Method for obtaining estimates of a model parameter so as to characterise the evolution of a subsurface volume over a time period

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.