A method for characterising the evolution of a subsurface volume over time, said 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, said first seismic data corresponding to a first time, at least some of said 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, said second seismic data corresponding to a second time, at least some of said second seismic data being obtained with a non-zero offset; performing a ray tracing operation to determine ray paths which approximate the paths taken by each received seismic signal between its transmission and reception, said determined ray paths including those having said non-zero offsets; performing an inversion to obtain estimates of changes having occurred between said first time and said second time in terms of at least one model parameter; wherein for said inversion: said first seismic data and said second seismic data is not processed to be equivalent to zero-offset data prior to said inversion; it is assumed that the ray paths determined in the ray tracing operation are the same for said first survey and said second survey; and an objective function is used which is not dependent on the distance travelled by the received seismic signal. 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, said first seismic data corresponding to a first time, at least some of said 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, said second seismic data corresponding to a second time, at least some of said second seismic data being obtained with a non-zero offset; performing a ray tracing operation to determine ray paths which approximate the paths taken by each received seismic signal between its transmission and reception, said determined ray paths including those having said non-zero offsets; performing an inversion to obtain estimates of changes having occurred between said first time and said second time in terms of at least one model parameter; wherein for said inversion: said first seismic data and said second seismic data is not processed to be equivalent to zero-offset data prior to said inversion; it is assumed that the ray paths determined in the ray tracing operation are the same for said first survey and said second survey; and an objective function is used which is not dependent on the distance travelled by the received seismic signal. said first seismic data and said second seismic data is not processed to be equivalent to zero-offset data prior to said inversion; it is assumed that the ray paths determined in the ray tracing operation are the same for said first survey and said second survey; and an objective function is used which is not dependent on the distance travelled by the received seismic signal. <EOS>