对修订现行工业CT相关国家标准的建议

2012年以来我国发布了一系列关于工业CT的国家标准。虽然起到了相当的积极作用,但是仍然暴露出一些明显的不足之处。本文就名词术语标准的制定、空间分辨率的检测条件、对比度分辨率、低对比度空间分辨率、加强对于圆盘模体测定技术的研究和测定透射散射比等问题提出若干修订的建议。     

医用CT仪上基于衰减的曝光量和峰值电压自动控制基本等式

为了给患者以最少的辐射剂量而不影响诊断,现行医用CT仪都有基于噪声指标的自动曝光量控制。峰值电压对于图像质量和辐射剂量影响都很大,因此自动峰值电压控制很重要。本文从图像质量和辐射剂量优化的第一原理出发,给出基于衰减的曝光量和峰值电压同时自动控制的一般等式。此等式可用于单独曝光量自动控制或者单独峰值电压自动控制。对于单独暴光量自动控制,此等式要求对患者体厚度毎增加一厘米,曝光量要增加3.8%。对于单独峰值电压自动控制,此等式要求对患者体厚度每增加一厘米,峰值电压要平均增加1.53%。如果患者体厚度是常数,此等式要求平均百分比曝光量增加是百分比峰值电压减少的2.49倍。这些等式应该可以在现行医用CT仪上用于曝光量和峰值电压的自动控制。     

基于背景噪声研究青藏高原东北缘瑞利波相速度和方位各向异性

本研究收集了甘肃、青海、宁夏等118个宽频带数字地震台站的连续波形资料,利用噪声互相关,经过计算和筛选,在5~38 s范围内,共得到5773条瑞利波相速度频散曲线.然后采用1°×1°的网格划分,反演获得青藏高原东北缘相速度和方位各向异性分布.结果表明：短周期8~12 s内,鄂尔多斯从低速异常变为高速异常;该周期范围内各向异性结果与区域断裂走向有很好的一致性.18~25 s周期内,祁连地块、松潘-甘孜地块、羌塘地块低速异常范围逐渐变大,随周期增加地壳低速异常与人工探测结果相符;鄂尔多斯表现为速度随周期增加逐渐变大,说明其中下地壳速度相对偏高,不存在低速异常;该周期范围内的各向异性特征表现为,祁连地块和松潘甘孜地块大致呈NW-SE方向,而青藏高原内部快波方向显示了顺时针旋转的形态.在30~35 s范围内面波速度主要受莫霍面深度和莫霍面附近介质速度的影响,与地壳厚度分布有非常好的吻合.综合不同方法获得的各向异性研究结果,支持印度-欧亚板块的碰撞使青藏高原东北缘地壳发生缩短和逐渐隆升的观点,认为整个岩石圈的垂直缩短变形是青藏高原东北缘的主要形成机制.

     

(ϖμ/ϖT) P of the lower mantle

We estimate (/T) _P of the lower mantle at seismic frequencies using two distinct approaches by combining ambient laboratory measurements on lower mantle minerals with seismic data. In the first approach, an upper bound is estimated for |(/T) _P | by comparing the shear modulus () profile of PREM with laboratory room-temperature data of extrapolated to high pressures. The second approach employs a seismic tomography constraint ( lnV _S / lnV _P ) _P =1.8–2, which directly relates (/T) _P with (K _S /T) _P . An average (K _S /T) _P can be obtained by comparing the well-established room-temperature compression data for lower mantle minerals with theK _S profile of PREM along several possible adiabats. Both (K _S /T) and (/T) depend on silicon content [or (Mg+Fe)/Sil of the model. For various compositions, the two approaches predict rather distinct (/T) _P vs. (K _S /T) _P curves, which intersect at a composition similar to pyrolite with (/T) _P =–0.02 to –0.035 and (K _S /T) _P =–0.015 to –0.020 GPa/K. The pure perovskite model, on the other hand, yields grossly inconsistent results using the two approaches. We conclude that both vertical and lateral variations in seismic velocities are consistent with variation due to pressure, temperature, and phase transformations of a uniform composition. Additional physical properties of a pyrolite lower mantle are further predicted. Lateral temperature variations are predicted to be about 100–250 K, and the ratio of ( lnp/ lnV _S ) _P around 0.13 and 0.26. All of these parameters increase slightly with depth if the ratio of ( lnV _S / lnV _P ) _P remains constant throughout the lower mantle. These predicted values are in excellent agreement with geodynamic analyses, in which the ratios ( ln / lnV _S ) _P and ( / lnV _S ) _P are free parameters arbitrarily adjusted to fit the tomography and geoid data.     

