Study On High-Resolution GPR Technique Through Reducing The Influences Of Absorption And Attenuation

Ground Penetrating Radar (GPR) is a Geophysical method which can confirm the distribution of the medium inner using of the high frequency electromagnetic wave. By now, GPR has developed for one century from the beginning of the twenty century. GPR has a broad application in the practice or field surveying for its high resolution excellence. With the deep development of the GPR technology and the range of the application, more high resolution should be required. For the more severity attenuation and dispersion of the GPR wave when it propagates in the loss or heterogeneous medium, the high resolution excellence of the GPR decrease so much, at the same time, which deeply restrict the development of the GPR technique. As well as the high resolution seismic exploration, high resolution GPR technique is a system project, which include the base theory study (in particular the electromagnetic wave (EW) attenuation mechanisms, the dispersion characteristic), methods research (such as the data collection, processing and the interpretation) and the instrument develop (especially the GPR antennas study ). This thesis tries to have a study on the high resolution GPR system project which aims at reducing the influence of the absorption and the attenuation. The thesis mostly includes three parts: the first, we develop one experiment penetrating survey antenna which can be use to study on the GPR wave attenuation; and the second, using the panel electromagnetic wave theory and the direct wave method, we have a study on the monochromatic frequency and the GPR wavelet attenuation separately. And the last one, we present a way which can be used to enhance the GPR data resolution. The content of the three parts followed:1. At present, the GPR antenna mostly designed for the surveying on the ground. However, when we study on the GPR wave attenuation, the antenna had better to be putted into the medium, such as the water and sand. Moreover, the commercial antenna is usually expensive and large, so that it can not freely have the attenuation survey in the lab, and can't popularize all kinds of GPR researches. Because of the reasons above, it is so necessary to design an antenna that is convenient for the attenuation's penetrating survey. We firstly consider the half wave symmetric dipole antenna because it is small, simple design and easy analysis and so on. To the half wave symmetric dipole antenna design and making, we detailed introduce every parameter's choice and its computation of the antenna in the applied point of view. These parameters involve the material, figure, length, radius, width of the connect and the protect layer of the antenna and etc. Through the analysis of the result of electromagnetic simulation and the practice surveying effect, the antenna made by the way of the thesis's introduction is confirmed that it has the excellence of the wide band and high radian efficiency, and it can be used to study on the attenuation of the GPR wavelet.2. Study on the monochromatic frequency attenuation and dispersion. As we all know, the complex propagation constant can deduce the formulation of the absorption coefficient and phase coefficient. The research of this part is to conclude the relation of the GPR wave attenuation or dispersion to the frequency, conductivity and the dielectric permittivity. By the way of combination of the logarithm reference frame and the isoline map, we make the description of the relation be more intuitional and detailed. Another, we have some analysis of the loss tangent, and point out the loss tangent change to the frequency. At the same time, which will be a guidance to the practice surveying.3. Through the direct wave method, we have a study on the GPR wavelet attenuation in the numerical simulation and the practice penetrating surveying. Studying on the GPR wavelet, we can find that the changes of the GPR wavelet's amplitude with transformation of the distance, conductivity, dielectric permittivity and the antenna's main frequency. Besides, we also obtain the relation between the radar wavelet center frequency or the frequency band width and the four parameters above. In all, about the study of the radar wavelet attenuation, the thesis mainly study in the way of 2D numerical simulation, and the practice penetrating survey data is only prove the result of the numerical simulation. Though we have not too much research on the wavelet attenuation in practice antenna penetrating survey, the two methods have their excellence and shortcoming each other, and they are supplement each other:The study on the GPR wavelet attenuation in practice penetrating survey which have the excellence of closing the practice, simple operation, easy to analysis and understand. But it has the shortcoming, too. For example, it will take off some time and charge to design and make the antenna; on the other hand, how we can get the better GPR data when we have a penetrating survey on the material. It is not difficult to put the antenna into the medium such as water and sands. But if we put it into the hardy rock, that will be some difficult. All of these problems should be solved in future.To the GPR numerical simulation method, it has the excellence of little expensive, convenient operate and freely change parameters, etc. However, the GPR simulation technique is not so perfect up to now, especially the GPR 3D numerical simulation technique still need to be developed further under the complicated conditions. Under the limitation of the simulate technique, the GPR wave wavelet attenuation study using the simulate technique exist some shortage. For example, a lot of medium's electric parameters can not be measured accurately in the complicated conditions underground. Thus, as we are making the numerical simulation, so many parameters can't be input correctly, so that the result or effect of the simulation is not accord with the practice instance. Another, for the restriction of the computer technique (comprise the software and hardware), it usually takes a long time to have a GPR simulation. Compare 3D numerical simulation, the 2D numerical simulation is better consummate and little time be used. Moreover, the result of the 2D simulation is more believable, so that we adopt the method of 2D simulation. But, the 2D simulation technique only can simulate some simple model. The 3D simulation is close to the real instance, it can be used to simulate some 3D target. But the 3D simulation technique comparatively takes time, even to some simple simulation. Furthermore, because of the simulation technique limitation, the finally result of the simulation is not so satisfying.4. In order to compensate the GPR wave loss because of the absorption and attenuation when propagating in the conductive medium, this thesis will present a new frequency compensation method, i.e. FC frequency compensation technique. FC frequency compensation technology is benefit form the high frequency compensation using the micro- seismogram logs in seismic exploration. It is base on the stratum system response model. Through fetching the inverse filter of the stratum system response, the absorption that the radar wave traverses the stratum can be compensated, eventually reach the aim of improving the data resolution. And because the inverse filter is computed from the original GPR data, the compensation physics meaning is definitude and better comprehended.The best central part of the FC frequency compensation technique is computing a good inverse filter. And the key of inverse filter computation is how to select a better input and output wavelet. About the wavelet selection, the thesis give a serial measures to ensure the correction of the selection. Besides, if we can't find the appropriate wavelet in the radar record, the inverse filter may be computed by the way of bore well and GPR numerical simulation assistant methods. In a word, the thesis provides a set of methods over the computation of the inverse filter.However, if we can not get a good inverse filter finally, here, the frequency compensation can be implemented using the method of multi-compensation. On the particular analysis in this thesis, we usually can compute one good inverse filter as the FC frequency compensation technique used correctly. Another, utilizing the method of multi-compensation or comprising different inverse filter, we can commonly enhance the GPR resolution in some degree.In order to have a better and more complete comprehension of the FC frequency compensation technique, we compare the FC technique to the deconvolution (mainly the spiking deconvolution and predictive deconvolution) and analyze them. In general, as the radar wavelet's attenuation and dispersion in the lots of conductive medium are better serious, the effect of the spiking or predictive deconvolution is not satisfied. But the FC technique is a pertinence and stretchy frequency compensation method which can enhance the GPR data resolution effectively. Because the FC frequency compensation technique operate simply and has a marked effect, it can be involve to the GPR common processing as one method of enhance the data resolution, and better serve the GPR daily survey.