1.1 目的和应用- 本指南涵盖了使用地震折射法评估地下条件的设备、现场程序和解释方法。本指南中所述的地震折射测量适用于绘制各种用途的地下条件，包括地质、岩土、水文和环境 ( 1. ) 矿产勘探、石油勘探和考古调查。地震折射法用于绘制地质条件，包括基岩深度或地下水位、地层、岩性、结构和裂缝或所有这些。计算的地震波速与材料的力学性质有关。 因此，根据地震速度和其他地质信息对材料（岩石类型、风化程度和可裂性）进行表征。 1.1.1 岩土行业使用英制或国际单位制。 1.2 限制: 1.2.1 本指南概述了使用压缩的地震折射法( P )波浪。它没有涉及地震折射理论、现场程序或数据解释的细节。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。建议地震折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以及文中引用的适当ASTM标准 2.1 . 1.2.2 本指南仅限于在陆地上进行地震折射测量的常用方法。地震折射法可适用于陆地、钻孔内和水中的许多特殊用途。然而，本指南不包括对地震折射测量的这些其他适应性的讨论。 1.2.3 在某些情况下，需要测量剪切波以满足项目要求。地震剪切波的测量是地震折射的一个子集。本指南不包括此主题，仅侧重于 P 波浪测量。 1.2.4 本指南中建议的地震折射法方法是常用的、广泛接受的和经过验证的； 然而，可以替代技术上合理的地震折射法的其他方法或修改。 1.2.5 本文讨论了地震折射法的技术局限性和干扰 D420 , D653 , D2845 , D4428/D4428M , D5088 , D5730 , D5753 , D6235 和 D6429 . 1.3 注意事项: 1.3.1 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，并考虑使用炸药时的安全和监管影响。 1.3.2 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。 1.4 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。 1.5 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ASTM标准不代表或取代必须根据其判断给定专业服务的充分性的谨慎标准，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。 1.6 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 本指南总结了用于使用地震折射法测定地下土壤和岩石或工程材料的深度、厚度和地震速度的设备、现场程序和解释方法。 5.1.2 通过地震折射法测量地下条件需要地震能量源、触发电缆（或无线电链路）、检波器、检波器电缆和地震仪（参见 图1 ). 图1 显示两层土壤/岩石系统（α）中直接和折射地震波路径的12通道地震仪的现场布局 c =临界角） 5.1.3 检波器和震源必须与土壤或岩石紧密接触。检波器通常位于一条直线上，有时称为检波器排列。地震源可以是大锤、撞击地面的机械装置或其他类型的冲击源。 炸药用于深层折射或需要更大能量的特殊条件。检波器将地面振动转换为电信号。该电信号由地震仪记录和处理。地震波的传播时间（从震源到检波器）由地震波形确定。 图2 显示了使用单个检波器的地震仪记录。 图3 显示了使用12个检波器的地震仪记录。 图2 来自单个检波器的典型地震波形 注1: 箭头表示第一个纵波的到达。 图3 显示爆炸声源产生良好初至的十二通道模拟地震仪记录( 2. ) 5.1.4 地震能源产生弹性波，从震源穿过土壤或岩石。当地震波到达具有不同地震速度的两种材料之间的界面时，波根据斯奈尔定律折射 ( 3. , 4. ) . 当入射角等于界面处的临界角时，折射波沿着两种材料之间的界面移动，将能量传输回表面( 图1 ). 该界面称为折射器。 5.1.5 地震能源会产生大量弹性波。因为压缩 P -波具有最高的地震速度，它是到达每个检波器的第一个波（参见 图2 和 图3 ). 5.1.6 这个 P -波速 五、 p 以以下方式取决于体积模量、剪切模量和密度 ( 3. ) : 哪里: 五、 p = 纵波速度， K = 体积模量， μ = 剪切模量，以及 ρ = 密集 5.1.7 地震仪记录来自地震源的能量到达每个检波器( 图3 ). 行程时间（地震所需的时间 P -从地震能量源传输到检波器的波由每个波形确定。时间单位通常为毫秒（1毫秒=0.001秒）。 5.1.8 根据震源和检波器之间的距离绘制行程时间，以绘制时间-距离图。 图4 显示了水平双层地球的震源和检波器布局以及由此产生的理想时距图。 图4 ( 一 )地震射线路径和( b )具有平行边界的两层地球的时间-距离图( 2. ) 5.1.9 地震能量源和检波器之间的地震波传播时间是它们之间的距离、折射器的深度和波通过的材料的地震速度的函数。 5.1.10 折射器的深度是使用震源到检波器的几何形状（间距和高程）计算的，确定视在地震速度（是时间-距离图中绘制线的斜率的倒数），以及时间-距离图上的截距时间或交叉距离（见 图4 ). 文献中推导了拦截时间和交叉距离深度公式 ( 5- 4. ) . 这些推导很简单，因为测量了地震波的传播时间，根据时间-距离图计算了每层中的速度，并且已知光路几何结构。这些解释公式基于以下假设:( 1. )层之间的边界是水平或以恒定角度倾斜的平面( 2. )没有地表起伏( 3. )每一层都是均匀和各向同性的( 4. )各层的地震速度随着深度的增加而增加，并且( 5. )中间层必须具有足够的速度对比度、厚度和横向范围，以便检测。 参考 ( 2. ) 对两层和三层情况下的这些方程进行了极好的总结。两层情况的公式（参见 图4 )如下所示。 5.1.10.1 截距时间公式: 哪里: z = 折射率2的深度， t 一、 = 拦截时间， 五、 2. = 第二层的地震速度，以及 五、 1. = 第一层地震速度。 5.1.10.2 交叉距离公式: 哪里: z、 五、 2. 和 五、 1. 如上文所定义，以及 x c =交叉距离。 5.1.10.3 三到四层通常是通过地震折射测量可以解决的最多的层。 图5 显示了理想三层情况下的震源和检波器布局以及生成的时间-距离图。 图5 ( 一 )地震射线路径和( b )具有平行边界的三层模型的时间-距离图( 2. ) 注1: 虽然这些方程适用于手动计算，但通常用于分析地震道的商用软件中使用了更先进的算法。 5.1.11 折射法用于定义一个或多个折射器顶部的深度或轮廓，或两者，例如地下水位或基岩的深度。 5.1.12 能量源通常位于检波器排列的两端或附近；在每个方向上进行折射测量。这些被称为正向和反向测量，有时被错误地称为反向测量，从中可以绘制单独的时间-距离图。 图6 显示了倾斜折射仪的震源和检波器布局以及由此产生的时间-距离图。仅从这两个测量值中的任何一个获得的折射器速度就是折射器的视速度。这两种测量对于求解真实地震速度和地层倾角都是必要的 ( 2. ) 除非有其他数据表明水平层状土壤。这两个视在速度测量值和截距时间或交叉距离用于计算折射器的真实速度、深度和倾角。注意，使用这种方法只能获得平面折射器的两个深度（参见 图7 ). 通过使用更复杂的数据收集和解释方法，在每个检波器下获得折射深度。 