Wavefront healing in random media
- 出版日期：2003.
- 页数：141 p. :
- 第一责任说明：Adam Mirza Baig.
- 分类号：a263.2
MARC全文
02h0020789 20101216112238.0 cr un||||||||| 101018s2003 xx ||||f|||d||||||||eng | CNY371.35 (UnM)AAI3103031 UnM UnM NGL a263.2 Baig, Adam Mirza. Wavefront healing in random media [electronic resource] / Adam Mirza Baig. 2003. 141 p. : digital, PDF file. Adviser: F. A. Dahlen. ; Source: Dissertation Abstracts International, Volume: 64-08, Section: B, page: 3713. Thesis (Ph.D.) -- Princeton University, 2003. In this thesis, we present the results of detailed numerical and theoretical studies on wave propagation in weakly heterogeneous random media, paying particular attention to the cases where the length scales of the heterogeneity are of the same order as the wavelength of the probing wave. It is in this regime that the diffractive phenomenon of wavefront healing becomes significant and the high-frequency approximation of ray theory (or geometrical optics) becomes questionable. By using finite-frequency kernels to calculate quantities like the traveltime delays or amplitude fluctuations of waves in these media, we are able to overcome the ray-theoretical limitations on the anomaly's length scale. In fact, the misfit of ray theory is controlled by the ratio of the scale length, a, to the maximum width of the Fresnel zone, lL , where l is the dominant wavelength of the wave, and L is the propagation distance of the wave. Ray-theoretical traveltimes become invalid whenever anomalies have scale lengths a ≲ 0.5 lL and amplitudes are invalid for a ≲lL . In contrast, our finite-frequency kernels do not suffer these restrictions and produce traveltimes and amplitudes in much better agreement with the actual measurements. Using these kernels, we go on to calculate various statistical properties of finite-frequency waves in random media, namely the traveltime variance, the amplitude variance, and the mean traveltime. The validity of the linear approximations we use appear to be controlled by the value of the theoretical amplitude variance, 〈δA2〉/ A2: whenever this quantity is greater than 0.1, non-linearities become significant for the amplitude fluctuations, though the linearity of traveltimes is a valid assumption over a larger range of parameter space. Since the mean traveltime is frequency dependent, a heterogeneous medium will appear to be dispersive. By comparing the predicted dispersion from this effect to the dispersion unaccounted for in the Earth, we place constraints on the S-velocity heterogeneity spectrum of the Earth's mantle. Wave-motion, Theory of. ; Seismic waves. aDahlen, F. A. aCN bNGL http://proquest.calis.edu.cn/umi/detail_usmark.jsp?searchword=pub_number%3DAAI3103031&singlesearch=no&channelid=%CF%B8%C0%C0&record=1 NGL Bs650 rCNY371.35 ; h1 xhbs1003