1.1 目的和应用: 1.1.1 本指南总结了使用地震反射法评估浅层地下条件的技术、设备、现场程序、数据处理和解释方法。 1.1.2 本指南中所述的地震反射测量适用于绘制各种用途（包括地质用途）的浅层地下条件 ( 1. ) 、岩土工程、水文地质 ( 2. ) ，和环境 ( 3. ) . 2. 地震反射法用于绘制、探测和描绘地质条件，包括基岩表面、限制层（隔水层）、断层、岩性地层、孔隙、地下水位、裂缝系统和地层几何形状（褶皱）。地震法的初步应用- 反射法是绘制岩性单元的横向连续性，通常是检测地下声学特性的变化。 1.1.3 本指南将重点介绍应用于近地表的地震反射方法。近地表地震反射应用基于与深层地震反射测量相同的原则，但公认的做法可能在几个方面有所不同。近地表地震反射数据通常具有高分辨率（主频高于80 Hz），图像深度约为6米至数百米。偶尔会进行小于6 m的调查，但这些应视为实验性的。 1.2 限制: 1.2.1 本指南概述了浅层地震反射方法，但未涉及地震理论、现场程序、数据处理或数据解释的细节。为此目的，包含了大量参考文献，并被视为本指南的重要组成部分。建议地震反射法的用户熟悉本指南中的相关材料、文中引用的参考文献和指南 D420 , D653 , D2845 , D4428/D4428M 实践 D5088 ，导向装置 D5608 , D5730 , D5753 , D6235 和 D6429 . 1.2.2 本指南仅限于在陆地上进行的二维（2-D）浅层地震反射测量。地震反射法可适用于多种特殊用途: 陆上、钻孔内、水上和三维（3-D）。然而，本指南不包括对反射测量的这些专门调整的讨论。 1.2.3 本指南提供的信息有助于理解地震反射法的概念和在广泛的岩土、工程和地下水问题中的应用。 1.2.4 本指南中建议的地震反射法方法是常用的、广泛接受的和经过验证的；然而，技术上合理的地震反射方法的其他方法或修改也同样适用。 1.2.5 本文讨论了地震反射法的技术局限性 5.4 . 1.2.6 本指南讨论了压缩( P )和剪切( S )波浪反射方法。在适用的情况下，本指南将指出这两种方法之间的区别。 1.3 本指南提供了有组织的信息收集或一系列选项，并不推荐具体的行动方案。本文件不能取代教育或经验，应与专业判断一起使用。并非本指南的所有方面都适用于所有情况。本指南不代表或取代必须根据其判断给定专业服务是否充分的谨慎标准，也不应在不考虑项目的许多独特方面的情况下应用本文件。本指南标题中的“标准”一词仅表示该文件已通过ASTM共识程序获得批准。 1.4 以国际单位制表示的数值视为标准值。括号中给出的数值为英寸-磅单位，仅供参考，不被视为标准值。 1.5 注意事项: 1.5.1 本指南的用户有责任遵循设备制造商建议中的任何预防措施，制定适当的健康和安全实践，并考虑使用炸药或任何高能（机械或化学）源时的安全和监管影响。 1.5.2 如果该方法适用于有危险材料、操作或设备的场所，则本指南的用户有责任在使用前制定适当的安全和健康做法，并确定任何法规的适用性。 1.5.3 本标准并非旨在解决与其使用相关的所有安全问题（如有）。本标准的用户有责任在使用前制定适当的安全、健康和环境实践，并确定监管限制的适用性。 1.6 本国际标准是根据世界贸易组织技术性贸易壁垒（TBT）委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认标准化原则制定的。 ====意义和用途====== 5.1 概念: 5.1.1 本指南总结了用于检测、圈定或绘制浅层地下特征以及使用地震法进行地层几何形状或地层学相对变化的基本设备、现场程序和解释方法- 反射法。该方法的常见应用包括绘制基岩顶部、描绘河床或地层几何形状、识别地下材料特性的变化、检测孔隙或断裂带、绘制断层、定义地下水位顶部、绘制限制层以及估计地下材料中的弹性波速。人员要求如实践中所述 D3740 . 5.1.2 使用地震反射法进行地下测量需要一个震源、多个地震传感器、多通道地震仪以及每个传感器之间的适当连接（无线电或硬线）( 图1 ，还显示了可选的滚动开关）。 地震传感器之间的地震能量传播时间取决于波类型、传播路径和材料的地震速度。 反射体波的传播路径（压缩( P )和剪切( S )波）由地下材料速度和由声阻抗（速度和密度的乘积）变化定义的界面几何形状控制。两层之间的声阻抗差异导致隔层边界上的阻抗对比度，并确定边界的反射率（反射系数）；例如，反射多少能量和传输多少能量( 等式3 ). 在正常发病率下: 哪里: R = 反射率=反射系数， 五、 1. 五、 2. = 第1层和第2层的速度， ρ 1. ρ 2. = 第1层和第2层的密度， Vρ = 声阻抗，以及 A. = 阻抗对比度。 斯内尔定律( 等式4 )描述入射、折射和反射地震波之间的关系: 哪里: 一、 = 入射角， r = 反射角，以及 t = 折射角度。 在由速度和密度乘积（声阻抗）的变化表示的每个边界处，入射地震波产生反射波 P ，反射 S ，已传输 P ，并传输 S 波动该过程由Zoeppritz方程描述（例如，Telford等人。 ( 4. ) ). 5.1.3.2 通过分析和识别不同地震传感器上的地震能量到达模式，可以估计反射系数（反射器）的深度和反射系数与地球表面之间的平均速度。每个地震传感器记录的地震波的模拟显示通常在地震图上以摆动轨迹格式显示( 图2 )并表示与地震传感器（检波器或加速计）和震源的方向和类型一致的粒子运动（速度或加速度）。 （A） 速度是适用于材料的范围内的平均值 ( 5. ) . （B） 声阻抗是速度乘以密度，特别是压缩波；剪切波的等效值称为地震阻抗（单位为kg/s·m） 2. ). （C） 研究超浅近地表的研究人员报告了亚音速。 （A） 第1层打开 图1 . （B） 第2层打开 图1 . （C） R 在里面 等式3 ，绝对值 R =1总反射率。 5.2.1 地震反射法图像改变了地下层的声（地震）阻抗和特征，代表了地下材料特性的变化。 虽然地震反射技术取决于非零反射系数的存在，但解释人员必须根据当地条件和其他数据的知识解释地震反射数据，并得出地质上可行的解决方案。反射波形的变化可以指示地下的变化，例如岩性（岩石或土壤类型）、岩石稠度（即，断裂、风化、合格）、饱和度（流体或气体含量）、孔隙度、地质结构（几何变形）或密度（压实）。 5.2.2 反射系数或反射率- 反射率是对具有不同声阻抗值的材料之间的边界（界面）预期返回的能量的测量。 声阻抗较大的材料叠加在声阻抗较小的材料上，将导致负反射率和反射子波的相关相位反转。直观地说，小波极性遵循反射系数，当较快或密度较大的层覆盖较慢或密度较小（例如，干砂上的粘土）层时，反射系数为负值，当较慢或密度较小的层覆盖较快或密度较大（例如，石灰岩上的砾石）层时，反射系数为正值。反射率为1意味着所有能量将在界面上反射。 5.3 设备- 用于地表地震测量的地球物理设备可分为三大类:震源、地震传感器和地震仪。震源产生的地震波以脉冲或编码波列形式在地面传播。 地震传感器可以测量加速度、速度、位移或压力的变化。地震仪通过调节模拟信号，然后将模拟信号转换为数字格式（a/D），测量、转换和保存来自地震传感器的电信号。这些数字数据以预定的标准格式存储。有各种各样的地震测量设备可供选择，地震反射测量设备的选择应满足测量目标。 5.3.1 来源- 地震震源有两种基本类型:脉冲震源和编码震源。脉冲源将其所有能量（势能、动能、化学或某些组合）瞬间（即通常不到几毫秒）转移到地球。 冲击源类型包括炸药、落锤和抛射物。编码源以预定的方式在给定的时间间隔内传递能量（扫频或脉冲调制为时间的函数）。源能量特性高度依赖于近地表条件和源类型 ( 6- 9 ) . 在地震反射测量中，一致、宽带震源能量性能非常重要。信源有效性的主要度量是根据记录信号估计的信噪比和分辨率。 5.3.1.1 地震震源的选择应基于勘测目标、场地表面和地质条件及限制、勘测经济性、震源重复性、先前震源性能、勘测场地可能的总能量和带宽（基于先前研究或场地特定实验）以及安全性。 5.3.1.2 对于给定的总地震能量，编码地震源通常不会像脉冲源那样对环境造成干扰。变幅背景噪声（如过往汽车、飞机、行人交通等）影响编码源收集的数据质量，而不是冲击源。编码源需要额外的处理步骤，以将时变信号波列压缩到更易于解释的等效脉冲。这通常使用相关或移位和堆栈技术来实现。 5.3.1.3 在大多数情况下，与地面源相比，埋置的小型炸药将产生更高的频率和更宽的带宽数据。然而，与其他爆炸源相比，爆炸源通常具有使用限制、法规和更多安全考虑。 大多数爆炸物和抛射物源设计为侵入性，而重量下降和大多数编码源通常与地面直接接触，因此是非侵入性的。 5.3.1.4 震动、冲击或驱动地面使主要粒子运动水平于地面的源是剪切波源。震动、冲击或驱动地面使主要粒子运动垂直于地面的源是压缩源。许多震源可用于产生剪切波和纵波能量。 5.3.2 地震传感器- 地震传感器将机械质点运动转换为电信号。有三种不同类型的地震传感器: 加速度计、检波器（偶尔称为地震计）和水听器。 5.3.2.1 加速计是测量粒子加速度的装置。加速度计通常需要前置放大器来调节信号，然后再传输到地震仪。与检波器或水听器相比，加速计通常具有更宽的灵敏度带宽和更大的高重力容限。加速计具有首选的灵敏度方向。 5.3.2.2 检波器由一个固定的圆柱形磁铁组成，磁铁周围有一个线圈，线圈连接在弹簧上，可以相对于磁铁自由移动。检波器测量粒子速度，因此产生一个信号，该信号是加速度计测量的加速度的导数。 检波器通常坚固耐用，具有与其固有频率和线圈阻抗成比例的独特响应特性。固有频率与弹簧常数有关，线圈阻抗是线圈中导线绕组数量的函数。 5.3.2.3 水听器用于测量在液体中传播的地震信号。由于剪切波不通过水传播，水听器只对纵波作出响应。然而，剪切波可以在水/土界面转换为纵波，并提供剪切波的间接测量。水听器是一种压敏器件，通常由一个或多个压电元件构成，压电元件随压力而变形。 5.3.2.4 检波器和加速计可用于陆地上的纵波或横波测量。地震传感器的方向决定了地震传感器对不同粒子运动的响应和灵敏度。一些地震传感器是全方位的，对平行于传感器运动轴的粒子运动敏感，而与传感器的空间方向无关。其他地震传感器设计用于一个方向或另一个方向( P 或 S ). 剪切波地震传感器对垂直于传播方向（震源和地震传感器之间的线）的粒子运动敏感，对垂直方向敏感( SV公司 )或水平( 上海 )横波运动。