Multimodal Layered Transdimensional Inversion of Seismic Dispersion Curves With Depth Constraints

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2018-12, Vol.19 (12), p.4957-4971
• Main Authors: Killingbeck, S. F., Livermore, P. W., Booth, A. D., West, L. J.
• Format: Article
• Language: English
• Subjects: Boreholes; Depth; Dispersion; Distribution; Dynamics; Geophysical data; Glacier flow; Glaciers; Inversions; Markov chains; Modelling; Probability distribution; Probability theory; Radar; Resolution; Seismic activity; Seismic velocities; Seismic wave velocities; Seismic waves; Shear wave velocities; Solutions; Statistical methods; Surface water waves; Velocity; Wave data; Wave dispersion; Wave velocity
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2018GC008000

MuLTI (Multimodal Layered Transdimensional Inversion) is a Markov chain Monte Carlo implementation of Bayesian inversion for the probability distribution of shear wave velocity (Vs) as a function of depth. Based on Multichannel Analysis of Surface Wave methods, it requires as data (i) a Rayleigh‐wave dispersion curve and (ii) additional layer depth constraints, the latter we show significantly improve resolution compared to conventional unconstrained inversions. Such depth constraints may be drawn from any source (e.g., boreholes, complementary geophysical data) provided they also represent a seismic interface. We apply MuLTI to a Norwegian glacier, Midtdalsbreen, in which ground‐penetrating radar was used to constrain internal layers of snow, ice, and subglacial sediments, with transitions at 2 and 25.5 m, and whose Vs is assumed to be in the range 500–1,700, 1,700–1,950, and 200–2,800 m/s, respectively. Synthetic modeling demonstrates that MuLTI recovers the true model of Vs variation with depth. Significantly, compared to inversions without depth constraints, in this synthetic case MuLTI not only reduces by about a factor of 10 the error between the true and the best fitting model, but also reduces by a factor of 2 the vertically averaged spread of the distribution of Vs based on the 95% credible intervals. We further show that using frequencies above about 100 Hz lead to unconverged solutions due to mode ambiguities associated with fine spatial structures. For our acquired data on Midtdalsbreen, we use 14‐100 Hz data for which MuLTI produces a large‐scale converged inversion. Plain Language Summary Geophysical inversion is used to infer plausible subsurface features from surface measurements. However, inversions based on data sets acquired with a single geophysical technique often have poor resolution due to many different subsurface models fitting the data within the error tolerance. This study presents a novel method, Multimodal Layered Transdimensional Inversion, MuLTI, for inverting seismic surface wave data with constraints on depths of internal layers to obtain a more accurate and reliable interpretation of the subsurface. Here our depth constraints are drawn from ground‐penetrating radar horizon observations. MuLTI has been tested on an example data set from a glaciated environment to determine the seismic wave velocity of the subglacial sediment, which has important implications for glacier flow dynamics. By constraining the subsurface with gro