Surface wave mode coupling and the validity of the path average approximation in surface waveform inversions: an empirical assessment

Datta, Arjun and Priestley, Keith F. and Roecker, Steve and Chapman, Chris H. (2017) Surface wave mode coupling and the validity of the path average approximation in surface waveform inversions: an empirical assessment. Geophysical journal international, 211 (2). pp. 1070-1098. ISSN Online ISSN 1365-246X Print ISSN 0956-540X DOI 10.1093/gji/ggx345

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We employ an empirical approach to study the phenomenon of surface wave mode conversion due to lateral heterogeneity, and, as an example, assess its impact on a specific waveform inversion methodology used for surface wave tomography. Finite difference modelling in 2-D media, using a method that allows modelling of a single surface wave mode at a time, is combined with frequency domain decomposition of the wavefield onto a basis of local mode eigenfunctions, to illuminate mode conversion as a function of frequency and heterogeneity parameters. Synthetic waveforms generated by the modelling are inverted to study the effects of mode conversion on the inversion process. For heterogeneities in the upper mantle depth range of ∼40–300 km, we find that heterogeneity strengths of about 5 per cent (with sharp lateral boundaries), or lateral boundary length scales of 10–15 times the seismic wavelength (with 10 per cent maximum strength) produce significant mode conversion at periods of 30 s and shorter. These are significant in the sense that, depending on source strength, converted mode amplitudes can be well above typical noise levels in seismology. Correspondingly, waveform inversion with higher modes reveals the inadequacy of the path average approximation at these periods, with the potential for errors as large as 7 per cent in inferred group velocities, which will translate into errors in the inverted shear-velocity structure.

Item Type: Article
Uncontrolled Keywords: 2017AREP; IA72
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
08 - Green Open Access
Journal or Publication Title: Geophysical journal international
Volume: 211
Page Range: pp. 1070-1098
Identification Number: 10.1093/gji/ggx345
Depositing User: Sarah Humbert
Date Deposited: 19 Jun 2017 09:10
Last Modified: 01 Mar 2018 14:39

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