青藏高原典型多年冻土场地地面脉动特征

CHARACTERISTICS OF MICROSEISM AT TYPICAL PERMAFROST SITES IN QINGHAI—TIBET PLATEAU

通过对青藏高原典型多年冻土场地地面脉动的现场测试,分析研究该地区冻土场地地面脉动的频谱特性、卓越频率随多年冻土地温分区、上限深度的变化特征以及多年冻土区与典型融土区场地地面脉动卓越频率的差异。地面脉动一定程度上反映了场地浅层土体的宏观动力特性,这种特性与场地土体强度特征相联系。研究结果表明,青藏高原多年冻土区场地地面脉动频谱曲线呈现出较为显著的单主峰特征,其垂向分量的卓越频率为3.3～4.8 Hz,水平分量的卓越频率为2.3～4.7 Hz;多年冻土区场地地面脉动垂向分量的卓越频率值与冻土场地多年冻土上限深度和地温有较好的对应关系,垂向分量的卓越频率随地温升高而降低,随多年冻土上限抬升而增大;多年冻土区与典型融土区(西部黄土地区)场地地面脉动卓越频率的量值范围差异明显,前者卓越频率范围较窄而后者较宽,且总体上前者地面脉动卓越频率值大于后者。

Based on microseism tests at the typical permafrost sites along the Qinghai-Tibet Railway, the characteristics of frequency spectra of the permafrost sites are determined. The changes of the predominant frequencies with ground temperatures and permafrost tables, and the differences in frequency spectra between permafrost sites and loess sites are analyzed. Statistical curves of frequency spectra by Fourier analysis at these typical permafrost sites in the plateau are the shapes with single dominant peak value. The predominant frequencies of microseism＇s vertical components are between 3.3 Hz and 4.8 Hz; and the horizontal predominant frequencies are between 2.3 Hz and 4.7 Hz. The microseism is a macroreaction of lithologic characteristics, especially physico-mechanical features of those shallow strata, which depends on the strength of permafrost sites; and it will be changed with the ground temperatures and permafrost tables. Analytical results show that the predominant frequencies of microseism＇s vertical components are correlated with ground temperatures and permafrost tables at the permafrost sites： the higher the ground temperature is, the lower the vertical predominant frequency is; the higher the permafrost table is, the greater the vertical predominant frequency is. It indicates that there is a close relation between predominant frequencies and the strengths of permafrost sites. Moreover, a comparative result of microseism＇s predominant frequencies in permafrost sites and loess sites reveals that there is an obvious difference in predominant frequencies of microseism between the two different areas： the ranges of predominant frequencies at the permafrost sites are narrower than those of the loess sites; and there is a higher value of predominant frequency of microseism in the permafrost sites.