纵波地震传感器对平行于传播方向（震源和地震传感器之间的线）的粒子运动敏感，因此地震传感器的运动轴需要处于垂直位置。 5.3.3 地震仪- 地震仪测量地震传感器产生的电压随时间的变化，并将其与震源同步。地震仪具有不同的地震道数量和一系列电子规格。应根据调查目标选择合适的地震仪。现代多通道地震仪是基于计算机的，需要最小程度的微调来调整场地特征的差异或变化。影响记录数据精度或质量的可调地震仪采集设置通常限于采样率、记录长度、模拟滤波器设置、前置放大器增益和记录通道数。现代大动态范围（>16位）地震仪对可选模拟滤波器和增益调整的需求有限。 地震仪以标准格式（例如，SEGY、SEGD、SEG2）存储数字数据，这些格式通常取决于存储介质的类型和系统的主要设计应用。地震仪可以是单个单元（集中式），所有记录通道（特别是模拟电路和A/D转换器）都位于单个位置，也可以在测量区域周围分布多个自主地震仪。分布式地震仪的特点是位于检波器附近的几个小型分散数字化模块（每个模块1-24个地震道），以减少长电缆地震传感器的信号损失。来自每个分布式模块的数字数据被传输到中央系统，在那里收集、编目和存储来自多个分布式单元的数据。 5.3.4 震源和地震传感器耦合- 地震传感器和震源必须与地面耦合。根据地面条件以及震源和地震传感器配置，这种耦合的范围可以从简单地停留在地面上（例如，陆地拖缆、重量下降、振动器）到侵入性地面渗透或掩埋（例如，尖桩、埋藏的炸药、孔底的射弹交付）。水听器通过浸入湖泊、溪流、钻孔、沟渠等的水中而与地面耦合。 5.3.5 支持组件- 其他设备包括滚动开关、电缆、时断系统（地震仪和震源之间的无线电或硬线遥测）、质量控制（QC）和故障排除设备（地震传感器连续性、接地泄漏、电缆泄漏、地震仪畸变和噪声阈值、电缆和地震传感器短路插头）以及土地测量设备。 5.4 限制和干扰: 5.4.1 地球物理方法固有的一般局限性: 5.4.1.1 所有地球物理方法的一个基本限制是，给定的一组数据不能唯一地表示一组地下条件。仅凭地球物理测量无法唯一解决所有歧义，需要一些额外的信息，如钻孔测量。由于地球物理方法的固有局限性，地震反射测量不能完全代表地下地质条件。与其他地质信息适当结合，地震反射测量可以成为获取详细地下信息的有效、准确和经济有效的方法。 所有地球物理测量测量地球的物理性质（例如，速度、电导率、密度、敏感性），但需要与现场的地质和水文相关。反射测量不直接测量材料特定特征（例如颜色、纹理和粒度）或岩性（例如石灰石、页岩、砂岩、玄武岩或片岩），除非这些岩性可能具有不同的速度和密度。 5.4.1.2 所有地表地球物理方法固有地受到信号衰减和分辨率随深度降低的限制。 5.4.2 地震反射法的特定限制: 5.4.2.1 地震反射法的理论局限性与非- 零反射系数、地震能量特征、地震特性（速度和衰减）以及与记录几何相关的地层几何。在均质地球中，不会产生反射，因此无法记录反射。当在地球表面进行反射测量时，如果地球内存在反射系数非零的层，则只能从地球内返回反射。例如，由岩性变化定义的层，其速度或密度没有可测量的变化，无法用地震反射法成像。地震数据集的河床或物体分辨率的理论极限与反射能量的频率含量有关（见 8.4 ). 5.4.2.2 成功成像倾斜超过45度的地质层可能需要非标准部署震源和地震传感器。 5.4.2.3 分辨率（在中讨论 8.4 )信噪比是决定地震反射法实际局限性的关键因素。震源配置、震源和地震传感器耦合、近地表材料、记录系统规格、地震事件的相对振幅以及相干震源产生的地震噪声的到达几何形状都是定义地震反射方法实际局限性的因素。 (1) 高度衰减的近地表材料，如干砂和砾石，可能会对地震能量深度的分辨率潜力和信号强度产生不利影响 ( 10 ) . 衰减是指地震能量通过地球材料传播时迅速减少，通常在高频下最为明显。衰减材料可能会阻碍调查目标的实现。 (2) 虽然可以增强原始野外数据上不可见的信号，但最安全的做法是通过CMP叠加的所有处理步骤，从原始野外数据跟踪已处理地震反射剖面上的所有相干事件。可以对噪声进行处理，使其在CMP堆叠部分上看起来是相干的。 (3) 水质差异似乎不会充分改变速度和密度，从而无法通过地震反射法进行检测 ( 11 ) . 5.4.3 自然和文化条件造成的干扰: 5.4.3.1 地震反射法对各种来源的机械和电气噪声非常敏感。生物、地质、大气和文化因素都会产生噪音。 (1) 生物来源- 生物噪声源包括地面和地下洞穴中动物的振动，以及树木、杂草和草在风中抖动。可能引起噪音的动物包括老鼠、蜥蜴、牛、马、狗和鸟。动物，尤其是牲畜，可以在高分辨率数据上的较长偏移距记录道上产生比地震信号大几个数量级的地震振动。 (2) 地质来源- 噪音的地质来源包括岩石滑坡、地震、裂缝、断层或其他不连续性产生的散射能量以及流动的水（例如，瀑布、河流急流、井中的瀑布）。 (3) 大气源- 大气噪声源包括风振地震传感器或电缆、雷电、落在地震传感器上的雨水、积雪融化并从树木和屋顶上掉落，以及风振表面结构（例如建筑物、杆塔、标志）。 (4) 文化来源- 文化噪声源包括电力线（即50 Hz、60 Hz和相关谐波）、车辆（例如汽车、摩托车、火车、飞机、直升机、ATV）、空调、割草机、小型发动机动力工具、施工设备以及在地震线附近移动的机组人员和行人。从雷达装置、无线电发射器或信标传输的射频（RF）和其他电磁（EM）信号可能以比震源大几倍的幅度出现在地震数据上- 生成的地震信号。 5.4.3.2 在地震反射测量的设计和操作过程中，应考虑生物、地质、大气和文化噪声源及其与测量区域的距离，尤其是噪声的特征和受噪声影响区域的大小。由于与地球耦合和能量衰减相关的未知因素，每个干扰并不总是可预测的。 5.4.4 源噪声引起的干扰: 5.4.4.1 地震源同时产生信号和噪声。信号是用于解释地下条件的任何能量。噪声是任何未用于解释地下条件或降低信号可解释性的记录能量。 地滚（表面波）、直达波、折射、衍射、空气耦合波和反射倍数都是在地震反射剖面期间记录的地震图上观察到的常见类型的源噪声( 图3 ). 图3 从一条地震线上的两个不同位置获得现场记录 注1: 两个记录上都显示了反射到达。 (1) 地面滚动- 地滚是一种出现在反射地震图上的表面波（参见 无花果。2和 3. ). 地滚由震源产生，并以低速、高振幅的色散波沿地表传播。地滚可以控制近偏移距地震传感器，使近偏移距处的反射分离变得困难。 地滚可能被误解为反射到达，尤其是在使用不正确的偏移或检波器间隔的情况下。 (2) 直达波- 最先到达距离震源最近的传感器的地震能量称为直达波。直达波是直接从震源地震传感器穿过地球最上层的体波。 (3) 折射- 折射地震能量沿着速度对比（接触分离两种不同的材料）传播，以与对比上方和下方的速度相关的角度返回表面，并且线性相位速度等于速度对比下方材料的地震速度。折射通常是到达传感器、从震源开始的第一个（在时间上）相干地震能量- 传感器偏移超过直达波能量首先到达的位置。有关折射及其作为地球物理成像工具的使用的更详细讨论，请参阅指南 D5777 . (4) 衍射- 衍射是从不连续的地下层（断层、裂缝）或地下层或物体终止点（透镜体、通道、漂石）散射的能量。在进行反射测量时，绕射通常被视为地震噪声。 (5) 空气耦合波- 空气耦合波是通过空气传播的声波，激励地震传感器附近的地面，然后由地震传感器记录。震源产生的空气波以大约330米/秒（空气中的声速）的线速度（距震源的距离¸到达时间）到达地震图。 飞机产生的文化噪声是一种空气耦合波。空气耦合波可以从地表物体反射，在某些情况下，与地震图上地球内部各层的反射非常相似。空气耦合波可以产生假道间相干，并被误解为反射。 (6) 反射倍数- 反射倍数是在地下几层之间回响的反射。层间的多次反射或混响是反射，因此出现在具有所有反射特征的地震图上。地震图上的多次波的到达模式和循环性质以及低于预期的正常移动可以最好地区分多次波- 输出速度。 5.5 替代方法- 上述限制可能会妨碍使用地震反射法。其他地球物理（见指南 D6429 )或者，当信噪比过低或分辨率不足以实现测量目标时，可能需要使用非地球物理方法来调查地下条件。

1.1 Purpose and Application: 1.1.1 This guide summarizes the technique, equipment, field procedures, data processing, and interpretation methods for the assessment of shallow subsurface conditions using the seismic-reflection method. 1.1.2 Seismic reflection measurements as described in this guide are applicable in mapping shallow subsurface conditions for various uses including geologic ( 1 ) , geotechnical, hydrogeologic ( 2 ) , and environmental ( 3 ) . 2 The seismic-reflection method is used to map, detect, and delineate geologic conditions including the bedrock surface, confining layers (aquitards), faults, lithologic stratigraphy, voids, water table, fracture systems, and layer geometry (folds). The primary application of the seismic-reflection method is the mapping of lateral continuity of lithologic units and, in general, detection of change in acoustic properties in the subsurface. 