Plateau seismic survey

Songpan-Aba area and Qiangtang area in Tibet are the representative areas where the strategic constituency project chooses to carry out earthquake investigation and tackle key problems in the plateau area. The geophysical exploration difficulties in these two areas mainly include:

Seismic survey lines pass through complex terrain such as mountains, deserts, river valleys, grasslands, swamps, etc., with relatively large topographic relief and criss-crossing gullies. The extensive exposure of old strata and the widespread existence of near-surface frozen soil layer have serious shielding, absorption and scattering effects on seismic wave energy; The underground geological conditions are complex, the strata fold is serious, the occurrence is changeable, the dip angle of strata is large, faults are developed, and the old and new strata cut each other.

Qiangtang basin is a new exploration area with special surface and complex underground geological conditions. The quality of seismic data varies greatly, and the overall signal-to-noise ratio is very low.

(1) Early earthquake investigation work

1. Overview of previous seismic exploration

Geophysical work in Songpan-Aba area is not very developed. Due to the influence of stratigraphic folds and surface coverage, only a few seismic acquisition profiles have no obvious characteristics of reflected wave groups, and only a few wave groups can be tracked and compared continuously on the profiles. From the analysis of the data collected in the past, it can be seen that the seismic geological conditions in the middle and shallow layers in this area are poor, mainly because the reflected energy in the middle and shallow layers (within 4.5s) is weak or there is no obvious reflection. Deep-ultra-deep (5 ~ 8s) reflected waves are strong and widely distributed. According to previous geological data, the deep strata in this area are dominated by Paleozoic, and the Mesozoic and Cenozoic are missing. The thickness of Paleozoic is more than10000m, and faults are developed around the basin with large dip angle. According to the outcrop data in this area, the density and velocity difference of strata below Triassic in this area is small, which makes the wave impedance interface not obvious, which may be one of the reasons why the shallow and middle reflection is not obvious.

The degree of petroleum seismic exploration in Qinghai-Tibet Plateau is extremely low. Only Qiangtang Basin is marine, and its work level is relatively high. The workload of 2D seismic exploration is 2,640 line kilometers, of which the density of survey network in Wan 'an Lake area reaches 4km×4km. However, due to the low signal-to-noise ratio, there are not many seismic profiles that can be used for interpretation, which affects the effect of oil and gas exploration, so that the evaluation of oil and gas resources in Qiangtang Basin is mainly based on the analysis and synthesis of surface geological data, and its credibility is not high. Therefore, improving or improving the quality of seismic profile has become one of the key issues to make a breakthrough on the basis of predecessors.

2. Pre-earthquake investigation methods

(1) seismic acquisition method

From February 2002 to June 2003, the exploration and development branch of southern Songpan-Aba area of Sinopec organized Shengli Geophysical Bureau and other units to tackle the key problems of field seismic acquisition, and gained a certain understanding of the seismic and geological conditions in this area. The main acquisition parameters are shown in Table 9-3.

Source type: explosion source (single well excitation)

Well depth: 14 ~ 25m

Number of channels received: 240 channels, a few 480 channels and 600 channels.

Recording format: SEGD

Track spacing: 50m, at least 20m.

Maximum layout length: 5975m.

Record length: 12s

Sampling interval: 2ms, several1ms.

Coverage times: 60 times, with a few times ranging from 120 to 300 times.

PetroChina entered Qiangtang Basin for the first time in 1995, using vibroseis and explosion sources, and obtained usable or even good seismic records in some areas, but the quality of most data was extremely poor. On the premise of insufficient excitation energy, the test results of track spacing and high coverage times are not obvious. In the case that the overall quality of the original data is not high, although the detailed processing was carried out under the technical conditions at that time, the profiles processed by different companies or those processed successively by the same company are different, so a regional benchmark profile has not been obtained so far, and only a few profiles are used for petroleum structure interpretation. 1998, depth-III respectively carried out deep reflection seismic tests in Bangor, Lempola, Doma and Shuanghu sections. Using well blasting, the well depth 15 ~ 18m can obtain effective reflection information, which shows that increasing the well depth and increasing the excitation amount appropriately may be one of the effective ways to improve the quality of seismic data in the Qinghai-Tibet Plateau (Qiangtang Basin).

Table 9-3 Table of Seismic Acquisition Parameters in Qiangtang Area of the former Ministry of Petroleum

(2) Processing technology

In Songpan-Aba area, the main pretreatment techniques are: chromatographic inversion static correction combined with relative refraction static correction; After linear correction, the refraction multiples are suppressed by weighted subtraction of mean value in FK domain, and the reflection multiples are suppressed by combining velocity analysis and prediction deconvolution. FK domain average weighted subtraction to suppress linear coherent noise: iterative velocity analysis and residual static correction for many times to improve the continuity and signal-to-noise ratio of reflection in-phase axis.

According to the characteristics of Qiangtang basin data, the former petroleum department thought that low signal-to-noise ratio and large static correction were two major problems, and summarized the following understandings from the processing method test and processing process:

1) The quality of seismic data collected in Qiangtang Basin is generally poor due to special surface conditions, complex underground geological structure and harsh environmental factors. Before treatment, a comprehensive investigation and analysis should be carried out in order to take targeted technical measures and achieve better results.

