Calibrating the 2016 iris wavefields experiment nodal sensors for amplitude statics and orientation errors

Abstract

We used teleseismic P and S waves recorded in the course of the 2016 Incorporated Research Institutions for Seismology (IRIS) community-planned experiment in northern Oklahoma, to estimate amplitude correction factors (ACFs) and orientation correction fac-tors (OCFs) for the gradiometer's three-component Fairfield nodal sensors and two other gradiometer-styled subarray nodal sensors. These subarrays were embedded in the 13 km aperture nodal array that was also fielded during the 2016 IRIS experiment. The array cal-ibration method we used in this study is based on the premise that a common wavefield should be recorded over a small-aperture array using teleseismic observation. In situ esti-mates of ACF for the gradiometer vary by 2.3% (standard deviation) for the vertical com-ponents and, typically, variability is less than 4.3% for the horizontal components; associated OCFs generally dispersed by 3°. For the two subarrays, the vertical-component ACF usually vary up to 2.4%; their horizontal-component ACFs largely spread up to 3.6%. OCFs for the subarrays generally disperse by 6.5°. ACF and OCF estimates for the gradiom-eter are seen to be stable across frequency bands having high signal coherence and/or sig- nal-to-noise ratio. Gradiometry analyses of calibrated and uncalibrated gradiometer records from a local event revealed notable improvements in accuracy of attributes obtained from analyzing the calibrated horizontal-component waveforms in the light of catalog epicenter- derived azimuth. The improved waveform relative amplitudes after calibration, coupled with the enhanced wave attribute accuracy, suggests that instrument calibration for ampli-tude statics and orientation errors should be encouraged prior to doing gradiometry analysis in future studies.

Publication Title

Bulletin of the Seismological Society of America

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