Calibrating Dense Spatial Arrays for Amplitude Statics and Orientation Errors

Abstract

AbstractAn iterative procedure is designed to calibrate dense arrays of three‐component seismic instruments for station amplitude statics and horizontal component orientation errors. Station amplitude statics refers to differences in sensor and digitizer gains and amplification effects due to local site and installation conditions. Amplitude statics and orientation errors can seriously affect computation of the wavefield gradient tensor that is the basis of seismic strain and rotation computations as well as analysis of wave field attributes through wave gradiometry since wave spatial gradients rely on first‐order differences of the wavefield between sensors in an array. The technique is based on the assumption of a common wave field observed over a small array using teleseismic earthquake observations. In situ calibration of the broad band, 800‐m aperture array at Pinyon Flat, California, achieves relative amplitude precision for vertical components of the array from 0.16% to 0.25% and horizontal components from 0.16% to 0.61%. Relative sensor orientations can be determined to a precision better than 0.35°. Comparison of horizontal strains computed from array observations of a regional M4.9 earthquake and strains from the Gladwin Tensor borehole strain meter at Pinyon Flat shows excellent correspondence over the frequency band of 0.05 to 2 Hz. Agreement of borehole measured strains and seismic array strains for regional/local seismic events gives confidence that a borehole strain meter and single, three‐component, colocated seismometer can be used as a “point array” for deducing wavefield attributes from wave gradiometry.