Physical properties and sedimentology on core GeoB1510-1

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 (ca. 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 = kfn. 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 ca. 700 dB/m at 400 kHz and a power of n = 1-2 in coarse-grained sands to few decibels per meter and n <= 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波纵波（ultrasonic 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。在含水饱和沉积物中，对弹性波传播起主导调控作用的物理参数为纵波衰减系数（P wave attenuation coefficient），与之相关的沉积学参数则为粒度分布（grain size distribution）。本数据集包含一套高分辨率超声波透射地震记录（ultrasonic transmission seismograms，频率范围约50~500 kHz），该类记录可通过信号形态与频率组分反映岩心深度方向的粒度变化。强衰减的纵波可用于识别粗粒有孔虫软泥（foraminiferal ooze）层位，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波形。地震记录的彩色编码像素图与瞬时频率（instantaneous frequencies）图像可呈现岩性与衰减特征的全波形成像结果。本文提出一种改进的谱差法（modified spectral difference method），用于求解衰减系数及其幂律关系α = kfⁿ。将该方法应用于基于品质因子Q恒定模型（constant Q model）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。沿岩心深度方向的分析可得到一条衰减测井曲线（attenuation log）：在400 kHz频率下，粗粒砂岩段的衰减系数可达约700 dB/m，幂指数n为1~2；而细粒黏土段的衰减系数仅为数分贝每米，幂指数n≤0.5。通过最小二乘拟合（least squares fit）得到的二次多项式，可表征平均粒度与衰减系数之间的相互关系。利用该多项式预测平均粒度时，所得结果与沉积学实测值几乎完全吻合。