羌塘盆地油气二维地震勘探进展综述

Current advances of the 2D seismic exploration in the Qiangtang Basin: An overview

关键词:  羌塘盆地  地震勘探  可控震源  高密度  高覆盖  宽线  干扰波

Key words:  Qiangtang Basin  seismic exploration  vibroseis  high-density  high-fold  wide-line  interference wave

摘要:

本文详细讨论了羌塘盆地二十多年来二维地震勘探所取得的进展。羌塘盆地除发育背景干扰外，还发育多种面波散射、线性干扰、折射波和多次折射波；检波器大组合能压制背景和面波散射干扰，但不能压制速度较高的线性干扰、折射波和多次折射波。最佳激发因素为常规可控震源振动台次3台1次，驱动幅度70%，扫描频率6～84Hz，扫描长度18s；低频可控震源振动台次2台1次，驱动幅度60%，扫描频率1.5～84Hz，扫描长度16s；大吨位可控震源振动台次2台1次，驱动幅度70%，扫描频率6～84Hz，扫描长度16s。炸药震源为单井高速层下7m激发，最浅井深18m，药量18kg；组合方式激发为2口井×15m×12kg或3口井×12m×8kg。尽管可控震源单炮的能量、信噪比、频谱及子波一致性与炸药震源相比较并不占优，但可控震源激发在高密度高覆盖采集条件下仍能获得等同于或明显优于井炮激发质量的地震剖面资料。从“环保、安全、经济、高效”上考虑，羌塘盆地宜采用可控震源和井炮联合的宽线高密度高覆盖采集方案，3L3S或2L3S，960次以上覆盖为可控震源最佳观测系统；2L3S，360次左右覆盖为井炮震源最佳观测系统。北羌塘坳陷构造稳定，容易获取高品质地震资料，南羌塘坳陷构造过于复杂，资料信噪比低，可能不太适合开展地震勘探工作。文章最后还讨论了冻土层静校正和激发接收方面存在的问题及解决方案。

Abstract:

The present paper gives a detailed overview of current advances of the 2D seismic exploration in the Qiangtang Basin during the last twenty years or more. Besides the ambient noises, there exist several kinds of ground rolls, linear interferences, refraction waves and multiple refraction waves in the Basin. Both the ambient noises and ground rolls can be effectively suppressed when the geophone array length equals to 160 m long. However, as for the linear interferences, refraction waves and multiple refraction waves, the idealized suppressed results can't be acquired even when the geophone array length equals to 160 m long. The optimum excitation parameters for general vibroseis are indicated that the vibration patterns display the three-diesel vibrating every time, the drive level is 70%, the sweep frequencies range from 6 Hz to 84 Hz, and the sweep length is 18s. The optimum excitation parameters of low frequency vibroseis indicate that the vibration patterns display two-diesel vibrating every time, the drive level is 60%, the sweep frequencies range from 1.5Hz to 84 Hz, and the sweep length is 16s. The optimum excitation parameters of large-tonnage vibroseis are indicated that the vibration patterns display two-diesel vibrating every time, the drive level is 70%, the sweep frequencies range from 6Hz to 84 Hz, and the sweep length is 16s. The optimum excitation parameters of explosive sources include single-hole excitation at the depth of 7 m below the high-velocity layers (the least depth of 18 m for a shallow well) with the explosive volume of 18 kg in weight, two-hole array shooting at the depth of 15 m below the high-velocity layers with the explosive volume of 12 kg in weight, or three-hole array shooting at the depth of 12 m below the high-velocity layers with the explosive volume of 8 kg operated under unfavourable surface conditions in the Basin. Although the energy levels, signal-to-noise ratios, bandwidth and wavelet coherence of shooting recorded by vibroseis are not as good as those of explosive sources, the data acquired from the seismic profiles by using the high-density and high-fold wide line seismic acquisition technique may be identical to or superior to those of borehole explosives. Being an environmentally friendly, more safe, more economical, and high efficient operation mode, the high-density and high-fold wide line seismic acquisition technique with a combination of vibroseis and borehole explosives shot is much more suitable to the Qiangtang Basin. The layout chart by the vibroseis can be designed as 3L3S or 2L3S with a minimum fold of 960 times. The layout chart by borehole explosive shooting can be designed as 2L3S with a minimum fold of 360 times. Because of stable geo-tectonic conditions, the northern Qiangtang depression is more suitable for the seismic exploration while the southern Qiangtang depression may not be suitable for the seismic exploration due to its more complicated geo-tectonic conditions. Finally, the problems and suggestions about how to resolve the tundra static correlation and increase exciting and receiving efficiency are also proposed in this paper.