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Method of locating buried active fault by composite drilling section doubling exploration

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Abstract

In this paper, an optimized drilling exploration method, the doubling section method, was summarized after many composite drilling section explorations of buried active fault in urban areas. Operation steps of this method are as follows; Firstly,drill a borehole at each of the two ends of the drilling section to make sure that fault is between the two boreholes, then ,drill the third borehole at the middle of the two holes; and secondly,confirm again the segment where the fault is and drill the next borehole in the middle of it. By repeating the similar practice,the accurate location of fault can be constrained progressively. Meanwhile, this paper also uses a quantitative indicator, the key horizon gradient between two boreholes,instead of stratigraphic throw,to determine the location of buried fault and puts forward two criterions: 1 ) the fault is located between two boreholes if the key horizon gradients between these two boreholes are positive and increase with depth ; and 2 ) the fault is located where the key horizon gradients between two boreholes increase obviously relative to the previous values and that of adjacent segments, besides the increase with depth. While in contrast, the key horizon gradient in a normal fault segment decreases obviously. Application cases show that the method can determine precisely the location of buried active fault.

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... The composite section exploration in Xietan Village (location shown in Figure 2A,B) was used to detect the Hetan-Guotan buried fault, and the composite section exploration in Lizui Village (location shown in Figure 2A,C) was used to detect the Zhenjing-Zhenbei buried fault. During the construction, the "folding method" is used to approach the positioning fault from the outside to the inside [23], emphasizing dynamic construction and dynamic analysis. The position of the fault is found through continuous trials. ...
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By using shallow seismic exploration, composite drilling section exploration and sample dating test, we have obtained precise positions, burial depths of uppermost point and activity characteristics of Hetan-Guotan buried fault and Zhenjing-Zhenbei buried fault in Zhongwei Basin. The results show that the latest active period of Hetan-Guotan buried fault is the middle-late Middle Pleistocene, and the latest active period of Zhenjing-Zhenbei buried fault is the Early and Middle Pleistocene. The two buried faults became inactive at the end of the Middle Pleistocene and have been inactive since the Late Pleistocene.
... According to the nature of the activity and geometric characteristics of fault F 5 , the obvious mutation point of the reflection lineups on the shallow SRS profile was selected, and combined borehole section profiling on both sides of the breakpoint was performed. The borehole arrangement followed a doubling exploration pattern [37,38], with at least three boreholes in the hanging wall and three boreholes in the footwall of the fault, and the spacing between boreholes was no more than 10 m. ...
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Yinchuan Basin is a graben-like downfaulted Cenozoic era basin located on the west edge of Ordos Massif. Its activity is violent and deposition is very thick. Yinchuan City is located in the middle of Yinchuan Basin. The seismic petroleum exploration shows that a buried active fault lies in the east of Yinchuan City, named as the Yinchuan buried fault, which strikes NNE and dips west, with a total length of more than 80km. Because the seismic petroleum exploration did not gain any explained signals at the depth ranging from 0 to 400m, so whether the Yinchuan buried fault is active or not in the late Quaternary and its exact surface projective location hasn't been known yet. It has been a "worry" in the urban planning and development of Yinchuan for a long time. Under the financial support of the national and local governments, we launched the project entitled "The prospecting of active fault and earthquake risk assessment in Yinchuan City". In order to facilitate the exploration, we selected Xinqushao village in the southeast suburb of Yinchuan City to be the site for the integrated test exploration of the Yinchuan buried fault before the exploration, based on the information obtained from the seismic petroleum exploration. Considering that the thick Quaternary sediment in Yinchuan reaches to 1609m, and that the depositional environment is the Yellow River flood plain and the lateral change of lithology is complex, we adopted in the test exploration the train of thoughts of "inferring an unknown fact from a known fact, and from deep to shallow and directly to the top". The experimentation has been developed step by step according the working order of multilevel seismic exploration→composite geological profile drilling→trenching. Along the same measuring line at Xinqushao, first, we adopted the seismic reflection exploration of primary wave in three levels with the group interval of 10m→5m→1m to catch the master fault of the Yinchuan buried fault, and by tracing upward layer by layer in the order of the three exploration ranges, i. e. 1400 ∼ 400m→600 ∼ 80m→150 ∼ 20m, the position of the master fault at ° 20m depth under the ground and its offset trace were primarily identified. And then, along the master fault and within the range of 100m at its both sides, 9 boreholes of 20.5 ∼ 100m were arranged for the composite geological profile drilling. The resulting information about the throws of the master fault was obtained, they are 20.34m, 9.66m and 2.25m respectively at the depth of 43.75m, 20.33m and 13.04m from the ground, and the buried depth of the upper offset point ≤8.34m. At the same time, using the intact core specimen from the fault plane of the borehole No. 7, we calculated the dip angle of the fault as 71°at the depth of 55.27m and figured out the exact position of its extension to the earth's surface. Finally, a large-scale trial trench, which is 40 meters long, 8 ∼ 12 meters wide and 6 meters deep, was arranged across the master fault. The trenching revealed that the actual buried depth of the upper offset point of the master fault is 1.5m and there are seismic remains, such as offsets of 5 stages, sand liquefaction and surface rupture, etc. Among the 5 stages offsets, 4 events occurred prior to 3170 ± 80 a BP, belonging to the mid to late Holocene paleo-earthquakes. The age of the last event cannot be determined and it is inferred to be the result of the M8.0 Yinchuan-Pingluo earthquake in 1737. In a word, through the comprehensive test exploration, we find that the Yinchuan buried fault is a Holocene active fault, which lays solid base for the next exploration.
