Magnetic gradient survey: Measuring the rate of change of magnetic field in space (in ng/m or nit/km) is called magnetic field gradient measurement. Vertical gradient measurement and horizontal gradient measurement are applied in production practice.
The receiving portion of the vertical gradient measurement is two vertically placed probes. If measured on an aircraft, the vertical distance of the probe can be designed to be large, about 30 to 60 meters. The measuring instrument generally uses a very high sensitivity optical pump magnetometer, and the two probes simultaneously read. If the surface is measured at the surface, the vertical distance of the probe is about 2 meters, and the measuring instrument is mainly based on the nuclear spin-in magnetometer. In the surface measurement, in order to overcome the mutual interference of the magnetic fields generated when the two probes are excited, the upper and lower probes are often used to make two excitation readings at intervals of 2 seconds. Dividing the two probe readings by the distance between the probes is the vertical gradient at that point. The vertical gradient change of the local magnetic anomaly is more pronounced than the regional anomaly, so the vertical gradient measurement is suitable for implementation in areas with strong regional interference in order to obtain useful local anomalies. Horizontal gradient measurements are common at sea, and the receiving portion is also two probes, but placed horizontally. Record the readings of both probes simultaneously. The instantaneous difference is the horizontal gradient. Horizontal gradient measurements are useful for identifying day changes.