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In order to research whether it is suitable to set a geological disposal repository for high-level radioactive nuclear waste into one target granite body, two active source seismic profiles were arranged near a small town named Tamusu, Western China. The study area is with complex surface conditions, thus the seismic exploration encountered a variettraveltimey of technical difficulties such as crossing obstacles, de-noising harmful scattered waves, and building complex near-surface velocity mod- els. In order to address those problems, techniques including cross-obstacle seismic geometry design, angle-domain harmful scattered noise removal, and an acoustic wave equation-based inversion method jointly utilizing both the and waveform of first arrival waves were adopted. The final seismic images clearly exhibit the target rock’s unconformable contact boundary and its top interface beneath the sedimentary and weathered layers. On this basis, it could be confirmed that the target rock is not thin or has been transported by geological process from somewhere else, but a native and massive rock. There are a few small size fractures whose space distribution could be revealed by seismic images within the rock. The fractures should be kept away. Based on current research, it could be considered that active source seismic exploration is demanded during the sitting process of the geological disposal repository for nuclear waste. The seismic acquisition and processing techniques proposed in the present paper would offer a good reference value for similar researches in the future. Keywords Geological disposal repository · Nuclear waste · Granite body · Active source seismic exploration · Near-surface velocity inversion 1 Introduction forms of life, countries across the globe have attached great importance to their safe disposal (Guo et al. 2001). The dis- High-level radioactive nuclear waste generally refers to the posal technology normally includes marine disposal, island waste products containing high-level radioactive materi- disposal, etc. At present, one widely adopted scheme for als which are generated by the nuclear power industry and safe disposal of high-level radioactive nuclear waste is sup- are inevitable by-products due to nuclear power plants. As posed to be deep geological disposal (Alberdi et al. 2000), they may seriously threaten the health of human and other which, in other words, means that the high-level radioactive nuclear waste is buried within a stable geological body at a depth of 500–1000 m to separate them from the biosphere Edited by Jie Hao permanently (Su et al. 2011). The engineered underground facility for burying the nuclear waste is called geological * Yi-Ke Liu ykliu@mail.iggcas.ac.cn disposal repository. In various countries, different types of rocks would be selected for such repositories. China mainly School of Earth Sciences and Engineering, Nanjing focuses on the large granite masses (Wang et al. 2006). University, Nanjing 210008, China For a comprehensive assessment of one target granite Key Laboratory of Petroleum Resource Research, Institute body, synthetical studies of the tectonics, hydrology, and of Geology and Geophysics, Chinese Academy of Sciences, geophysics are required. Active source seismic exploration Beijing 100029, China 3 is one of the most important technical approaches (Chen Geophysical Exploration Center, China Earthquake et al. 2017). Administration, Zhengzhou 450002, China Vol:.(1234567890) 1 3 Petroleum Science (2021) 18:742–757 743 On one pre-selected geological disposal repository 2 Seismic acquisition and the cross‑obstacle located on the eastern edge of the Badanjilin Desert, our seismic geometry design seismic exploration work was carried out. The objectives of the proposed exploration were to answer the following ques- Based on the seismic exploration objectives, the main seis- tions: whether the rock is thin, where the rock’s boundaries mic working parameters were set as: 4 m trace interval, 24 m are and their spatial distribution, if the faults interpreted by shot interval, 300 traces receiving (the receiver array length remote sensing technology (Fig. 1) exist. of one single-shot was 1.2 km), fold number ≥ 25, 1 ms sam- The seismic exploration research focusing on one large pling rate, and 3 s record length. high-velocity granitic pluton is uncommon. During the In the course of seismic