Qingyun Di

Multi-geophysical Investigation of Geological Structures in a Pre-selected High-level Radioactive Waste Disposal Area in Northwestern China

A potential high-level radioactive waste (HLRW) disposal site in northwestern China was investigated to determine its suitability for such a use. The site is primarily covered with welldeveloped metamorphic granite rocks. The primary targets for geological repositories are three granite rock masses (I, II, and III). Only surface geological data were available from previous studies. The objective of this study was to evaluate the quality of the rock mass and identify any weak geological structures that could jeopardize this future underground repository. We used gravity and aeromagnetic data on a large scale to study the regional geological structures within and around the three rock masses. Subsequently, in 2009, a controlled source audio-frequency magnetotelluric (CSAMT) survey was conducted to study rock mass I in more detail. This paper introduces three-dimensional (3-D) tomography imaging of gravity and aeromagnetic data. 3-D tomography imaging was carried out on previously collected gravity and aeromagnetic data and, using the results of different depth slices, we evaluated the rock mass quality and interpreted the geology. The aeromagnetic depth slices show that at about 1-km deep in rock masses I and III there is a high magnetic susceptibility body, possibly caused

Migration of ground‐penetrating radar data with a finite‐element method that considers attenuation and dispersion

We use a numerical model to study the effects of attenuation and dispersion upon the migration of ground-penetrating radar (GPR) profiles. A finite-element method (FEM) is developed that incorporates attenuation and is used to generate synthetic GPR profiles with random noise. These profiles are then migrated with and without the attenuation term using our FEM codes. The misfit between the position of interfaces in the model and the position of corresponding interfaces in the migrated profile is greatly decreased when the attenuation term is considered in the migration process. The improvement in resolution results from the use of the group velocity rather than any phase velocity. Consequently, for dispersive media the attenuation term of high-frequency GPR waves cannot be ignored in migration.

Geophysical Evidence through a CSAMT Survey of the Deep Geological Structure at a Potential Radioactive Waste Site at Beishan, Gansu, China

This paper describes a controlled-source audio-frequency magnetotellurics (CSAMT) survey conducted in August 2009 over a potential high-level radioactive waste (HLRW) disposal site in northwestern China. The site is primarily covered with well-developed metamorphic rocks. The purpose of the CSAMT survey was to map the outcropped faults to depth and identify any hidden faults or weakened zone in the subsurface. The site is located in the arid Beishan area, Gansu Province. Substantial challenges were encountered in acquiring quality electrical field data because of the highly resistive ground. Satisfactory electrode contact conditions were generally maintained by applying salt-saturated water to both the transmitting electrodes and the receiving electrodes. The excitation frequency ranges from 9,600 Hz to 1 Hz with a target depth of investigation of 1,000 m. The CSAMT data were processed in several steps. Low-pass filtering was applied to remove the static effect caused by the local electrical inhomogeneities near the ground surface. An optimum filter length was found through experiment to yield the maximum static effect reduction. The pre-processed data are inverted for geoelectrical cross sections using a 2-D inversion method. Inversion artifacts were suppressed by imposing a model smoothness constraint. The inversion reveals several important results. First, the inversion cross sections correctly recognized the fractures and deformation bands mapped at the surface. The cross sections also identified four new faults that were not observed in the geological survey. The inversion profiles suggested that the narrow factures and deformation bands observed along survey line 1 extend to a great depth. The profile helped identify a possible weak mineralization zone along survey line 2. Considering the regional tectonic stress direction, the lower resistivity zone suggests that the faults parallel to survey line 2 are subject to an extensional or transtensional force that produces a broad and broken alteration zone. Although subject to further drilling confirmation, these interpretation results greatly enhance the understanding of the deep geological hazards at the Beishan site.

Geophysical exploration for a long deep tunnel to divert water from the Yangtze to the Yellow River, China

To identify problematic geological structures which would be encountered when driving tunnels in the high mountainous area of South West China, a joint geophysical and engineering geological study was made. The tunnels will form part of a project to divert water from the Yangtze to the Yellow Rivers. A controlled source audio-frequency magnetotelluric method (CSAMT) was carried out in 2004 between the Ma-ke and Jia-qu rivers in high, steep terrain. The paper discusses the method of data collection, processing, and analysis for the West route. The faults/fractures identified will need to be taken into account in the design and construction of the tunnel.RésuméAfin d’identifier des structures géologiques contraignantes qui pourraient être rencontrées lors du creusement de tunnels au travers des régions montagneuses de la Chine du sud-ouest, une étude couplée de géophysique et de géologie de l’ingénieur a été réalisée. Les tunnels feront partie du projet de détournement des eaux du bassin du Yangtze vers celui du Fleuve Jaune. Une méthode magnéto-tellurique (CSAMT) a été mise en œuvre en 2004 entre les rivières de Ma-ke et Jia-qu dans une région d’altitude présentant des pentes raides. L’article discute des méthodes de récupération, traitement et analyse des données pour l’itinéraire de l’Ouest. Les failles et fractures identifiées devront être prises en compte dans la conception et la construction du tunnel.