图6 ( 一 )地震射线路径和( b )具有倾斜边界的两层模型的时间-距离图( 2. ) 图7 时间-距离图( 一 )和解释地震剖面( b ) ( 7. ) 5.1.13 地质、工程、水文和环境应用的大多数折射测量都是为了确定折射深度小于100米（约300英尺）。然而，如果有足够的能量，可以在300米（1000英尺）及以上的深度进行折射测量 ( 5. ) . 5.2 测量参数和代表值: 5.2.1 地震折射法提供了压缩速度 P -地下材料中的波浪。虽然 P -波速是土壤或岩石类型的良好指示器，但不是唯一的指示器。 表1 结果表明，每种类型的沉积物或岩石都具有广泛的地震速度范围，其中许多范围重叠。虽然地震折射技术测量地球材料中地震波的地震速度，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折射数据，并得出地质上可行的解决方案。 5.2.2 P -在以下情况下，波速通常更大: 5.2.2.1 致密岩石比轻质岩石； 5.2.2.2 较老的岩石比较年轻的岩石； 5.2.2.3 火成岩比沉积岩； 5.2.2.4 固体岩石比有裂缝或裂缝的岩石更坚硬； 5.2.2.5 未风化岩石比风化岩石； 5.2.2.6 固结沉积物比未固结沉积物； 5.2.2.7 水饱和松散沉积物比干燥松散沉积物；和 5.2.2.8 湿土比干土好。 5.3 设备- 用于地表地震折射测量的地球物理设备包括地震仪、检波器、检波器电缆、能源和触发电缆或无线电链路。有各种各样的地震地球物理设备可供使用，应选择用于地震折射测量的设备，以满足测量目标。 5.3. 1. 地震仪- 不同制造商可提供各种各样的地震仪。它们的范围从相对简单的单通道单元到非常复杂的多通道单元。大多数工程地震仪以数字方式采集、记录和显示地震波。 5.3.1.1 单通道地震仪- 单道地震仪是最简单的地震折射仪，通常与单检波器一起使用。检波器通常放置在固定位置，并在距检波器越来越远的位置用锤子敲击地面。第一地震波到达时间( 图2 和 图3 )在地震波形的仪器显示屏上识别。 对于一些简单的地质条件和小型项目，单通道单元是令人满意的。单通道系统也用于测量岩石样品或工程材料的地震速度。 5.3.1.2 多通道地震仪- 多通道地震仪使用6、12、24、48或更多检波器。使用多通道地震仪，同时记录所有检波器的地震波形（参见 图3 ). 5.3.1.3 波形的同时显示使操作员能够观察数据的趋势，并有助于可靠地选择首次到达时间。这在地震噪音大的地区和地质条件复杂的地区很有用。 可以使用计算机程序帮助口译员选择首次到达时间。 5.3.1.4 信号增强- 大多数地震仪都可以使用滤波和叠加来增强信号，从而提高信噪比。在嘈杂区域或使用小能源工作时，它是一种辅助工具。信号叠加是通过添加折射地震信号来实现的。该过程通过对相干地震信号的振幅求和来提高信噪比，同时通过平均来降低随机噪声的振幅。 5.3.2 检波器和电缆: 5.3.2.1 检波器转换 P -将波能量转换为地震仪记录的电压。 对于折射工作，检波器的频率从8 Hz到14 Hz不等。检波器连接至连接至地震仪的检波器电缆（请参阅 图1 ). 检波器电缆具有每个检波器的电气连接点（输出），通常沿电缆均匀分布。根据描述折射器表面和折射器深度所需的详细程度，检波器放置的间距约为1米至数百米（2或3英尺至数百英尺）。可以在电缆的炮端调整检波器间隔，以在浅层地下提供额外的地震速度信息。 5.3.2.2 如果检波器和电缆之间的连接不防水，则必须小心确保它们不会被湿草、雨水等短路。浅水区域需要特殊的防水检波器（沼泽检波器）、检波器电缆和连接器。 5.3.3 能源: 5.3.3.1 地震折射能量源的选择取决于调查深度和地质条件。地震折射测量中通常使用四种能源:大锤、机械落锤或冲击装置、射弹（枪）源和炸药。 5.3.3.2 对于5至10 m（15至30 ft）的浅层调查，可使用4至7 kg（10至15 lb）的大锤。 使用地震仪的信号增强功能进行三到五次锤击通常就足够了。地面上的撞击板用于改善锤子与土壤之间的能量耦合。 5.3.3.3 对于干燥和松散材料的深入调查，需要更多的地震能量，可以选择机械化震源或射弹（炮）震源。射弹源在地表或地表以下放电。机械震源使用大重量（约100至500 lb或45至225 kg）在动力作用下下落或向下驱动。由于其尺寸，机械重量下降通常安装在拖车上。 5.3.3.4 少量炸药可大幅提高能量水平。 为了减少能量损失和安全起见，通常埋置炸药。如果回填并夯实，在1至2 m（3至6 ft）处埋少量炸药（小于1 lb或0.5 kg）对浅层调查（小于300 ft或100 m）有效。对于更大的调查深度（低于300英尺或100米），需要更大的炸药装药（大于1磅或0.5千克），通常埋深为2米（6英尺）或以上。使用炸药需要经过专门培训的人员和特殊程序。 5.3.4 计时- 碰撞时的正时信号( t =0）发送到地震仪（参见 图1 ). 冲击时间( t =0）通过机械开关、压电装置或检波器（或加速计）或来自爆破装置的信号进行检测。 应使用特殊的地震雷管进行精确计时。 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，一组给定的数据不能与一组独特的地下条件相关联。在大多数情况下，仅凭地表地球物理测量无法解决所有模糊问题，需要一些额外的信息，如钻孔数据。由于地球物理方法的固有局限性，地震折射测量不是对地下条件的完整评估。与其他地质信息适当结合，地震折射测量是一种有效、准确和经济的方法- 获取地下信息的有效方法。 5.4.1.2 所有地表地球物理方法固有地受到分辨率随深度降低的限制。 5.4.2 地震折射法的特定限制: 5.4.2.1 当在层状地球上进行折射测量时，假定各层的地震速度是均匀的、各向同性的。如果地下层中的实际情况与该理想模型显著偏离，则任何解释也会偏离理想。随着地层倾角的增加，深度计算中引入了越来越大的误差。误差是倾角和倾斜层之间速度对比的函数 ( 8. , 9 ) . 5.4.2.2 地震折射测量固有的另一个限制称为盲区问题 ( 3. , 2. , 10 ) . 上覆材料的地震速度与折射体的地震速度之间必须有足够的对比度，以便检测折射体。一些重要的地质或水文地质边界没有现场可测量的地震速度对比，因此无法使用该技术进行检测。 5.4.2.3 一层也必须有足够的厚度才能被检测到 ( 10 ) . 5.4.2.4 如果一层的地震速度低于其上一层的地震速度（速度反转），则无法检测到低地震速度层。 因此，深层的计算深度大于实际深度（尽管最常见的地质条件是地震速度随深度增加，但有时会发生地震速度反转）。在某些情况下，可以使用解释方法来解决这个问题 ( 11 ) . 5.4.3 自然和文化条件造成的干扰: 5.4.3.1 地震折射法对各种来源的地面振动（时变噪声）很敏感。地质和文化因素也会产生不必要的噪音。 5.4.3.2 环境源- 环境噪声源包括由风、水运动（例如，附近海滩上的波浪破碎）、自然地震活动或检波器上的降雨引起的任何地面振动。 5.4.3.3 地质来源- 地质噪声源包括由于地下层地震速度的横向和垂直变化（例如，土壤中存在大漂石）导致的旅行时间的意外变化。 5.4.3.4 文化来源- 文化噪声源包括现场工作人员、附近车辆、施工设备、飞机或爆破引起的振动。文化因素，如测线下方或附近的埋藏结构，也可能导致行程时间的意外变化。附近的电力线可能会在长检波器电缆中产生噪声。 5.4.3.5 在设计和执行折射测量的过程中，应考虑环境、地质和文化噪声的来源，并注意其发生时间和位置。 干扰并不总是可预测的，因为它取决于噪声的大小以及检波器和震源的几何形状和间距。 5.5 替代方法- 上述限制可能会阻止使用地震折射法，并且可能需要其他地球物理或非地球物理方法来调查地下条件（见指南） D5753 ).

