TY - JOUR
AU - Deng,, Zhiwen
AB - Abstract Because of the complex surface and subsurface conditions, as well as the poor shooting and receiving conditions encountered in 3D seismic acquisition in mountainous areas, it is difficult to use the data acquired with low signal-to-noise ratio for static correction and imaging of steeply dipping underground structures. This paper develops a suite of techniques that is suitable for data acquisition in complex mountainous areas. In the proposed method, the spatial sampling interval and the layout are determined from existing seismic profiles, and satellite remote sensing data are also used. A combination of seismic sources is used in the survey and optimal static correction methods are also investigated. 3D seismic acquisition, satellite remote sensing, static correction 1 Introduction The following difficulties, which lead to distinct field operation and static correction problems, are encountered during seismic exploration in complex mountainous areas. Violently fluctuating surfaces, complex near-surface layers, outcrops of badly weathered strata and poor traffic conditions. Poor shooting and receiving conditions. On an original single-shot record, the perturbing waves, i.e. the multiple refraction wave, the surface wave, random noise and high-frequency interference are well developed and variable. The effective reflection energy is relatively weak and the signal-to-noise (S/N) ratio is low. A complex subsurface. Overthrust outcrops of badly weathered steeply dipping strata lead to multiple solutions of velocity picking and reversion in the time direction. Hence, it is difficult to determine the stacking velocity field accurately. Difficulties in processing are also increased. After many years investigating the use of various different technologies for seismic exploration in complex mountainous areas, the Bureau of Geophysical Prospecting, China National Petroleum Corporation (BGP-CNPC) has developed a suite of 2D technologies suitable for seismic acquisition, processing and interpretation in mountainous areas. However, this 2D seismic survey method still has several disadvantages when resolving steeply dipping overthrust structures and there is no successful 3D seismic technology available for complex mountainous areas. Therefore, BGP has carried out a series of technical investigations into 3D seismic exploration in mountainous regions and has produced a suite of methods suitable for 3D seismic surveying in this type of area. This paper describes the major acquisition technology and is based on a 3D seismic survey in the KL2 gasfield. 2 Target-specific technical design The design parameters for 3D seismic acquisition include CMP bin size, trace interval, fold, maximum source–receiver offset, receiver line spacing, source line spacing, and offset and layout. The general method for determining these parameters is to read the relevant geophysical parameters from existing seismic profiles and then calculate the selected range using theoretical expressions. The following illustrates a technical design for a specific target, by describing the CMP bin size and layout selected for the KL2 3D seismic acquisition. 2.1 CMP bin size The size of the CMP bin is one of the most critical parameters in a 3D seismic survey. Not only does it relate closely to data acquisition quality but also to cost effectiveness. The traditional method is to calculate the CMP bin size from seismic stack profiles. A direct and simple method which can be applied to obtain the required group interval without acquiring the seismic wave velocity is to calculate the group interval on the basis of maximum frequency. Figure 1 is a reflection record of a seismic profile. Consider n CMP channels. If the field trace interval is Δx1 then the CMP trace interval is Δx1/2 and the distance between n CMP channels is Δl = (n - 1)Δx1/2. Assuming the time difference between the n CMP traces is Δt and the maximum frequency is fmax then the CMP bin size, Δx, can be calculated using 1 which leads to 2 The CMP bin size in the experimental region described in this paper is 15 m × 30 m. Figure 1 Open in new tabDownload slide Seismic reflection profile used to calculate the 3D CMP bin size. Δt and Δl denote the time difference and the distance between two specified traces, respectively. Figure 1 Open in new tabDownload slide Seismic reflection profile used to calculate the 3D CMP bin size. Δt and Δl denote the time difference and the distance between two specified traces, respectively. 2.2 Layout An 8L12S brick-wall-type layout was designed and a flexible observation system laid out. Figure 2 is the N–S stacking profile through the structure. The target layer of the seismic exploration is an anticlinal structure. To ensure down-dip shooting, up-dip receiving and a sufficient fold number, non-symmetrical observations were carried out on both wings of the structure and symmetrical ones were carried out on the top of the structure. The longitudinal layout was as follows: south wing 3585-15-30-15-5025, top 4305-15-30-15-4305 and north wing 5025-15-30-15-3585. Figure 2 Open in new tabDownload slide N–S stack section. Figure 2 Open in new tabDownload slide N–S stack section. Compared to the traditional layout (figure 3(a)), the brick-wall-type layout (figure 3(b)) has the following advantages: offsets are distributed more evenly (figure 4); shot-points are distributed evenly; and the static correction result is good. Figure 3 Open in new tabDownload slide (a) The 8L12S traditional layout; (b) the L12S brick-wall-type layout. Figure 3 Open in new tabDownload slide (a) The 8L12S traditional layout; (b) the L12S brick-wall-type layout. Figure 4 Open in new tabDownload slide Offset distributions of the 8L12S layout. (a) Traditional layout; (b) brick-wall-type layout. Figure 4 Open in new tabDownload slide Offset distributions of the 8L12S layout. (a) Traditional layout; (b) brick-wall-type layout. 