Anisotropic P-wave travel-time tomography implementing Thomsen's weak approximation in TOMO3D

Adrià Meléndez,Clara Estela Jiménez,Valentí Sallarès,César R Ranero

doi:10.5194/se-10-1857-2019

Abstract

Abstract. We present the implementation of Thomsen's weak anisotropy approximation for vertical transverse isotropy (VTI) media within TOMO3D, our code for 2-D and 3-D joint refraction and reflection travel-time tomographic inversion. In addition to the inversion of seismic P-wave velocity and reflector depth, the code can now retrieve models of Thomsen's parameters (δ and ε). Here, we test this new implementation following four different strategies on a canonical synthetic experiment in ideal conditions with the purpose of estimating the maximum capabilities and potential weak points of our modeling tool and strategies. First, we study the sensitivity of travel times to the presence of a 25 % anomaly in each of the parameters. Next, we invert for two combinations of parameters (v, δ, ε and v, δ, v⊥), following two inversion strategies, simultaneous and sequential, and compare the results to study their performance and discuss their advantages and disadvantages. Simultaneous inversion is the preferred strategy and the parameter combination (v, δ, ε) produces the best overall results. The only advantage of the parameter combination (v, δ, v⊥) is a better recovery of the magnitude of v. In each case, we derive the fourth parameter from the equation relating ε, v⊥ and v. Recovery of v, ε and v⊥ is satisfactory, whereas δ proves to be impossible to recover even in the most favorable scenario. However, this does not hinder the recovery of the other parameters, and we show that it is still possible to obtain a rough approximation of the δ distribution in the medium by sampling a reasonable range of homogeneous initial δ models and averaging the final δ models that are satisfactory in terms of data fit.

Highlights

An isotropic velocity field is the rare exception in the Earth subsurface
A homogeneous model of δ = 0 was fixed throughout the inversions. These tests were unsuccessful, with noticeably poorer results than when considering a dependence on δ (Table 8). They were useful in proving that even if a detailed δ model is not necessary to successfully retrieve the other parameters, at least a rough approximation of the δ field is needed to recover the other parameters, e.g., the background δ model that we used as the initial model in inversions displayed in Figs. 5 and 6
We tested whether it would be possible to obtain at least this rough approximation of δ in the medium, valid in the sense that it allows for the successful recovery of the rest www.solid-earth.net/10/1857/2019/

Summary

Introduction

An isotropic velocity field is the rare exception in the Earth subsurface. Anisotropy is a multiscale phenomenon, and its causes are diverse. A homogeneous model of δ = 0 was fixed throughout the inversions These tests were unsuccessful, with noticeably poorer results than when considering a dependence on δ (Table 8). The narrowing of the initial δ distribution to a smaller subrange of mean δ values for the inverted models is indicative of a good general convergence trend

Objectives

Findings

Discussion

Conclusion