1. Chicxulub revealed with new seismic and gravity data Jo Morgan, Anneka Smith, Elinor Styles, Imperial College London, UK Anusha Surendra, Penny Barton, University of Cambridge, UK

2. Some observations Preliminary travel-time inversion Correlation between reflection, velocity, gravity and magnetic data Model assessment Purpose: to examine model reliability Details 2005 seismic and gravity experiment

3. New seismic data New gravity data Funded by NERC and NSF

4. Data examples Clear arrivals – small uncertainty Refraction data 3,800,000 travel-times first-arrivals for 1/3 of data picked to-date Reflection data Processing complete Reduced travel-time (secs) TWTT (secs) 0 2 0 2 Horizontal distance (~10 km) Tertiary Peak ring Dipping reflectors

5. Seismic inversion Determine a velocity model that “best-fits” the travel-times Blue dots: land stations Red are ray paths (looking from above) 80 km 65 km Anneka Smith Subset of rays into western land stations

6. Anusha Surendra Example Part of inverted velocity model from marine seismometers Green surface is 6 kms -1 velocity boundary Peak ring is visible on marine reflection profiles Topographic peak ring is directly above a low-velocity-zone at depth

7. Structures visible on marine reflection profiles show relationship to velocity gravity and magnetic data Bouguer gravity Horizontal gradient gravityMagnetic data E. Styles M. Pilkington innermost slumped block peak ring

8. Problem: Non-uniqueness A range of models will fit the data equally as well Solution: Model assessment In final model, need to distinguish between structure that is required by the data and structure that is consistent with the data Model assessment

9. “Basic” model assessment Use ray coverage to indicate areas in the model that are well constrained (dark shading)

10. “Better” model assessment: Checkerboard Testing Add checkerboard of high and low velocity anomalies to final velocity model Determine travel-times through this “checkerboard” model Use inversion to recover the checkerboard

11. Determine how well checkerboard is recovered (semblance) Identify areas of good recovery (dark) Gradually decrease size of checkerboard until it is too small to recover

12. More model assessment Test effect of starting model starting model, inversion parameters etc Test for uncertainties in parameters Run synthetics tests to investigate resolution – example below Finally get “reliable” velocity model Can identify parts of model required by data Peggy Vermeesch synthetic model central upliftinverted model (1996 data)

13. Use drill core, reliable velocity model, and other geophysical data to: Obtain well-constrained structural/lithological model Melt volume, sample stratigraphic uplift, determine nature of peak ring… ICDP/IODP workshop – future drilling of Chicxulub Postdam 11-12 September 2006 Call for participants