The Radioactive Marker Sub is a short joint of pipe designed to hold a radioactive “pip-tag”
for accurately placing downhole tools and tubing conveyed perforating guns (TCP) on depth.
After running in-hole using the driller’s depth, a gamma ray correlation device is run through
tubing to detect the actual depth of the pip-tag. The final setting depth of the tools or TCP
guns can be adjusted based on the actual depth reading versus the desired depth.