Define near-surface dead zone and strategies to mitigate it.

• A. The region near the weld where echoes are unpredictable due to material anisotropy; mitigate by increasing probe frequency only.
• B. The near-surface region where echoes are unreliable due to standoff and coupling; mitigation is to use higher power and longer dwell time.
• C. The region just beyond the near-surface dead zone where reflections are strongest; mitigation by lowering the coupling medium.
• D. The near-surface region where echoes are unreliable due to standoff and coupling; mitigate with immersion, different frequency, or alternative beam geometry.

Answer: (D)