浅谈地震数据处理中反射波的高频成分

地震数据采集得到的地震波是由反射波、规则干扰和噪声叠合而成的复合波,数据处理将淹没在干扰和噪声之中的反射波提取出来,再经过频率补偿使反射波高频成分的微弱振幅得以提升,从而得到主频高、频带宽的高分辨率数据。记录下来的反射波频率的高低,不能用“60dB高频死亡线”来衡量,用原始数据滤波扫描也得不到反射波频率的正确范围,而用采集+处理综合动态范围和地层吸收衰减模型可对反射波频率范围作出较客观的估计。对处理后的数据进行滤波扫描,才能得到正确的反射波频率范围。塔里木盆地沙漠区2ms采样间隔的常规处理数据高通滤波扫描结果表明,反射波频率可高达尼奎斯特频率(约240Hz),证明井中激发、地面接收采集到的数据已记录了这样的反射波高频成分。特征子波反褶积处理的实例证明了这样的微弱振幅高频成分是展宽高频段的有用信号。然而1ms采样间隔的高精度数据经叠前时间偏移处理,其反射波高频仅达到120Hz,甚至更低至60Hz。之所以如此,是叠前时间偏移前的滤波和频率衰减所致,这种做法变相地将1ms采样的高精度数据当作4ms甚至8ms采样间隔数据使用。此外,抽稀时间和空间采样间隔的算法和处理措施会使反射波高频成分成倍降低。

A seismic wave obtained via data acquisition is a composite wave made of reflection waves,various interferences,and noise.Using frequency compensation,the amplitude of reflection waves and their high-frequency components—which are drowned in the interference and noise—can be significantly strengthened.Therefore,high-resolution data with a high dominant frequency and a wide frequency band can be obtained.The frequency of a reflected wave cannot be evaluated owing to an existing 60dB high-frequency “death limit”.Therefore,the correct frequency range of the reflected wave cannot be captured via scanning with raw-data filtering.However,this range can be estimated objectively by exploiting the dynamic range resulting from integrating raw and processed data,with an attenuation model.The correct frequency range of the reflected wave was obtained by filtering and scanning the processed data.High-pass filter scanning of conventional processed data (using a 2ms sampling interval) from a desert area in the Tarim Basin resulted in a frequency of the reflected wave close to the Nyquist frequency (approximately 240Hz).This demonstrates that the high-frequency component of the reflection wave was correctly captured by ground receivers.Via wavelet deconvolution,it was demonstrated that high-frequency components with weak amplitudes can prove to be useful in broadening the high-frequency band.However,high-precision data (with a sampling interval of 1ms) processed via prestack time migration resulted in a reflected wave frequency of only 120Hz,or even lower than 60Hz.This is due to the filtering and frequency decay that occur before the processing of the prestack time migration,which causes the resampling of high-precision data at 4 or 8ms intervals.Other processing techniques and algorithms,such as sparse sampling intervals,reduce the frequency of the reflected waves exponentially.