A computer-implemented method for monitoring a reservoir in an underground formation by generating a four dimensional seismic signal based on multiple sets of seismic data representing a subterranean formation, the method comprising: receiving (110, 120) a first common transmission point, CTP, gather and a second CTP gather, the first and second CTP gathers corresponding to at least one CTP, the first CTP gather comprising a first at least one plurality of before CTP traces and a first at least one plurality of cross CTP traces, and the second CTP gather comprising a second at least one plurality of before CTP traces and a second at least one plurality of cross CTP traces, wherein a CTP gather comprises a plurality of traces caused by reflections of acoustic waves transmitting towards and through a CTP, a before CTP trace is a trace corresponding to a portion of an acoustic wave reflected before the wave crossed the CTP, a cross CTP trace is a trace corresponding to a portion of an acoustic wave reflected after the wave crossed the CTP, the first CTP gather corresponds to a baseline seismic survey conducted at a first time, and the second CTP gather corresponds to a monitor seismic survey conducted at a second time, the second time occurring after the first time; for the first CTP gather, aggregating the first at least one plurality of before CTP traces to generate a first at least one aggregate before CTP trace and aggregating the first at least one plurality of cross CTP traces to generate a first at least one aggregate cross CTP trace; for the second CTP gather, aggregating the second at least one plurality of before CTP traces to generate a second at least one aggregate before CTP trace and aggregating the second at least one plurality of cross CTP traces to generate a second at least one aggregate cross CTP trace; determining a first time difference between the first at least one aggregate before CTP trace and the second at least one aggregate before CTP trace; determining a second time difference between the first at least one aggregate cross CTP trace and the second at least one aggregate cross CTP trace; determining a third time difference based on the first time difference and the second time difference; and determining an amount of change of hydrocarbon fluid levels in the reservoir between the baseline survey and the monitor survey based at least partially on the third time difference. receiving (110, 120) a first common transmission point, CTP, gather and a second CTP gather, the first and second CTP gathers corresponding to at least one CTP, the first CTP gather comprising a first at least one plurality of before CTP traces and a first at least one plurality of cross CTP traces, and the second CTP gather comprising a second at least one plurality of before CTP traces and a second at least one plurality of cross CTP traces, wherein a CTP gather comprises a plurality of traces caused by reflections of acoustic waves transmitting towards and through a CTP, a before CTP trace is a trace corresponding to a portion of an acoustic wave reflected before the wave crossed the CTP, a cross CTP trace is a trace corresponding to a portion of an acoustic wave reflected after the wave crossed the CTP, the first CTP gather corresponds to a baseline seismic survey conducted at a first time, and the second CTP gather corresponds to a monitor seismic survey conducted at a second time, the second time occurring after the first time; a CTP gather comprises a plurality of traces caused by reflections of acoustic waves transmitting towards and through a CTP, a before CTP trace is a trace corresponding to a portion of an acoustic wave reflected before the wave crossed the CTP, a cross CTP trace is a trace corresponding to a portion of an acoustic wave reflected after the wave crossed the CTP, the first CTP gather corresponds to a baseline seismic survey conducted at a first time, and the second CTP gather corresponds to a monitor seismic survey conducted at a second time, the second time occurring after the first time; for the first CTP gather, aggregating the first at least one plurality of before CTP traces to generate a first at least one aggregate before CTP trace and aggregating the first at least one plurality of cross CTP traces to generate a first at least one aggregate cross CTP trace; for the second CTP gather, aggregating the second at least one plurality of before CTP traces to generate a second at least one aggregate before CTP trace and aggregating the second at least one plurality of cross CTP traces to generate a second at least one aggregate cross CTP trace; determining a first time difference between the first at least one aggregate before CTP trace and the second at least one aggregate before CTP trace; determining a second time difference between the first at least one aggregate cross CTP trace and the second at least one aggregate cross CTP trace; determining a third time difference based on the first time difference and the second time difference; and determining an amount of change of hydrocarbon fluid levels in the reservoir between the baseline survey and the monitor survey based at least partially on the third time difference.