1.1.3 This guide will focus on the seismic-reflection method as it is applied to the near surface. Near-surface seismic reflection applications are based on the same principles as those used for deeper seismic reflection surveying, but accepted practices can differ in several respects. Near-surface seismic-reflection data are generally high-resolution (dominant frequency above 80 Hz) and image depths from around 6 m to as much as several hundred meters. Investigations shallower than 6 m have occasionally been undertaken, but these should be considered experimental. 1.2 Limitations: 1.2.1 This guide provides an overview of the shallow seismic-reflection method, but it does not address the details of seismic theory, field procedures, data processing, or interpretation of the data. Numerous references are included for that purpose and are considered an essential part of this guide. It is recommended that the user of the seismic-reflection method be familiar with the relevant material in this guide, the references cited in the text, and Guides D420 , D653 , D2845 , D4428/D4428M , Practice D5088 , Guides D5608 , D5730 , D5753 , D6235 , and D6429 . 1.2.2 This guide is limited to two-dimensional (2-D) shallow seismic-reflection measurements made on land. The seismic-reflection method can be adapted for a wide variety of special uses: on land, within a borehole, on water, and in three dimensions (3-D). However, a discussion of these specialized adaptations of reflection measurements is not included in this guide. 1.2.3 This guide provides information to help understand the concepts and application of the seismic-reflection method to a wide range of geotechnical, engineering, and groundwater problems. 1.2.4 The approaches suggested in this guide for the seismic-reflection method are commonly used, widely accepted, and proven; however, other approaches or modifications to the seismic-reflection method that are technically sound may be equally suited. 1.2.5 Technical limitations of the seismic-reflection method are discussed in 5.4 . 1.2.6 This guide discusses both compressional ( P ) and shear ( S ) wave reflection methods. Where applicable, the distinctions between the two methods will be pointed out in this guide. 1.3 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This guide is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration for a project’s many unique aspects. The word “Standard” in the title of this guide means only that the document has been approved through the ASTM consensus process. 1.4 The values stated in SI units are regarded as standard. The values given in parentheses are inch-pound units, which are provided for information only and are not considered standard. 1.5 Precautions: 1.5.1 It is the responsibility of the user of this guide to follow any precautions within the equipment manufacturer’s recommendations, establish appropriate health and safety practices, and consider the safety and regulatory implications when explosives or any high-energy (mechanical or chemical) sources are used. 1.5.2 If the method is applied at sites with hazardous materials, operations, or equipment, it is the responsibility of the user of this guide to establish appropriate safety and health practices and determine the applicability of any regulations prior to use. 1.5.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee. ====== Significance And Use ====== 5.1 Concepts: 5.1.1 This guide summarizes the basic equipment, field procedures, and interpretation methods used for detecting, delineating, or mapping shallow subsurface features and relative changes in layer geometry or stratigraphy using the seismic-reflection method. Common applications of the method include mapping the top of bedrock, delineating bed or layer geometries, identifying changes in subsurface material properties, detecting voids or fracture zones, mapping faults, defining the top of the water table, mapping confining layers, and estimating of elastic-wave velocity in subsurface materials. Personnel requirements are as discussed in Practice D3740 . 