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Comparative experiment on seismic sources in high-resolution seismic exploration for urban active faults
Abstract
Research on a large number of seismic events at home and abroad has indicated that tremendous earthquake hazards in urban areas are mostly attributed to earthquakes caused by active faults buried beneath the cities. The identification of urban buried active faults, therefore, is an important and urgent task. High-resolution seismic exploration is an effective geophysical technique that can be used to identify urban buried active fault at present. High-resolution seismic exploration for urban buried active faults is a sophisticated and systematic project, which involves excitation and receiving techniques, observational system, as well as seismic data processing and interpretation. The seismic source is of the first importance among the other problems that should be solved during the exploration. High-resolution seismic exploration for urban active fault calls for specific performance of the seismic source, because of peculiar environment in urban areas and particular characteristics of urban buried faults. For examples, relatively small offset of the fault requires a wider source spectrum, while strong disturbances in urban areas need a higher anti-jamming capability of the source. A comparative experiment on various types of sources, including vibroseis, vacuum accelerating weight drop, hammer-blow, air gun and explosive is carried out along the traverse across the Bayishuiku Fault. The features of various source spectrums are obtained by using spectrum analysis technique. The comparison of time-stacked sections obtained by using vibroseis, vacuum accelerating weight-drop and hammer blow from the traverse across the Bayishuiku Fault in Fuzhou City is presented in this paper. The effectiveness of various seismic sources in the exploration of urban buried active faults is discussed in detail.
Detecting subsurface fault structure is important for evaluating potential earthquake risks associated with active faults. In this study, we propose a new method to detect faults using reflected surface waves observed in ambient noise cross correlation functions. Ambient noise tomography using direct surface waves obtained from ambient noise interferometry has been widely used to characterize active fault zones. In cases where a strong velocity contrast exists across the fault interface, fault-reflected surface waves are expected. We test this idea using a linear array deployed in the Suqian segment of Tanlu fault zone in Eastern China. The fault-reflected surface waves can be clearly seen in the cross-correlation functions of the ambient noise data, and the spatial position of the fault on the surface is close to the stations where the reflected signals first appear. Potentially reflected surface waves could also be used to infer the dip angle, fault zone thickness and the degree of velocity contrast across the fault by comparing synthetic and observed waveforms.
The boom-type roadheader is the main equipment for realizing the mechanization of coal drifting in coal mines, and it is an indispensable production equipment in major coal-producing countries. Substantial vibrations are generated during the operation of a roadheader; these vibrations carry substantial energy and, thus, can be regarded as a potential source and used for seismic advance detection purposes in mine drifts. Compared with a conventional exploration source, a roadheader source produces a complex continuous random signal. The shape of a seismic wavelet is uncertain and its duration is relatively long; thus, it must be processed into a conventional pulse signal before it can be used for subsequent processing and analysis. Therefore, based on the advantages of seismic interferometry in random signal processing, two seismic interference techniques, namely, deconvolution and cross-correlation, are introduced for constructing a compound interference algorithm. On the basis of a theoretically derived formula, a random signal impulse processing experiment is conducted using field-acquired source signals from a roadheader; this approach resolves the problem that cross-correlation alone cannot yield ideal results. Hence, a feasible algorithm for the impulse processing of a random signal, namely, the compound interference algorithm, is proposed. The algorithm deconvolves each seismic trace to obtain the reference trace and other receiver traces after compressing the wavelet. Then, the reference trace and each receiver trace are cross-correlated, and the wavelet time delay information of each correlated wavelet pulse, namely, the wavelet time delay information of the receiver trace relative to the reference trace, is obtained. Accordingly, the direct wave and reflected waves are recognized. To evaluate the performance of the algorithm, an algorithm application experiment is conducted for another group of random source signals that were collected by a roadheader under different coal drift conditions. Again, the algorithm processing results are consistent with the single-shot record characteristics of an explosive source. Consequently, the proposed algorithm can satisfy the requirements for engineering exploration and analysis. A comprehensive analysis further demonstrates that the compound interference algorithm is both feasible and effective and that the processed seismic signals can be used for subsequent processing and interpretation.
The investigation and study of fault activities are a basic work for urban earthquake prevention and disasters reduction. In order to find out the location, characteristics and activities of the Laoyachen Fault in Zhengzhou, the high-resolution shallow seismic P and S wave survey profiling across the Laoyachen Fault was carried out at the end of 2006, and different seismic sources along with combinations of diverse observation geometries with different parameters were used. The fine structures in different depths beneath the profile were obtained and the patterns as well as characteristics of the Laoyachen Fault were determined. The results show that the Laoyachen Fault, running in NW and dipping in NE, is a normal fault and its dip angle is about 60°-70°, which incises strata of Eocene, Permian, Carboniferous or Ordovician epoch and goes up to the top boundary of Eocene stratum at the 800-850m depth. There is no any reflector of offset stratum found in Q + N strata. The borehole geological sections across steep slopes of earth surface present that the layers inferred from reflected seismic wave groups of shallow seismic profile are well correspondent with boring geological layers. The borehole results reveal that the three reference laminas, i. e. the boundary between Malan loess and silt with clay soil at about 21 m in depth, the calcareous gravel clay layer of 53.9m deep, and the calcareous silt layer of 61.9m deep, all have not depth variations at the two sides of surface steep slopes and are situated almost at the same ground surface elevations, which suggests that the steep slopes at the earth's surface should not result from the activities of Laoyachen Fault. In this study, through shallow seismic P wave and S wave exploration as well as combination of joint borehole geological sections, not only the location and characteristics of Laoyachen Fault was determined, but the geological and seismological evidences for the fault activity estimations were provided.
