Generating a velocity model for a subsurface structure using refraction travel time tomography

1. A computer-implemented method for determining velocity model for a subsurface structure, comprising:

generating, by at least one hardware processor, a first velocity model for the subsurface structure by performing a refraction traveltime tomography procedure based on an initial velocity model;

generating, by the at least one hardware processor, a first refined velocity model based on the first velocity model and a time-domain structure skeleton model, wherein the time-domain structure skeleton model is determined based on reflection seismic data of the subsurface structure, and wherein the generating first refined velocity model based on the first velocity model and the time-domain structure skeleton model comprises:

dividing the subsurface structure into a plurality of regions based on the time-domain structure skeleton model; and

for each region that is divided based on the time-domain structure skeleton model, replacing velocity values in the first velocity model with an average velocity value of a respective region; and

generating an image of the subsurface structure based on the first refined velocity model.

2. The method of claim 1 , wherein the first velocity model and the first refined velocity model are generated at a first iteration, the method further comprising:

repeating the generations of the first velocity model and the first refined velocity model in a plurality of iterations until a stopping criterion is determined to be met, wherein at each iteration, the refined first velocity model generated at a previous iteration is used as the initial velocity model to generate the first velocity model at a current iteration.

3. The method of claim 2 , wherein the stopping criterion is determined to be met based on a comparison between the first velocity models generated at consecutive iterations.

4. The method of claim 1 , wherein generating the first refined velocity model comprises:

converting the first velocity model into a first time domain velocity model;

generating a first refined time domain velocity model based on the time-domain structure skeleton model; and

converting the first refined time domain velocity model into a depth domain to generate the first refined velocity model.

5. The method of claim 1 , wherein performing the refraction traveltime tomography procedure comprises:

calculating a weighting factor based on an angle between a ray segment and a vertical direction; and

calculating a gradient based on the weighting factor.

6. The method of claim 1 , wherein the initial velocity model is a homogenous velocity model having a same velocity value throughout the subsurface structure.

7. A non-transitory computer-readable medium storing instructions which, when executed, cause a computer to perform operations comprising:

generating, by at least one hardware processor, a first velocity model for a subsurface structure by performing a refraction traveltime tomography procedure based on an initial velocity model;

generating, by the at least one hardware processor, a first refined velocity model based on the first velocity model and a time-domain structure skeleton model, wherein the time-domain structure skeleton model is determined based on reflection seismic data of the subsurface structure, and wherein the generating first refined velocity model based on the first velocity model and the time-domain structure skeleton model comprises

dividing the subsurface structure into a plurality of regions based on the time-domain structure skeleton model; and

for each region that is divided based on the time-domain structure skeleton model, replacing velocity values in the first velocity model with an average velocity value of a respective region; and

generating an image of the subsurface structure based on the first refined velocity model.

8. The non-transitory computer-readable medium of claim 7 , wherein the first velocity model and the first refined velocity model are generated at a first iteration, the operations further comprising:

repeating the generations of the first velocity model and the first refined velocity model in a plurality of iterations until a stopping criterion is determined to be met, wherein at each iteration, the refined first velocity model generated at a previous iteration is used as the initial velocity model to generate the first velocity model at a current iteration.

9. The non-transitory computer-readable medium of claim 8 , wherein the stopping criterion is determined to be met based on a comparison between the first velocity models generated at consecutive iterations.

10. The non-transitory computer-readable medium of claim 7 , wherein generating the first refined velocity model comprises:

converting the first velocity model into a first time domain velocity model;

generating a first refined time domain velocity model based on the time-domain structure skeleton model; and

converting the first refined time domain velocity model into a depth domain to generate the first refined velocity model.

11. The non-transitory computer-readable medium of claim 7 , wherein performing the refraction traveltime tomography procedure comprises:

calculating a weighting factor based on an angle between a ray segment and a vertical direction; and

calculating a gradient based on the weighting factor.

12. The non-transitory computer-readable medium of claim 7 , wherein the initial velocity model is a homogenous velocity model having a same velocity value throughout the subsurface structure.

13. A device, comprising:

at least one hardware processor; and

a non-transitory computer-readable storage medium coupled to the at least one hardware processor and storing programming instructions for execution by the at least one hardware processor, wherein the programming instructions, when executed, cause the at least one hardware processor to perform operations comprising:

generating a first velocity model for a subsurface structure by performing a refraction traveltime tomography procedure based on an initial velocity model;

generating a first refined velocity model based on the first velocity model and a time-domain structure skeleton model, wherein the time-domain structure skeleton model is determined based on reflection seismic data of the subsurface structure, and wherein the generating first refined velocity model based on the first velocity model and the time-domain structure skeleton model comprises:

dividing the subsurface structure into a plurality of regions based on the time-domain structure skeleton model; and

for each region that is divided based on the time-domain structure skeleton model, replacing velocity values in the first velocity model with an average velocity value of a respective region; and

generating an image of the subsurface structure based on the first refined velocity model.

14. The device of claim 13 , wherein the first velocity model and the first refined velocity model are generated at a first iteration, the operations further comprising:

repeating the generations of the first velocity model and the first refined velocity model in a plurality of iterations until a stopping criterion is determined to be met, wherein at each iteration, the refined first velocity model generated at a previous iteration is used as the initial velocity model to generate the first velocity model at a current iteration.

15. The device of claim 14 , wherein the stopping criterion is determined to be met based on a comparison between the first velocity models generated at consecutive iterations.

16. The device of claim 13 , wherein generating the first refined velocity model comprises:

converting the first velocity model into a first time domain velocity model;

generating a first refined time domain velocity model based on the time-domain structure skeleton model; and

converting the first refined time domain velocity model into a depth domain to generate the first refined velocity model.

17. The device of claim 13 , wherein performing the refraction traveltime tomography procedure comprises:

calculating a weighting factor based on an angle between a ray segment and a vertical direction; and

calculating a gradient based on the weighting factor.