2.3 Exploration area Some errors enter the calculation of the full coverage area with the general formula before and after migration. This is the result of the large formation velocity and dip spatial variation. The diffracted wave before migration is hyperbolic. An excellent result could be obtained if only 75% of the energy of the hyperbola is migrated. Therefore, the pre-migration full coverage range (figure 5(a)) can be determined using the range of the diffracted wave on the stack profile and the post-migration full-coverage range (figure 5(b)) can be determined using the range of the post-migration reflection on the stack profile. Then the pre-migration and post-migration full-coverage ranges (figure 6) for the exploration target can be determined using the location of these points on the isogram. In figure 6, the outer black border defines the pre-migration full-coverage exploration area, whereas the inner black border defines the post-migration full-coverage exploration area. Figure 5 Open in new tabDownload slide (a) Pre-migration and (b) post-migration full-coverage exploration areas determined using the stacking and migration profiles. Figure 5 Open in new tabDownload slide (a) Pre-migration and (b) post-migration full-coverage exploration areas determined using the stacking and migration profiles. Figure 6 Open in new tabDownload slide Map showing the full coverage of the exploration area. Figure 6 Open in new tabDownload slide Map showing the full coverage of the exploration area. 3 Use of satellite remote sensing data The mountainous area where the KL2 3D seismic exploration was carried out has a very complex topography and a large elevation difference. Near-surface longitudinal and transverse velocities and lithology change rapidly. It is impossible to determine the topography of this large mountainous area in detail by traditional surveying. Hence, it is necessary in addition to use high-precision satellite remote sensing data. For the demonstration and operation of the technical method for 3D seismic acquisition in the KL2 area, a large number of low-precision satellite remote sensing 3D vector data were used in the following four aspects of the 3D design. The identification of surface features and classification of different shooting lithology divisions in the area of operation (figure 7). Using both satellite remote sensing data (figure 8) and on-site survey results, the KL2 3D seismic operation area was divided into the south Gobi area (A), a small loessal alluvial fan on the southern front of the mountain (B), a small pebble area within an alluvial channel (C), an alluvial channel (D), an area of non-compact pebble-bearing loess (E), the outlet sandstone of the mountain (F), the interlayered sandstones and mudstones of the mountain (G), and a small pebble area in the north (H). In combination with the data quality maps, the advance identification of those areas with poorer shooting conditions and obstacles (high steep cliffs etc). The S/N ratio data can be improved by making full use of areas with good shooting conditions and avoiding those with poor ones, as well as changing layouts in those areas with obstacles. The provision of a reference for the actual field stake out and point choice so that the routine and point layout could be undertaken simultaneously. The organization of the field operation, for instance choosing bases, logistic points and transportation routines. Figure 7 Open in new tabDownload slide Classification of surface lithology using satellite remote sensing data. Figure 7 Open in new tabDownload slide Classification of surface lithology using satellite remote sensing data. Figure 8 Open in new tabDownload slide Choosing the shooting position in the pre-design with satellite remote sensing data. Figure 8 Open in new tabDownload slide Choosing the shooting position in the pre-design with satellite remote sensing data. 4 Combination of various seismic sources On the basis of the satellite remote sensing data and the ground geological survey, we determined the topography and lithology of the KL2 mountainous seismic area in detail. According to the different surface conditions, the area of operation was divided into different shooting areas and a suitable shooting method was chosen for each. A mountain drill rig was used in the non-compact pebble-bearing loess, the outlet sandstone and the interlayered sandstone–mudstones of the mountain. Truck-mounted drill rigs were used in the alluvial channel and in the small pebble area in the alluvial channel. Vibrators were used in the south Gobi area, the small loessal alluvial fan on the southern front of the mountain and in the small pebble area in the north. The layout for each area was adjusted to be large enough for 30 shots. Phase conversion is necessary in data processing for correlating the wavelet phases of dynamic and vibrator shootings. At present, two methods for picking the wavelet phase conversion operators can be used. One method is to pick the operator directly from the single-shot records acquired through dynamic and vibrator shootings at the same point. The other is to pick the operator from the stacking profile shooting through dynamic and vibrator shootings in the same section. For data with low S/N ratios, the second method is better for generating a more precise operator. Because of the complex surface conditions in the area, the second method is generally preferable. Since the operation was divided into many divisions for vibrator shootings, only one or two divisions in the whole area are needed to obtain the repeat stack profiles during the 3D