Lots of different ideas stay on whether the Dongbeiwang-Xiaotangshan Fault in the northwest of Beijing exists or how it displays and acts. This paper analyzes the present seismic prospecting data and drill logs from exploration of active faults in Beijing urban areas, and reaches conclusion that the Dongbeiwang-Xiaotangshan Fault does exist as a subsurface fault and it is 40km long, trending NNE. Divided by the Nankou-Sunhe Fault, we name its northeast segment as the Xiaotangshan Fault and the southwest segment as the Dongbeiwang Fault. The Xiaotangshan Fault is active in early Pleistocene but not in middle Pleistocene. The Dongbeiwang Fault is active in the end of mid-Pleistocene but not in late Pleistocene.
This paper gives a brief account of the significance of urban active fault exploration and presents a general review of active fault exploration around the world. Earthquakes provoked by active faults directly beneath the metropolises can cause serious disasters to the cities. If urban active faults are precisely determined and effective precautions are taken, losses at the time of earthquake occurrence can be greatly reduced. We elaborate on various kinds of possible geophysical methods for seismic active fault exploration and their main features. We also discuss the scope of application of related geophysical methods and the major problems that they can solve in the different periods of active fault exploration, such as regional survey or preliminary investigation, detailed exploration or precise location, and the identification of seismogenic structures.
Vibrations induced by train moving on railway tracks could arouse elastic waves in the shallow crust, which radiate on the ground surface in a long distance around the train lines. We recorded some of the ground waves along the Datong-Qinling railroad in Northern China, with two sets of broad-band and one set of short-period seismometers. 38 shock events on 9 sites were clearly recorded at a distance of 0-2.15km to the railroad. Observations show that the vibrations induced by a running train would cause significant ground trembling in a distance of 2-3km, and the flat frequency of the seismometers covers 0.05 to 20Hz. The broad frequency range and repeatability of the trembling indicates that the vibration could be used as a seismic source to detect shallow structures in the crust. It is expected that a new kind of exploration source with related new observing systems, new data processing methods and new crustal models will bring lights to the traditional exploration seismology.
The method and principle of common offset seismic survey as well as the field data gathering and processing technique were introduced briefly. Through two urban active fault survey examples in Fuzhou and Shenyang, the efficiency and limitation about using common offset seismic reflection technique to carry out urban active fault survey were probed. The results show that this technique has the properties of high resolving power, better reconstruction of subsurface structures, and real-time analyzing and interpreting of the investigating results on site. This method can be used to quickly locate the investigating objects accurately in the areas with thinner Q overburdens and strong bedrock interface fluctuations.
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.
The methods and schemes of detecting buried fault structures using microtremor survey method are introduced, by which two branch faults of the Tanlu fault zone are explored. Comparison with the shallow seismic exploration results indicates that the spatial locating ability of microtremor survey method is better to detect the buried fault structure, the location error of fault caused by lithological differences is close to that from the results of the shallow seismic exploration. For its deep exploration ability, the microtremor survey method can reveal the spatial distribution characteristics as well as contact relationship between the strata and the faults beneath the microtremor survey arrays, which is important to identify and confirm the main rupture surface of faults. However, the microtremor survey method is difficult to reveal or describe the detail features of faults. To explore the strata or faults, it is the best to combine the shallow seismic exploration method with the microtremor survey method to study the relationship between deep and shallow faults. The microtremor survey method can be used to detect fault roughly, and then the shallow seismic method can be used to determine accurate position of faults after knowing the general location and spatial characteristic of faults.
Vibrations induced by a moving train can excite elastic waves in the shallow crust, which spread on the ground surface to a long distance around the train lines. We recorded some of the ground waves along the Datong-Qinling railroad in Northern China, with two sets of broad-band and one set of short-period seismometers. 38 shock events on 9 sites were clearly recorded at a distance of 0–2.15km to the railroad. Observations show that the vibrations induced by a running train can cause significant ground trembling at a distance of 2–3km, and the flat frequency of the seismometers covers 0.05Hz to 20Hz. The broad frequency range and repeatability of the trembling indicate that the vibration can be used as a seismic source to detect shallow structures in the crust. It is expected that a new kind of exploration source with related new observation systems, new data processing methods and new crustal models will bring lights to the traditional exploration seismology.
Seismic exploration is an effective geophysical means for detecting buried faults. The spread geometries of seismic exploration are the main factors to affect the exploration results as well as the accuracy of fault location. Based on the shallow seismic exploration data from an active fault survey in Yinchuan city and in combination with the petroleum seismic exploration profiles and drilling data, this paper proposes an upward fault-tracing method for determining the geometry of the target fault and its upper breakpoint in thick Quaternary sediments. The result shows that the combination of high resolution shallow seismic exploration and borehole geological section is an effective way for locating the buried fault and determining its activeness.
The chenaisorption of atomic hydrogen and deuterium on clean and Cl-covered Ag(111) has been studied using temperature programmed desorption (TPD), work function change (δφ) measurements and ultra-violet photoelectron spectroscopy (UPS). The desorption of molecular hydrogen and deuterium follows a fractional-order kinetic description with an activation energy of 5.5 kcal/mol (H2) and 6.4 kcal/mol (D2). Based on these results and the absence of dissociative chemisorption at 100 K, an upper limit for the Ag-D(H) bond of 53. (52) kcal mol−1 is derived. UPS reveals an H(1s)-derived bonding state at about 7.3 eV below the Fermi level. There are no significant changes in the shape of the Ag d-band, consistent with its, at most, very weak involvement in the D-Ag bond. Saturation exposure of the clean Ag(111) surface to atomic D results in a Δφ of −0.17 eV. Surface Cl alters the bonding and reduces the saturation coverage of D, results that are interpreted in terms of each Cl controlling a large number of D(a) adsorption sites and leading to widely varying local surface coverages.