1.1 Purpose and Application— This guide covers the equipment, field procedures, and interpretation methods for the assessment of subsurface conditions using the seismic refraction method. Seismic refraction measurements as described in this guide are applicable in mapping subsurface conditions for various uses including geologic, geotechnical, hydrologic, environmental ( 1 ) , mineral exploration, petroleum exploration, and archaeological investigations. The seismic refraction method is used to map geologic conditions including depth of bedrock, or the water table, stratigraphy, lithology, structure, and fractures or all of these. The calculated seismic wave velocity is related to mechanical material properties. Therefore, characterization of the material (type of rock, degree of weathering, and rippability) is made on the basis of seismic velocity and other geologic information. 1.1.1 The geotechnical industry uses English or SI units. 1.2 Limitations: 1.2.1 This guide provides an overview of the seismic refraction method using compressional ( P ) waves. It does not address the details of the seismic refraction theory, field procedures, 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 refraction method be familiar with the relevant material in this guide and the references cited in the text and with appropriate ASTM standards cited in 2.1 . 1.2.2 This guide is limited to the commonly used approach to seismic refraction measurements made on land. The seismic refraction method can be adapted for a number of special uses, on land, within a borehole and on water. However, a discussion of these other adaptations of seismic refraction measurements is not included in this guide. 1.2.3 There are certain cases in which shear waves need to be measured to satisfy project requirements. The measurement of seismic shear waves is a subset of seismic refraction. This guide is not intended to include this topic and focuses only on P wave measurements. 1.2.4 The approaches suggested in this guide for the seismic refraction method are commonly used, widely accepted, and proven; however, other approaches or modifications to the seismic refraction method that are technically sound may be substituted. 1.2.5 Technical limitations and interferences of the seismic refraction method are discussed in D420 , D653 , D2845 , D4428/D4428M , D5088 , D5730 , D5753 , D6235 , and D6429 . 1.3 Precautions: 1.3.1 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 are used. 1.3.2 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. 1.4 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. 1.5 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 ASTM standard 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 of 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. 1.6 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 equipment, field procedures, and interpretation methods used for the determination of the depth, thickness and the seismic velocity of subsurface soil and rock or engineered materials, using the seismic refraction method. 