GEOPHYSICAL TECHNIQUES IN DETECTION TO RIVER EMBANKMENTS

Geophysical technologies are very effective in environmental, engineering and groundwater applications. Parameters of delineating nature of near surface materials such as compressional wave velocity, shear wave velocity can be obtained using shallow seismic methods. Electric methods are primary approaches for investigating groundwater and detecting leakage. Both of the methods are applied to detect embankment in hope of obtaining evidence of the strength and moisture inside the body. A technological experiment has been done for detecting and discovering the hidden troubles in the embankment of Yangtze River, Songzi, Hubei, China in 2003. Surface wave and DC multi channel array resistivity sounding techniques were used to detect hidden trouble inside and under dike like pipe seeps. This paper discusses the exploration strategy and the effect of geological characteristics. A practical approach of combining seismic and electric resistivity measurements was applied to locate potential pipe seeps in embankment in the experiment. The method presents a potential leak factor based on the shear wave velocity and the resistivity of the medium to evaluate anomalies. An anomaly found in a segment of embankment detected was verified, where occurred a pipe seep during the 98’ flooding.

The implementation of an improved NPML absorbing boundary condition in elastic wave modeling

In elastic wave forward modeling, absorbing boundary conditions （ABC） are used to mitigate undesired reflections from the model truncation boundaries. The perfectly matched layer （PML） has proved to be the best available ABC. However, the traditional splitting PML （SPML） ABC has some serious disadvantages： for example, global SPML ABCs require much more computing memory, although the implementation is easy. The implementation of local SPML ABCs also has some difficulties, since edges and corners must be considered. The traditional non-splitting perfectly matched layer （NPML） ABC has complex computation because of the convolution. In this paper, based on non-splitting perfectly matched layer （NPML） ABCs combined with the complex frequency-shifted stretching function （CFS）, we introduce a novel numerical implementation method for PML absorbing boundary conditions with simple calculation equations, small memory requirement, and easy programming.

The distribution of deep source rocks in the GS Sag:joint MT-gravity modeling and constrained inversion

The coal-bearing strata of the deep Upper Paleozoic in the GS Sag have high hydrocarbon potential. Because of the absence of seismic data, we use electromagnetic （MT） and gravity data jointly to delineate the distribution of deep targets based on well logging and geological data. First, a preliminary geological model is established by using three-dimensional （3D） MT inversion results. Second, using the formation density and gravity anomalies, the preliminary geological model is modified by interactive inversion of the gravity data. Then, we conduct MT-constrained inversion based on the modified model to obtain an optimal geological model until the deviations at all stations are minimized. Finally, the geological model and a seismic profile in the middle of the sag is analysed. We determine that the deep reflections of the seismic profile correspond to the Upper Paleozoic that reaches thickness up to 800 m. The processing of field data suggests that the joint MT-gravity modeling and constrained inversion can reduce the multiple solutions for single geophysical data and thus improve the recognition of deep formations. The MT-constrained inversion is consistent with the geological features in the seismic section. This suggests that the joint MT and gravity modeling and constrained inversion can be used to delineate deep targets in similar basins.

Primary Investigations on Yangtze River Bank Slope Stability in Wanzhou for the Three Gorges Reservoir Project

This paper investigates the main factors contributing to bank slope failures, such as the structure of rock and soil, water level change, bank slope gradient, vegetation, weathering and human activities. Based on these investigations, the bank slope failure models are analyzed. The stability of bank slopes in Wanzhou is investigated using geological surveying, 2D resistivity imaging surveying, excavated trenches and other methods. Finally, the disasters of bank slope failures in Wanzhou were investigated in detail. The results show that instability problems might occur in 60.38 km of bank slopes when the water level rises to 175 m. It is suggested that 37.8 km of unstable bank slopes should be stabilized, and 14.2 km of unstable banks should be moved or avoided after further geological surveying and reconnaissance. These results provide scientific basis and reliable data for the government to develop the third geodisaster prevention plan for the Three Gorges reservoir.

Application of Multifocusing Method for an Irregular Topography Imaging

A stack of records becomes one of the main steps in modern seismic data processing. In the stack procedure, the crucial operation is time correction. Conventional methods, e.g. , normal moveout （NMO） and dip moveout （DMO） stacks require a sufficiently accurate macro-velocity model, whereas a multifocusing imaging method does not depend on a macro-velocity model. The multifocusing method proposed by Gelchinsky et al. belongs to a group of methods that can be characterized as macro-model-independent imaging methods. The multifocusing method represents a transformation of 2-D multicoverage reflection data into a simulated zero-offset stack profile. This transformation is based on a completely data-derived spatial stacking operator, and includes stacking large supergathers of seismic traces, each of which can span many CMP gathers. By extending the multifocusing moveout formula to explicitly account for non-zero elevations of the source and receiver, the multifocusing imaging method can yield appropriate results when seismic data are acquired over an irregular topography. In recent years, many applications of multifocusing imaging over an irregular topography have demonstrated its advantages in comparison with conventional CMP processing. This paper illustrates the corresponding formulas for a synthetic data example modeled by the wave equation finite difference method. The result of the synthetic example is very encouraging. By stacking large supergathers and applying multifocusing moveout correction, the reflectors are aligned very well and the S/N is greatly improved. We have also applied multifocusing imaging over an irregular topography to a real data example. The elevation of the data acquisition area varies considerably. Applying multifocusing imaging, a substantial improvement of the simulated section was achieved, compared with a conventional CMP stacked section.

Inverse Spline Interpolation for All-time Resistivity of Central-Loop TEM

A convenient numerical calculation method (inverse spline interpolation) for all-time apparent resistivity intransient electromagnetic method (TEM) is proposed in this paper. Characteristic of early and late normalized inductiveelectromotive force was investigated. According to the turning point, the transient process is divided into the earlyphase, the turning point, and the late phase. Afterwards, apparent resistivity is obtained through inverse spline interpo-lation in the early and the late phases, respectively. Finally, the resistivities of the early-time and the late-time wereconnected together by the turning point. The result shows that the inverse spline method is feasible and the method alsolays a foundation for initial model construction in the TEM automatic inversion.

THE APPLICATION OF GROUND PENETRATING RADAR IN THE HIGHWAY ROADBED INVESTIGATION

Ground penetrating radar (GPR) is a new nondestructive geophysical method for road quality investigation. In this article, a section of the highway joints with the third Yangtze Bridge in Wuhan have been investigated using the RAMAC/GPR system developed by the MAL? GeoScience Company. Various antennas with different frequency were used in the exploration. And the data gathered by the unshielded antenna with frequency in 400 MHz is very well. Then the field data was processed using methods such as trace equalization, F K filtering, deconvolution filtering and so on. Through the processing, the GPR profile shows the underground structure more clearly. From the processed GPR profile, the sinking and slipping condition of the roadbed can be easily found. Most parts of the roadbed are well, but several parts of the roadbed have sunken or slipped seriously. Contrasts to the shallow seismic section, the GPR profile accords with it very well. It indicates that the GPR method is available and has the high efficiency in the highway roadbed investigation.