Grain size analysis of sediment core GeoB1510-2

Detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic P wave seismograms recorded automatically during full waveform core logging. The physical parameter which predominantly affects the elastic wave propagation in water-saturated sediments is the P wave attenuation coefficient. The related sedimentological parameter is the grain size distribution. A set of high-resolution ultrasonic transmission seismograms (-50-500 kHz), which indicate downcore variations in the grain size by their signal shape and frequency content, are presented. Layers of coarse-grained foraminiferal ooze can be identified by highly attenuated P waves, whereas almost unattenuated waves are recorded in fine-grained areas of nannofossil ooze. Color -encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation. A modified spectral difference method is introduced to determine the attenuation coefficient and its power law a = kF. Applied to synthetic seismograms derived using a "constant Q" model, even low attenuation coefficients can be quantified. A downcore analysis gives an attenuation log which ranges from -700 dB/m at 400 kHz and a power of n=1-2 in coarse-grained sands to few decibels per meter and n :s; 0.5 in fine-grained clays. A least squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient. When it is used to predict the mean grain size, an almost perfect coincidence with the values derived from sedimentological measurements is achieved.通过对全波形岩心测井（full waveform core logging）过程中自动记录的数字化超声P波（P wave）地震记录进行分析，可获取沉积物物性与微观结构的详细信息。在饱水沉积物中，对弹性波传播起主导影响的物理参数为P波衰减系数，与之相关的沉积学参数为粒度分布（grain size distribution）。本文展示了一组频率范围为-50~500 kHz的高分辨率超声透射式地震记录，其信号形态与频率成分可反映岩心内部的粒度变化。可通过强衰减的P波识别粗粒有孔虫软泥（foraminiferal ooze）层，而在颗石藻软泥（nannofossil ooze）的细粒区域则可记录到几乎无衰减的声波。地震记录与瞬时频率的颜色编码像素图可呈现岩性与衰减特性的全波形图像。本文提出了一种改进的谱差法（spectral difference method），用于求解衰减系数及其幂律关系a = kF。将该方法应用于基于"常数Q（constant Q）"模型生成的合成地震记录时，即便极低的衰减系数也可被定量计算。岩心内部分析得到的衰减测井曲线（attenuation log）显示，在400 kHz频率下，粗粒砂层的衰减系数可达-700 dB/m，幂指数n为1~2；而细粒黏土层的衰减系数仅为数分贝每米，幂指数n≤0.5。通过二次多项式的最小二乘拟合（least squares fit），可表征平均粒度与衰减系数之间的相互关系。利用该关系预测平均粒度时，所得结果与沉积学测量得到的数值几乎完全吻合。