Can Seismic Strain Waves Be Measured Quantitatively by Borehole Tensor Strainmeters?

Abstract

Abstract It has been confirmed by comparison with particle velocities of surface waves of wave normal perpendicular to borehole axis measured by collocated seismic broadband array or borehole seismometers that high-sensitivity borehole tensor strainmeters could measure surface strain waves qualitatively. However, quantitative measurability of strain waves of wave normal being oblique to borehole axis by borehole tensor strainmeters is still a challenging problem. In this article, we introduce a scattering model of oblique-incidence seismic waves by an empty borehole to investigate the influence of incident-wave normal and polarization on theoretical-frequency-response bandwidth of borehole tensor strainmeters. Within the theoretical bandwidth of borehole tensor strainmeters, zero-frequency gains are introduced to build quantitative relation between instrumental strain waves of borehole tensor strainmeters and oblique-incidence seismic strain waves. Better understanding the influence of incident-wave normal and polarization on the relation between instrumental strain and strain components induced by the incident strain wave shows that incident-wave normal and polarization must be assessed by other instrumentation to promise quantitative measurement of incident strain waves by borehole tensor strainmeters, which would benefit in situ calibration of borehole tensor strainmeters.