acquisition, seismic spread research, we found that the study area was with complex must overcome certain obstacles. As shown in Fig. 2d, landforms which included three distinct types: semi-desert one large rock outcrop emerges from the ground (stake grasslands, areas of shifting dunes, and areas of outcrops 4748–5084 m of the TMS2 seismic line). If its negative (Fig. 2). In addition, the elevations along the seismic lines influences were ignored, it would engender the lack of seis- change dramatically (Fig. 3). Thus, in the course of seismic mic data, and there would be a gap existing between stake data acquisition and processing, we encountered difficulties 4750–5100 m in the final reflection imaging profile. Given (Chen et al. 2015) of crossing obstacles, removing harmful this realization, we presented a patent for invention (Li et al. scattered waves, and building complex near-surface velocity ZL201710342906.5) whose function aims at mitigating the models. To address those problems, targeted techniques such harmful effect of obstacles on the seismic line via rede- as cross-obstacle seismic geometry design, angle-domain signing the routine seismic geometry. As shown in Fig. 4a, vector resolution based scattered wave removal (Liu 2013; new shot and receiver positions were set (Fig. 4a). One of He et al. 2015), and an acoustic wave equation-based inver- the rules for setting those new positions was to assure the sion method jointly utilizing both the travel time and wave- real fold numbers on each Common Mid-Point (red points) form of first arrival waves (Luo and Schuster 1991; Jiang and always above the mean level (blue points) in the most eco- Zhang 2016; Yao et al. 2019), were adopted. nomical way. As shown in Fig. 4b, there was no gap in the Legend Interpreted Faults Mafic Dikes Seismic Lines Geological Work Area TMS1 TMS2 102°30′00″E 103°00′00″E 103°30′00″E Fig. 1 Graph of remote sensing interpretation of the seismic work area 1 3 40°30′00″N 41°00′00″N 744 Petroleum Science (2021) 18:742–757 Fig. 2 Pictures of landforms along the seismic lines relevant position, thus the negative influence of the obstacle of emergence angles between reflected waves and scattered was effectively weakened. It was due to this technique, the waves to de-noise. In addition, it could be used in pre-stack geological task of continuously tracking the rock’s top inter- processing. face could be realized. After applying the abovementioned method on common- shot-gathers, the useful reflected waves and the harmful scat- tered waves were separated satisfactorily (Fig. 5b, c). Conse- 3 Removing complex near‑surface scattered quently, the quality of the seismic imaging profile, especially waves via an angle‑domain vector the left and top portion of the profile, was improved signifi - resolution de‑noising method cantly (Fig. 6). When active source seismic exploration work is carried out in a complex near-surface area, the acquired seismic records 4 Building complex near‑surface velocity normally contain many harmful scattered waves. The Tamusu models utilizing both the travel time research area is also a case in point. The scattered waves are and waveform of first arrival waves usually mixed with the effective signal (Fig. 5a), and by our research, they could not be reasonably filtered out via the tra- According to previous experience, the accuracy of near- ditional filtering methods of the time and frequency domain surface velocity would influence the total quality of seismic or wavelet domain (Li et al. 2016). profiles (Xia 2014), especially in a study area with complex To de-noise the harmful complex near-surface scattered surface conditions. waves, an angle-domain vector resolution de-noising method Thus, an acoustic wave equation-based high-resolution was adopted. This method could make use of the difference near-surface velocity inversion method, which could jointly 1 3 Petroleum Science (2021) 18:742–757 745 (a) TMS1 elevations 0100020003000 4000 5000 6000 7000 8000 9000 10000 Distance, m (b) TMS2 elevations 01000 2000 3000 4000 5000 60007000 8000 9000 1000011000 1200013000 1400015000 16000 Distance, m Fig. 3 Elevations along the TMS1 (a) and TMS2 (b) lines utilize both the travel time and waveform of first arrival d x , t +Δ;x d x , t;x r s r s obs cal (2) E = dt waves, was adopted in the present research. The fundamen- t t tal theory of this method was proposed by Luo and Schuster in 1991. Its basic procedure includes: firstly, get