5.1.2 Measurement of subsurface conditions by the seismic refraction method requires a seismic energy source, trigger cable (or radio link), geophones, geophone cable, and a seismograph (see Fig. 1 ). FIG. 1 Field Layout of a Twelve-Channel Seismograph Showing the Path of Direct and Refracted Seismic Waves in a Two-Layer Soil/Rock System (α c = Critical Angle) 5.1.3 The geophone(s) and the seismic source must be placed in firm contact with the soil or rock. The geophones are usually located in a line, sometimes referred to as a geophone spread. The seismic source may be a sledge hammer, a mechanical device that strikes the ground, or some other type of impulse source. Explosives are used for deeper refractors or special conditions that require greater energy. Geophones convert the ground vibrations into an electrical signal. This electrical signal is recorded and processed by the seismograph. The travel time of the seismic wave (from the source to the geophone) is determined from the seismic wave form. Fig. 2 shows a seismograph record using a single geophone. Fig. 3 shows a seismograph record using twelve geophones. FIG. 2 A Typical Seismic Waveform from a Single Geophone Note 1: Arrow marks arrival of first compressional wave. FIG. 3 Twelve-Channel Analog Seismograph Record Showing Good First Breaks Produced by an Explosive Sound Source ( 2 ) 5.1.4 The seismic energy source generates elastic waves that travel through the soil or rock from the source. When the seismic wave reaches the interface between two materials of different seismic velocities, the waves are refracted according to Snell's Law ( 3 , 4 ) . When the angle of incidence equals the critical angle at the interface, the refracted wave moves along the interface between two materials, transmitting energy back to the surface ( Fig. 1 ). This interface is referred to as a refractor. 5.1.5 A number of elastic waves are produced by a seismic energy source. Because the compressional P -wave has the highest seismic velocity, it is the first wave to arrive at each geophone (see Fig. 2 and Fig. 3 ). 5.1.6 The P -wave velocity V p is dependent upon the bulk modulus, the shear modulus and the density in the following manner ( 3 ) : where: V p = compressional wave velocity, K = bulk modulus, μ = shear modulus, and ρ = density. 5.1.7 The arrival of energy from the seismic source at each geophone is recorded by the seismograph ( Fig. 3 ). The travel time (the time it takes for the seismic P -wave to travel from the seismic energy source to the geophone(s)) is determined from each waveform. The unit of time is usually milliseconds (1 ms = 0.001 s). 5.1.8 The travel times are plotted against the distance between the source and the geophone to make a time distance plot. Fig. 4 shows the source and geophone layout and the resulting idealized time distance plot for a horizontal two-layered earth. FIG. 4 ( a ) Seismic Raypaths and ( b ) Time-Distance Plot for a Two-Layer Earth With Parallel Boundaries ( 2 ) 5.1.9 The travel time of the seismic wave between the seismic energy source and a geophone(s) is a function of the distance between them, the depth of the refractor and the seismic velocities of the materials through which the wave passes. 