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利用零偏移距垂直地震剖面(VSP)资料,我们能预测井底总深度以下的地质构造。VSP资料应用中的关键问题是分辨率及延伸的距离。在利用VSP资料进行钻前预测中,波阻抗曲线是非常有用的。VSP合成道首先必须与地表地震剖面以及测井曲线合成地震道准确相连。对VSP合成道进行反演就可得到波阻抗曲线。但是,在进行反演之前,我们需要:(1)对合成道进行井底总深度之下的衰减效应校正;(2)输入速度,以提供低频信息。对井底总深度之下的VSP资料进行指数函数增益放大,就能够补偿衰减所造成的振幅损失。利用VSP速度校正地震速度就能够得到必要的低频信息。利用野外数据集及总深度之上的数据集就可说明这些情况。我们把根据VSP资料得到的结果与测井资料进行了比较。 根据测井资料进行VSP模拟可以估计资料的分辨率。研究表明,从延伸资料中获得的分辨率在50～75英尺(15～23米)的级别上。对于资料中所具有的频率而言,这相当于1/4地震波长。此外,这也表明这些资料的最大延伸距离可轻易地达到2000英尺(600米),也可能达到4000英尺(1200米)。 为了说明起见,我们把本文所述及研究的方法应用于一个近海VSP数据集中,求得一条波阻抗曲线。利用测井资料校正这条曲线后,在井底总深度之下正确地确定了目的砂层的底,成功地预测了砂层的厚?
Using zero offset vertical seismic profiles (VSP) data, we can predict the geological formation below the total depth of the bottom hole. The key issue in VSP data applications is the resolution and the extended distance. The wave impedance curve is very useful in pre-drilling prediction using VSP data. The VSP Synthesizer must first be accurately linked to the surface seismic profiling and the synthetic trace of the well logs. The wave impedance curve can be obtained by inverting the VSP synthesis path. However, prior to inversion, we need to: (1) correct the attenuation effect below the total depth of the well; and (2) input the velocity to provide low-frequency information. VSP data below the total depth of the well exponentiation gain amplification, can compensate for the attenuation caused by the loss of amplitude. The use of VSP velocity correction seismic velocity will be able to get the necessary low-frequency information. These are illustrated by the use of a dataset above the field data set and total depth. We compare the results from the VSP data with well logs. VSP simulation based on log data can estimate the resolution of the data. Research has shown that the resolution obtained from the extended data is on the order of 50 to 75 feet (15 to 23 meters). For frequencies in the data, this corresponds to 1/4 seismic wavelength. In addition, it shows that the maximum extension of these data can easily reach 2,000 feet (600 meters) and possibly 4,000 feet (1,200 meters). For the sake of illustration, we apply the method described and studied in this article to an offshore VSP dataset and obtain a wave impedance curve. After calibrating this curve with log data, the bottom of the target sand layer is correctly determined below the total depth of the bottom hole and the thickness of the sand layer is successfully predicted.