Detection properties of phase velocities with SPAC arrays including structural boundary
Abstract
Microtremor survey method (MSM) is a technique to estimate subsurface velocity structures by inverting phase velocities of the surface waves in the microtremors. All the existing inversion techniques for the MSM have been deduced under an implicit assumption of horizontal velocity structure. Velocity structures in shallow sediment are, however, practically very inhomogeneous, and the assumption may not be suitable in many cases of the MSM applications. Therefore we have examined the behavior of estimated phase velocities with the SPAC technique when arrays intersect a structural boundary, because errors in velocity estimation in the MSM for inhomogeneous velocity cases have not been elucidated. An example of theoretical analysis is shown in Figs, where phase differences of two sensors which placed across a structural boundary have been analytically expressed and then the SPAC technique have been performed using complex coherence functions (Shiraishi et. al., 2006). Fig.1 shows the geometry of a SPAC array along with a velocity boundary. Here, we assumed different horizontal velocity model for right and left sides. Fig.2 shows estimated phase velocities for various locations of structural boundaries. This result shows that the estimated velocities gradually vary according to the location of boundaries and it seems that the phase velocities determined by the occupying rate of each structure along the length of the array. Therefore, in the case of the boundary placed inside the center of the array, the estimated velocities would be adopted intermediate value of the two different structures and the error is maximized. We have also examined these properties with three dimensional simulations and have confirmed similar features. Furthermore, we have examined some classifying measures of discontinuous structures within the array. We believe that these results will contribute to derive a new estimation technique for the MSM of covering discontinuous or gradient structures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNS43A1783S
- Keywords:
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- 0999 EXPLORATION GEOPHYSICS General or miscellaneous;
- 7299 SEISMOLOGY General or miscellaneous