A Blind Test of Traveltime and Waveform Inversion Colin A. Zelt 1, R. Gerhard Pratt 2, Andrew Brenders 2, Sara Hanson-Hedgecock 1 and John A. Hole 3 1 Rice University, Houston, TX, USA 2 Queen's University, Kingston, ON, Canada 3 Virginia Tech, Blacksburg, VT, USA

use traveltime inversion to estimate large- scale model (and starting model for waveform inversion) – resolution  fresnel zone use waveform inversion to estimate high- resolution model – resolution  wavelength Methodology

Comparison of traveltime and waveform inversion for crosswell synthetic data

Talk Outline 1.True model and data 2.Traveltime inversion 3.Waveform inversion

True Model

The Data 51 shots 1390 receivers 2-11 Hz 2-D viscoelastic FD code (Robertsson et al. 1994) No noise

Example shot gather

1-D starting model 1 st arrival traveltime tomography Simultaneous refl+refr traveltime inversion for lower half of model 1-D starting model Waveform tomography

Refraction and reflection traveltime tomography combination of smooth tomography and layered model to invert first arrivals and reflections

1st arrival Model 1st arrival + refl/refr layered Model

True Model Traveltime Model

Waveform tomography 2-D acoustic modeling Forward and inverse steps in frequency domain (computationally efficient) iterative least-squares minimization of data residuals (frequency components) proceed from low to high frequency gradient direction from multiplication of forward and back-propagated waveforms in frequency domain

Waveform tomography (continued) like pre-stack, reverse time migration, but formulated in terms of velocity, not reflectivity, and iterative (Born approximation) invert for source signature require “good” starting model (from traveltimes or other method of velocity analysis) reference: Pratt 1999, Geophysics, 64,

Waveform tomography (application) Invert frequencies from 0.8 to 7 Hz (wavelengths of 5-10 km to km) Window data, initially 3 s, later 6 s PC platform, Intel Pentium 4, 2.8 GHz processor, 4 GB RAM, RedHat Linux days total CPU time

True Model Starting Model

Starting Model: waveform tomography

Final Model (2 Hz): waveform tomography

Final Model (4 Hz): waveform tomography

Final Model (7 Hz): waveform tomography

True Model

Waveform Model

Traveltime Model Waveform Model

Data comparison Input data Predicted data

Conclusions Model/data available: terra.rice.edu/department/faculty/zelt/ccss Waveform tomography shows great potential Traveltime model and/or low frequency data important Future work: elastic and 3-D See EOS feature article May 3, 2005 issue Waveform references: Brenders & Pratt 2007, GJI, 168,