厚层火山岩地震响应研究

Study of seismic response for thick-layered volcanic rock

在辽河拗陷M地区,火山岩厚度差异最大达到570m。为了准确地了解厚层火山岩的波动特征,本文采用波 动方程交错网格有限差分正演方法模拟辽河拗陷M地区层状火山岩的地震响应。分析表明,由于火山岩具有高 速度、高密度的特点,厚层火山岩对构造解释的精度影响极大,它对下伏地层的影响巳不仅是散射(或屏蔽作 用),在地震成像形态上呈现“拗陷变浅、隆起变凹”的现象。具体表现为:①火山岩厚度大与厚度小(或火山岩不 存在)的区域相比,前者由于火山岩的高速会“吃掉”的旅行时达到177ms,其中在厚层火山岩附近,导致下伏地 层的隆起在地震成像形态上幅度变大,下伏地层的凹陷在地震成像形态上幅度交小;②在无火山岩或火山岩较 薄的部位,导致下伏地层的隆起在地震成像形态上幅度会变小;③如果不考虑火山岩在空间的分布情况,使用 统一的时-深量板进行时-深转换,厚度为500-600m的火山岩对下伏地层可造成200-300m的深度解释 误差。

In M area of Liaohe depression, the beggest difference of thickness in volcanic rocks can reach to 570m, in order to accurately know the wave characters of thick volcanic rocks, the paper used staggering grid finite-difference algorithm of wave equation to forward simulate the seismic response of layered volcanic rocks in M area of Liaohe depression. The analyses showed that because the volcanic rocks are characteristics of high velocity and high density,the thick volcanic rock has great influence on the precision of structural interpretation , its influence on underneath formations is not only the scattering (or shield function), but also appears the phenomenon: ' depression becoming shallower and uplift becoming sag'. The exact contents are as follows:(1)in comparison with the area having thin volcanic rocks (or without volcanic rock),in the area having thick volcanic rocks,the 'eaten up' travel-time can reach 177ms because of high velocity in volcanic rock,among which it can lead the uplift amplitude of underneath formations to become greater and sag amplitude--smaller in seismic imaging configuration nearby the thick volcanic rocks ;(2)in the position without volcanic rock or of thin volcanic rock, it leads the uplift amplitude of underneath formations to become smaller in seismic imaging configuration; (3)if without considering the distribution of volcanic rocks in space,the volcanic rocks 500-600m thick can cause 200 -300m interpretation errors in depth for underneath formations when the time-depth conversion is implemented by using uniform time-depth template.