Method for seismic acquisition and processing
A simultaneous sources seismic acquisition method is described that introduces notch diversity to improve separating the unknown contributions of one or more sources from a commonly acquired set of wavefield signals while still allowing for optimal reconstruction properties in certain diamond-shaped regions. In particular, notch diversity is obtained by heteroscale encoding.
1. A wavefield acquisition and processing method, comprising: obtaining wavefield recordings based on activation of at least two sources along one or more activation lines or grids while varying the source activation times of the sources such that at least one shifted replica of a related version of the wavefield of at least one of the sources appears in the frequency- wavenumber domain; separating a contribution of at least one of the at least two sources to the obtained wavefield recordings as generated by the at least two sources individually in absence of other sources; generating subsurface representations of structures or Earth media properties using the separated contribution of the at least one of the at least two sources; and outputting the generated subsurface representations, wherein the varying of the source activation times of the at least two sources comprises time shifts that vary from one source activation location to another source activation location and that are representable as a product of at least two separate source activation time functions corresponding to variations in the activation times on at least two different length scales along the one or more activation lines or grids.
2. The method of claim 1 , wherein the varying of the source activation times on at least two different length scales are chosen before or during the acquisition.
3. The methods of claim 1 , wherein the at least one shifted replica of the related version of the wavefield of the at least one of the sources is a wavefield with limited, cone-shaped support.
4. The method of claim 1 , wherein the varying of the source activation times on the at least two different length scales comprises variations on a first scale including a periodic part, and variations on a second scale that introduce further notch diversity with avoidance of non-diverse notches associated with the periodic part.
5. The method of claim 4 , wherein the variations on the first scale give rise to substantially non-overlapping diamond-shaped regions in the frequency-wavenumber domain and the variations on the second scale are chosen to be slow as a function of space such that the corresponding separate source activation time functions have relatively small numerical support in a spatial transform domain.
6. The method of claim 1 , wherein the variations in the source activation times on the at least two different length scales enables computation of a stable inverse matrix, which is required in the separation of the contribution.
7. The method of claim 1 , wherein the variations in the source activation times on the at least two different length scales enables changing, away from zero, a determinant of a matrix whose inverse is required in the separation of the contribution.
8. The method of claim 1 , wherein the step of separating the contribution is carried out in the frequency-space domain.
9. The method of claim 5 , wherein the variations on the second scale are smooth enough so that convolutions they cause in the frequency-wavenumber domain do not substantially change the non-overlapping diamond-shaped regions in the frequency-wavenumber associated with varying the source activation times only on a single first scale.
10. The method of claim 1 , wherein the obtained wavefield recordings for multilple receivers are processed jointly in a multi-dimensional or higher-dimensional space where aliasing ambiguity is reduced.
11. The method of claim 1 , wherein the separating step further comprises seismic data processing steps including at least one of imaging, migration, and demultiple for which dealiasing allows for better data quality.
12. The method of claim 1 , wherein the obtained wavefield recordings comprise multiple components.
13. The method of claim 12 , wherein one or more of the multiple components have a contribution separated independently, or two or more of the multiple components have a contribution separated jointly.
14. The method of claim 12 , wherein one or more of the multiple components are combined before one or more products of the combination have a contribution separated.
15. The method of claim 1 applied to land seismic data, marine seismic data, seabed seismic data, permanent monitoring seismic data, time-lapse seismic data, transition zone seismic data or borehole seismic data with near surface or downhole placed receivers, and sources such as VSP, 3D VSP, or distributed acoustic sensing seismic data.
16. The method of claim 1 , wherein one of the at least two different length scales corresponds to two, three, four, five, six, seven, or eight times a distance between source activation locations along one or more directions of the activation lines or grids, or a representative average of distances between source activation locations.
17. The method of claim 16 , wherein a second or higher length scale of the at least two different length scales corresponds to N times a distance between source activation locations along the one or more directions of the activation lines or grids, or a representative average of distances between source activation locations, and wherein N is an integer larger than a total number of source activation points along one or more directions of the source activation lines or grids.
18. The method of claim 1 , wherein the variations in the activation times on the at least two different length scales along the one or more activation lines or grids result in the source activation times being non-periodic.