A comparison of inversion schemes 1 - 3

This section compares the results of using different subsets of the data (mainly refractions or all the data or refractions then reflections) to determine the optimum inversion scheme.

Figure 5.32 shows a randomly selected shot gather with maximum offsets of 1 km. Figure 5.32b shows the equivalent shot gather from the field data. It comprises both reflections and refractions, with the latter dominating at the early times. The dashed red line on this gather represents the position of the bottom mute used for some of the inversions. Figure 5.32a shows the equivalent gather form the synthetic data generated using the starting model. The dataset comprises mainly transmitted arrivals. The kinematics of the transmitted arrivals is fairly accurate, with the start data and the field data within half a cycle for most of the phases present. The starting data though does not comprise any reasonable reflected arrivals. This starting model is deemed sufficiently accurate for the inversion using mainly transmitted arrivals.

Figure 5.33 shows the equivalent synthetic data generated using the FWI models obtained for the three scenarios with different subsets of the data. Figure 5.33a shows the synthetic FWI data obtained if the inversions used a conventional scheme with mainly transmitted arrivals. The dashed red line represents the bottom mute applied to the field data. The inversion has changed the kinematics of the refracted arrivals such that the arrival times are closer to that of the field data for offsets ranging from 500 to 1000 m. However there are no significant improvements in the refraction data for offsets less than 500 m. The reflected arrivals below the mute are not recovered as expected, as these arrivals were not incorporated into the inversions.

Figure 5.33b shows the equivalent synthetic FWI data obtained if a windowed technique was used such that the early iterations were biased to mainly transmitted arrivals, and the later iterations incorporated both transmissions and reflections. The dashed red line represents the mute applied to window the field data for early iterations. The FWI has improved the kinematics of transmitted arrivals at all offsets (up to 1000 m), such that the match to the field data is improved in comparison to the FWI scenario that favoured the transmitted arrivals. Additionally, reflected arrivals not present in the starting data were introduced into the FWI data. These FWI reflections are extremely well correlated with the reflections in the field data. For any given trace the kinematics of all phase are almost perfectly matched to that of the field data. The amplitudes though for a give trace and phase are not identical for the two datasets. But the amplitude-offset relationship for a given reflection is very similar to that of the field data.

Figure 5.33c shows the equivalent synthetic FWI data obtained if all arrivals were incorporated into the FWI from the start. The kinematics of the transmitted arrivals at offsets greater than 500 m is improved, but not as significantly as the FWI scenario that were biased towards the transmitted

arrivals. The kinematics of the transmitted arrivals at offsets less than 500 m show no significant improvements. Reflected arrivals not present in the starting data are also recovered by the FWI.

Using the opening window technique, both the reflections and refractions (Figure 5.33b) best correlate with the field data in comparison to the other two scenarios suggesting that this technique is the most effective. Using all the data for the inversion, there appears to be a tradeoff between the improvement of the transmitted and the reflected arrivals. Using mainly refracted arrivals the inversion improves these arrivals.

Figure 5.34 shows a horizontal depth slice at 125 m below sea surface, through the final FWI velocity models for the inversion scenarios using different subsets of the data. Figure 5.34a shows the FWI model recovered from using refractions then reflections, and Figure 5.34b shows the FWI results using all the data. Figure 5.16 shows the equivalent FWI model with mainly refracted arrivals. The starting model at this depth was smooth (Figure 5.8b). The recovered FWI models are similar for all 3 tests. The background velocity is updated to values of approximately 1700 m/s, but highest when using refractions only, lowest when using all the data from the start of the inversions. The most prominent feature for the three scenarios is the high-velocity meandering channels. The velocities of these channels though are quite varied. Focusing mainly on the refracted arrivals, the velocities reach as high as 2056 m/s, while using a windowed technique from refractions to reflections, the velocities reach a maximum of 1926 m/s. Also visible on Figure 5.34 and Figure 5.16 is a series of low-velocity channels that are equally well-resolved as the high-velocity channels, but less visible as the velocities are very similar to that of the background velocity. Similarly these channels have the lowest velocities for the inversions that focused mainly on the refracted arrivals. The gross structure and fine details of the channels are almost identical for the three FWI scenarios.

f

Figure 5.32 Randomly selected shot record through: (a) the synthetic data generated using the starting model and (b) the field data. The dashed red line represents bottom mute applied to preprocess the field data. The shot gathers are equivalent to the gathers in Figure 5.33.

Figure 5.33 Randomly selected shot record through the synthetic data generated using the different FWI model obtained from using: (a) mainly refractions; (b) refractions followed by reflections; and (c) all the data (refractions and reflections). The dashed red line represents bottom mute applied to preprocess the field data. The shot gathers are equivalent to the gathers in Figure 5.32.

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(b) FWI data with refractions then reflections

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(c) FWI data with all arrivals

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Figure 5.34 Horizontal depth slice at 125 m through the different FWI model obtained using: (a) inversion scheme 3;

and (b) inversion scheme 2. These slices are coincident with the slices in Figure 5.16 and Figure 5.8.

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