When using the surface wave method, what effect can patches of oil or dirt have?

Using the surface wave method in nondestructive testing can be sensitive to the presence of contaminants like oil or dirt on the material's surface. When these patches are present, they can interact with the sound waves emitted during the test.

The presence of oil or dirt can lead to attenuation of the sound waves, meaning that the energy of the sound waves is reduced as they pass through or interact with these contaminants. This reduction in energy can result in a weaker signal being detected on the screen. Consequently, the quality of the signal may be compromised, leading to inaccurate readings or interpretation of the data.

Additionally, the presence of contaminants can also cause variations in the acoustic properties at the surface. This can lead to mixed signals appearing on the display, indicating the presence of these irregularities. As a result, both the attenuation of sound and the appearance of unexpected indications on the screen can occur due to the patches of oil or dirt, making it crucial to ensure a clean testing surface for accurate results.