Micromagnetics is the petroleumexploration form of magnetic surveying for recording and interpreting all frequencies of the magnetic field, from the shortest wavelength anomalies sourced in the shallowest sedimentary formations to the longest wavelength anomalies sourced at or beneath the basement surface. This is accomplished by using handheld magnetometers on the ground or by using airborne magnetometers flown at a terrain clearance in the order of 150 m (500 ft). The objective is to map all magnetic indications of structure from the top of the sedimentary column to crystalline basement.
The earliest magnetometers allowed measurement accuracies of 10 to 25 nT (gammas). In 1936, W. P. Jenny wrote that the method had been “revolutionized” by improvements in the instruments and in field techniques that allowed accuracies of ± 2 nT. Magnetometers involved in the early mapping of sedimentary responses were mechanical, tripod-mounted, magnetic balances that measured either the vertical or horizontal component of the Earth's field. These and later mechanical devices, eventually improved to allow accuracies of ± 1 nT, were the mainstays for surface work until the coming of the proton magnetometer in the 1960s, with its 1-nT sensitivity that eventually was improved to 0.1 nT.
In 1948, the Gulf Airborne Fluxgate Magnetometer, developed during World War II for antisubmarine warfare, became available for commercial use.