ALBERT

Description: Subway tunnel construction plays a significant role in the development of urban underground space. Advance surface grouting treatment of adverse subsurface geological bodies is an effective measure to avoid the accidents during subway construction. This paper describes the successful application of geological prospecting and grouting assessment at the tunnel site of Qingdao subway line No. 1. The study area is located in a coastal bedrock fissure water distribution area. It was decided to conduct surface grouting to reduce the high risk of water inrush. Integrated investigation consisting of electrical resistivity tomography (ERT), seismic ahead prospecting, surface vertical drilling and cross-hole ERT was conducted to ascertain the geological conditions and guide the grouting scheme design. Surface ERT and seismic ahead prospecting were adopted to provide a rough survey of water bearing conditions and fractured zones. Then, detailed imaging was conducted using cross-hole ERT inversion with the inequality constraint based on the information obtained using surface vertical drilling. The shape and location of three water bearing structures were identified. Based on the integrated investigation results, an appropriate grouting scheme was designed. After the application of grouting, cross-hole ERT was carried out again to assess the effect of grouting by comparing the vibration of twice inversion results of cross-hole ERT and resistivity tests of the grout sample. Such information serves as a basis for guiding geotechnical excavations in complex urban environments and avoids the risk of construction accidents.

Description: Improvements in detecting underground spaces in urban areas require improvements to the radiation efficiency and resolving power of the artificial source. Due to the use of bipolar pulse and the low efficiency of the artificial source, the existing artificial sources have difficulty in accurately imaging complex urban underground spaces of varying depths and sizes, to complete the task of “transparent detection” of urban underground spaces to depths of two hundred meters. In this paper, a new type of transient electromagnetic radiation based on a high-performance artificial source and differential pulse is proposed. At the same time, multi-resolution theory and multi-scale information extraction technology are combined to further improve the resolution of the radiation field in imaging complex urban underground structures of varying sizes. With this new method, we are able to achieve our goal of transparent detection of ‘two hundred meters' depths of urban underground space.

Description: Many of most serious disasters in coal mines around the world are attributed to underground flooding from water-bearing goafs. To ensure the operational safety in the deeper coal seams, water-bearing goafs must be clearly mapped to prevent water inrush accidents. Existing geophysical methods cannot fully meet this requirement. Building on the merits of the transient electromagnetic method (TEM) at the surface and in the mine, we propose a surface-to-underground transient electromagnetic method (SUTEM). We first analyze the differences between the surface TEM and SUTEM with a uniform half-space model. Secondly, we establish a geophysical model for water-bearing goafs and simulate the response signals at different depths with a vector finite-element method. Finally, we conduct a field experiment of SUTEM. The results of the simulation and experiment show: 1) When the surveying line is below the water-bearing goaf, the abnormal response information is mainly focused on the ascending segment of the electromotive force (EMF) curve, and the relative difference (RD) increases with the value of h/z decrease; 2) Appling SUTEM, we can accurately map the lateral position of the water-bearing goaf with the RD.

Description: With the significant development of China's metro construction, the development of urban underground karst poses a serious threat to related tunnel construction and public safety, with frequent occurrences of mud and water inrushes during tunnel construction and urban ground subsidence events. Because of the complex, urban, and shallow geological conditions and construction environments, conventional geophysical methods cannot meet the requirements for high-precision detection of small-scale and inhomogeneous complex geological bodies. Based on numerical simulation, herein we comprehensively applied both cross-borehole electrical resistivity tomography (ERT) and cross-borehole seismic computed tomography (CT) to urban underground karst surveys of the Hangzhou-Fuyang intercity railway. The results showed that: 1) under limited urban construction conditions, the use of advanced geophysical monitoring equipment greatly improved construction efficiency; 2) utilizing drilling geological results to calibrate the abnormal geophysical field attribute parameters (including wave velocity and resistivity) improved the accuracy of karst exploration and reduce defective geophysical multi-explanation effects; 3) applying the joint comparative explanation of both velocity and resistivity profiles can distinguish and explain karst and fracture development zones; 4) 550 pairs of velocity and resistivity profiles were obtained which revealed 258 karst cave anomalies and 5 fracture development zones which integrated detection accuracy exceeded the 1 m level. Thus, the high-precision joint cross-borehole tomography technology was shown to be useful for guiding intercity railway construction.

Description: In this study, for the purpose of accurately detecting fissured disaster sources and diversion channels during tunnel construction, an array source transient electromagnetic device is proposed. Then, an apparent resistance conversion method and an approximate inversion algorithm for the proposed transient electromagnetic device are presented. First, a finite-difference time-domain (FDTD) method is used for the forward modeling of the proposed array source transient electromagnetic device, and the electromagnetic responses of the cracks and water-conducting channels are obtained. When compared with the transient electromagnetic square responses of the traditional loop sources, it is found that the data confirmed that the responses to the anomalies of the proposed array source transient electromagnetic device are more apparent. Then, the transformation from the array source transient electromagnetic field to the apparent resistivity is realized according to the principle of inverse function. In order to clearly identify the boundaries of the targeted anomalous bodies, a multi-source S-inversion algorithm is used to realize the array source transient electromagnetic differential imaging. The method is validated using a theoretical model. It was determined that the results of the differential imaging had effectively identified the boundaries of the anomalous bodies, and the apparent resistivity imaging had successfully determined the resistivity distributions of the anomalous bodies. Furthermore, a combination of the aforementioned methods is used to effectively identify the faults and water-flowing fractures in the model. This study's proposed algorithm was applied to the actually measured data, and the interpretation results were found to be consistent with the excavation results, which fully demonstrated the feasibility of the proposed method.