5.1.10 The depth of a refractor is calculated using the source to geophone geometry (spacing and elevation), determining the apparent seismic velocities (which are the reciprocals of the slopes of the plotted lines in the time distance plot), and the intercept time or crossover distances on the time distance plot (see Fig. 4 ). Intercept time and crossover distance-depth formulas have been derived in the literature ( 5- 4 ) . These derivations are straightforward inasmuch as the travel time of the seismic wave is measured, the velocity in each layer is calculated from the time-distance plot, and the raypath geometry is known. These interpretation formulas are based on the following assumptions: ( 1 ) the boundaries between layers are planes that are either horizontal or dipping at a constant angle, ( 2 ) there is no land-surface relief, ( 3 ) each layer is homogeneous and isotropic, ( 4 ) the seismic velocity of the layers increases with depth, and ( 5 ) intermediate layers must be of sufficient velocity contrast, thickness and lateral extent to be detected. Reference ( 2 ) provides an excellent summary of these equations for two and three layer cases. The formulas for a two-layered case (see Fig. 4 ) are given below. 5.1.10.1 Intercept-time formula: where: z = depth of refractor two, t i = intercept time, V 2 = seismic velocity in layer two, and V 1 = seismic velocity in layer one. 5.1.10.2 Crossover distance formula: where: z, V 2 and V 1 are as defined above and x c = crossover distance. 5.1.10.3 Three to four layers are usually the most that can be resolved by seismic refraction measurements. Fig. 5 shows the source and geophone layout and the resulting time distance plot for an idealized three-layer case. FIG. 5 ( a ) Seismic Raypaths and ( b ) Time-Distance Plot for a Three-Layer Model With Parallel Boundaries ( 2 ) Note 1: While these equations are suitable for hand calculations, more advanced algorithms are used in commercially available software that is generally used to analyze seismic traces. 5.1.11 The refraction method is used to define the depth to or profile of the top of one or more refractors, or both, for example, depth of water table or bedrock. 5.1.12 The source of energy is usually located at or near each end of the geophone spread; a refraction measurement is made in each direction. These are referred to as forward and reverse measurements, sometimes incorrectly called reciprocal measurements, from which separate time distance plots are made. Fig. 6 shows the source and geophone layout and the resulting time distance plot for a dipping refractor. The velocity obtained for the refractor from either of these two measurements alone is the apparent velocity of the refractor. Both measurements are necessary to resolve the true seismic velocity and the dip of layers ( 2 ) unless other data are available that indicate a horizontal layered earth. These two apparent velocity measurements and the intercept time or crossover distance are used to calculate the true velocity, depth and dip of the refractor. Note that only two depths