Full-waveform analysis of core-mantle boundary structure using adjoint methods
Authors: Borgeaud Anselme F.E. , Koroni Maria , Deschamps Frédéric 
Published: March 2020

&lt;div&gt;This study presents a new approach for investigating the structure of the core-mantle boundary (CMB)&lt;/div&gt;&lt;div&gt;topography based on full-waveforms and adjoint methods. We compute intermediate period (10-20 seconds)&lt;/div&gt;&lt;div&gt;spectral-element seismograms using existing models of core-mantle boundary topography and we analyse the&lt;/div&gt;&lt;div&gt;sensitivity of relevant seismic phases. Our study adds new information about effects of CMB structure on&lt;/div&gt;&lt;div&gt;exact synthetics and observable traveltimes of seismic body waves by means of sensitivity kernels. It also&lt;/div&gt;&lt;div&gt;highlights the difficulty of imaging the boundary due to the strong trade-off between velocity and topography&lt;/div&gt;&lt;div&gt;variations, addressed by many previous investigators.&lt;/div&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;div&gt;Given the significance of CMB and its importance for many disciplines in geophysical research, there have&lt;/div&gt;&lt;div&gt;been many studies trying to understand and geographically map the variations of topography and velocity&lt;/div&gt;&lt;div&gt;above this boundary. However, the vast mantle area wherein seismic waves travel before and after they&lt;/div&gt;&lt;div&gt;interact with the CMB makes the identification of desired seismic phases somehow difficult. In addition, the&lt;/div&gt;&lt;div&gt;observable traveltimes can be hard to interpret as a result of the boundary&amp;#8217;s topography only, due to the&lt;/div&gt;&lt;div&gt;approximate inverse methods and limited modelling methodologies. Despite considerable progress made the&lt;/div&gt;&lt;div&gt;past years, there is still a necessity for improving the understanding of effects of core-mantle boundary and&lt;/div&gt;&lt;div&gt;D&amp;#8243; structure on recorded waveforms.&lt;/div&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;div&gt;For our analyses, we perform comparisons between time shifts due to topography made on full-waveform&lt;/div&gt;&lt;div&gt;synthetics to ray theoretical predictions in order to assess methods usually deployed for imaging CMB.&lt;/div&gt;&lt;div&gt;Then, we calculate the corresponding sensitivity kernel for time windows around the theoretical arrival of&lt;/div&gt;&lt;div&gt;each phase. We focus on diffracted, core reflected and refracted &lt;em&gt;P&lt;/em&gt; and &lt;em&gt;S&lt;/em&gt; waves. The sensitivity kernels&lt;/div&gt;&lt;div&gt;depict the finite-frequency nature of these waves and possible contributions from other phases unpredictable&lt;/div&gt;&lt;div&gt;by ray theory. Results show that for most phases ray theory performs acceptably with some accuracy loss,&lt;/div&gt;&lt;div&gt;however comparisons of the effect of velocity variations to topography on traveltimes are discouraging due&lt;/div&gt;&lt;div&gt;to the low sensitivity to the latter.&lt;/div&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;div&gt;The conclusions drawn by our traveltime and sensitivity analyses are twofold. Firstly, using spectral-&lt;/div&gt;&lt;div&gt;element waveforms, the seismic phases which are frequently found in literature can be thoroughly investigated&lt;/div&gt;&lt;div&gt;and better understood, since their traveltime sensitivity through mantle and core is explicitly shown. The&lt;/div&gt;&lt;div&gt;full-waveform analysis allows us to assess the usability of phases which are informative for core-mantle&lt;/div&gt;&lt;div&gt;boundary structure and its topography. Secondly, we propose that using the analysed phases simultaneously&lt;/div&gt;&lt;div&gt;in a full-waveform inversion scheme will improve imaging of the CMB, while also allowing to jointly invert&lt;/div&gt;&lt;div&gt;for velocity variations along the D&amp;#8243; layer, which is generally poorly understood. From this study, we want&lt;/div&gt;&lt;div&gt;to promote advanced techniques of full-waveform inversion for improving CMB and lower mantle models.&lt;/div&gt;