5.1.2 Subsurface measurements using the seismic-reflection method require a seismic source, multiple seismic sensors, multi-channel seismograph, and appropriate connections (radio or hardwire) between each ( Fig. 1 , also showing optional roll-along switch). Seismic energy propagation time between seismic sensors depends on wave type, travel path, and seismic velocity of the material. The travel path of reflected body waves (compressional ( P ) and shear ( S ) waves) is controlled by subsurface material velocity and geometry of interfaces defined by acoustic impedance (product of velocity and density) changes. A difference in acoustic impedance between two layers results in an impedance contrast across the boundary separating the layers and determines the reflectivity (reflection coefficient) of the boundary; for example, how much energy is reflected versus how much is transmitted ( Eq 3 ). At normal incidence: where: R = reflectivity = reflection coefficient, V 1 V 2 = velocity of layers 1 and 2, ρ 1 ρ 2 = density of layers 1 and 2, Vρ = acoustic impedance, and A = impedance contrast. Snell’s law ( Eq 4 ) describes the relationship between incident, refracted, and reflected seismic waves: where: i = incident angle, r = reflected angle, and t = refracted angle. At each boundary represented by a change in the product of velocity and density (acoustic impedance), the incident seismic wave generates a reflected P , reflected S , transmitted P , and transmitted S wave. This process is described by the Zoeppritz equations (for example, Telford et al. ( 4 ) ). 5.1.3.2 Analysis and recognition of seismic energy arrival patterns at different seismic sensors allows estimation of depths to reflection coefficients (reflectors) and average velocity between the reflection coefficient and the earth’s surface. Analog display of the seismic waves recorded by each seismic sensor is generally in wiggle trace format on the seismogram ( Fig. 2 ) and represents the particle motion (velocity or acceleration) consistent with the orientation and type of the seismic sensor (geophone or accelerometer) and source. (A) Velocities are mean for a range appropriate for the material ( 5 ) . (B) Acoustic impedance is velocity multiplied by density, specifically for compressional waves; the equivalent for shear waves is referred to as seismic impedance (units of kg/s·m 2 ). (C) Subsonic velocities have been reported by researchers studying the ultra-shallow near surface. (A) Layer 1 on Fig. 1 . (B) Layer 2 on Fig. 1 . (C) R in Eq 3 , Absolute value R = 1 total reflectance. 5.2.1 The seismic-reflection method images changes in the acoustic (seismic) impedance of subsurface layers and features, which represent changes in subsurface material properties. While the seismic reflection technique depends on the existence of non-zero reflection coefficients, it is the interpreter who, based on knowledge of the local conditions and other data, must interpret the seismic-reflection data and arrive at a geologically feasible solution. Changes in reflected waveform can be indicative of changes in the subsurface such as lithology (rock or soil type), rock consistency (that is, fractured, weathered, competent), saturation (fluid or gas content), porosity, geologic structure (geometric distortion), or density (compaction). 5.2.2 Reflection Coefficient or Reflectivity— Reflectivity is a measure of energy expected to return from a boundary (interface) between materials with different acoustic impedance values. Materials with larger acoustic impedances overlying materials with smaller acoustic impedances will result in a negative reflectivity and an associated phase reversal of the reflected wavelet. Intuitively, wavelet polarity follows reflection coefficients that are negative when faster or denser layers overlie slower or less dense (for example, clay over dry sand) layers and positive when slower or less dense layers overlie faster or denser (for example, gravel over limestone) layers. A reflectivity of one means all energy will be reflected at the interface. 5.3 Equipment— Geophysical equipment used for surface seismic measurement can be divided into three general categories: source, seismic sensors, and seismograph. Sources generate seismic waves that propagate through the ground as either an impulsive or a coded wavetrain. Seismic sensors can measure changes in acceleration, velocity, displacement, or pressure. Seismographs measure, convert, and save the electric signal from the seismic sensors by conditioning the analog signal and then converting the analog signal to a digital format (A/D). These digital data are stored in a predetermined standardized format. A wide variety of seismic surveying equipment is available and the choice of equipment for a seismic reflection survey should be made to meet the objectives of the survey. 