During testing by the surface wave method, what may patches of oil or dirt on the surface cause?

The surface wave method relies on effective sound transmission through the coupling medium, which is often the surface of the material being tested. When there are patches of oil or dirt on the surface, they interfere with this necessary coupling. Such contaminants can lead to two primary issues: attenuation of the sound waves and potential indications on the testing display.

Attenuation occurs because oil or dirt can disrupt the mechanical coupling between the transducer and the test material, reducing the amount of sound energy that successfully travels into the material. This results in lower amplitude signals and potentially weaker returns.

Additionally, depending on the distribution and nature of the contaminants, the display screen may show irregular signals. For example, the presence of patches may cause variability or noise in the signal, creating indications that do not truly represent the material's internal condition.

Thus, the combination of these effects—sound attenuation and altered indications on the display—supports the correctness of the answer that identifies both attenuation of sound and its impact on the screen during testing.