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Application of CSAMT to buried active faults investigation
Abstract
There are multiple Quaternary buried active faults in the eastern Beijing plain region based on the results obtained from geological and geophysical studies. In order to investigate the condition of bedrock surface undulation, geometry of the buried faults and the extension of fault planes in the eastern Beij'ing plain area,two CSAMT profiles were completed in Miaojuan,Shunyi district and Sunhe,Chaoyan district in early 2010. The paper presents the advantages of CSAMT in exploration of buried active faults,the data collection procedure and the methods used for data processing. Combining with regional geological data,we make integrate interpretation to the geological structures of the study region. The results show that the CSAMT method can be used effectively in the exploration of buried faults. It can reveal the location,dip,displacement,and size of faults in the survey area, which provide reliable and elemental information for geological analysis. This method has become an important geophysical tool in buried fault detection and plays an increasingly important role in exploration of active faults in urban areas.
... In 2010, Li Xiaochang surveyed the structure of a block of Longwei Lake in the north ginger pond basin on the Qinghai-Tibet Plateau using continuous electromagnetic profiling [5]. Also in 2010, Dong Zeyi et al. detected a series of active concealed faults in the Beijing Eastern Plain using controlled source audio-frequency electromagnetic method [6]. However, the application of multi-frequency electromagnetic profiles in the investigation of the distribution of bronze at archaeological sites is still very rare. ...
... The two sets of ruins are similar in size, type, quantity of relics, and geographic scope. From the large number of precious relics unearthed in and the large buildings around it, It is highly likely that Jinsha ruins was a political, economic, and cultural center of the ancient Shu State between late Shang and Western Zhou dynasty [1,6,7].The paper describes the use of multi-frequency electromagnetic profiling to detect the distribution of bronze ware in the ritual areas of the Jinsha ruins. ...
Like the Sanxingdui ruins, the Jinsha ruins are an important archaeological site in the Chengdu Plain. Excavation of these ruins may offer important evidence for the origins and evolution of the ancient Shu civilization. In order to protect historical relics during excavation, it is necessary to use a nondestructive approach when determining the distribution of those relics. Multi-frequency electromagnetic profiling is one nondestructive geophysical exploration technique. With this method, the metal bodies underground can be detected based on the different conductivities of rocks and minerals in the crust. In the present paper, multi-frequency electromagnetic profiling was used to study the distribution of bronze in the sacrifice area of the Jinsha ruins, and the data were processed with multi electromagnetic dipole forward modeling. The distribution of bronze derived from multifrequency electromagnetic profiling was in accordance with the results of site excavation, proving that multi-frequency electromagnetic profiling method is an effective and nondestructive means of assessing the distribution of bronze ware in archaeological ruins.
In this part, the main characteristics of the giant ore fields and deposits formed in every tectonic domain will be discussed. This part mainly aims to research the giant ore-field structures for the endogenic metallic deposits. The main characteristics of the super-large, giant ore fields and deposits are listed in the Appendix. This monograph pays more attention to the ore-field structures of endogenic metallic deposits and their ore-controlling tectonics, which may be very useful to explore the ore fields and deposits in the future.
The Daixian Basin, a typical asymmetric half-graben basin, lies in the northern Shanxi Fault zone. It is bounded by the northeast trending Wutaishan northern piedmont fault zone and Hengshan southern piedmont fault zone. To investigate the shallow electric structures beneath this basin and the boundary faults, a CSAMT profile was applied which runs northwesterly through the basin. CSAMT data were recorded at 246 locations along the 12.55km-long profile. At each location data were recorded from 8533.333 to 1.333333 Hz and processed to give estimates of apparent resistivity and phase. The profile is through the Wutaishan Fault in the southeast and arrives at the Hengshan piedmont loess hilly region in the northwest. Some data are of low signal-to-noise ratio due to ractive noises from power lines, pipelines, railways, highways and power substations. But useful data can be acquired 200meters away from the interference sources. The curves of apparent resistivity and phase have an obvious segmentation character. The shape of the curve of each section is consistent to the adjacent section and the segmentation shows good correlation with geomorphologic features. NLCG inversion was performed on the CSAMT data along this profile to reveal the electric structure within the depth of 1km from the surface in the basin. The result shows that the Wutaishan northern piedmont fault zone consists of a set of north-dipping normal faults which decline down stepwise, verging toward the Daixian Basin. Because the CSAMT profile does not run through the whole Hengshan southern piedmont fault zone, only partial features of it are revealed, i. e. the southeast-dipping high-resistivity foot wall bedrock and low-resistivity hanging wall alluvial deposits are underlain by thin-bed low-resistivity flood deposits. Besides, it is found that the Daixian Basin is laterally inhomogeneous, corresponding to its topography, where the tilting alluvial-flood plain is underlain by low-resistivity layer and the resistivity below the alluvial plain is slightly higher. The application of CSAMT to the Daixian Basin indicates that this method can be used to determine the location, dip and size of faults, and provide reliable basic data for survey of active faults.