5.3.1 Sources— Seismic sources come in two basic types: impulsive and coded. Impulsive sources transfer all their energy (potential, kinetic, chemical, or some combination) to the earth instantaneously (that is, usually in less than a few milliseconds). Impulsive source types include explosives, weight drops, and projectiles. Coded sources deliver their energy over a given time interval in a predetermined fashion (swept frequency or impulse modulated as a function of time). Source energy characteristics are highly dependent on near-surface conditions and source type ( 6- 9 ) . Consistent, broad bandwidth source energy performance is important in seismic reflection surveying. The primary measure of source effectiveness is the measure of signal-to-noise ratio and resolution potential as estimated from the recorded signal. 5.3.1.1 Selection of the seismic source should be based upon the objectives of the survey, site surface and geologic conditions and limitations, survey economics, source repeatability, previous source performance, total energy and bandwidth possible at survey site (based on previous studies or site specific experiments), and safety. 5.3.1.2 Coded seismic sources will generally not disturb the environment as much as impulsive sources for a given total amount of seismic energy. Variable amplitude background noise (such as passing cars, airplanes, pedestrian traffic, etc.) affects the quality of data collected with coded sources less than for impulsive sources. Coded sources require an extra processing step to compress the time-variable signal wavetrain down to a more readily interpretable pulse equivalent. This is generally done using correlation or shift and stack techniques. 5.3.1.3 In most settings, buried small explosive charges will result in higher frequency and broader bandwidth data, in comparison to surface sources. However, explosive sources generally come with use restrictions, regulations, and more safety considerations than other sources. Most explosive and projectile sources are designed to be invasive, while weight drop and most coded sources are generally in direct contact with the ground surface and therefore are non-invasive. 5.3.1.4 Sources that shake, impact, or drive the ground so that the dominant particle motion is horizontal to the surface of the ground are shear-wave sources. Sources that shake, impact, or drive the ground so that the dominant particle motion is vertical to the surface of the ground are compressional sources. Many sources can be used for generating both shear and compressional wave energy. 5.3.2 Seismic Sensors— Seismic sensors convert mechanical particle motion to electric signals. There are three different types of seismic sensors: accelerometers, geophones (occasionally referred to as seismometers), and hydrophones. 5.3.2.1 Accelerometers are devices that measure particle acceleration. Accelerometers generally require pre-amplifiers to condition signal prior to transmission to the seismograph. Accelerometers generally have a broader bandwidth of sensitivity and a greater tolerance for high G-forces than geophones or hydrophones. Accelerometers have a preferred direction of sensitivity. 5.3.2.2 Geophones consist of a stationary cylindrical magnet surrounded by a coil of wire that is attached to springs and free to move relative to the magnet. Geophones measure particle velocity and therefore produce a signal that is the derivative of the acceleration measured by accelerometers. Geophones are generally robust, durable, and have unique response characteristics proportional to their natural frequency and coil impedance. The natural frequency is related to the spring constant and the coil impedance is a function of the number of wire windings in the coil. 5.3.2.3 Hydrophones are used when measuring seismic signals propagating in liquids. Because shear waves are not transmitted through water, hydrophones only respond to compressional waves. However, shear waves can be converted to compressional waves at the water/earth interface and provide an indirect measurement of shear waves. Hydrophones are pressure-sensitive devices that are usually constructed of one or more piezoelectric elements that distort with pressure. 