立体气枪阵列延迟激发震源特性及在浅海区OBS探测中的应用

利用中、小容量气枪组成的立体气枪阵列延迟激发震源和海底地震仪(OBS)在我国北部浅海海域开展了人工地震深部地球物理探测试验.基于水深条件和压制水体虚反射、提升低频能量的需要,使气枪震源有足够的输出能量和高品质子波特性,研究了立体气枪阵列延迟激发震源工作机理,经远场子波理论模拟优选了组合参数并进行了海上试验工作.结果表明,中、小容量气枪组成的立体气枪阵列延迟激发震源,适应了浅水海域的激发环境,降低了由虚反射造成的局部陷波和干扰作用,有效地改善了OBS信号的品质,获得了Ps,Pg,PmP,Pn等多种震相.创新了由中、小容量气枪组成的立体气枪阵列延迟激发震源在浅海区OBS探测中的应用,也填补了南黄海海域深地震探测数据的空白,为南黄海、渤海深部地壳结构研究及含油气盆地形成演化研究提供了重要的基础资料.

Tridimensional air-gun array with delay fired source signal characteristics and the application in OBS exploration in shallow seas

The Bohai Sea is in the North China block and the South Yellow Sea is located in the Yangtze block. These seas are rich in oil and gas resources, related with block convergence and plate movement. The aim is to carry out seismic deep OBS detection in the Southern Yellow Sea and the Bohai Sea to get the seismic reflection and refraction information of lithosphere and the Moho surface for the study on the crust-mantle structure characteristics, the contact relation of the North China block and the Yangtze block and the tectonic effect of the plate movement and its control on the basin formation, evolution and hydrocarbon accumulation. The deep crust OBS exploration in the deep sea uses a conventional air-gun array which is made of several sub-arrays each containing a number of large volume air-guns. But in the shallow sea,it is difficult to play an effective role due to the gun depth limit and serious seafloor ghost reflection and source water ghost. To meet the needs for attenuation of water ghost reflection and enhancement of the low frequency content of seismic wavelet, based on the water depth condition and analysis of the principle of multi-level air-gun array and delay fired source applied in oil and gas exploration in the deep sea, we develop a method to delay the sub-array fired time for the tridimensional air-gun array source composed of a number of smaller volume air-guns. Compared with the multi-levels air-gun array,we apply a smaller depth difference and flexible sub-array sinking for sub air-gun array in order to adapt to the harsh environment for shooting the air-gun in the shallow water, thus we can get better effect for bubble suppression and ghost attenuation. A pattern of flexible delay fired time is designed for improving to output energy of the source and the quality of the wavelet. We design different combination schemes for tridimensional air-gun array and delay fired source and model its far-field wavelets. The amplitude-frequence distribution of our designed tridimensional air-gun array with delay fired source is more smooth than that of a conventional one, and more energy can be extended in the low frequency bands. Based on water depth conditions in the OBS exploration area,we applied respectively different tridimensional air-gun array and delay fired source pattern in the Bohai Sea and the Southern Yellow Sea. Compared with the deep crust OBS exploration using conventional large volume air-gun array in the Bohai Sea in 2012,the quality of the OBS data accquired by using new source is improved obviously, the seismic signal has a stronger amplitude, richer phase,higher resolution, better continuity and higher S/N ratio. The maximum propagation distance of source signal is over 150 km, which is more than that of the conventional large volume air-gun array. The deep crust OBS exploration which is the first one in the South Yellow Sea indicates the OBSs can record rich seismic phases including Ps,Pg,PmP and Pn both in the uplift areas where distributed the high velocity layers which had shielding effect on the seismic signals and the depression areas with large deposition thickness. The OBSs record quality is higher and the effective propagation distance of the air-gun signal is over 140 km.The deep crust OBS exploration test using tridimensional air-gun array with delay fired source technique shows that this method can generate seismic signals with long propagation distance and deep penetration in the shallow water, which can effectively suppress the source ghost effect and improve OBSs data quality. The method provides an ideal artificial source for marine seismic exploration, which can help study deep lithospheric structure, composition and status, and multichannel seismic detection for deep target layers. An important function for the tridimensional air-gun array and delay fired source technique is to attenuate the source ghost and to enhance low frequency energy. But the air-gun far field signals are also affected by the new design. Firstly, the source ghost is an effective attenuator of bubble pulses,the tridimensional air-gun array and delay fired source technique deliver a lower peak-to-bubble ratio (PTB) than a conventional air-gun array. Secondly, since the guns are towed deeper, they fire under higher hydrostatic pressure, which would reduce bubble sizes and oscillation periods and result in decrease of the high frequency contents. Thirdly, the difference of the bubble sizes caused by different array depths leads to less flexibility in array design and bubble pulse attenuation. Therefore, the far field wavelet modeling is the key to ensure an effective tridimensional air-gun array with delay fired source design. To avoid the side effects, we must carefully carry out the simulation and analysis of far-field wavelets to develop an optimal design for tridimensional air-gun array and delay fired source.