基于相对熵准则的CMP道集Q扫描

CMP gather Q scanning based on relative entropy criterion

地下介质的大地滤波效应(主要包括薄层叠合效应、散射效应和非弹性效应等)使地震信号振幅变小、频带变窄、主频降低、相位畸变,这可用等效Q值引起的效应表示。合理估计等效Q值并进行相应的Q值补偿对提高地震资料的分辨率十分重要。吸收衰减是一个累积效应,当地层Q值一定时,地震子波的衰减现象随传播距离/时间的增加和频率的增大而愈明显,即深层、远炮检距反射子波的能量和高频成分衰减更严重。为此,假设地层为水平层状、反射同相轴满足双曲线规律,已知地下均方根速度模型,利用CMP道集,沿双曲线轨迹开时窗以提取单个衰减子波;然后选取一组Q值,在频率域对衰减子波振幅谱做Q值补偿处理,依据不同炮检距处补偿后地震子波振幅谱一致性最佳原则估计水平地层的等效Q值。该方法需要一个合适的度量准则,因此对比、分析了相似系数、质心频率和KL散度、JS散度等准则。实验表明,在选取合适频带范围(如0~60 Hz)后,JS散度准则对规范化后的振幅谱一致性度量的敏感性更高,抗噪能力更强;在合适的信噪比条件下,基于相对熵准则的CMP道集扫描估计品质因子方法可以得到比较稳定的等效Q值估计结果。

The geodetic filtering effects(mainly thin layer overlapping effect,scattering effect,and inelastic effect)of underground media will attenuate seismic signal amplitudes,narrow frequency bands,reduce the main frequency,and distort phases,and they can be represented by the effect caused by equivalent Q value.It is very important to estimate the equivalent Q value reasonably and make the corresponding Q compensation to improve the seismic data resolution.The absorption attenuation is a cumulative effect.When the Q value of the stratum is constant,the attenuation of seismic wavelet becomes more and more obvious with the increase in the propagation distance/time and frequency.In other words,the amplitude and high‑frequency components of deep and far offset reflection wavelets are attenuated more seriously.In this paper,it is assumed that the stratum is horizontally layered,and the reflection events conform to the hyperbolic law;the underground root mean square velocity model is known,and a single attenuation wavelet can be extracted along the time window of the hyperbolic trajectory by using the CMP gather.Then a set of Q values is selected to compensate for the amplitude spectrum of the attenuation wavelet in the frequency domain by Q values.According to the principle of optimum consistency of amplitude spectrum of compensated seismic wavelet at different offsets,the equivalent Q value of horizontal strata is estimated.This method needs an excellent measurement criterion.Therefore,the measurement criteria,such as similarity coefficient,centroid frequency,KL divergence,and JS divergence are compared.Experimental results show that the JS divergence criterion is more sensitive to the normalized amplitude spectrum consistency measurement and has a stronger anti‑noise ability when the appropriate frequency band range(e.g.,0~60 Hz)is selected.The quality factor estimation method by CMP gather scanning based on relative entropy criterion can obtain relatively stable equivalent Q value estimation results when the signal‑to‑noise ratio is suitable.