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从垂直地震剖面能得到一组理想的数据,利用这组数据,可以对地震频带范围内的衰减进行测定。我们根据从窗口中截取出来的直达波计算每个深度上的傅立叶振幅谱,根据这些振幅谱之间的比例,就可以估算出各层的衰减系数。假定在地震波的整个频带范围内,品质因素与频率无关,我们就可以用线性最小二乘方技术计算出每一层的品质因素Q. 在垂直地震剖面中测量到的视衰减包含了岩石中的内在消耗,传输损失,层间多次反射以及散射效应等各种因素。所有这些,都会使振幅发生变化,因此,那种认为视衰减完全是由消耗所造成的观点是不正确的。为了阐明上述各种因素对衰减所起的作用,相应地作了一系列的模型试验。物理模型的内容包括速度,密度及地层衰减剖面。频率域垂直地震剖面合成地震记录是将单频的(monochr omatic)上行波、下行波以及与它们有关的所有的多次波的传播能量加在一起而成的。这就相当于将检波器相继埋置在各地层中时一维波动方程的解。利用吸收系数及速度数值,很容易算出与每一个傅立叶频率分量相对应的复波数目。根据声测井及密度测井资料,可以得到合成垂直地震剖面数据,这些数据本身就包含了所测定的衰减。将合成垂直地震剖面上直达波的波谱,旅行时,波形及振幅与真实的垂直地震剖面上的有关数据进行对比,经过若干次迭代之后,就可以得到一个高精度的Q值剖面。在合成中,我们往往是利用Q值提取算法对不同的Q值进行计算的。从计算中我们就可以看出,与垂直地震剖面间距比较起来,当对应于固定Q值的深度间隔足够大时,我们就能够可靠地对衰减进行测定。我们用从野外得到的垂直地震剖面对衰减进行了测定。以此作为例证,对所述方法作了进一步的说明.
A set of ideal data can be obtained from the vertical seismic section. Using this set of data, the attenuation in the seismic band can be measured. We calculate the Fourier amplitude at each depth based on the direct waves taken from the window and estimate the attenuation coefficient for each layer based on the ratio between these amplitude spectra. Assuming that the quality factor is frequency independent over the entire frequency range of the seismic wave, we can use the linear least squares technique to calculate the quality factor for each layer Q. The apparent attenuation measured in a vertical seismic profile includes the Internal consumption, transmission loss, multi-layer reflection and scattering effects and other factors. All of these will change the amplitude, so that the view that the attenuation is caused solely by consumption is not correct. In order to clarify the effect of the above factors on attenuation, a series of model tests have been made accordingly. The physical model includes velocity, density and formation attenuation profiles. Frequency Domain Vertical Seismic Profile Synthetic seismograms are made by combining the monochromic upgoing waves, the descending waves, and the propagation energies of all the multiple waves associated with them. This is equivalent to the one-dimensional wave equation solution when the geophones are buried one after another in each stratum. Using the absorption coefficient and the velocity value, it is easy to calculate the number of complex waves corresponding to each Fourier frequency component. Based on acoustic and density logging data, synthetic vertical seismic section data are available, and the data itself contains the measured attenuation. Comparing the spectrum, travel time, wave shape and amplitude of the direct wave on the synthetic vertical seismic section with the relevant data of the real vertical seismic section, after a few iterations, a high-precision Q-value section can be obtained. In the synthesis, we often use the Q value extraction algorithm to calculate different Q values. From the calculations we can see that we can reliably measure the attenuation when the depth interval corresponding to a fixed Q value is large enough in comparison with the vertical seismic profile spacing. We used the vertical seismic profile obtained from the field to determine the attenuation. Taking this as an example, the method is further described.