5.3.2.4 Geophones and accelerometers can be used for compressional or shear wave surveys on land. Orientation of the seismic sensor determines the seismic sensor response and sensitivity to different particle motion. Some seismic sensors are omnidirectional and are sensitive to particle motion parallel to the motion axis of the sensor, regardless of the sensor’s spatial orientation direction. Others seismic sensors are designed to be used in one orientation or the other ( P or S ). Shear wave seismic sensors are sensitive to particle motion perpendicular to the direction of propagation (line between source and seismic sensors) and are sensitive to vertical ( SV ) or horizontal ( SH ) transverse wave motion. Compressional wave seismic sensors are sensitive to particle motion parallel to the direction of propagation (line between source and seismic sensor) and thus the motion axis of the seismic sensor needs to be in a vertical position. 5.3.3 Seismographs— Seismographs measure the voltages generated by seismic sensors as a function of time and synchronize them with the seismic source. Seismographs have differing numbers of channels and a range of electronic specifications. The choice of an appropriate seismograph should be based on survey objectives. Modern multichannel seismographs are computer based and require minimal fine-tuning to adjust for differences or changes in site characteristics. Adjustable seismograph acquisition settings that will affect the accuracy or quality of recorded data are generally limited to sampling rate, record length, analog filter settings, pre-amplifier gains, and number of recording channels. There is limited need for selectable analog filters and gain adjustments with modern, large dynamic range (>16 bits) seismographs. Seismographs store digital data in standard formats (for example, SEGY, SEGD, SEG2) that are generally dependent on the type of storage medium and the primary design application of the system. Seismographs can be single units (centralized), with all recording channels (specifically analog circuitry and A/D converters) at a single location, or several autonomous seismographs can be distributed around the survey area. Distributed seismographs are characterized by several small decentralized digitizing modules (1–24 channels each) located close to the geophones to reduce signal loss over long-cable seismic sensors. Digital data from each distributed module are transmitted to a central system where data from multiple distributed units are collected, cataloged, and stored. 5.3.4 Source and Seismic Sensor Coupling— The seismic sensors and sources must be coupled to the ground. Depending on ground conditions and source and seismic sensor configuration, this coupling can range from simply resting on the ground surface (for example, land streamers, weight drop, vibrator) to invasive ground penetration or burial (for example, spike, buried explosives, projectile delivery at bottom of a hole). Hydrophones couple to the ground through submersion in water in a lake, stream, borehole, ditch, etc. 5.3.5 Supporting Components— Additional equipment includes a roll-along switch, cables, time-break system (radio or hardwire telemetry between seismograph and source), quality control (QC) and troubleshooting equipment (seismic sensor continuity, earth leakage, cable leakage, seismograph distortion and noise thresholds, cable and seismic sensor shorting plug), and land surveying equipment. 5.4 Limitations and Interferences: 5.4.1 General Limitations Inherent to Geophysical Methods: 5.4.1.1 A fundamental limitation of all geophysical methods is that a given set of data does not uniquely represent a set of subsurface conditions. Geophysical measurements alone cannot uniquely resolve all ambiguities, and some additional information, such as borehole measurements, is required. Because of this inherent limitation in geophysical methods, a seismic-reflection survey will not completely represent subsurface geological conditions. Properly integrated with other geologic information, seismic-reflection surveying can be an effective, accurate, and cost-effective method of obtaining detailed subsurface information. All geophysical surveys measure physical properties of the earth (for example, velocity, conductivity, density, susceptibility) but require correlation to the geology and hydrology of a site. Reflection surveys do not directly measure material-specific characteristics (such as color, texture, and grain size), or lithologies (such as limestone, shale, sandstone, basalt, or schist), except to the extent that these lithologies may have different velocities and densities. 