Down Conversion of Ambient Seismic Noise As A Tool To Detect Non-Linearity And Estimate Instrument Noise Levels In A Gravity Meter

Abstract:

Nonlinearities in a gravity meter’s sensor can cause down conversion of seismic noise to lower frequencies, obscuring low frequency signals of interest. We have found that a plot of the noise power in a selected quiet bandwidth between 0.01-0.03 Hz as a function of the overall seismic power provides a key Power Assessment Tool (PAT) for visualization of instrument noise. Changing levels of background seismic noise ordinarily make estimations of the noise level of the instrument itself difficult. The PAT plot can be used to detect non-linearity in a gravity sensor by displaying a correlation between noise in the quiet band and the overall level of seismic noise. In the absence of a non-linearity the PAT plot provides a reliable upper estimate of instrument noise level with a single instrument in a noisy location. Collocated superconducting gravity meter and gPhone gravity meter data records collected at Table Mountain Gravity Observatory exhibit very similar behavior using the PAT plot and indicate an upper bound on the instrument noise of both instruments of between 0.2-0.3 μGal/√(Hz) in a quiet noise band between 0.01-0.03 Hz. These noise levels are consistent with the stated instrument noise of the superconducting gravity meters and also agree with noise levels obtained from differenced spectra between the collocated gPhone gravity meters. This confirms the utility of the PAT plot in estimating the noise level of a gravity sensor.