Journal of Computational Acoustics, Vol. 9, No. 4 (2001) 1407–1416 c  IMACS JOINT 3D TRAVELTIME INVERSION OF P, S AND CONVERTED WAVES * GIULIANA ROSSI and ALDO VESNAVER Istituto Nazionale di Oceanografia e di Geofisica Sperimentale — OGS, Borgo Grotta Gigante 42/c, 34010 Sgonico Trieste, Italy Received 18 June 1999 Revised 22 January 2001 Converted waves can play a basic role in the traveltime inversion of seismic waves. The sought velocity fields of P and S waves are almost decoupled, when considering pure P and S arrivals: their only connection are the possible common reflecting interfaces in the Earth. Converted waves provide new equations in the linear system to be inverted, which directly relates the two velocity fields. Since the new equations do not introduce additional unknowns, they increase the system rank or its redundancy, so making its solutions better constrained and robust. 1. Introduction Seismic tomography can reconstruct a macromodel for the Earth in depth, i.e. the major reflecting interfaces between the geological layers and the spatial variations of the veloc- ity of P and S waves. This information is very important for oil and gas exploration and production because it is related to the distribution of fluids within the Earth. Thus, it al- lows locating new hydrocarbon reservoirs and optimizing the production of the ones already known. Recently, Basick, 1 Jones and Gaiser 2 and Gaiser 3 proved that P and S velocities obtained by tomography from converted waves can enhance the prestack depth images for complex salt bodies and subbasalt formations. In a pioneering paper, Sena and Toks¨ oz 4 studied the feasibility of detecting anisotropy in elastic media and allowing for it in the Kirchhoff migration. A basic problem when dealing with different wave types in an elas- tic medium is their recognition and separation. When just one scalar field is recorded, P and S wave can be separated by exploiting their different velocities and amplitude versus offset (AVO) behavior. For converted waves, however, these differences may be too small and do not allow for a reliable interpretation: thus, a recording with all three components of the wave motion can be necessary in some cases. In this way, we can detect the wave polarization at the receiver and reduce significantly the interpretation ambiguities. The use of three-component seismograms triplicates the input data size: however, we do not get a proportional benefit unless we invert all available wave types jointly. In fact, the inversion * Presented at ICTCA’99, the 4th International Conference on Theoretical and Computational Acoustics, May 1999, Trieste, Italy. 1407 J. Comp. Acous. 2001.09:1407-1416. Downloaded from www.worldscientific.com by NATIONAL INSTITUTE OF OCEANOGRAPHY & EXPERIMENTAL GEOPHYSICS on 11/05/12. For personal use only.