| At present,domestic research and development of seismometer are mainly on the stage of laboratory research and development without well-developed products for application to practical use.However,the technology of seismometer overseas,having developed mature and strong,as the development of digital integrated circuit,are growing full-digitally and without any error transmission.Because of the consumption of a large sum of funding on the introduction of MEMS digital demodulator and the pursuit of narrowing the technological gap and keeping pace with world-advanced sensing technology,our country must carry out the research and development of MEMS digital demodulator technology independently,which has significant social and economic influences on raising the level of domestic equipment manufacturing and that of safety inspection in road engineering.To make the data of digital demodulator reveal the stratum structure correctly,the priority is to carry out forward modeling on the Rayleigh wave.The forward modeling means working out the theoretical data or curve of Rayleigh wave's transmission through theoretical calculation and according to the properties of the known soil.The forward modeling of Rayleigh wave lays a theoretical foundation for its inversion,which helps to raise the inversion speed and accuracy in weak foundation and holes.This article is mainly about the relations between the wave's characteristics in transmission and the stratum structure when the wave transmits in weak stratum.The first step of the research is to simulate the data through finite difference software FLAC3 D and MATLAB when the wave transmits in weak covering and weak interlayer,and then the second step is to imitate the data through finite difference software MATLAB when Rayleigh wave transmits between stratum with holes.All factors below are taken into consideration:(1)the influence of the transverse hardness distribution of weak covering on time-domain signal of Rayleigh wave;(2)the impact of the vertical hardness distribution of covering on the dispersion curve of Rayleigh wave;(3)the impact of the vertical hardness distribution of weak interlayer on the time-domain signal and dispersion curve of Rayleigh wave;(4)the influence of with or without holes and the distance between holes and the earth' surface on the dispersion curve of Rayleigh wave.By numerical simulation all achievements below have been gotten:(1)As for weak covering,the time-domain signals of Rayleigh wave are slightly steep.The thicker the weak covering,the lower speed of the wave,the rapid energy consumption of Rayleigh wave.If the time-domain signal transmitted by time-domain signal of Rayleigh wave changes at phase axis,the stratum of testing area is transversely uneven,with soft-hard dividing line in the turning point in phase axis.(2)As for weak interlayer,with a certain thickness of the covering,the phase velocity of the dispersion curve of Rayleigh wave will slow down at the inflection point as the increase of the thickness of weak interlayer.(3)Because the changes in thickness of weak interlayer very slightly affect the time-domain signal when there existing weak interlayer,it's difficult to judge the distribution of weak interlayer through time-domain signal.As for weak covering,the VR on the dispersion curve of Rayleigh wave shows a declining trend wholly;At low-speed section,VR lowers down very rapidly;But at high-speed section,the VR basically remains unchanged displaying an approximately horizontal line.In certain cases,the thickness of the weak interlayer is fixed,the thickness of earth surface has a greater influence on in the middle frequency of curve of Rayleigh wave' transmission(VR-f),the phase velocity VR of Rayleigh wave at the jumping point increases with the increase of the thickness of surface layer.(4)In the stratum model with underground holes the dispersion curve of Rayleigh wave is divided into 3 sections that are not continuous.In the stratum model with underground holes the horizontal ordinate of the first sectional line on the dispersion curve of Rayleigh wave increases with the increase of distance between the hole and the earth surface.All the knowledge above will lay a theoretical foundation for the inversion of Rayleigh wave in the weak stratum. |
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