The feasibility problem of application of CSAMT to prospecting active fault in the West Route of South-to-North Water Transfer Project is studied. The study results show that CSAMT can be applied to prospect the characteristics of deep region of active fault for engineering design. Some beneficial conclusion and advice about geological prospect are obtained.
Displacements along active faults buried directly beneath major cities create devastating earthquakes that seriously threaten human lives and properties. Exploration and seismic hazard assessment of active faults in urban areas are thus important for disaster mitigation in major cities. It is also a new field for active tectonic studies. The kernel of this work includes determining of the exact location of active faults, dating the ages of last tectonic activity, relating the shallow level faults to structures in the crustal depth, assessing seismic hazard and potential for surface offsets, and formulating countermeasures for disaster mitigations. We use the following simple phrases to express these key scientific problems: Where are the faults? Are they active? How deep are they? Will they create earthquake? Will they form surface offsets? What are the countermeasures? This paper explains these key scientific problems in detail.
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.
Of the geoelectromagnetic techniques, multi-electrode Direct Current (DC) and Transient Electromagnetic (TEM) methods are the most powerful tools for shallow soundings. For city active fault detection, urban noise is a key problem in the use of electrical or electromagnetic methods. Effects of some major noises on DC and TEM are discussed on the basis of the experiments carried out in Fuzhou City in 2001. The experiments show that underground noise (pipes, cables, etc.) are most harmful to DC soundings, while for TEM, in addition to the underground noise, aerial noise (power lines, metal sheds, etc.) will also lead to serious effects. Even so, effective soundings can be obtained providing that the noise is not too strong and some proper countermeasures are taken. In the experiments, specific measurement environments, including aerial and underground power lines and cables, water supply pipelines, roads, metal sheds, waste disposal sites, etc., were chosen as the urban noise sources. A set of RESECS instruments from DMT, Germany, were used for DC test, and EM-47/EM-67 by Geonics, Canada, for TEM test. The results of the experiments show that for DC soundings if the underground noise is not too strong, an effective record generally can be obtained, and especially the results would be much improved if the sampling time window of the instrument could be adjusted according to the noise distribution. We strongly recommend, therefore, that an instrument with real-time display of injection current and measurement potential, having adjustable time window be used for the active fault detection in urban areas. The configuration of electrodes is also important in some cases. For TEM soundings, it is better to set the measurement traverse at least 50m away from power lines, roads, cables, and big pipes. If the pipes are not big and not densely distributed within the transmitter loop, good data can be obtained at the sites several meters away from the pipes. In an area with electric current channeling, different configurations of the transmitter loops of different dimensions should be tested before the measurement.
Geological and geophysical studies suggest that there are many buried active faults and buried Quaternary basins below the Beijing plain. In order to investigate shallow crustal structures, geometry of buried faults, fault activity and the relationship between shallow and deep structures in this area, a mid-deep seismic reflection profile with 60-fold and several shallow seismic reflection profiles across buried faults were completed in the northwestern region of Beijing plain in 2006. The results show that the depth of crystalline basement is about 3~6 km along the profile. The seismic reflection section shows a set of strong reflected phases with good lateral continuity above the crystalline basement, which should be the sedimentary rock systems of Cenozoic, Mesozoic, and Paleozoic era. Beneath the crystalline basement, there is a series of weak reflected events with poor continuity possibly representing the crystallized rock system with comparatively strong metamorphism or other non-layered rock systems. The faults revealed by mid-deep and shallow seismic reflection profiles present consistent upper-lower relationship and have been significantly active since Quaternary, which play an important role in controlling the rift valley-horst like basin and range tectonics as well as the thicknesses of Cenozoic strata in this region. The study results can further improve the understanding of the neotectonic activity in the Beijing area, and the used exploration method and technology can serve as a reference for the survey of deep-shallow structures in other areas.
It is very important to explore weak geological bodies and structure with geophysical and engineering geological exploration and integral interpretation to detect the geological structure, rock properties and the water bearing characteristics that may affect the tunnel safety. We did such kind of investigation in 2004 for the 20km long deep shield tunnel between the Make river and Jiaqu river for the west route project of diverging water from the Yangtze river to the Yellow river. This paper presents the advantages of CSAMT tunnel exploration, data collection procedure and data processing techniques under conditions of rough topography and complex media. According to the geophysical and engineering geological exploration results, we describe the geological structure characteristics of rocks in the 20km tunnel and point out the faults, fractures and some anomalies that may cause dangers to the tunnel construction.