simulated where Δ x ;x is the time difference of the first r s data (Yao et al. 2016) by solving acoustic wave equation arrival waves of observed data and simulated data. and work out a kind of pseudo-residual data via a link func- d x , t;x represents the calculated simulated data. cal r s tion which relates the simulated data and the observed data; d x , t +Δ;x represents the observed data, which obs r s secondly, we are supposed to minimize the pseudo-residual have been corrected by Δ . d x , t;x represents the pres r s data with the classical full-waveform inversion engine (Yao pseudo-residual data. E is the energy term related to the and Wu 2017; Li et al. 2020; Liu et al. 2020). time derivative of d x , t;x and d x , t +Δ;x ; cal r s obs r s The abovementioned method has been applied to cross- 3. Backward-propagate the pseudo-residual data to calcu- hole seismic data effectively. In the present research, we late the pseudo-residual wavefield, meanwhile recon- adopt it to process ground-observation seismic data on struct the source wavefield; the complex near-surface area. The detailed algorithm is 4. Calculate the correction gradient direction g() of the elaborated as follows: velocity model via Eq. (3), and calculate the update direction via a kind of conjugate gradient method (e.g., 1. Calculate the simulated seismic data via solving the Conjugate Descent scheme, Fletcher–Reeves scheme, acoustic wave equation; Hestenes–Stiefel scheme, etc.); 2. Calculate the pseudo-residual data of first arrival waves p , t;x p , t;x via the following equations, s s r s g() =− dt (3) t t v () s∈S d x , t;x 2 cal r s (1) d x , t;x =− Δ x ;x pres r s r s where p , t;x is the reconstructed forward-propagat- E t s s ing source wavefield. p , t;x is the backward-prop- r s and agating pseudo residual wavefield. v() represents the current velocity model; 1 3 Elevation, m Elevation, m 266 6000 265 5976 264 5952 263 5928 262 5904 261 5880 260 5856 257 5784 256 5760 255 5736 254 5712 253 5688 252 5664 251 5640 250 5616 249 5592 248 5568 247 5544 246 5520 244 5472 243 5448 242 5424 241 5400 240 5376 239 5352 238 5328 237 5304 235 5256 233 5208 230 5184 227 5160 224 5136 221 5112 218 5088 216 5040 213 4968 210 4944 207 4920 204 4728 201 4704 198 4680 197 4656 196 4632 195 4608 194 4584 193 4560 192 4536 191 4512 190 4488 189 4464 188 4440 187 4416 186 4392 746 Petroleum Science (2021) 18:742–757 (a) 200 800 1400 2000 2600 3200 3800 4400 5000 5600 6200 6800 7400 8000 8600 9200 9800 (b) 0.20 0.40 0.60 Fig. 4 The redesigned cross-obstacle seismic geometry (a) and the final profile (b) utilizing the new geometry 5. Take the sum of the square of the pseudo-residual data In addition to the above, there were some details required of all the shots as the objective function, which is math- manual intervention when real data were processed. The ematically expressed as, total processing procedure of the seismic data of Tamusu is described below. C = d x , t;x dt (4) r s pres s∈S 4.1 Pick up the travel time of first arrival waves and calculate the iteration step length via a three-point parabola method. In the total inversion process, travel time of first arrival 6. Update the model and go to the next iteration. waves of the observed data needs to be picked up once. But because real data normally includes a mass of noises and As shown in Fig. 3, the ground surface of the Tamusu abnormal values, this process needs human intervention to area is rugged. To address the issue of an irregular free- ensure the results’ quality. The travel time of first arrival surface, we adopt an algorithm called the acoustic- waves of the TMS1 and TMS2 seismic lines are shown in elastic boundary approach (Xu et al. 2007) which deals Fig. 8a, b. with the model’s top free-surface by modifying the den- sity and lame constants on special positions. As shown 4.2 Waveform data preparation in Fig. 7, the red and blue triangles separately indicate the positions on which and should be modified x z The frequency band of the raw data of Tamusu is about ( = 0.5, = 0.5 ) in a staggered-grid finite-difference x z 16–80 Hz, which is slightly higher for the waveform inver- model. The black points indicate where should be set sion research. Hence, the filtering work should be done to 0 ( = 0). extract the low-frequency components of the observed data. 1 3 TWT, s Petroleum Science (2021) 18:742–757 747 Traces 121416181 101 121 141 161 181 201 221 241 261 281 (a) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Traces 121416181 101 121 141 161 181 201 221 241 261 281 (b) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Traces 121416181 101 121 141 161 181 201 221 241 261 281 (c) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Fig. 5 Results of removing scattered noises: original common-shot gather (a), de-noised common-shot gather (b), and the separated noises (c) According to the research, we finally chose a band-pass typical data processing results of seismic data of TMS1 filter with the parameters of (4.8–24.48 Hz). To avoid (FFID = 101) and TMS2 (FFID = 201) are shown in Fig. 9. the disturbance that came from background noises and late-arrival waves, we only left the first arrival wave. The 1 3 Time, s Time, s Time, s 748 Petroleum Science (2021) 18:742–757 Distance, m 800 2000 3200 4400 5600 6800 8000 9200 1040011600 1280014000 (a) 0.25 0.50 0.75 Distance, m 800 2000 3200 4400 5600 6800 8000 9200 1040011600 1280014000 (b) 0.25 0.50 0.75 Fig. 6 The top portion of the reflection imaging profiles obtained from the original common-shot gathers (a) and the de-noised common-shot gathers (b) x on horizontal freesurface on vertical freesurface 35 3000 Normal grid position on freesurface 41 Normal grid position 45 0 620625 630635 640645 650655 660665 670 X-Grids Fig. 7 Model parameter modification scheme for irregular free-surface of acoustic-elastic boundary approach iterations, and the calculation of the TMS2 line stopped 4.3 Initial models’ building after 71 iterations. The updated velocity models obtained in the first, middle, and last iterations are displayed sepa- To carry out the joint inversion of travel time and wave- form of first arrival waves, initial velocity models were rately in Fig. 11. built via an MSFM (multi-stencil fast marching) based ray-tracing inversion algorithm (Lu et al. 2013). The initial 4.5 Analysis of the pseudo‑residual of first arrival waves models built for the TMS1 and TMS2 seismic lines are displayed in Fig. 10a, b. To check the inversion processing, parts of pseudo-resid- ual data and the corresponding observed data and simu- lated data are displayed in Fig. 12. It could be found that 4.4 Velocity model updating process the value of initial velocity models built via the ray-tracing inversion algorithm tended to be small, because in the first Started with the initial models mentioned above, the inversion calculation of the TMS1 line stopped after 68 iteration, the arrival-time of the simulated data (blue line) felled behind that of the real data (red line). As iteration 1 3 TWT, s TWT, s Z-Grids Petroleum Science (2021) 18:742–757 749 0.5 (a) Travel time of first arrival waves of TMS1 0.4 0.3 0.2 0.1 02000400060008000 10000 12000 14000 16000 Station locations, m 0.5 (b) Travel time of first arrival waves of TMS2 0.4 0.3 0.2 0.1 02000400060008000 10000 12000 14000 16000 Station locations, m Fig. 8 Manually picked travel time of the first arrival waves of the TMS1 (a) and TMS2 (b) lines Traces Traces Traces Traces Traces Traces 60 120 180 240 60 120 180 240 60 120 180 240 60 120180 24060120 180240 60 120180 240 (a) (b) Original data Filtered data First arrival Original data Filtered data First arrival TMS1, ffid101 TMS1, ffid101 TMS1, ffid101 TMS2, ffid201 TMS2, ffid201 TMS2, ffid201 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 0.8 0.8 -22 0 -11 0 -11 0 -22 0 -11 0 -11 0 11 11 ×10 ×10 Fig. 9 Examples of filtering and extracting first arrival waves: single shot of TMS1 (a), single shot of TMS2 (b). Left: raw data; middle: filted data; right: first arrival waves; cyan line: travel time picked by manual 1 3 Time, s Time, s Time, s Time, s 750 Petroleum Science (2021) 18:742–757 MX = 5104, dx = 2 m 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 (a) Initial velocity model of TMS1 MX = 8501, dx = 2 m 2000 4000 6000 8000 10000120001400016000 (b) 1270 4000 Initial velocity model of TMS2 2500 Fig. 10 Initial velocity models for the TMS1 (a) and TMS2 (b) lines went on, the difference between the red line and the blue tomographic static correction, migration, etc., could benefit line decreased gradually. from the models as well. The green line represents the waveform matching results which were obtained through the calculation of waveform cross-correlation. In the first iteration, they were not continu-5 Seismic interpretation ous and regular enough. This phenomenon could be attrib- uted to the waveform of first arrival