I just finished reading a book by Peter Thiel, “Zero to One.” He starts the book with a question he always asks people he interviews for a job: “What important truth do very few people agree with you on?” It is a contrarian question that is related to the title of his book, he argues that while incremental innovations, making existing things better, is going from 1 to n, inventing new technology is going from 0 to 1. That made me think what would be my answer to his question, and one possible result is Smart Migrations vs. Full Waveform Inversion and Reverse Time Migration.
While a large number of researchers in our industry have joined the stampede on Full Waveform Inversion (FWI) and Reverse Time Migration (RTM), high end imaging tools requiring more and more computer resources, fewer have followed John Sherwood’s work on improving Beam Migration and Beam Tomography. I believe the combination of Fast Beam Migration and Beam Tomography can lead to high resolution velocity models similar to the ones obtained using FWI, but 100 to 1000 times faster. I will explain why in more detail later, but first some background on Smart Migrations, a class of algorithms that uses information inside the prestack data to greatly speed up the turnaround time for large 3-D seismic projects.
Smart Migrations are a new generation of depth imaging algorithms, two orders of magnitude faster than the standard Kirchhoff depth migration. These clever algorithms, combined with very fast migration velocity analysis tools, can reduce the turnaround time for large 3-D seismic projects from six to eight months down to one to two months. Great tools to have for Fast Track projects. These Smart Migrations algorithms use the information inside the prestack data for migration as opposed to standard depth migration algorithms like Kirchhoff migration or reverse time migration (RTM), which assume every image point in the subsurface is a diffractor and make no a priori assumptions about the migrated image structure. Instead, Smart Migrations analyze the input data, build a database of locally coherent events, and use these events to guide the direction of the migration operator. This knowledge of the input data can speed up the processing time by 100 to 500 times. Kirchhoff and RTM are Brute Force Migrations, at the opposite end of the computation power spectrum from Smart Migrations.
The Smart Migrations class of algorithms is a perfect response to the industry need to handle large amounts of data in short time. One early and successful commercial implementation of a Smart Migration class algorithm is the Fast Beam Migration (FBM) developed by John Sherwood at Applied Geophysical Services. The speed of FBM is achieved in two steps:
1. A factor of 10 in speedup is achieved using beam forming, or beam decomposition of the input data, where the number of input data is reduced by a factor of 10.
2. A factor of 10 to 100 in speedup is obtained by spreading each input trace or beam over a beam instead of a full aperture-volume.
The first step of the FBM decomposes the seismic data in wavelets, 100 to 200 ms in time duration. The beam formation reduces the input data volume by a factor of 10 and is an important factor in the speedup achieved with FBM. For larger reduction factors, data is not invertible; the original data cannot be fully recovered from the data beam. But at a larger data compression rate, the beams increase the signal-to-noise ratio and retain the coherent energy in the data. The migrated image appears almost free of artifacts due to this signal-to-noise enhancement and makes the image easier to interpret in complex areas.