Borehole Radar Tomography Software System Study And Development

GPR(Ground Penetrating Radar) is a kind of efficiently and high resolution technique for shallow layer geophysical explore. It sends high-frequency electromagnetic waves and makes use of the difference of the electrical property of underground medium to analyze and deduce the structure and character of the medium, based on kinematics and dynamics character of the wave, such as swing, wave form and frequency. In spite of high resolution , the detection range of GPR is limited, which is confined to within 100 meters below the surface. So as to solve the problem that the detection range of GPR is small, the borehole radar places the transmitting and receiving devices in boreholes. This technique now is increasingly applied.Cross-hole tomography is a major mode of borehole radar. It is a new geophysical detection technique which uses the travel time or amplitude data of the electromagnetic waves between the boreholes to inverse velocity distribution or attenuation coefficient distribution of the detection region. This technique is that transmitting antenna is placed in a borehole, receiving antenna in another to receive the direct wave .It gets a mount of data, according to electromagnetic waves in different media at different speeds or different attenuation coefficient .Then it is inversed by the computer, and gets the detection area profiles of velocity or attenuation distribution. It can be studied vividly and intuitively that the causes of complex geological phenomena and local structure. This technique has obvious advantage on the bedrock of the fault , the rupture course , underground karst cave and other underground objects of unknown. Cross-hole radar tomography consists of velocity tomography and attenuation tomography .This paper focuses on the velocity tomography study. Firstly, the exploition demain between the holes is discretized in cells with constant physical properties. For each transmitter-receiver pair , the length of each segment of ray in every cell is calculated(that generate the coefficient matrix L). Secondly, the first travel time of the direct waves is extracted. At last ,the slowness field is obtained by"inversion"of the segment length matrix multiplied by the measured time vector.1. In the aspect of the extraction of the first travel time of the direct waves, this paper studies manual extraction and two kinds of automatic extraction (the maximum SNR method and the maximum energy ratio method). Three methods have different features. Manual extraction method can accurately extract first break when signal-to-noise ratio is relatively small, but slowly. Automatic extraction is efficient, but precision is less than manual extraction. Therefore, these three methods can be used to complement each other through human-computer interaction.2. In the aspect of tomography inversion algorithm, this paper mainly introduces LSQR(Least Square QR Method), GMRES(Generalized Minimal Residual Method), CG(Conjugate gradient method), BICGSTAB(Bi-stable conjugate gradient method), ART(Aigebraic Reconstruction Technique) and SIRT(Simultaneous Iterative Reconstruction Technique). Then the algorithms are verified by the actual models simulated by GprMax software. All algorithms fulfill Practical demand.3. In the aspect of the fators influencing the tomography result, this paper consists of the angle of ray coverage , regularization factor and the coupling between devices arrangement and direction of abnormal. When the ray coverage in the 40 degrees to 70 degrees, that is ground penetrating radar antenna beam width half-power range, the image effect is the best .When less than 40 degrees, the longitudinal extent of abnormal usually large than the actual ,instead, the image distortion .when larger than 70 degrees, due to the serious interference the image cannot be distinguished .The fundamental equations of tomography are usually ill-posed equations, requiring regularization .Regularization factor is also of significance on the inverse image of the smoothness and flatness .The appropriate regularization factors have important effects on the inversion. Specific parameters depend on the actual circumstances. Arrangement of transmitting and receiving devices also have important implications on imaging results ,.Because of restrictions by the position of the boreholes, projection ray can not make a full range of abnormal body. When the anomalous body horizontal, the inversion of the image is consistent with the model, and the result is the best .When the anomalous body vertical, the inversion of the image in the horizontal direction is more width than the actual, owing to anomalous body homogenized in the horizontal direction.Finally, we have developed borehole radar tomography software system, the software can easily and convenient process tomography data and the final results of the inversion are displayed in graph, which can be intuitive analysis of the regional geological structure.