How to read a drilling log

What the log records

A drilling log is the borehole’s birth certificate. It records the formations passed through and at what depth, where water was struck, the position of casing and screen, and the yield measured at the end of development.

Why the strata matter

The sequence of formations tells you whether the supply is likely to be seasonal. A borehole drawing from weathered overburden behaves differently in a dry year from one drawing from fractured basement, and the log is the only place that distinction is written down.

The number to check first

Yield at completion — and specifically whether it was measured after development or during drilling. A figure taken while the hole is still being flushed is not a yield, and a figure quoted without saying when it was taken should be treated as no figure at all.

What to do with it

Keep it. When a pump fails five years later, the log is what tells the next engineer whether the borehole is worth rehabilitating or whether the problem is below the pump.