A method of measuring seismic signals generated by marine acoustic sources, the method comprising: towing with a marine vessel one or more streamers connected to one or more seismic sources and seismic receivers; deploying at least one seismic sensor unit from a surface of a body of water to a location on a floor of the body of water within a survey area, the at least one seismic sensor unit including a seismic sensor and a recording device configured to receive and store seismic data; recording, by the recording device, seismic signals detected by the seismic sensor over a selected time period, the seismic signals including a real measured far field signature generated by the said one or more seismic sources during a marine seismic survey; performing the marine seismic survey, the marine seismic survey including generating a seismic source signal by the said one or more seismic sources at a plurality of locations above one or more shot points in the survey area and receiving a return signal reflected from an earth formation at the said one or more seismic receivers located above the survey area; identifying the far field signature of the said one or more seismic sources for each of the plurality of locations based on the seismic signals detected by the seismic sensor; processing seismic data collected from the return signal, the processing including designaturing the seismic data based on the far field signature to appropriately relate the far field signature to the correct shot point and time; the method being characterised by automatically actuating a retrieval device disposed at the at least one seismic sensor unit to cause the at least one seismic sensor unit to rise to the surface, in response to one of: expiration of the selected time period and an actuation signal from the surface; and retrieving the at least one seismic sensor unit from the surface; wherein the at least one seismic sensor unit includes an actuator configured to automatically actuate the recording device to record the seismic signals in response to the at least one seismic sensor unit coming in contact with the floor; the at least one seismic sensor unit includes a plurality of seismic sensor units mounted on a cable attached to a float so as to be deployed at multiple depths in the body of water above the location; and recording includes recording the far field signature at the multiple depths. towing with a marine vessel one or more streamers connected to one or more seismic sources and seismic receivers; deploying at least one seismic sensor unit from a surface of a body of water to a location on a floor of the body of water within a survey area, the at least one seismic sensor unit including a seismic sensor and a recording device configured to receive and store seismic data; recording, by the recording device, seismic signals detected by the seismic sensor over a selected time period, the seismic signals including a real measured far field signature generated by the said one or more seismic sources during a marine seismic survey; performing the marine seismic survey, the marine seismic survey including generating a seismic source signal by the said one or more seismic sources at a plurality of locations above one or more shot points in the survey area and receiving a return signal reflected from an earth formation at the said one or more seismic receivers located above the survey area; identifying the far field signature of the said one or more seismic sources for each of the plurality of locations based on the seismic signals detected by the seismic sensor; processing seismic data collected from the return signal, the processing including designaturing the seismic data based on the far field signature to appropriately relate the far field signature to the correct shot point and time; the method being characterised by automatically actuating a retrieval device disposed at the at least one seismic sensor unit to cause the at least one seismic sensor unit to rise to the surface, in response to one of: expiration of the selected time period and an actuation signal from the surface; and retrieving the at least one seismic sensor unit from the surface; wherein the at least one seismic sensor unit includes an actuator configured to automatically actuate the recording device to record the seismic signals in response to the at least one seismic sensor unit coming in contact with the floor; the at least one seismic sensor unit includes a plurality of seismic sensor units mounted on a cable attached to a float so as to be deployed at multiple depths in the body of water above the location; and recording includes recording the far field signature at the multiple depths.