2) The intensity and range of pre-stack noise should be properly controlled so as to effectively suppress the noise without destroying the effective wave.

3) Through experiments and analysis, the effects of several static calibration methods are compared, and the general technology with strong pertinence is adopted.

4) Post-stack denoising should be appropriate, and attention should be paid to fidelity. Excessive modification will make the characteristics of profile wave group not obvious.

5) The processing flow of the same batch of data should be consistent, otherwise the data in the same block will be difficult to interpret.

3. Understanding of previous earthquake investigations

From the analysis of previous seismic work, large signal-to-noise ratio and static correction are two major problems faced by seismic exploration in Qiangtang Basin. In 2004, 2006 and 2007, a series of improvement measures were taken, such as increasing the number of wells, selecting the best charge, increasing the arrangement length and path spacing, and achieved certain results.

(2) Seismic survey in selected districts

On the basis of previous earthquake research in Qiangtang basin, the strategic selection project focuses on improving the signal-to-noise ratio of seismic data, and according to the characteristics of surface waves, refracted waves, multiple refracted waves and high-frequency random noise in this area, the main attack strategy in acquisition construction is drawn up.

1. Seismic acquisition method

See Table 9-4 for specific acquisition parameters.

Table 9-4 List of Construction Parameters Collected from Earthquake Survey in 2009

1) In view of the low recording signal-to-noise ratio, the countermeasures are: detector area combination; Increase the number of detectors and suppress random interference. The specific receiving parameters are as follows: combination form: combination along the rectangular area of 36 detectors; Combined base distance: LX = LX = 4mly =11m; +0m； Intra-group distance: Δ x = 4m Δ y =1m; Intra-group height difference: less than1m; ; Buried depth: not less than 20cm.

2) In view of the great vertical and horizontal changes of the surface structure, the countermeasures are as follows: to encrypt the investigation points of the surface structure and control the changes of groundwater level. The specific excitation parameters are as follows: excitation depth: 3 m below the top interface of high-speed layer; The minimum drilling depth shall not be less than 15m (except gravel floodplain); The minimum drilling depth of gravel floodplain shall not be less than12m; ; Excitation dose: 18kg.

3) In view of the shallow burial of the main target layer, 48 ~ 60 times of coverage were used in the past, which resulted in long array length and large shot distance, resulting in low effective coverage times of the main target layer and poor imaging effect. The countermeasures are: increasing the stacking times to ensure the stacking effect; Reduce the distance of shot point and increase the effective coverage times; In order to obtain more geological information, a variable line element observation system is adopted.

4) In view of the harsh climatic environment such as long gale time and frequent rain, snow and hail in this area, buried geophones; Monitor external noise with instruments, and don't shoot when the interference is large; Low noise construction should be carried out in the priority period.

5) In view of the large amount of gravel well water in the river beach and the difficulty in well completion, clay powder and caustic soda are used to make mud for well drilling.

Comparing the acquisition and construction parameters of the earthquake survey in Qiangtang area in 2009 and 2008, it can be seen that the main goal of the work in 2009 is to improve the signal-to-noise ratio of the acquired data by increasing the coverage times, as shown in Table 9-5.

Table 9-5 Comparison Table of Construction Parameters Collected from Earthquake Survey in 2008 and 2009

2. Seismic data processing

In view of the low signal-to-noise ratio of seismic data in Qiangtang basin, the understanding of seismic geological conditions in this area and the collection and analysis of original data were strengthened, and the following targeted processing techniques were adopted in combination with the previous experience of low signal-to-noise ratio data processing, which effectively improved the data quality.

(1) static correction processing technology

Static correction of long wave and medium wave adopts chromatographic static correction method. The surface consistent static correction solves the residual static correction of short wavelength.

(2) Multi-system and multi-method joint denoising.

Combined with the best denoising methods of different processing systems, the signal-to-noise ratio of prestack data is effectively improved.

(3) Surface consistency and processing technology for improving resolution.

On the basis of experiments, the post-stack residual amplitude is analyzed and compensated by gain curve analysis and time-frequency analysis, which makes the wave group characteristics clear and the intensity characteristics obvious. In addition, the surface consistent multi-channel deconvolution is used to eliminate the influence of surface factors on wavelet, improve the stacking effect and improve the profile quality.

3. The effect of earthquake investigation

The shallow and middle reflection wave groups in the profile obtained by earthquake research are complete, the main reflection wave groups have good continuity, natural transition, easy identification and tracking, and the structural characteristics are obviously reliable, which can basically complete the geological task of understanding basement fluctuation and dividing structural pattern, as shown in Figure 9- 15.

Through the detailed interpretation of the seismic data in the selected area, we have basically understood the stratigraphic distribution, the distribution characteristics of Jurassic and Triassic main target layers, the structural pattern and style of overlying strata, the buried depth of basement, the shape of basement and the characteristics of faults, which laid an important foundation for further evaluation of oil and gas resources in Tibet.

Fig. 9- 15ts 2009-03 seismic geological interpretation profile