5.4.1.2 All surface geophysical methods are inherently limited by signal attenuation and decreasing resolution with depth. 5.4.2 Limitations Specific to the Seismic-Reflection Method: 5.4.2.1 Theoretical limitations of the seismic-reflection method are related to the presence of a non-zero reflection coefficient, seismic energy characteristics, seismic properties (velocity and attenuation), and layer geometries relative to recording geometries. In a homogenous earth, no reflections are produced and therefore none can be recorded. When reflection measurements are made at the surface of the earth, reflections can only be returned from within the earth if layers with non-zero reflection coefficients are present within the earth. Layers, for example, defined by changes in lithology without measurable changes in either velocity or density cannot be imaged with the seismic reflection method. Theoretical limits on bed or object-resolving capabilities of a seismic data set are related to frequency content of the reflected energy (see 8.4 ). 5.4.2.2 Successful imaging of geologic layers dipping at greater than 45 degrees may require non-standard deployments of sources and seismic sensors. 5.4.2.3 Resolution (discussed in 8.4 ) and signal-to-noise ratios are critical factors in determining the practical limitations of the seismic-reflection method. Source configuration, source and seismic sensor coupling, near-surface materials, specification of the recording systems, relative amplitude of seismic events, and arrival geometry of coherent source-generated seismic noise are all factors in defining the practical limitations of seismic-reflection method. (1) Highly attenuative near-surface materials such as dry sand and gravel, can adversely affect the resolution potential and signal strength with depth of seismic energy ( 10 ) . Attenuation is rapid reduction of seismic energy as it propagates through an earth material, usually most pronounced at high frequencies. Attenuative materials can prevent survey objectives from being met. (2) While it is possible to enhance signal not visible on raw field data, it is safest to track all coherent events on processed seismic reflection sections from raw field data through all processing steps to CMP stack. Noise can be processed to appear coherent on CMP stacked sections. (3) Differences in water quality do not appear to change the velocity and density sufficiently that they can be detected by the seismic-reflection method ( 11 ) . 5.4.3 Interferences Caused by Natural and by Cultural Conditions: 5.4.3.1 The seismic-reflection method is sensitive to mechanical and electrical noise from a variety of sources. Biologic, geologic, atmospheric, and cultural factors can all produce noise. (1) Biologic Sources— Biologic sources of noise include vibrations from animals both on the ground surface and underground in burrows as well as trees, weeds, and grasses shaking from wind. Examples of animals that can cause noise include mice, lizards, cattle, horses, dogs, and birds. Animals, especially livestock, can produce seismic vibrations several orders of magnitude greater than seismic signals at longer offset traces on high-resolution data. (2) Geologic Sources— Geologic sources of noise include rockslides, earthquakes, scattered energy from fractures, faults or other discontinuities, and moving water (for example, water falls, river rapids, water cascading in wells). (3) Atmospheric Sources— Atmospheric sources of noise include wind shaking seismic sensors or cables, lightning, rain falling on seismic sensors, snow accumulations melting and falling from trees and roofs, and wind shaking surface structures (for example, buildings, poles, signs). (4) Cultural Sources— Cultural sources of noise include power lines (that is, 50 Hz, 60 Hz, and related harmonics), vehicles (for example, cars, motorcycles, trains, planes, helicopters, ATVs), air conditioners, lawn mowers, small engine-powered tools, construction equipment, and people—both crew members and pedestrians—moving in proximity to the seismic line. Radio Frequency (RF) and other electromagnetic (EM) signals transmitted from radar installations, radio transmitters, or beacons can appear on seismic data at amplitudes several times larger than source-generated seismic signals. 