BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Talks.cam//talks.cam.ac.uk// X-WR-CALNAME:Talks.cam BEGIN:VEVENT SUMMARY:Quantitative passive imaging by iterated back propagation with app lications to helioseismic holography - Thorsten Hohage (Georg-August-Unive rsität Göttingen) DTSTART:20230619T103000Z DTEND:20230619T113000Z UID:TALK200404@talks.cam.ac.uk DESCRIPTION:Passive imaging problems\, arising in seismology\, ocean acous tics\, ultrasonics\, and local helioseismology\, among others\, consist in finding coefficients of wave equations from correlation data of measured randomly excited waves. Most of the methods used in this field so far only yield qualitative results\, but no quantitative information. The present study has mainly been motivated by helioseismic holography\, a powerful me thod for feature detection in the interior of at far-side of the Sun\, whi ch is successfully used in space weather prediction\, among others. In hel ioseismic holography\, as for many other methods\, improvements are mainly achieved by clever choices of the backward propagators. In this work we s how that commonly used back-propagators can be interpreted as the adjoints of Fré\;chet derivative of forward operators mapping unknown interi or coefficients to covariance operators of wave fields on the surface. Bas ed on this interpretation\, we can achieve improvements and convergence fo r nonlinear inverse problems by iteration rather than improved back-propag ation\, and in particular turn helioseismic holography into a quantitative imaging method. Our approach uses the full information contained in cross -correlation data implicitly without the need of pre-computing cross-corre lations explicitly\, which is typically infeasible due to the high dimensi onality. We show results of the proposed method for the estimation of meri dional flows and differential rotation of the Sun.Finally\, we show unique ness results for some passive imaging problems in the Sun. \; LOCATION:Seminar Room 1\, Newton Institute END:VEVENT END:VCALENDAR