Description: Over the past decade, helicopter-borne transient electromagnetic (HTEM) systems have been rapidly developed. A new HTEM prototype (referred to as a CAS-HTEM) has been developed by the Chinese Academy of Sciences. In terms of hardware, the CAS-HTEM system uses an inflatable structure to carry the transmitting loop, which significantly reduces the weight of the system and makes it easier to transport. A dual gain receiver was innovated to extend the dynamic range of the system. In addition, an observation circuit for transmitting voltage waveform is introduced, so that the derivative waveform of transmitting current with higher SNR could be calculated. In terms of data processing, more reliable early time data could be obtained by band-limited effect removal; a weighted stacking algorithm is introduced to reduce the narrow band noise more effectively and increase the sensitivity of data to the anomaly location; a method based on τ-domain transform is used for late time signal processing. The results of the field test which was carried out in Inner Mongolia were found to be consistent with the drill data, which effectively verified the performance of this HTEM prototype.

Description: A controlled-source audio-frequency magnetotelluric (CSAMT) survey was used to detect geological structures beneath the thick quaternary formation in Taiyuan, Shanxi Province, northern China. Two main CSAMT survey lines with 182 survey sites were recorded. A two-dimensional (2D) inversion technique was used to interpret the CSAMT data. The inversion results suggested that: 1) there are four main buried faults named F1, F2, F3, and F4 with dip angles about 65° across the survey line from west to east, the fault displacements of these faults are about 230 m, 180 m, 220 m and 200 m, respectively; 2) the depth of the bedrocks decrease from 1600 to 500 m along the survey lines; and 3) from top to bottom, there are four major layers in the survey area that include the upper layer with the resistivity less than 40 ohm-m represents unconsolidated sediments in the Quaternary formation, a second layer with the resistivity range from 40 to 120 ohm-m represents mudstone and sandstone, a third layer with the resistivity range from 120 to 280 ohm-m represents coal measure strata in the Permian and Carboniferous and a bottom layer with the resistivity higher than 280 Ω·m represents limestone in Ordovician. The CSAMT method is an effective technique for exploring buried fault of several hundred meters deep in metropolitan environment.

Description: Powerline noise is one of the most common contaminating types of the observed transient electromagnetic signal. For the conventional TEM method using the bipolar square wave as transmitter waveform, synchronous sampling is the main technology for powerline noise suppression, and notching filtering is sometimes used also. When the transmitter wave has been encoded based on pseudo-random binary sequence, it has proven difficult to achieve the effect by using the above-mentioned conventional methods for the suppression of powerline noise. This is due to the fact that the duration of each logic states of the pseudo-coded transmitter waveform is normally inconsistent. In this study, a method for the suppression of powerline noise is proposed, which is based on the independent component analysis method (ICA). In order to introduce the observation and processing details of this method more clearly, we attempt to apply this method to the time-domain electromagnetic method with coded source researched and developed by the institute of geology and geophysics of Chinese academy of sciences on the basis of the MTEM method. In terms of specific processes, the electrical measurements need to be simultaneously observed in the inline and crossline directions firstly, and then the data will be input into the processing procedures based on ICA method to realize the effective separation of the powerline noise and the useful signals. The processing results of the simulation data and field data show that the method proposed in this paper can suppress the powerline noise effectively, and the processing results build a good data foundation for the subsequent processing.

Description: Urban geophysics ups the ante in the world of applied geophysics, which requires innovative thinking and seemingly off-the-wall approaches, if for no other reason than the settings. Ambient-noise-tomography (ANT) can play a pivotal role in yielding subsurfa2ce information in urban areas, which is capable of dealing with challenges related to these scenarios ( e.g., human activities and low signal-to-noise ratio). In this study, the ANT was conducted to investigate the near-surface shear-velocity structure in the surrounding area of the Baotu Spring Park in downtown Jinan, Shandong Province, China. Quiet clear Rayleigh waves have been obtained by the cross-correlation, which indicates that strong human activities, such as moving vehicles and municipal engineering constructions, can produce approximately isotropic distribution of noise sources for high-frequency signals. The direct surface-wave tomographic method with period-dependent ray-tracing was used to invert all surface-wave dispersion data in the period band 0.2-1.5 s simultaneously for 3D variations of shear-velocity (Vs) structure. Our results show a good correspondence to the geological features with thinner Quaternary sediments, the geological structural characteristic of the limestone surrounded by the igneous which has the highest velocity than that of the limestone in the study area, and several concealed faults of which specific location has been detected at depth. The results demonstrate that it is possible to successfully use ANT with high-frequency signal in an urban environment provided a detailed planning and execution is implemented.

Description: Worldwide, slip on earthquake faults causes numerous disasters, resulting in large losses in human life and built structures. To minimize future losses associated with earthquakes along such faults, it is important to precisely locate the faults relative to the built environment and to determine the subsurface geometry of the faults. In Beijing, China, we used shallow-depth geophysical methods to evaluate the location and subsurface geometry of the Huangzhuang-Gaoliying fault (HGF), one of the principal tectonic faults of Beijing area. We used seismic reflection and refraction tomography, multi-channel analysis of surface waves (MASW), and paleoseismic trenching to characterize the north section of HGF near the Gaoliying section of Beijing. Our seismic images indicated that there are at least two strands of the HGF that are distributed over an approximately 200-m-wide zone. We identified a principal fault strand (F1) that is observed in all the seismic images, as well as in a paleoseismic trench. The F1 strikes approximately N49°E and dips southeastward at 70° to 75°. Over the past few years, surface ruptures have occurred along the HGF in several locations, but it is unclear if the surface ruptures were the result of tectonic slip on the HGF or were related to land subsidence along the fault.