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Shallow seismic surveying is a common method applied to the research of blind fault activity at the Quaternary stratum region, especially in urban area. The bifurcating, uniting, bending, intermitting and ending off of reflection waves on stacked time section are important signs which are often used to judge the existence of blind fault. At the northern area of Songhuajiang River in Harbin City, we finished two seismic reflection profiles. On the shallow seismic reflection profiles of Lugangtun and Jubaotun, the T0 wave group corresponding to the interface between Cretaceous sand-mud stone and lower Pleistocene gravel is clear. T0-1, reflecting wave group corresponding to the contact surface between gravel and clay, or fine sand and gravel, is also clear. Based on the stacking seismic profiles and borehole data, we conclude that the Ashihe fault offset the bottom stratum of lower Pleistocene, and Binzhou fault cut off the bottom stratum of upper Pleistocene. By building high quality composite drilling geological profiles, dating the samples from bores, and contrasting stratum from one bore to the other, we consider that the Ashihe fault does not offset the Quaternary formation, the fault may only exist in Cretaceous sand-mud stone. The Binzhou fault is only an early Pleistocene fault; it doesn't offset the late Pleistocene formation. Finally, we think that the bifurcating, bending, intermitting and ending off of seismic reflection wave group on Luegangtun and Jubaotun seismic profiles are not the result of fault activity, but the phase change of Quaternary formation.
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This paper makes a comprehensive analysis of the recent progress of the seismic active fault prospecting in Lanzhou city. Based on the satellite and aerial photos interpretation, geological and geomorphic investigation, geochemistry prospecting, shallow seismic investigation, resistivity imaging, drilling, especially large-scale trenching along the 7 active fault zones in Lanzhou city, we have achieved very important progress and gained new knowledge about the recent activity of main active faults and deformation features in Lanzhou Basin. The main conclusions are summarized bellow: (1) The Jinchengguan Fault is a thru st fault, constituting the northern boundary of the Lanzhou Tertiary Basin. It is revealed by geophysical prospecting and drilling that the newest strata offset by the Jinchengguan Fault are the early-Pleistocene sandstone and conglomerate, and that the overlying second and third terraces of the Yellow River remain intact. So, it's an early and middle Pleistocene active fault. ( 2) The Liujiabu Fault and Shengouqiao Fault constitute the northern and western boundaries of the Qilihe Subsidence, respectively. Revealed by geophysical prospecting, drilling and large trenching, they are not faults but lithologic boundaries of different rocks between Pliocene and early Pleistocene. (3) The Leitanhe Fault is the eastern boundary of Qi lihe Subsidence, a boundary fault separating the Tertiary Lanzhou Basin into the east and west basins. According to the geophysical prospecting and drilling, the Leitanhe Fault is a thrust fault and its newest activity age is early and middle Pleistocene. It is not active since late Quaternary and does not cut the third terrace of the Yellow River. (4) The Siergou Fault is the southwestern boundary of Lanzhou Basin, a thrust fault too. It's an early and middle Pleistocene active fault and does not offset the forth terrace of Yellow River. While the Xijincun Fault is much nearer to the south margin of Lanzhou Basin and forms the southern boundary of the Tertiary Lanzhou Basin. It's an early Pleistocene fault. (5) The northern margin of Maxianshan Mountains faul t is a major seismic fault on the southern margin of Lanzhou Basin, and its movement is characterized by segmentation. The east segment, the Neiguanying sub-fault, is a late Pleistocene fault. The middle segment, the Maxianshan and Qidaoliang faults, are active during late Pleistocene and early Holocene. The west segment, the Wusushan sub fault, is active during late Pleistocene and Holocene, and it's also the seismic fault of the M7 Lanzhou earthquake. On the whole, we correct the previous recogn itions about the activity times of 4 faults, i. e. the Jinchengguan Fault, Leitanhe Fault, Siergou Fault and Xijicun Fault. They are all early and middle Pleistocene instead of late Pleistocene active faults. Especially, we find that the Liujiabu Fault and Sbengouqiao Fault directly across Lanzhou city are not late Pleistocene or Holocene active faults but lithologic boundaries between Pliocene mudstone and early Pleistocene conglomerate. The results are very important for the urban planning and engineering construction, and will produce obvious economical and social benefits.
Article
We have conducted an integrated survey and investigation on the Quaternary activity of the Liaocheng-Lankao buried fault. Methods include geochemical exploration, shallow seismic exploration, drilling geological profile and neo-strata dating. The object is to determine the accurate location of the fault, dislocation amount of each time period since Quaternary and the offset age of the last time of dislocation. The results show that the dislocation of the fault extends upward to the depth 20m or so below the surface. This fault has been active in early Holocene time. The average slip rate of the fault is 0.12mm/a.