of the planar refractor are obtained using this approach (see Fig. 7 ). Depth of the refractor is obtained under each geophone by using a more sophisticated data collection and interpretation approach. FIG. 6 ( a ) Seismic Raypaths and ( b ) Time-Distance Plot for a Two-Layer Model With A Dipping Boundary ( 2 ) FIG. 7 Time Distance Plot ( a ) and Interpreted Seismic Section ( b ) ( 7 ) 5.1.13 Most refraction surveys for geologic, engineering, hydrologic and environmental applications are carried out to determine depths of refractors that are less than 100 m (about 300 ft). However, with sufficient energy, refraction measurements can be made to depths of 300 m (1000 ft) and more ( 5 ) . 5.2 Parameter Measured and Representative Values: 5.2.1 The seismic refraction method provides the velocity of compressional P -waves in subsurface materials. Although the P -wave velocity is a good indicator of the type of soil or rock, it is not a unique indicator. Table 1 shows that each type of sediment or rock has a wide range of seismic velocities, and many of these ranges overlap. While the seismic refraction technique measures the seismic velocity of seismic waves in earth materials, it is the interpreter who, based on knowledge of the local conditions and other data, must interpret the seismic refraction data and arrive at a geologically feasible solution. 5.2.2 P -wave velocities are generally greater for: 5.2.2.1 Denser rocks than lighter rocks; 5.2.2.2 Older rocks than younger rocks; 5.2.2.3 Igneous rocks than sedimentary rocks; 5.2.2.4 Solid rocks than rocks with cracks or fractures; 5.2.2.5 Unweathered rocks than weathered rocks; 5.2.2.6 Consolidated sediments than unconsolidated sediments; 5.2.2.7 Water-saturated unconsolidated sediments than dry unconsolidated sediments; and 5.2.2.8 Wet soils than dry soils. 5.3 Equipment— Geophysical equipment used for surface seismic refraction measurement includes a seismograph, geophones, geophone cable, an energy source and a trigger cable or radio link. A wide variety of seismic geophysical equipment is available and the choice of equipment for a seismic refraction survey should be made in order to meet the objectives of the survey. 5.3.1 Seismographs— A wide variety of seismographs are available from different manufacturers. They range from relatively simple, single-channel units to very sophisticated multichannel units. Most engineering seismographs sample, record and display the seismic wave digitally. 5.3.1.1 Single Channel Seismograph— A single channel seismograph is the simplest seismic refraction instrument and is normally used with a single geophone. The geophone is usually placed at a fixed location and the ground is struck with the hammer at increasing distances from the geophone. First seismic wave arrival times ( Fig. 2 and Fig. 3 ) are identified on the instrument display of the seismic waveform. For some simple geologic conditions and small projects a single-channel unit is satisfactory. Single channel systems are also used to measure the seismic velocity of rock samples or engineered materials. 