Small-scale array experiments in seismic-wave gradiometry
Three small-scale refraction and gradiometry experiments were performed to investigate whether off-the-shelf exploration geophones and seismographs can be used to perform meaningful gradiometry measurements. Relative calibration of the geophones was attempted through huddle tests and spectral measurements of ambient ground motions. The results show that relative gains cannot be completely characterized because of geophone/ground interaction. Numerical tests show that typically observed 4.5% gain errors introduce a standard deviation of 0:03 s=km and 1.97° about the correct input slowness and azimuth, respectively, for 100,000 realizations of synthetic array data. A standard linear refraction experiment was performed to investigate the slowness of P and Rayleigh waves from hammer sources to compare with measurements taken from two gradiometer designs. One design consists of four 6-instrument gradiometers in a linear array to investigate the spatial and temporal variation of horizontal slowness and propagation azimuth for sources close to the array as well as to test the location abilities of the entire gradiometer array. Its location estimate for a shot 10 m from the center of the array using the two closest cells was close to the actual source position with a 1.38-m error. The gradiometers were able to correctly determine the slowness values of the P and surface waves identified in the refraction profile. A second gradiometer experiment involves superimposed cells to explore precision in calculation of spatial gradients. Significant increases in the precision of the wave attributes occur when a larger number of averaged time-shifted waveforms are used as the reference station displacement in place of a single-reference-station displacement waveform. The concept of center-station correlation is introduced to avoid spurious amplitude errors from drastically affecting the wave parameter estimates. We conclude that the off-the-shelf equipment can be used to construct small dense gradiometer arrays that can be used to infer wave attributes that are important for the interpretation of wavefields in exploration seismology experiments.
Seismological Research Letters
Barker, L., & Langston, C. (2016). Small-scale array experiments in seismic-wave gradiometry. Seismological Research Letters, 87 (5), 1091-1103. https://doi.org/10.1785/0220160040