A new method of alternating shooting of two different seismic sources for deep geophysical surveys

Abstract

Abstract In the latest geophysical survey crossing the Ninety East Ridge of the Indian Ocean, a new method was employed to perform proportional double seismic source excitation and synchronously receive signals from the sea surface and the seabed. The two seismic sources used for excitation were two sets of gun arrays with different energies and dominant frequencies, a G gun array and a Bolt gun array. The G gun array consisted of 3 G.II guns with a total capacity of 450 in3 and a dominant frequency of 20–100 Hz. The Bolt gun array consisted of 4 Bolt 1500LL air guns with a total capacity of 6000 in3 and a dominant frequency of 10–40 Hz. The seismic receiving system comprised a 480-channel seismic streamer towed from the sea surface and 21 ocean bottom seismometers (OBS). During offshore operations, the integrated navigation system produced equidistant trigger signals at an interval of 50 m. The trigger signals were distributed to the G gun array and Bolt gun array at a ratio of 3:1 after passing through a pulse signal proportional distributor. The two sets of gun arrays fired alternatingly at a given ratio. The receiving equipment on the sea surface and seabed simultaneously received the seismic signals excited by the two sets of gun arrays. After targeted data processing, in addition to the seismic profile generated by the conventional G gun seismic source, the deep seismic profile generated by the Bolt gun seismic source and the survey profile of the active-source OBS were obtained simultaneously. The penetration depths of the three sets of profiles reach 2 km, 6 km, and 30 km, respectively, greatly improving the efficiency of offshore deep-sea seismic surveys.