Theory And Application Of Interface Location With Reflected Elastic Waves In An Un-orientational Detecting System

Position detecting is the core of target searching, locating and tracking. In full space, only orientational detecting techniques, such as radar, can provide the accurate coordinates. However, the azimuth angle of the target cannot be provided accurately by traditional detecting techniques using un-orientational systems, which is named as the non-uniqueness of the azimuth in this article. Thus, this study is focused on how to eliminate the non-uniqueness of the azimuth in position detecting with the reflected elastic waves excited and received un-orientationally.First, the non-uniqueness was studied from a view of wave motion. The results indicate that the non-uniqueness exists only when the radiation field is symmetrical. Thus, only if un-orientational instruments are used in omnibearing detecting, the non-uniqueness will be taken into account. In practice, the non-uniqueness disappears in catching the dip angle, because one can determine whether an interface is updip or downdip with the arrival time of the reflected waves and the position relations between the source and the receivers. However, the nonuniqueness must be considered in catching the azimuth angle. The reason is that the polarization direction is often used to obtain the propagation direction and then the azimuth angle although the relationship between the propagation direction and the polarization direction is not a one-to-one correspondence. Generally, two solutions separated by 180° for the azimuth angle will be left after traditional signal processing, whether the detecting is in three- or two-dimensional spaces. To determine the azimuth angle uniquely, additional information is needed. As the known information in detecting techniques with reflected waves, the information about the source will help us eliminate the un-uniqueness, if certain rules on the reflection are investigated throughly.Secondly, the sign relations of the reflection coefficients(RCs) between displacement and normal stress were established: in the directions vertical to the line from the source to the receiver, the normal stresses RCs are opposite in sign to those of the displacement components RCs. A reflected wave will have the same shape like its incident wave if the reflection coefficient(RC) is positive. Otherwise, the reflected waveform will be reversed. Thus, the sign relationships reveal the waveform changes after a reflection, and then it will be possible to help us infer the reflected wave vector from the waveform changes.Thirdly, the shape change of elastic waves in propagating was studied. The results indicate that in single-well imaging, only reflections can reverse the shape of a wave, even when the media are dissipative and have some dispersion. Hence, it is feasible to analyze a reflection according to whether the waveform is reversed or not.Fourthly, a scheme to eliminate the non-uniqueness in detecting or imaging was proposed and its application prospect was analyzed. Although this scheme needs one more signal recorder, it provides the azimuth angle accurately only with the traditional acoustic records. Compared with traditional un-orientational techniques, the advantage of this scheme is that the azimuth angle can be determined uniquely. Compared with orientational techniques, the advantage is that the full space scan can be completed at any time.Finally, with the data generated by Finite Difference Time domain(FDTD) procedures, the scheme was tested for two types of models. The first model includes only one interface and the second model derives from single-well imaing. The test results verify that the technique provided in this dissertation can eliminate the nonuniqueness of the azimuth obviously. Especially, tests on single-well imaging indicate that the technique can be used in real downhole detecting.