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A seismic refraction survey shows rock head at 8 m but a borehole finds it at 14 m. The likely explanation is

  • A.the rock being unusually strong at that location
  • B.the survey line being too long for the depth involved
  • C.a hidden low velocity layer beneath a stiffer crust
  • D.an error in the borehole depth measurement

Answer: C

A weathered zone beneath a hard cap violates the method's assumption of increasing velocity, so the interface it reports is the cap rather than true rock head. Boreholes calibrate the geophysics precisely to catch this.

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