| Ground Penetrating Radar (GPR) is a nondestructive testing tool for evaluating pavements. It was firstly used in Europe and America in the late of 1980s, and it was introduced to our country at the beginning of 1990s. As a new technology, GPR hasn't been used on monitoring the quality of pavement for a long time, but it has such high accuracy of measuring that it can meet the requirement of error. Also, GPR works quickly and continually, so it has the potential to be widely applied. GPR can be used not only for measuring pavement thickness, but also for the detection of voids in roadbeds, the cracks of pavements and their trends. Moreover, we can use GPR to identify pavement compactness, moisture content, the content of stabilized soil, and the stripping of asphalt surfacing layer, etc. Some countries have started to and apply such technologies. From above, we can see that GPR technology has many advantages, and it can be used for quality supervision and maintenance of highway construction projects, especially for superhighways. So it is spreading gradually at home and abroad.GPR operates by using electromagnetic waves of high frequency which present in the form of wide frequency band and short pulse. Such signals are transmitted into the pavement by the emitter in the antenna. When the waves come across an interface of two layers with different electrical properties, some of the electromagnetic energy will be reflected. The other part of energy will penetrate through the interface and get into the under layer. The reflected electromagnetic energy is collected by the receiver in the antenna, and then it is sampled and digitalized to be processed. After analyzing such signals, we can get much information which characterizes pavement layers.The signals collected directly from the receiving antenna are very complicated. They are displayed as waveforms in time domain. The reflected waves contain important information of the pavement. They consist of thereflected waves from inside and at the bottom of antenna, from pavement surface and interfaces between layers. In addition, there is random noise in them. So the reflected signals are nonstationary and nonlinear. Just because of the complication of such signals, it is most important to analyze and process them (such as filtering, removing noise, extracting characteristic, etc), so that we can get information of requirement and continue to do some subsequent work, such as computing the thickness of pavement layers.It is FFT that the existing software of GPR applies to analyze data. As Fourier Transform can't be used to analyze local characteristic of signals, some problems are difficult to be resolved for the nonstationary GPR waves. For example, when we test thickness of thin layers or identify slight holes in a pavement, the collected signals are not too regular to be easily distinguished. Wavelet transform (WT) set up a new area in applied mathematics at present, which develops very quickly. It is a kind of time-scale (time-frequency) analysis method. It has such good localization characteristic in both time and frequency domain that it is suitable for processing nonstationary signals. Nowadays, it is widely used in practical applications.According to the instability of GPR signals, this paper will use WT to analyze and process them after comparing the existing signal processing methods. There are continual wavelet transform (CWT) and discrete wavelet transform (DWT), and the latter is applied in this paper. This paper covers:1. It introduces GPR system, and it mainly describes the developing status of GPR technology, the compositions and principles of GPR, and the correlative testing parameters and performance indexes.2. Several methods are analyzed in detail and compared in this paper. There are many methods that can be used to process signals, such as temporal analysis, frequency-domain analysis and time-frequency analysis, and this paper analyzes Fourier Transform, Short-time Fourier Transform and WaveletTransform. Additionally, it poi... |
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