5.4.3.2 During the design and operation of a seismic reflection survey, sources of biologic, geologic, atmospheric, and cultural noise and their proximity to the survey area should be considered, especially the characteristic of the noise and size of the area affected by the noise. The interference of each is not always predictable because of unknowns associated with earth coupling and energy attenuation. 5.4.4 Interference Caused by Source-Generated Noise: 5.4.4.1 Seismic sources generate both signal and noise. Signal is any energy that is to be used to interpret subsurface conditions. Noise is any recorded energy that is not used to interpret subsurface conditions or diminishes the interpretability of signal. Ground roll (surface waves), direct waves, refractions, diffractions, air-coupled waves, and reflection multiples are all common types of source-generated noise observed on a seismogram recorded during seismic reflection profiling ( Fig. 3 ). FIG. 3 Gained Field Records from Two Different Positions on One Seismic Line Note 1: The reflection arrivals are shown on both records. (1) Ground Roll— Ground roll is a type of surface wave that appears on a reflection seismogram (see Figs. 2 and 3 ). Ground roll is generated by the source and propagates along the ground surface as a lower velocity, higher amplitude, dispersive wave. Ground roll can dominate near-offset seismic sensors, making separation of reflections at close offsets difficult. Ground roll can be misinterpreted as reflection arrivals, especially if the incorrect offsets or geophone interval are used. (2) Direct Waves— The seismic energy arriving first in time at the sensors closest to the source is known as the direct wave. Direct waves are body waves that travel directly from the source seismic sensor through the uppermost layer of the earth. (3) Refractions— Refracted seismic energy travels along a velocity contrast (contact separating two different materials) returning to the surface at an angle related to the velocity above and below the contrast and with a linear phase velocity equal to the seismic velocity of the material below the velocity contrast. Refractions are generally the first (in time) coherent seismic energy to arrive at a sensor, beginning a source-to-sensor offset beyond those where direct wave energy arrives first. For a more detailed discussion of refractions and their use as a geophysical imaging tool, see Guide D5777 . (4) Diffraction— Diffractions are energy scattered from discontinuous subsurface layers (faults, fractures) or points where subsurface layers or objects terminate (lens, channel, boulder). Diffractions are generally considered seismic noise when undertaking a reflection survey. (5) Air-coupled Waves— Air-coupled waves are sound waves traveling through the air, exciting the ground near the seismic sensor and then recorded by the seismic sensor. Air waves generated by the source arrive on seismograms with a linear velocity (distance from source¸ arrival time) of ~330 m/s (velocity of sound in air). Cultural noise generated by aircraft is a form of air-coupled wave. Air-coupled waves can reflect from surface objects and in some cases appear very similar to reflections from layers within the earth on seismograms. Air-coupled waves can alias to produce false trace-to-trace coherency and be misinterpreted as reflections. (6) Reflection Multiples— Reflection multiples are reflections that reverberate between several layers in the subsurface. Multiple reflections or reverberations between layers are reflections and therefore appear on seismograms with all the characteristics of reflections. Multiples can best be distinguished by their arrival pattern and cyclic nature on seismograms and their lower than expected normal move-out velocity. 5.5 Alternative Methods— Limitations discussed above may preclude the use of the seismic-reflection method. Other geophysical (see Guide D6429 ) or non-geophysical methods may be required to investigate subsurface conditions when signal-to-noise ratio is too low or the resolution potential is insufficient for the survey objectives.