond itself, gamma ray being a correlation aid at best and the other two being unrelated open hole formation evaluation tools, so all three are the correct selections for methods NOT used in CBL investigation.
Final Answer:
\[ \boxed{\text{A: Gamma ray, B: Neutron and C: Electrical logging are NOT used in CBL}} \]