seismic operation. However, the chosen profile must be of high quality with a good event, and the longitudinal full coverage length of the repeat stack section should not be smaller than 10 CMPs. 5 Static correction 3D static correction for a complex mountainous surface is well known to be a difficult problem. There are many technical difficulties: near-surface investigation precision is poor, it is difficult to establish the near-surface model and choose the filling velocity and datum plane, and the single static correction method cannot satisfy the demands of the data processing. In processing the KL2 3D data, we have applied two different measures in order to (1) improve the precision of the static correction, (2) improve the precision of the near-surface data by using various investigation methods and (3) apply a technology to combine the static database. These measures are the near-surface modelling static correction and first-break static correction methods. 5.1 Near-surface modelling static correction method The mountainous area has a large elevation difference and the high-velocity top interface fluctuates violently, hence a horizontal datum plane and an intermediate reference plane were introduced in the static correction calculation (figure 9). The intermediate reference plane denotes the smooth curved surface under the actual high-velocity top surface. Figure 9 is a sketch for static correction in which the surface model is made up of three layers: a low-velocity layer, a deceleration layer and a high-velocity layer. The static correction, Δt, is now given by 3 where h0 and v0 are, respectively, the thickness and velocity of the low-velocity layer, h1 and v1 are the thickness and velocity of the deceleration layer respectively, vb and vt are the stripping velocity and the filling velocity respectively, hg is the separation of the deceleration layer and the reference plane, and e and eg are the elevations of the surface and intermediate reference plane respectively. Figure 9 Open in new tabDownload slide Sketch showing the static correction in the mountainous area. Figure 9 Open in new tabDownload slide Sketch showing the static correction in the mountainous area. The filling velocity is the velocity of the medium between the horizontal datum plane and the intermediate reference plane. It ranges from 1500 m s-1 to 3500 m s-1. The test processing was carried out every 500 m s-1; the processed test profile shows that the result at 1500–2000 m s-1 is worse and the result at 2500–3500 m s-1 is better. The filling velocity was finally determined as 2500 m s-1 (figure 10) so that it could be compared with the 2D profile. Figure 10 Open in new tabDownload slide Selecting the filling velocity. Figure 10 Open in new tabDownload slide Selecting the filling velocity. The stripping velocity is the velocity between the actual high-velocity layer and the top of the smoothed high-velocity layer, which will affect the precision of the static correction. In order to choose the stripping velocity accurately, a velocity test was performed every 500 m s-1. The processed test profiles show that the result between 2500 m s-1 and 3500 m s-1 is somewhat better. In order to improve the accuracy of the static correction, the following velocity-variation test was undertaken: a velocity database was set up with the velocities from refraction in the high-velocity layer; velocities were picked from this database to ensure the closure of the whole area. The processing profile shows that the velocity-variation stripping result is ideal (figure 11). Figure 11 Open in new tabDownload slide Selecting the stripping velocity. Figure 11 Open in new tabDownload slide Selecting the stripping velocity. For smoothing purposes, the intermediate reference plane should be a smooth plane below the actual high-velocity top layer (Bailey and Weyer 1999). The test result in figure 12 illustrates that the smoothing with a radius of 2500 m is excellent. Figure 12 Open in new tabDownload slide Processing test for the smoothing radius of the intermediate reference plane. Figure 12 Open in new tabDownload slide Processing test for the smoothing radius of the intermediate reference plane. 5.2 Establishing a static correction database The surface data and static correction databases were set up to ensure the accuracy and closure of the plane: The available 2D static and refraction data were used to form a grid according to thickness and velocity interpolation. These establish databases of thickness, and low-velocity and high-velocity layers, which were then used to draw a plane variation map. During the operation, supplemented by the newly acquired refraction and up-hole data, the accuracy of the static correction data was greatly improved and high-precision databases of thickness, and low-velocity and high-velocity layers were finally obtained. 5.3 First-break reflection static correction The critical steps in obtaining an accurate static correction value are to recover the surface construction, establish an accurate near-surface model and calculate an accurate near-surface velocity and thickness. At present, the methods commonly used to establish the surface model are the control point interpolation and first-break refraction methods. The control point interpolation method establishes a near-surface model through interpolation by taking the refraction and up-hole data as control points and referencing the variation of the elevation or thickness in the near-surface layer. This method is particularly suitable for dealing with medium- and long-wave problems. The first-break reflection method inverts the near-surface model with a seismic reflection first break. This method is especially useful for high-frequency static problems. The mountainous area has a complex