Authigenic carbonates from the eastern Black Sea as an archive for shallow gas hydrate dynamics – Results from the combination of CT imaging with mineralogical and stable isotope analyses

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area (“Batumi seep area”) located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups.     

基于精细数据处理的叠前深度偏移在琼东南盆地陵水凹陷坡折区的应用

琼东南盆地陵水凹陷,受坡折区崎岖海底和复杂地质构造的影响,导致该地区速度模型构建精度不高,从而严重影响到成像品质。针对坡折区的精确速度模型构建难题,首先采用组合多次波衰减、宽频处理、叠前信噪比增强等针对性处理技术,得到宽频带高信噪比的叠前数据,在此基础上统计分析不同水深速度规律,应用断控和地质约束速度建模技术,建立初始速度模型,然后利用基于宽频数据的高分辨断控约束网格层析反演技术,经过多次迭代完成深度域各向异性速度建模,并实现了陵水凹陷坡折区的高精度叠前深度偏移成像。叠前深度偏移成像显示,基于精细速度模型的叠前深度偏移技术科明显提高坡折区信噪比,改善成像质量,同时可有效消除地层同相轴扭曲的现象,恢复了地下地层的真实构造形态,从而可为目标评价与钻探决策提供更可靠地震资料。     

吉林前郭5.8级震区上地壳速度结构

为了更好地研究吉林前郭地区的地震活动性,本文利用2013年10月至2015年2月吉林前郭5.8级震群周围17个地震台站记录到的405个地震事件资料,共获得2 218条P波走时数据和2 166条S波走时数据,采用地震双差层析成像技术,获取吉林前郭震区上地壳三维P波、S波速度精细结构。结果显示,震区上地壳速度分层明显,P波速度变化范围为5.8~7.2km/s,S波速度变化范围为3.4~4.0km/s,波速比(vP/vS)变化较小。震区有低速带穿过,低速带走向为北东向,倾角较大(近直立),纵向延伸至15km深度;震群主要位于低P波和S波速度区,对应较低的波速比。吉林前郭震区构造强度较低,受西太平洋板块俯冲产生应力场的影响,易于发生地震。     

3D structure of the Earth's crust beneath the northern part of the Bohemian Massif

The SUDETES 2003 wide-angle refraction/reflection experiment covered the area of the south-western Poland and the northern Bohemian Massif. The good quality data that were gathered combined with the data from previous experiments (POLONAISE'97, CELEBRATION 2000) allowed us to prepare a 3D seismic model of the crust and uppermost mantle for this area. We inverted travel times of both refracted and reflected P waves using the JIVE3D package. This allowed us to obtain a model of P-wave velocity distribution as well as the shape of major boundaries in the crust. We also present a detailed uncertainty analysis for both the boundary depths and the velocity field. In doing the uncertainty analysis we found an interesting, strong dependence between uncertainty and inversion scheme (order of used phases). We also compared the model with surface geology and found good correlation between velocity inhomogeneities in the uppermost crust (down to 2 km) and major geological units. The higher velocity lower crust (6.9–7.2 km/s) could result from remelting of the lower crust or magmatic underplating.     

频率域2.5-D井间波形层析成像及其实际应用

从频率域3-D声波波动方程出发,结合井间观测方式的特点,基于Tarantola广义反演理论,提出了一种频率域2.5-D井间波形层析成像方法.数值模型试验结果表明：该方法对薄层厚度的分辨能力能达到约主频波长的1/4,且分辨率显著高于走时层析成像,尤其垂直分辨率有实质改善.模拟资料的抗噪试验表明：在信噪比为0.8的情况下,随机噪声对波形层析成像的影响较小;而相干噪声对全波形层析成像的影响显著,特别是初至波附近的强振幅干扰影响更为严重.井间实际资料的试处理结果表明：波形层析成像能很好地刻画井间介质的分布情况与储层连通性,对于油藏开发阶段的方案实施具有指导意义.     

利用地震层析成象技术确定横向非均匀介质中的速度分布和界面位置

本文提出一种横向非均匀介质中的地震层析成象方法，由地震走时资料确定介质的速度分布和界面位置。计算中采用了参数分离、变阻尼、奇异值分解等技术，方程的解较稳定，且可得到解的分办。除反射波外，该方法还可同时利用分析波的资料。因此，适宜于折射及宽角度反射地震测深剖面的资料处理。数字试验结果表明，在模型内部，该方法能得到可靠的反演结果。在边界附近效果较差，受初始值影响较大。