Removal of the airwave effect by main-part decomposition of the anomalous field of MCSEM data

The airwave effect greatly influences the observational data from controlledsource electromagnetic exploration in shallow seas, which obscures the abnormal effects generated by exploration targets and, hence, affects the accuracy of the late exploration data interpretation. In this study, we propose a method to separate the main part from the anomalous field of marine controlled-source electromagnetic method (MCSEM) data based on Stratton–Chu integral transforms to eliminate the airwave effect, which dominates observed electromagnetic (EM) response in shallow seawater. This method of separating the main part from the anomalous field is a type of finite impulse response filter based on a discrete data set. Theoretical analysis proved that the method is stable and able to effectively depress noise. A numerical test indicated that the method could successfully eliminate the airwave effect from the observed EM signals generated by an air–water interface and a seawater layer. This technique is applicable for seawater models with either flat or rough seabeds.

Time-domain inversion of GPR data containing attenuation due to conductive losses

Many seismic data processing and inversion techniques have been applied to ground-penetrating radar (GPR) data without including the wave field attenuation caused by conductive ground. Neglecting this attenuation often reduces inversion resolution. This paper introduces a GPR inversion technique that accounts for the effects of attenuation. The inversion is formulated in the time domain with the synthetic GPR waveforms calculated by a finite-element method (FEM). The Jacobian matrix can be computed efficiently with the same FEM forward modeling procedure. Synthetic data tests show that the inversion can generate high-resolution subsurface velocity profiles even with data containing strong random noise. The inversion can resolve small objects not readily visible in the waveforms. Further, the inversion yields a dielectric constant that can help to determine the types of material filling underground cavities.

2D Multitransient Electromagnetic Response Modeling of South China Shale Gas Earth Model Using an Approximation of Finite Difference Time Domain with Uniaxial Perfectly Matched Layer

In this study, we introduced multitransient electromagnetic (MTEM) method as an effective tool for shale gas exploration. We combined the uniaxial perfectly matched layer (UPML) equation with the first derivative diffusion equation to solve for a finite difference time domain (FDTD) UPML equation, which was discretized to form an algorithm for 3D modeling of earth impulse response and used in modeling MTEM response over 2D South China shale gas model. We started with stepwise demonstration of the UPML and the FDTD algorithm as an effective tool. Subsequently, quantitative study on the convergence of MTEM earth impulse response was performed using different grid setup over a uniform earth material. This illustrates that accurate results can be obtained for specified range of offset. Furthermore, synthetic responses were generated for a set of geological scenarios. Lastly, the FDTD algorithm was used to model the MTEM response over a 2D shale gas earth model from South China using a PRBS source. The obtained apparent resistivity section from the MTEM response showed a similar geological setup with the modeled 2D South China shale gas section. This study confirmed the competence of MTEM method as an effective tool for unconventional shale gas prospecting and exploitation.

2-D finite element modeling for seismic wave response in media with sand bodies

Abstract Forward modeling plays an important role in structural trap exploration because of its usefulness in identifying seismic wave characteristics. Structural traps have been discovered in the Dongpu depression of Zhongyuan Oil Field of China. However, its proved reserves still fall far short of total estimated reserves, suggesting that there are undiscovered lithologic traps. According to the stratum properties revealed by borehole surveying and exploration seismograms, we have constructed the models of deposit sand body traps and conducted a complex seismic wave modeling study using the finite element method. Our models contain scattering bodies. The trap media is considered viscous–elastic and finite element equations are derived with artificial boundary terms. The absorbing effect of the artificial boundary terms we obtained is fairly good. The source is simulated by concentrated forces or explosive sources. We obtain the common source/receiver record profiles by putting secondary sources directly on the interface nodes. Direct waves and surface waves of single-shot seismograms can be removed with Huygens principle. We also put the filter of instrument response into the theoretical seismograms. In the paper, we have designed several models of sand bodies, and obtained their realistic seismic profiles with the finite element method. The corresponding records have been analyzed and the characteristics have been extracted. The method will provide an analytical tool for exploring lithologic traps effectively.

Field testing of the surface electromagnetic prospecting system

To test the performance of the Chinese whole-surface electromagnetic prospecting (SEP) system, system integrations, instrument performances, and large-scale production viabilities in Liaoning province and Inner Mongolia were measured via extensive field tests. Resultant electric fields, magnetic fields, apparent resistivities, impedance phases, and inversion profiles compared favorably with results of commercial equipment from other countries. The inversion results agreed well with the geologic information from boreholes. Field tests showed that the SEP system is stable, reliable, lightweight, and easy to operate, making it suitable and ready for real-field exploration.

Time domain electromagnetic migration imaging

This article is about the study of time domain electromagnetic data migration imaging. First, based on the electromagnetic diffusion theory and the boundary conditions, we deduce the analytical formula of electromagnetic migration field. Then, we complete the calculation of electromagnetic migration field filter by electromagnetic migration field theory in frequency domain. The analytical electromagnetic migration filter is discretized to achieve electromagnetic numerical migration filter to make it able to be applied into practical discrete sampling. By analyzing the error caused by of discretization procedure according to the theory of electromagnetic migration filter, we deduce discretization error correction formula. Moreover, we study and realize the transform from time domain to frequency domain, and make it able to be used in time domain electromagnetic migration imaging. In order to test migration imaging technology, whole space 2D or 3D geoelectric models are built with the transmiter-receiver distance varying from near field to several thousand kilometers. Using the technology presented by this paper to the numeric data, the reasonable position of underground exploration body can be recovered. Finally, we deduce 2D migration field analytical formula in time domain and discretize it, then complete the numeric calculation in time domain directly. We analyze the kernel function of time domain electromagnetic migration filter and its influence factors, and test the effect of time domain migration field imaging by several simple geoelectric models.