surface. The number of control points is limited and the lithology, velocity and thickness between these control points change dramatically. Thus the control point interpolation method would encounter some problems, while the first-break reflection method can supplement the modelling method because it calculates the static value among the control points. If the daily field first-break pick and the 3D seismic acquisition are carried out simultaneously, the calculation of the 3D first-break static correction value can be finished within a few days, and thus, in general, the completion of the entire data acquisition, the calculation of the static correction value for the 3D first-break reflection method, will also be timely. Since various shooting sources whose wavelet phases are different are often applied in the 3D acquisition, a wavelet conversion operator needs to be applied to correlate the wavelet inconsistencies in the shot point volume. In the KL2 static calculation, the zero-phase vibrator wavelet was minimized to satisfy the dynamic wavelet. Figure 13 shows a comparison between the refraction stack profiles of the 3D first break before and after the static corrections. From the sketch it can be seen that the high-frequency static problems have been well resolved and the continuity of the phase axis has been improved after the 3D first-break static correction. Figure 13 Open in new tabDownload slide Comparison of the refraction stack profiles of the 3D first break before and after the static corrections. Figure 13 Open in new tabDownload slide Comparison of the refraction stack profiles of the 3D first break before and after the static corrections. Figure 14 is a comparison of three stack profiles processed using the three 3D static correction methods. Of the three profiles, figure 14(c) has the best phase axis continuity, that in figure 14(b) is worse and that in figure 14(a) is the worst. Figure 14 Open in new tabDownload slide Comparison of the profiles processed using three different 3D static correction methods. (a) The modelling method; (b) the first-break reflection method; (c) the modelling-first break combination method. Figure 14 Open in new tabDownload slide Comparison of the profiles processed using three different 3D static correction methods. (a) The modelling method; (b) the first-break reflection method; (c) the modelling-first break combination method. The final processing of the data proved that the 3D static correction has a higher precision and that a clear result has been achieved. Figure 15 is the comparison between pre- and post-3D static correction stacking profiles. Figure 15(a) shows the case where no static correction has been applied and has a poor imaging result, while figure 15(b) shows that the 3D static processing has a good effect. Figure 15 Open in new tabDownload slide Comparison of (a) pre- and (b) post-3D static corrections. Figure 15 Open in new tabDownload slide Comparison of (a) pre- and (b) post-3D static corrections. 6 Conclusions Through the KL2 3D seismic exploration, a suite of acquisition parameters suitable for complex mountainous areas has been collected and an effective method for solving the 3D static correction problem has been found. The data obtained show that the original data have a high S/N ratio, the processed 3D data volume has many discernible geological features and the data quality is greatly improved in comparison with the previous 2D data. In addition, a ‘flat point’ phenomenon produced by the water–gas contact is observed. Figure 16 is a comparison between the 3D migration section after time-domain stacking and the 2D migration profile in the same location. Figure 17 is an in-line section of a post-stack 3D migration. Besides a high S/N ratio and good continuity, the ‘flat points’ of the water–gas contact are also visible. Figure 18 shows two structural maps of a top layer drawn using 3D and 2D seismic data respectively. It exhibits many small breaking blocks and is rich in distinguishable geological features, in contrast to the results of the 2D exploration. The reservoir calculated using the 3D seismic data is larger. This proves that 3D seismic exploration may provide a better economic benefit. Figure 16 Open in new tabDownload slide Comparison of the 3D and 2D migration profiles in the same location. Figure 16 Open in new tabDownload slide Comparison of the 3D and 2D migration profiles in the same location. Figure 17 Open in new tabDownload slide Migration profile along in-line section (the arrow indicates the location of the water-gas contact). Figure 17 Open in new tabDownload slide Migration profile along in-line section (the arrow indicates the location of the water-gas contact). Figure 18 Open in new tabDownload slide Top-layer structural maps in the KL2 gasfield, based on (a) the 2D seismic data and (b) the 3D seismic data. Figure 18 Open in new tabDownload slide Top-layer structural maps in the KL2 gasfield, based on (a) the 2D seismic data and (b) the 3D seismic data. Acknowledgment I would like to thank Mr Rongjun Qian for his technical direction and Yishu Wen for editing. Yudong Ni, Xuegiang Chen, Yonggui Hu, Zhenghua Li, Huanxin Bai and Zhihui Yan have also taken part in the research for this project. References Bailey B , Weyer J . , 1999 Imaging through a badly weathered near surface: a case history at Elk Hills 69th SEG Meeting Houston, USA (pg. 1365 - 8 ) Expanded Abstracts © 2004 Nanjing Institute of Geophysical Prospecting
TI - 3D seismic acquisition in a complex mountainous area
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2132/1/1/003
DA - 2004-03-12
UR - https://www.deepdyve.com/lp/oxford-university-press/3d-seismic-acquisition-in-a-complex-mountainous-area-02fC6TmqRK
SP - 17
VL - 1
IS - 1
DP - DeepDyve
ER -