In which zone does the amplitude of an indication from a given discontinuity diminish exponentially as the distance increases?

In which zone does the amplitude of an indication from a given discontinuity diminish exponentially as the distance increases?

• A. Near field zone
• B. Fresnel zone
• C. Dead zone
• D. Far field zone

Answer: (D)

In the far field zone, the wavefront is essentially planar and the echo from a fixed discontinuity becomes weaker as you move farther away. This happens because the energy spreads over a larger area (geometric spreading) and the material itself attenuates the wave as it travels, so the received amplitude diminishes with distance. This produces a more monotonic, distance-dependent decrease in the indication strength, consistent with the idea of the signal weakening as distance increases. Near the surface, the Fresnel (near-field) zone features complex interference patterns, so amplitudes can fluctuate and aren’t simply diminishing with distance. The dead zone refers to the region very close to the surface where indications may not be detectable due to transducer ringing, not to a distance-based exponential decay of a discontinuity’s indication.

In the far field zone, the wavefront is essentially planar and the echo from a fixed discontinuity becomes weaker as you move farther away. This happens because the energy spreads over a larger area (geometric spreading) and the material itself attenuates the wave as it travels, so the received amplitude diminishes with distance. This produces a more monotonic, distance-dependent decrease in the indication strength, consistent with the idea of the signal weakening as distance increases.

Near the surface, the Fresnel (near-field) zone features complex interference patterns, so amplitudes can fluctuate and aren’t simply diminishing with distance. The dead zone refers to the region very close to the surface where indications may not be detectable due to transducer ringing, not to a distance-based exponential decay of a discontinuity’s indication.