Print Email Facebook Twitter Surface-wave inversion for a P-velocity profile with a constant depth gradient of the squared slowness Title Surface-wave inversion for a P-velocity profile with a constant depth gradient of the squared slowness Author Ponomarenko, A. V. (St. Petersburg State University) Kashtan, B. M. (St. Petersburg State University) Troyan, V. N. (St. Petersburg State University) Mulder, W.A. (TU Delft Applied Geophysics and Petrophysics; Shell Global Solutions International B.V.) Date 2017-07-01 Abstract Surface waves are often used to estimate a near-surface shear-velocity profile. The inverse problem is solved for the locally one-dimensional problem of a set of homogeneous horizontal elastic layers. The result is a set of shear velocities, one for each layer. To obtain a P-wave velocity profile, the P-guided waves should be included in the inversion scheme. As an alternative to a multi-layered model, we consider a simple smooth acoustic constant-density velocity model, which has a negative constant vertical depth gradient of the squared P-wave slowness and is bounded by a free surface at the top and a homogeneous half-space at the bottom. The exact solution involves Airy functions and provides an analytical expression for the dispersion equation. If the v_{s}/v_{p} ratio is sufficiently small, the dispersion curves can be picked from the seismic data and inverted for the continuous P-wave velocity profile. The potential advantages of our model are its low computational cost and the fact that the result can serve as a smooth starting model for full-waveform inversion. For the latter, a smooth initial model is often preferred over a rough one. We test the inversion approach on synthetic elastic data computed for a single-layer P-wave model and on field data, both with a small v_{s}/v_{p} ratio. We find that a single-layer model can recover either the shallow or deeper part of the profile but not both, when compared with the result of a multi-layer inversion that we use as a reference. An extension of our analytic model to two layers above a homogeneous half-space, each with a constant vertical gradient of the squared P-wave slowness and connected in a continuous manner, improves the fit of the picked dispersion curves. The resulting profile resembles a smooth approximation of the multi-layered one but contains, of course, less detail. As it turns out, our method does not degrade as gracefully as, for instance, diving-wave tomography, and we can only hope to fit a subset of the dispersion curves. Therefore, the applicability of the method is limited to cases where the v_{s}/v_{p} ratio is small and the profile is sufficiently simple. A further extension of the two-layer model to more layers, each with a constant depth gradient of the squared slowness, might improve the fit of the modal structure but at an increased cost. Subject P-wave velocity profileSquared-slowness gradientSurface-wave inversion To reference this document use: http://resolver.tudelft.nl/uuid:88951b0d-8b6a-421d-a232-914a620a847e DOI https://doi.org/10.1111/1365-2478.12450 Embargo date 2018-07-31 ISSN 0016-8025 Source Geophysical Prospecting, 65 (4), 941-955 Part of collection Institutional Repository Document type journal article Rights © 2017 A. V. Ponomarenko, B. M. Kashtan, V. N. Troyan, W.A. Mulder Files PDF GP2015_2032nR2c.pdf 2.55 MB Close viewer /islandora/object/uuid:88951b0d-8b6a-421d-a232-914a620a847e/datastream/OBJ/view