What geophysical measuring technique would be used to assess the downhole vertical continuity of a clay stratum?

• A. Terrain conductivity
• B. Resistivity
• C. Ground penetrating radar
• D. Gamma ray logging

Answer: (C)

Assessing vertical continuity of a clay bed relies on imaging layer boundaries with depth and high vertical resolution. Ground-penetrating radar does this by sending short electromagnetic pulses down the borehole and recording reflections from interfaces where the dielectric properties change, such as the top and bottom of a clay layer. The travel time of these reflections translates to depth, and the pattern of reflections along depth shows whether the clay bed is continuous, where it thickens or thins, and if it dips or disruptions exist. This borehole imaging provides a clear, near-vertical view of the subsurface bedding around the well, making it well suited to determine whether the clay stratum remains intact down the hole. Other methods either map surface properties with less direct access to vertical stratigraphy, offer lower resolution for thin beds, or indicate clay content without revealing continuous bedding in depth.