5.3.1.2 Multi-Channel Seismograph— Multi-channel seismographs use 6, 12, 24, 48 or more geophones. With a multi-channel seismograph, the seismic wave forms are recorded simultaneously for all geophones (see Fig. 3 ). 5.3.1.3 The simultaneous display of waveforms enables the operator to observe trends in the data and helps in making reliable picks of first arrival times. This is useful in areas that are seismically noisy and in areas with complex geologic conditions. Computer programs are available that help the interpreter pick the first arrival time. 5.3.1.4 Signal Enhancement— Signal enhancement using filtering and stacking that improve the signal to noise ratio is available in most seismographs. It is an aid when working in noisy areas or with small energy sources. Signal stacking is accomplished by adding the refracted seismic signals for a number of impacts. This process increases the signal to noise ratio by summing the amplitude of the coherent seismic signals while reducing the amplitude of the random noise by averaging. 5.3.2 Geophone and Cable: 5.3.2.1 A geophone transforms the P -wave energy into a voltage that is recorded by the seismograph. For refraction work, the frequency of the geophones varies from 8 to 14 Hz. The geophones are connected to a geophone cable that is connected to the seismograph (see Fig. 1 ). The geophone cable has electrical connection points (take outs) for each geophone, usually located at uniform intervals along the cable. Geophone placements are spaced from about 1 m to hundreds of meters (2 or 3 ft to hundreds of feet) apart depending upon the level of detail needed to describe the surface of the refractor and the depth of the refractor(s). The geophone intervals may be adjusted at the shot end of a cable to provide additional seismic velocity information in the shallow subsurface. 5.3.2.2 If connections between geophones and cables are not waterproof, care must be taken to assure they will not be shorted out by wet grass, rain, etc. Special waterproof geophones (marsh geophones), geophone cables and connectors are required for areas covered with shallow water. 5.3.3 Energy Sources: 5.3.3.1 The selection of seismic refraction energy sources is dependent upon the depth of investigation and geologic conditions. Four types of energy sources are commonly used in seismic refraction surveys: sledge hammers, mechanical weight drop or impact devices, projectile (gun) sources, and explosives. 5.3.3.2 For shallow depths of investigation, 5 to 10 m (15 to 30 ft), a 4 to 7 kg (10 to 15 lb) sledge hammer may be used. Three to five hammer blows using signal enhancement capabilities of the seismograph will usually be sufficient. A strike plate on the ground is used to improve the coupling of energy from the hammer to the soil. 5.3.3.3 For deeper investigations in dry and loose materials, more seismic energy is required, and a mechanized or a projectile (gun) source may be selected. Projectile sources are discharged at or below the ground surface. Mechanical seismic sources use a large weight (of about 100 to 500 lb or 45 to 225 kg) that is dropped or driven downward under power. Mechanical weight drops are usually trailer mounted because of their size. 5.3.3.4 A small amount of explosives