Method for obtaining estimates of a model parameter so as to characterise the evolution of a subsurface volume over a time period
Abstract Disclosed is a method for characterizing the evolution of a subsurface volume over time. The method comprises providing first and second surveys of the subsurface volume. Each survey comprises seismic data acquired by transmitting seismic signals into the subsurface volume and subsequently detecting some or all of the seismic signals after reflection within the subsurface. The first seismic data of the first survey corresponds to a first time and the second seismic data of the second survey corresponds to a second time. At least some of the first seismic data and the second seismic data is obtained with a non-zero offset. An inversion is performed to obtain estimates of changes having occurred between the first time and the second time in terms of at least one model parameter; wherein for the inversion: the first seismic data and the second seismic data is not processed to be equivalent to zero-offset data prior to the inversion; and it is assumed that the path taken by each received seismic signal between its transmission and reception is the same for the first survey and the second survey.
1. A method for characterising the evolution of a subsurface volume over time, the method comprising:
providing a first survey of the subsurface volume with first seismic data acquired by transmitting seismic signals into the subsurface volume and subsequently detecting some or all of the seismic signals after reflection within the subsurface, the first seismic data corresponding to a first time, at least some of the first seismic data being obtained with a non-zero offset;
providing a second survey of the subsurface volume with second seismic data acquired by transmitting seismic signals into the subsurface volume and subsequently receiving some or all of the seismic signals after reflection within the subsurface volume, the second seismic data corresponding to a second time, at least some of the second seismic data being obtained with a non-zero offset;
performing an inversion to obtain estimates of changes having occurred between the first time and the second time in terms of at least one model parameter, wherein the inversion utilizes an objective function that is not dependent on distance travelled by the received seismic signal;
wherein for the inversion:
the first seismic data and the second seismic data is not processed to be equivalent to zero-offset data prior to the inversion; and
it is assumed that ray paths determined in a ray tracing operation are the same for the first survey and the second survey;
performing the ray tracing operation to determine ray paths which approximate the paths taken by each received seismic signal between its transmission and reception, the determined ray paths comprising those having the non-zero offsets; and
using the results of the method to aid hydrocarbon recovery from a reservoir.
2. The method as claimed in claim 1 , wherein the subsurface volume is discretized into cells and the objective function comprises a forward modelling operator comprising the travel times per cell for each of the ray paths.
3. The method as claimed in claim 2 wherein the ray tracing operation is performed only once and the forward modelling operator is used for all iterations of the inversion.
4. The method as claimed in claim 1 wherein the at least one model parameter comprises propagation velocity of the seismic signal.
5. The method as claimed in claim 1 wherein the inversion comprises finding propagation velocity changes which describe observed time shifts between the first seismic data and the second seismic data.
6. The method as claimed in claim 1 wherein the at least one model parameter comprises travel time of the seismic signal and/or time strain.
7. The method as claimed in claim 1 wherein the first seismic data and the second seismic data are obtained at a plurality of different non-zero offsets.
8. The method as claimed in claim 1 wherein the at least one model parameter is an anisotropic model parameter such that the inversion estimates the changes having occurred between the first time and the second time in terms of the at least one anisotropic model parameter.
9. The method as claimed in claim 8 comprising the step of decomposing the anisotropic model parameter as a function of the angle of the path taken by each received seismic signal.
10. The method as claimed in claim 8 wherein it is assumed that the anisotropic model parameter is elliptically anisotropic.
11. The method as claimed in claim 8 , wherein the first seismic data and the second seismic data is multi-azimuth seismic data, and the inversion estimates azimuthally varying changes having occurred between the first time and the second time in terms of the at least one anisotropic model parameter.
12. The method as claimed in claim 1 wherein the first seismic data comprises a first set of seismic traces and the second seismic data comprises a second set of seismic traces.
13. The method as claimed in claim 12 further comprising:
constructing an analytic filter operable to shift at least one seismic traces in dependence of the changes in the model parameter; and
performing the inversion by using the analytic filter to align the first survey and the second survey.
14. The method as claimed in claim 13 wherein the analytic filter comprises a plane wave destruction filter, and is a time domain analogue of a frequency domain phase shift operator operable to impose a constant time shift to a plane wave model of a seismic trace.
15. The method as claimed in claim 13 wherein the analytic filter is such that the model parameter is a function of the analytic filter instead of being a function of at least one of the second set of seismic traces.
16. A computer program comprising computer readable instructions which, when run on suitable computer apparatus, cause the computer apparatus to perform the method of claim 1 .
17. A computer program carrier comprising the computer program of claim 16 .