waves of the observed For seismic interpretation in the present research, it is data and the simulated data had large differences. As the iter - important to take distinctions into consideration between ation continued, the continuity and regularity of the green the high-velocity rock seismic exploration and those carried line were improved. What’s more, it could also be observed out on sedimentary environments (Yuan et al. 2017). Few that the amplitude of the pseudo-residuals became smaller studies of seismic imaging profiles on granite body have until hardly could be seen. All of these indicate the model’s been published in China. But generally speaking, because updating was in the right direction for reducing the travel the velocity of granite would normally exceed the values of time and waveform residuals. its surrounding rocks, the seismic events from rock bounda- ries, e.g., the side or top boundary, would be characterized 4.6 Analysis of the evolution of the objective by strong amplitude and good continuation. However, the function seismic events within the rock are normally disordered and with low-frequency, intermittent, worm-like forms. The As mentioned above, we have taken the sum of the square of unconformable contact relationships may be observed on pseudo-residual data of all the shots as the objective func- the flanks of the granite rock or between the rock with its tion. Figure 13 shows the evolution of the objective function, overlying strata. in which we could see the value decreases with the itera- According to regional geological research (Wang tion number increasing. The reason why the curve heightens et al. 1998; Liu and Zhang 2014), the study area could be at some points is due to the fact that we have set the top divided into two geologic zones: a Late Paleozoic plutonic and bottom limitations for the velocity model, when some zone in the northwest and a Mesozoic–Cenozoic basin in value excesses the limits, the value would be clipped and the southeast (Fig. 14). The Late Paleozoic plutonic zone the model would be smoothed again to remove the sharp consists primarily of Late Paleozoic plutonic rocks. And interfaces. there are a few residues of Early Paleozoic plutonic rocks Based on the features shown in Figs. 12 and 13, the final and Precambrian metamorphic crystalline basement, as updated models of TMS1 and TMS2 could be deemed reli- well as some Indo-Chinese plutonic rocks. The deposits able. The obtained near-surface velocity models could be of the Mesozoic–Cenozoic basin are composed primarily used as important references for the seismic interpretation. of Jurassic–Cretaceous sandy conglomerate. In the study Meanwhile, the subsequent seismic processing steps, such as area, the granite rock mass is always NEE extension and there are almost no fold structures. The major structural 1 3 MZ = 183, dz = 2 m MZ = 110, dz = 2 m Petroleum Science (2021) 18:742–757 751 Distance, m (a) 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Updated model (iters: 001) Distance, m 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Updated model (iters: 036) Distance, m 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Updated model (iters: 068) Distance, m (b) 2000 4000 6000 8000 10000 12000 14000 16000 1270 4000 Updated model (iters: 001) Distance, m 2000 4000 6000 8000 10000 12000 14000 16000 Updated model (iters: 036) Distance, m 2000 4000 6000 8000 10000 12000 14000 16000 Updated model (iters: 071) Fig. 11 Velocity model updating processes of the TMS1 (a) and TMS2 (b) lines features of the study area consist of faults with different 5.1 TMS1 seismic line strikes and ductile shear zones (Fig. 14). Figure 15 shows the interpretation results of the TMS1 seis- mic line, based on the near-surface velocity profile (Fig. 15a) and the reflection profile (Fig. 15b). 