A comparison of proton and self-calibrating rubidium magnetometers for tectonomagnetic studies.

Abstract

A network of 27 proton magnetometers (PM's), designed to detect stress related magnetic events of crustal origin, has been operating near active faults in California for the past decade. We present here comparative magnetic difference field data from PM's used in this net with that obtained from new selfcalibrating rubidium magnetometers (SCR's). The instruments were first compared over a 50-m baseline in an aseismic and magnetically quiet region in Colorado. For PM's having either a 0.25-nT or a 0.125-nT least count, the observed difference variations were 0.2-nT and 0.17-nT rms, respectively. For SCR's having a 0.001-nT least count and a 100-second averaging interval, the difference variation was 0.002-nT rms. Power spectra of these data indicate that the noise for the PM's is close to their least count limit. However, the SCR noise decreases at about 20 dB per decade until it approaches its least count limit 40dB below the PM limit, for periods less than 30 minutes. A similar experiment was conducted using collocated SCR and PM pairs separated by 13km along the San Andreas fault. Power spectra indicate that both systems are equivalent and are dominated by external noise at periods greater than 4 minutes. Below 4 minutes the PM noise approaches its least count limit while the SCR noise continues to decrease at about 20 dB per decade until it is 20 dB below the PM limit at a period of 30 seconds. Improved discrimination of magnetic transients caused by fault activity with periods of several minutes to perhaps an hour appears to be possible with higher sensitivity magnetometers.

A new optically pumped tandem magnetometer: principles and experiences

“…The measurement is derived from a nuclear effect, and consequently guarantees good absolute accuracy and excellent long-term stability, both of which are requirements for obtaining valid data over a time period of several decades. However, it does not provide a continuous signal, which means we can only obtain momentary information about the magnetic field [1,2]. Optically pumped magnetometers (OPMs) offer higher sensitivity due to a higher gyro-magnetic ratio, a continuous working regime, and the possibility of a fast response, which is important during periods of strong geomagnetic activity and for special applications.…”