海上气藏的叠前振幅检测

Marine gas pool indication using prestack seismic amplitude

由于海上地震勘探具有特殊的优越环境:表层地震地质条件单一,激发、接收条件稳定、高覆盖次数采集,这就为叠前的振幅分析提供了良好的基础。在正常情况下,地震记录经过振幅补偿处理,远、近道反射波的振幅趋于一致,而在具有含气砂体情况下,反射波的振幅则表现为由近道向远道逐渐增大的现象。AVO 技术就是利用这种振幅随炮检距的变化来检测气层的。普通道集记录不能直接用于 AVO 解释,只有经过提高信噪比,压制多次波和振幅补偿等特殊处理后的道集才能用于 AVO 解释。在振幅与炮检距的关系式中,除了反射纵波振幅参数之外,还包含有泊松比和横波振幅参数。因此,只要将上述特殊处理的道集作一些换算,就可以在获得垂直入射的纵波剖面的同时,获得相对泊松比剖面、泊松比振幅剖面和仿横波剖面。泊松比振幅剖面具有较强的检测气层的能力。

The prestack amplitude analysis is based on a good background be- cause marine seismic exploration is conducted in a specific advantageous environment,which includes simple seismic-geological condition of sur- face layer,stable shot-receiving condition and high-fold data acquisition. In normal case,the reflection amplitudes of far and near seismic traces tend to no difference after amplitude compensation processing of seismic records;but in the case of gas-bearing sand body,the reflection amp- litudes get stronger from near traces to far traces.AVO technique indicates gas-bearing formation by using the phenomenon that amplitude changes with offset.AVO technique does not adopt conventional seismic traces, but only uses the traces which have undergone some special processing steps such as S/N ratio improvement,multiple suppression,amplitude compensation,etc. Besides P wave reflection amplitude,the formula of amplitude and offset has Poisson's ratio and shear wave amplitude parameters;there- fore,the conversions of the specially processed seismic traces bring not only zero offset P-wave section but also relative Poisson's-ratio sec- tion,Poisson's-ratio amplitude section and pseudo shear-wave section. Poisson's-ratio amplitude section indicates gas reservoir quite well.