A computer-implemented method for inversion of measured geophysical data to determine a physical properties model for a subsurface region, comprising: (a) obtaining a group of two or more encoded gathers (60, 70; 260, 270) of the measured geophysical data (40, 50; 240, 250), wherein each gather is associated with a single generalized source or, using source-receiver reciprocity, with a single receiver, and wherein each gather is encoded with a different encoding function selected from a set of non-equivalent encoding functions (30; 230); (b) summing the encoded gathers in the group (80, 90; 280, 290) by summing all data records in each gather that correspond to a single receiver or, using reciprocity, to a single source, and repeating for each different receiver, resulting in a simultaneous encoded gather; (c) assuming a physical properties model (10; 210) of the subsurface region, said model providing values of at least one physical property at locations throughout the subsurface region; (d) inverting the measured geophysical data, one simultaneous encoded gather at a time, using the assumed physical properties model as an initial model, and iteratively updating (110; 310) said model to minimize a cost function measuring degree of misfit between model-simulated data and the measured geophysical data to generate an updated physical properties model (20; 220), wherein model adjustments are made using a gradient (100, 300) of the cost function with respect to at least one model parameter, which gradient is computed from a time integration of a product of encoded simultaneous-source data simulated forward in time and encoded simultaneous-source data simulated backward in time; and (e) downloading the updated physical properties model or saving it to computer storage, wherein the method further comprises using a different set of non-equivalent encoding functions (30, 230) for at least one iteration of the iterative updating of the model. (a) obtaining a group of two or more encoded gathers (60, 70; 260, 270) of the measured geophysical data (40, 50; 240, 250), wherein each gather is associated with a single generalized source or, using source-receiver reciprocity, with a single receiver, and wherein each gather is encoded with a different encoding function selected from a set of non-equivalent encoding functions (30; 230); (b) summing the encoded gathers in the group (80, 90; 280, 290) by summing all data records in each gather that correspond to a single receiver or, using reciprocity, to a single source, and repeating for each different receiver, resulting in a simultaneous encoded gather; (c) assuming a physical properties model (10; 210) of the subsurface region, said model providing values of at least one physical property at locations throughout the subsurface region; (d) inverting the measured geophysical data, one simultaneous encoded gather at a time, using the assumed physical properties model as an initial model, and iteratively updating (110; 310) said model to minimize a cost function measuring degree of misfit between model-simulated data and the measured geophysical data to generate an updated physical properties model (20; 220), wherein model adjustments are made using a gradient (100, 300) of the cost function with respect to at least one model parameter, which gradient is computed from a time integration of a product of encoded simultaneous-source data simulated forward in time and encoded simultaneous-source data simulated backward in time; and (e) downloading the updated physical properties model or saving it to computer storage, wherein the method further comprises using a different set of non-equivalent encoding functions (30, 230) for at least one iteration of the iterative updating of the model.