Physical Modeling Study Of Wave Propagation In Orthorhombic Anisotropic Medium

Geophysicists had already cognizance of that the pervasive velocity anisotropy with varying degree in the mantle or the sedimentary rock of upper crust is an important physical property . Almost all existed research works fasten on those transversely isotropic media with vertical or horizontal axis of symmetry(TI)., but rare are studies of orthotropy that is more common for rocks in sedimentary basin . It is significant for energy exploration , seismic prospecting , groundwater utilized and rock engineering to study the characteristics of seismic wavefields in those orthotropy media.In this thesis , the characteristics of wavefield in orthotropy media was investigated by means of seismic physics model technique. Two works guaranteed the fulfilment of this studies, (1)developing compressional - and shear - waves transducers with broadband which is key part of the system in the physics model experiment, and study of the special property and using skills of the transducers , groping the relative experience to develop transverse wave measurment and observation at laboratory. (2) establishing a suit of comprehensive testing system to observe the elastic wavefield in anisotropy media . Based on the summary of existed methods of testing transverse wave in anisotropy media , I present two new observation methods , and compare the using features of those testing methods of transverse wave by practical experiment in real material.Shear wave splitting was observed by employing torsion vibration shear- wave transducers with broadband , which show that shear wave, whatever it's vibration mode may be , can split in anisotropy media . When pure shear wave propagate in anisotropy media , even thought propagation direction is parallel to the direction of aligned cracks , experimental results exhibit that only two kinds of shear- wave velocity can be measured-fast shear wave and slow shear wave neglecting polarization direction .By using transducers with different central frequency and diameter, the approximate demands for the length of those samples and the transducers' frequency and diameter were presented in order to attain the phase or group velocities in the experiment . When the experimental frequency is above the critical frequency of 0.5 MHz , that the measured velocities are phase velocities must demand that propagation distance in the anisotropy media is less than two times of the transducer's diameter . When the experimental frequency is below 0.5 MHz , or transducer's diameter is less than 0.5mm , using the measured velocities must be careful.Common rock samples have been shown to have velocity orthotropy according to measured experimental results . The anisotropy of rock is cased mainly by three factors (1)aligned cracks. (2) the intrinsic anisotropy of mineral crystal as the results of crystallization. (3)multi-layer and lithological features (alignment of mineral grains and pore). Anisotropic textural direction can be determined correctly according to polarization testing .The materials by which to make orthotropic model with controlled crack parameter were attempted to develop at laboratory . Shear wave splitting was observed in three different directions in small scale samples . All those provide reliable experience in making physics model for further studies of the relationship between cracked media and the property of elastic wavefield in it.The features of phase and group velocity were advanced by means of preparing samples with different shaping . The phenomenon of "cusp" in group surface of quasi-shear wave (qSV) was observed . The inversed velocity surface according to calculated elastic constants in realorthotropic media is consistent with practical measured velocity surface .The P- wavefield was observed by adopting vertical transmission technique of longitudinal-exciting and transverse-receiving . the experimental results showed that quasi-compressional wave(qP) and quasi-shear wave (qSV) can exist concurrently with varying P-wave incidence , which is caused by vibration splitt...