Precision of geomagnetic field measurements in a tectonically active region.

A linear regression on standard deviations of differential proton magnetometer measurements at distances from a few meters to 50 kilometers indicates a standard deviation of hourly means that varies with site separation as σ=a+bd where a=0.07±0.08nT, b=0.01±0.003nT/km and d is the site separation in kilometers. At a few meters separation, for sites with low cultural noise in both seismically inactive and active regions, the standard deviation of hourly mean data has a mean of 0.12±0.03nT for instruments with 0.25nT sensitivity and 0.07±0.01nT for instruments with 0.125nT sensitivity. The least-count noise contributions are expected to be less than 0.06nT and 0.03nT, while undetermined instrument noise appears to contribute 0.10 and 0.06nT respectively. Instrument temperature sensitivity does not exceed 0.001nT/°C over a range from-6°C to 21°C. For typical site separations of 10 to 15km throughout the San Andreas fault, estimates of σ for hourly mean data range from 0.15 to 0.3nT depending on local magnetization characteristics. Spectral density estimates indicate difference field noise power decreases with increasing frequency at about 3 db/octave. Dominant spectral peaks occur at diurnal harmonics and at the tidal M2 frequency. Geomagnetic difference-field noise limits measurement capability at all frequencies below about 2 c.p.h. Where instrument precision starts to limit detection capability, instruments of higher sensitivity may be useful.