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

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.