1 3 Elevation, m Elevation, m Elevation, m Elevation, m Elevation, m Elevation, m 752 Petroleum Science (2021) 18:742–757 Traces Traces Traces Traces Traces Traces 60 120 180 240 60 120 180 240 60 120 180 240 60 120180 24060120 180240 60 120180 240 (a) (b) 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 TMS1, dobs101 TMS1, dcal101 TMS1, pres101 TMS2, dobs201 TMS2, dcal201 TMS2, pres201 (input data) (iters: 001) (iters: 001) (input data) (iters: 001) (iters: 001) 0.8 0.8 -10 1 -10 1 -10 1 -10 1 -10 1 -10 1 -6 -6 ×10 ×10 Traces Traces Traces Traces Traces Traces 60 120 180 240 60 120 180 240 60 120 180 240 60 120180 24060120 180240 60 120180 240 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 TMS1, dobs101 TMS1, dcal101 TMS1, pres101 TMS2, dobs201 TMS2, dcal201 TMS2, pres201 (input data) (iters: 036) (iters: 036) (input data) (iters: 036) (iters: 036) 0.8 0.8 -10 1 -10 1 -10 1 -10 1 -10 1 -10 1 -6 -6 ×10 ×10 Traces Traces Traces Traces Traces Traces 60 120 180 240 60 120 180 240 60 120 180 240 60 120180 24060120 180240 60 120180 240 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 TMS1, dobs101 TMS1, dcal101 TMS1, pres101 TMS2, dobs201 TMS2, dcal201 TMS2, pres201 (input data) (iters: 068) (iters: 068) (input data) (iters: 071) (iters: 071) 0.8 0.8 -10 1 -10 1 -10 1 -10 1 -10 1 -10 1 -6 -6 ×10 ×10 Fig. 12 Example analysis of the pseudo-residual of first arrival waves of the TMS1 (a) and TMS2 (b) lines left: observed data; middle: simu- lated data; right: pseudo-residual data; red line: first break of observed data; green line: first break of simulated data; cyan line: waveform march- ing result 1 3 Time, s Time, s Time, s Time, s Time, s Time, s Petroleum Science (2021) 18:742–757 753 -4 -3 ×10 ×10 6 8 (a) (b) 2 2 Sum of pres of TMS1 Sum of pres of TMS2 0 0 0204060800 20 40 60 80 Iterations Iterations Fig. 13 Evolution curves of the objective function of the TMS1 (a) and TMS2 (b) lines 103°00′ 103°04′ 103°08′ 103°12′103°16′ 103°20′ 40°50′ P3ο P ο P3 P3ο P ο P ο Chagantaolegai Delewulan T1 Huobenghuren P3 Hao Nuoergong P3ο Yihetukemu 3 P3 10.196 km Haoyaoer Aobao pl P3 Qp P ο 3 16.856 km Chaker Taolegai T P3ο P2ο P3ο P3 Huhetaolegai Huduge Haizi Adergen pal Qh pl+eol Qh Huhe Wenduoer P3 Yihe Erbeng 40°46′ T pal Qh T P3 P3 Pt3bpg P3 Har Zhaoenji Hanggale P3 Baga Erbeng T T Target Granite outcropping Zone C1 Pideritu TMS2 Pt3bpg Kouergen Huduge Pt bpg P3 TMS1 P C1 0 km Chagansai Pt bpg J j C1 1-2 40°42′ pal 3 Qh New Huduge P K1b Huhe Nuoergong P ο pl Qp3 P2 P Hadan Nuoergong 2 pl Pt phg Harzhagai 3 J1-2j Qp3 P3 Pt3sch K b J j 1 1-2 Argashun T Zhagaitu Pt3sch pl Qp3 P3 P Zhgn 2 pl Qp3 K b P K1b 1 pal Qh J j 1-2 Figure legend pl Qp pal 3 Qh K b Interpreted fault Geological drill Ductile shear zone 1 K b Yihe Hairhan 1 pl Qp3 J j 1-2 0 km Geological Ductile-Brittle pl pal Seismic line K1b Qh Qp3 Meso-Cenozoic Basin boundary deformation zone Baga Hairhan 40°38′ Fig. 14 Regional geological map of the seismic work area On the top of Fig. 15b, there is a high-energy reflection from 2000 m/s to about 4200 m/s. As can be seen, the upper wave group composed of two seismic events with good boundary of the high-velocity part in Fig. 15a is consistent lateral continuation. In Fig. 15a, at the depth of 40–50 m with the lateral variation of the strong seismic events on the below ground surface, the velocity value increases rapidly top of Fig. 15b. Comprehensively speaking, it could be spec- ulated that the high-energy reflections on the top of Fig. 15b derive from the top interface of the granite rock. Above the granite, there exist sedimentary and weathered strata, which 1 3 Objective function Objective function 754 Petroleum Science (2021) 18:742–757 pl+eol (a) S T Pt3bpg T Qh P ο P P ο N 3 3 3 Distance, m 100 700 1300 1900 2500 3100 3700 4300 4900 5500 6100 6700 7300 7900 8500 9100 9700 pl+eol (b) S T Pt3bpg T Qh P3ο P3 P3ο N Distance, m 100 7001300 1900 2500 3100 3700 4300 4900 5500 6100 6700 7300 7900 8500 9100 9700 0.20 0.40 0.60 R2 R1 0.80 1.00 Fig. 15 Seismic interpretation of the TMS1 line: near-surface velocity profile (a); reflection profile (b) shows low-speed. Below the top interface of the rock, the or interruptions of reflection events at the corresponding reflection events are weak and irregular (Fig. 15b), which position in Fig. 15b. These phenomena could indicate the could represent that the wave impedance differences within presence of a small fracture labeled as FP4. the rock are small. Based on the position of the faults, FP3 is speculated Based on the regional geology findings (Fig. 14), there to control the southeastern boundary of the Haizi Adergen was one fault (named Yihe Erbeng fault) across the middle ductile shear zone (Fig. 14), and FP2 is speculated to con- part of the TMS1 seismic line. It could be observed that the trol the southern boundary of the shallow graben which strong reflection wave group on the top of Fig. 15b obvi- is filled with Quaternary. The FP3 fault has been revealed ously disconnects beneath the stack 4599 m. In Fig. 15a, the as a normal fault in its upper part and a reverse