Summary

The maturing fields in the east of China have been in a high degree of prospecting, almost covered by 3-D exploration. At present, we use multiple-block-jointed static correction to do multi-survey joint processing, without considering the effects of the time-variant near-surface structure on the seismic data. Focusing on the time-variant near-surface structure, this paper introduces model analysis and forward model processing, and puts forward a new method of static correction for multi-survey joint processing in areas where the time-variant near-surface structure changes greatly.

Problem posing

3-D exploration has been carried out in maturing fields in the east of China since 1980s. Almost all favorable structural belts have been covered by 3-D exploration. In North China where the economy developed quickly, a large amount of groundwater is regarded as a necessity in industry and agriculture. As a result, the water table lowered quickly, and the amplitude reaches several meters to ten-plus meters, forming a large funnel of the water table (Fig 1, Table 1). The reasons for this phenomenon lie as follows: the water resource per capita in these areas is only 335m3 per year, less than one sixth of the national average. Moreover the surface water distributes odds in space-time, which results in the surface water becoming an important part of both the economic and social sustainable development in these areas. At present, the groundwater is more than seventy percent of the entire water resource consumption in Beijing, Shijiazhuang, Xingtai, Handan, Baoding, Hengshui, Langfang and Tangshan, for example. Ever since the 1960s, North China began large scale exploration of the underground water supply. The number of electro-mechanical wells has soared from 1800 in the 1960s to 700,000 in 1997. At the same time, the annual yield of the shallow groundwater is rising from 791.3×104 in the 1970s to an average of 1057.9 from 1985 to 1997. Long-term over-exploration of groundwater causes the underground water level to descend at average of 0.5-1.0 meters every year (Figure 2). In the past 50 years, a unified circulation system of ground water in North China Plain is changing into a new model which mainly features vertical movement in local regions. As a result, in space it forms funnels of groundwater with different sizes and different ranges while in the form of time, it shows variation with depth.

Due to the constant changes in the time variation of surface structure, static correction acquired in different years shows a large difference, which results in failing to close. The current method uses the newest 3-D data of surface structure and refers to the data acquired in the past, to build a unified model of surface structure. We think that the time variation of surface structure can be solved by statics. However, in those regions where the time variation of the surface structure changes greatly, it is difficult to settle the problems by statics.

An analysis of joint processing in the time variation regions of surface structure

In order to process the 3-D seismic data conveniently, the current method is to build a unified model of surface structure by smoothing the 3-D surface data collected in different years.

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