A method for inferring a subsurface model of velocity or other physical property, comprising: obtaining seismic data acquired in a survey by advancing a front source and a trailing rear source along a source line with a non-fixed spread of receivers, and activating each source at selected intervals along the source line, wherein every activation location for the rear source falls on a previous activation location for the front source, and wherein a selected field encoding is used for all source shots with the field encoding for the rear source always being the same as was previously applied to the front source at the same activation location, resulting in a series of shot records each simultaneously recording both sources but with each shot recorded by less than a full spread of receivers; and processing the shot records by steps comprising: (i) constructing from the shot records a plurality of pseudo super-shot records, constructed such that each has data from a full spread of receivers, wherein each pseudo super-shot record includes a plurality of source locations; (ii) encoding each pseudo super-shot record and stacking to form a simultaneous-source record of measured data; (iii) using a computer to simulate the simultaneous-source record of measured data, using the same encoding used in (ii) and also the field encoding, and using an assumed subsurface model of velocity or other physical property; and (iv) comparing the simulated simultaneous-source record with the simultaneous-source record of measured data, and determining from that an adjustment to the subsurface model of velocity or other physical property. obtaining seismic data acquired in a survey by advancing a front source and a trailing rear source along a source line with a non-fixed spread of receivers, and activating each source at selected intervals along the source line, wherein every activation location for the rear source falls on a previous activation location for the front source, and wherein a selected field encoding is used for all source shots with the field encoding for the rear source always being the same as was previously applied to the front source at the same activation location, resulting in a series of shot records each simultaneously recording both sources but with each shot recorded by less than a full spread of receivers; and processing the shot records by steps comprising: (i) constructing from the shot records a plurality of pseudo super-shot records, constructed such that each has data from a full spread of receivers, wherein each pseudo super-shot record includes a plurality of source locations; (ii) encoding each pseudo super-shot record and stacking to form a simultaneous-source record of measured data; (iii) using a computer to simulate the simultaneous-source record of measured data, using the same encoding used in (ii) and also the field encoding, and using an assumed subsurface model of velocity or other physical property; and (iv) comparing the simulated simultaneous-source record with the simultaneous-source record of measured data, and determining from that an adjustment to the subsurface model of velocity or other physical property. (i) constructing from the shot records a plurality of pseudo super-shot records, constructed such that each has data from a full spread of receivers, wherein each pseudo super-shot record includes a plurality of source locations; (ii) encoding each pseudo super-shot record and stacking to form a simultaneous-source record of measured data; (iii) using a computer to simulate the simultaneous-source record of measured data, using the same encoding used in (ii) and also the field encoding, and using an assumed subsurface model of velocity or other physical property; and (iv) comparing the simulated simultaneous-source record with the simultaneous-source record of measured data, and determining from that an adjustment to the subsurface model of velocity or other physical property.