fault in its high-speed portion nearly emerged from the ground at the lower part, which is considered to be related to multistage corresponding position. With all the above evidence consid- tectonization under a stress environment that had changed. ered, a southward-dipping reverse fault (labeled FP1) was A few arcuate reflection events such as R1 and R2 are interpreted. recognized in Fig. 15b, the phenomenon of which is con- Beyond that, the other three fault points (labeled sidered to be related to the nonuniform property inside FP2–FP4) could be interpreted in TMS1 profiles. There is the rock. a low-speed anomaly beneath the stack 6100–7100 m in Fig. 15a. Comparing this phenomenon with the reflection 5.2 TMS2 seismic line imaging profile (Fig. 15b), we could speculate that a north- ward-dipping fault (labeled FP2) and a southward-dipping Figure 16 shows the interpretation results of TMS2 seismic fault (labeled FP3) exist. FP2 extended downward to FP3, line, based on the near-surface velocity profile (Fig. 16a) and and the FP2, FP3 faults commonly controlled the Quater- the reflection profile (Fig. 16b). nary sedimentary sequence between them. In addition to What is interpreted at first is the southern unconform- this, there was a velocity reversal between FP2 and FP3 in ity (labeled AU3) of the target granite. We could see there Fig. 15a. Meanwhile, there are phenomena of distortions is a clear color edge below the stack 4240 m in Fig. 16a. 1 3 Elevation, m TWT, s Velocity, m/s Petroleum Science (2021) 18:742–757 755 pl pal pl S N Qp K b J j P ο P T P T Qh Qp P (a) 3 1 1-2 2 3 3 3 3 Distance, m 200 800 1400 2000 2600 3200 3800 4400 5000 5600 6200 6800 7400 8000 8600 9200 9800 10400110001160012200 1280013400 1400014600 1520015800 16400 pl pal pl S N Qp K b J j P ο P T P T Qh Qp P (b) 3 1 1-2 2 3 3 3 3 Distance, m 200 800 1400 2000 2600 3200 3800 4400 5000 5600 6200 6800 7400 8000 8600 9200 9800 10400110001160012200 1280013400 1400014600 1520015800 16400 FP7 FP9 AU1 0.20 FP5 AU2 FP8 0.40 AU3 R3 FP6 0.60 R2 0.80 R1 1.00 1.20 Fig. 16 Seismic interpretation of the TMS2 line: near-surface velocity profile (a); reflection profile (b) pl Besides, there is a group of strong, steep reflection events series of Pleistocene (Qp ). As the old strata (K b) crops 3 1 in Fig. 16b. On south of AU3, the reflections are numerous. out the ground, AU1 is discontinuous. What’s more, the neighboring reflections generally exhibit On the north side of AU3, there is a group of high-energy parallel or subparallel, all of which reveal that the reflection reflections with good lateral continuation on the top por - events on the south side of AU3 derive from a sedimentary tion of Fig. 16b. It could be speculated that the reflections environment. But on the north of AU3, the number of reflec- derive from the top of the granite. The fluctuation changes tions decreases obviously, meanwhile most of them seem of this reflection group are in perfect accordance with the disordered and weak. With the regional geological findings color edge of the high-speed portion in Fig. 16a. Meanwhile, considered, AU3 is interpreted as the southern boundary of there is good correspondence between the continuations of the target granite, the south of which is the Mesozoic–Ceno- the top reflection with the changes of outcrops’ lithology as zoic basin. shown in Fig. 14. Furthermore, it could be inferred that two unconformities There are some groups of arcuate reflection events (labeled AU1 and AU2) and two faults (labeled FP5 and (labeled R1–R3), which could be recognized in Fig. 16b. FP6) exist within the Mesozoic–Cenozoic basin. As shown This phenomenon may be related to the nonuniform property in Fig. 16b, the neighboring reflections are usually paral- within the rock mass. lel in local regions. However, on both sides of AU1 and Near the north end of the TMS2, based on Fig. 16a and b, AU2, clear crossing angles between the overlying and the three faults (labeled as FP7–FP9) are interpreted. nether reflection events are demonstrated. This phenomenon There is a distinct low-speed zone in Fig. 16a. As well is a typical characteristic of the angular unconformity on as a distinct dislocation phenomenon of reflections exists reflection imaging profiles. In addition, Fig. 16a exhibits beneath the stack 16,440 m in Fig. 16b. Meanwhile, the dis- rapid changes of velocity at the corresponding locations of tortions and interruptions phenomena of reflections could