WEMVA limitations

The most important limitation of WEMVA is the range of validity of its assumptions. In the case of the Born approximation in TIF WEMVA, phase differences caused by the true velocity anomalies in the image must be smaller than $\pi/4$. The corresponding velocity anomaly magnitudes vary with the range of frequencies present in the dataset and with the spatial extent of the anomalies.

We performed one WEMVA pass with 10 conjugate-gradient solver iterations on the ``WEMVA breaks'' dataset. In this instance the phase differences caused by the velocity anomalies are too large to be satisfactorily approximated in a Born manner. The velocity update thus obtained is presented in Figure . It does not converge toward the true anomalies like the corresponding update in the upper panel of Figure . Since this occurs when using the ideal image perturbation, inaccuracies in extracting the perturbation would only amplify the trend.

 

f5
Figure 5 Velocity model updated after one WEMVA iteration comprising 10 solver iterations on the ``WEMVA breaks'' dataset.

There are several possible avenues for avoiding either the Born approximation limitations or the Born approximation altogether: 1. obtaining a very good velocity starting guess with other velocity analysis methods; 2. creating the image perturbation in connection with residual migration Sava and Biondi (2001); 3. employing linearizations other than Born Sava and Fomel (2002); 4. inverting for the lower frequencies first Pratt (1999); 5. using DSO WEMVA Sava and Symes (2002) where the image perturbation is extracted directly from the image using a specific operator during the inversion, without the need to find a corrected image.