An optically-pumped rubidium 87 magnetometer for offshore magnetic surveys

An optically pumped rubidium 87 vapor magnetometer is described which measures the earth\´s magnetic field to one part in million. The sensitivity is about .01 gamma in the earth\´s field of 52,000 (0.52 oersteds). Rubidium 87 is preferred to rubidium 85 because the nuclear spin is 3/2 rather than 5/2. Thus the magnetometer described can "lock on" to a single hyperfine absorption line and not show orientation effects common in self-oscillating rubidium 85 magnetometers. Two magnetometers are used in offshore surveys, one fixed and one towed behind a vessel. The record of the fixed stations is subtracted from the moving station record thereby eliminating the time variations of H. Space variations are interpreted in terms of subsurface structures. In particular, negative magnetic anomalies may indicate salt domes usually having oil around their perimeters.