AU1 and AU2. Based on the research of regional tectonics, be observed. All of these phenomena reveal that one south- AU3 reflects the unconformable contact between the lower- ward-dipping fault (FP8) exists beneath the TMS2 seismic middle Jurassic Jijigou Formation (J j) and the middle Per- line. 1–2 mian quartz diorite (P δο). AU2 represents the unconformity According to the reflection wave features and the veloc- between the nether lower-middle Jurassic Jijigou Formation ity reversal phenomenon next to FP8, we could infer that (J j) and the overlying lower Cretaceous Bayingebi Forma- two small northward-dipping faults (labeled FP7 and FP9) 1–2 tion (K b). AU1 represents the unconformity between the exist, and they merge downward with FP8. As shown in nether strata (K b) and the overlying widespread proluvial the regional tectonic map (Fig. 14), the north of the TMS2 1 3 TWT, s Elevation, m Velocity, m/s 756 Petroleum Science (2021) 18:742–757 seismic line crosses the southwest corner of the Cretaceous target rock is not thin or has been transported by geo- Yihetukemu basin which is covered by Quaternary. Thus, logical process from somewhere else, but a native and FP7 and FP8 faults just correspond to the two boundaries of massive rock; ② a few small-scale local fractures whose the southwest corner of the basin. space distribution and characteristics were revealed by The reason why FP8 shows normal property in its upper the seismic profiles exist, and then keeping away from part and demonstrates reverse property in its lower part is these fractures is needed to be taken into account. considered to be related to multistage tectonization under different stress environments. Acknowledgements This research was supported by the National Key R&D Program of China (No. 2018YFC1503200), the Nuclear Waste Geological Disposal Project ([2013]727), the National Natural Sci- 6 Conclusions ence Foundation of China (Grant Nos. 41790463 and 41730425), the Spark Program of Earthquake Sciences of CEA (XH18063Y), and the Special Fund of GEC of CEA (YFGEC2017003, SFGEC2014006). The present research adopts seismic exploration technology, We sincerely thank the two anonymous reviewers of this article, for which has a long history for serving the petroleum industry, their earnest and patience, and have suggested the explicit, detailed, to address the issue of siting a geological disposal repository constructive revisions for our manuscripts. for high-level radioactive nuclear waste in the nuclear indus- try. The techniques presented in the paper, e.g., cross-obsta- Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- cle seismic geometry design, angle-domain near-surface tion, distribution and reproduction in any medium or format, as long scattered wave removal, and acoustic wave equation-based as you give appropriate credit to the original author(s) and the source, inversion method jointly utilizing both the and waveform of provide a link to the Creative Commons licence, and indicate if changes first arrival waves, have reference value for seismic explora- were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated tion on complex near-surface areas in Western or Southern otherwise in a credit line to the material. If material is not included in China. Based on the practice of seismic exploration on tar- the article’s Creative Commons licence and your intended use is not get granite rock mass, the following conclusions could be permitted by statutory regulation or exceeds the permitted use, you will developed:travel time need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . 1. To address the problems include crossing obstacles seis- mic acquisition, harmful scattered wave removal, and References complex near-surface area velocity inversion, several targeted techniques were adopted. It could be considered Alberdi J, Barcala JM, Campos R, et al. Full-scale engineered barriers that the experience derived from the present research experiment for a deep geological repository for high-level radioac- is beneficial and worthy of reference for future stud- tive waste in crystalline host rock (FEBEX project). Luxembourg: ies. 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Petroleum Science – Springer Journals
Published: Jun 4, 2021
Keywords: Geological disposal repository; Nuclear waste; Granite body; Active source seismic exploration; Near-surface velocity inversion
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