A computer-implemented method of characterising a subsurface discretised volume comprising a plurality of cells, said method comprising extracting a geobody from seismic data arranged within the discretised volume, said geobody comprising a subset of said plurality of cells, each cell of said subset having one or more properties indicative of a particular fluid phase, wherein said extracting a geobody comprises: determining a propagation probability value for each cell indicative of the probability that a front will propagate through said cell; beginning from a source within said discretised volume, using the propagation probability value to calculate a traveltime for each cell, said traveltime describing the time said front takes to travel from said source point to said cell; using said traveltimes to extract said geobody from said seismic data; and using the extracted geobody to aid hydrocarbon recovery from a reservoir by monitoring whether the volume and the geometry of the geobody corresponds to a target geobody volume and planned geometry relating to the volume of an injection fluid injected during said hydrocarbon recovery; and if not modifying an injection strategy relating to the manner of injection of the injection fluid used in said hydrocarbon recovery; wherein using said traveltimes to extract said geobody comprises: sorting said cells in accordance with their traveltime; and beginning with the cells having the shortest traveltime, cumulating the product of the cell volume and a cell volume fraction property until the target geobody volume is reached, the target geobody volume being based upon said volume of an injected fluid. determining a propagation probability value for each cell indicative of the probability that a front will propagate through said cell; beginning from a source within said discretised volume, using the propagation probability value to calculate a traveltime for each cell, said traveltime describing the time said front takes to travel from said source point to said cell; using said traveltimes to extract said geobody from said seismic data; and using the extracted geobody to aid hydrocarbon recovery from a reservoir by monitoring whether the volume and the geometry of the geobody corresponds to a target geobody volume and planned geometry relating to the volume of an injection fluid injected during said hydrocarbon recovery; and if not modifying an injection strategy relating to the manner of injection of the injection fluid used in said hydrocarbon recovery; wherein using said traveltimes to extract said geobody comprises: sorting said cells in accordance with their traveltime; and beginning with the cells having the shortest traveltime, cumulating the product of the cell volume and a cell volume fraction property until the target geobody volume is reached, the target geobody volume being based upon said volume of an injected fluid. sorting said cells in accordance with their traveltime; and beginning with the cells having the shortest traveltime, cumulating the product of the cell volume and a cell volume fraction property until the target geobody volume is reached, the target geobody volume being based upon said volume of an injected fluid. <EOS>