Geophysical and Space applications
Measurements of the geomagnetic field and its anomalies. These enable:
at a meter scale (a few meters): locating ferromagnetic objects underground or underwater such as unexploded ordnance or abandoned vessels with toxic waste
at a kilometer scale: identification of geological formations containing minerals or oil
at a hundred Km scale: investigating the Earth's outer mantle, the ionic currents in the ocean and the ionospheric dynamo (parameters studied for climate-change models)
at a thousand Km scale: investigating the geodynamo at depths of several thousand kilometers
There are also miscellaneous applications in warfare, including the identification of submarines.
(Includes studies of rocks - petrology)
"Magnetometry: Techniques, Recent Developments, Applications"
by Prof. Dmitri Budker
Figure 1: cover of the book "Optical Magnetometry"
Full table of contents:
Budker D., Romalis M., Nature Physics (3), p. 227–234 (2007)
Magnetometry with LGS technology
In a previous section, we referred to the "Laser Guide Stars". The publication by W. Happer et al (https://www.osapublishing.org/josaa/abstract.cfm?uri=josaa-11-1-263) is considered to be a major reference. W. Happer, a physicist who pioneered spin-polarized spectroscopy and the sodium Laser Guide Stars, had thought of using LGS for Magnetometry but never published on the subject. Below, preceded by a commentary, is the first publication on the subject by a team consisting of members of D. Budker's lab and also scientists of ESO.
"Physicists propose beaming laser at atmospheric sodium to measure global magnetic field"
What is the magnetometric sensitivity of the technique?
"The calculated magnetometric sensitivity, which is limited by currently available laser power and may therefore be expected to improve with advances in laser technology, is useful for the envisaged geophysical applications which require measurement of fields, e.g., in the 1–10 nT range for ocean circulation  and the tens of nanotesla range for the solar-quiet dynamo ; moreover, the dynamic range of the measurement is not subject to any simple physical limit, as the resonance technique works well and with similar sensitivity at any magnetic-field strength. Moreover, the sensitivity of this technique could be further enhanced by as much as five orders of magnitude by reflecting a laser from a rocket- or satellite-borne retroreflector, instead of being limited to the small fraction of fluorescence emitted toward the detecting telescope."
10 to the minus 9 enhanced by 5 orders of magnitude is 10 to the minus 14, one order below the femtotesla.
Please note that the brain magnetic field is in the order of picotesla and femtotesla (cf.
Figure 2: Facebook post from Laser Focus World
"Researchers at the Shanghai Institute of Optics and Find Mechanics (SIOM) have succeeded in developing a high-power 589 nm sodium laser pulsed at the Larmor frequency in the 200–350 kHz range for remote magnetometry, which translates to analysis of 100-km-scale variations at the ground: https://goo.gl/X4miT4"
Cited study at https://www.ncbi.nlm.nih.gov/pubmed/?term=29088161
Environmental magnetometric capabilities using light (lasers) are reported to have increased by an order of magnitude in 6 months.
Sensitivity reported to be at 28 nT/Hz½ (noise-equivalent power)