Why early magnetic detection worked best in shallow straits

Early magnetic anomaly detection had limited depth reach, so it was most useful in shallow, constrained waters such as the Strait of Gibraltar where submarines could not pass far below the patrol aircraft.

Question

How deeply could a U-boat dive and still be detected by an aircraft using early magnetic anomaly detection?

Short answer

Most U-boats could get below its useful range in deep water, but shallow choke points such as Gibraltar made the early system practical.

Notes

The wartime equipment was an early precursor to modern magnetic anomaly detection rather than a mature system. Its limited range made deep-ocean searches ineffective because a U-boat could pass below the detectable field. The Strait of Gibraltar offered a more favorable geometry: water depth and the narrow transit route brought submarines closer to Catalina patrol aircraft. The technology therefore worked as a specialized barrier sensor in constrained water, not as a universal substitute for radar, sonar, or visual search.