What type of logs can be used to locate the top of the gravel packed section and to detect voids in gravel packs?

• A. Focused gamma ray logs
• B. Neutron logs
• C. Unfocused gamma ray logs
• D. Sonic logs

Answer: (C)

Natural gamma ray logging helps distinguish lithology by how much natural radioactivity the zone contains. In a gravel-pack completion, the pack material is usually a clean, coarse gravel (lower gamma activity) compared with surrounding formation that may be richer in clays and other radiucky minerals (higher gamma activity). This contrast makes the top of the gravel pack stand out on the gamma ray log as a noticeable change in response.

Unfocused gamma ray logs sample a broader portion of the borehole vicinity, so their indication reflects the average lithology of the interval rather than just the near-wellbore zone. That broader view makes the gravel-pack interval easier to identify and helps reveal voids within the pack: where the gravel is incomplete or absent, the log response shifts toward the surrounding formation’s signature or away from the pack’s baseline, producing a detectable anomaly. In short, the unfocused log provides a clearer, more integrated indication of the pack boundary and any gaps inside the pack.