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超声P波地震图（digital ultrasonic P wave seismograms）进行分析，可获取沉积物属性（sediment properties）与微观结构（microstructure）的详细信息。在饱水沉积物（water-saturated sediments）中，对弹性波传播（elastic wave propagation）起主导影响的物理参数为P波衰减系数（P wave attenuation coefficient），与之相关的沉积学参数（sedimentological parameter）则为粒度分布（grain size distribution）。本文提供了一组频率范围约50~500 kHz的高分辨率超声透射地震图（high-resolution ultrasonic transmission seismograms），其信号形态（signal shape）与频率成分（frequency content）可反映岩心内部的粒度垂向变化（downcore variations）。粗粒有孔虫软泥（coarse-grained foraminiferal ooze）层可通过强衰减的P波识别，而在细粒颗石藻软泥（fine-grained nannofossil ooze）区域则记录到几乎无衰减的弹性波。彩色编码像素图（color-encoded pixel graphics）与瞬时频率（instantaneous frequencies）可呈现岩性（lithology）与衰减特性的全波形图像（full waveform images）。本文提出一种改进的谱差法（modified spectral difference method）以计算衰减系数及其幂律关系a = kfⁿ。将该方法应用于基于“品质因数Q恒定（constant Q）”模型生成的合成地震图时，即便极低的衰减系数也可被量化。岩心垂向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约为700 dB/m，幂指数n介于1~2之间；而细粒黏土（fine-grained clays）中衰减系数仅为数分贝每米（decibels per meter），且n ≤ 0.5。通过最小二乘拟合（least squares fit）得到的二次多项式（second degree polynomial）可描述平均粒度（mean grain size）与衰减系数之间的内在关联。将该关联用于预测平均粒度时，其结果与沉积学测量（sedimentological measurements）得到的数值几乎完全吻合。