provides a substantial increase in energy levels. Explosive charges are usually buried to reduce energy losses and for safety reasons. Burial of small amounts of explosives (less than 1 lb or 0.5 kg) at 1 to 2 m (3 to 6 ft) is effective for shallow depths of investigation (less than 300 ft or 100 m) if backfilled and tamped. For greater depths of investigation (below 300 ft or 100 m), larger explosives charges (greater than 1 lb or 0.5 kg) are required and usually are buried 2 m (6 ft) deep or more. Use of explosives requires specially-trained personnel and special procedures. 5.3.4 Timing— A timing signal at the time of impact ( t = 0) is sent to the seismograph (see Fig. 1 ). The time of impact ( t = 0) is detected with mechanical switches, piezoelectric devices or a geophone (or accelerometer), or with a signal from a blasting unit. Special seismic blasting caps should be used for accurate timing. 5.4 Limitations and Interference: 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 cannot be associated with a unique set of subsurface conditions. In most situations, surface geophysical measurements alone cannot resolve all ambiguities, and some additional information, such as borehole data, is required. Because of this inherent limitation in the geophysical methods, a seismic refraction survey is not a complete assessment of subsurface conditions. Properly integrated with other geologic information, seismic refraction surveying is an effective, accurate, and cost-effective method of obtaining subsurface information. 5.4.1.2 All surface geophysical methods are inherently limited by decreasing resolution with depth. 5.4.2 Limitations Specific to the Seismic Refraction Method: 5.4.2.1 When refraction measurements are made over a layered earth, the seismic velocity of the layers are assumed to be uniform and isotropic. If actual conditions in the subsurface layers deviate significantly from this idealized model, then any interpretation also deviates from the ideal. An increasing error is introduced in the depth calculations as the angle of dip of the layer increases. The error is a function of dip angle and the velocity contrast between dipping layers ( 8 , 9 ) . 5.4.2.2 Another limitation inherent to seismic refraction surveys is referred to as a blind-zone problem ( 3 , 2 , 10 ) . There must be a sufficient contrast between the seismic velocity of the overlying material and that of the refractor for the refractor to be detected. Some significant geologic or hydrogeologic boundaries have no field-measurable seismic velocity contrast across them and consequently cannot be detected with this technique. 5.4.2.3 A layer must also have a sufficient thickness in order to be detected ( 10 ) . 5.4.2.4 If a layer has a seismic velocity lower than that of the layer above it (a velocity reversal), the low seismic velocity layer cannot be detected. As a result, the computed depths of deeper layers are greater than the actual depths (although the most common geologic condition is that of increasing seismic velocity with depth, there are situations in which seismic velocity reversals occur). Interpretation methods are available to address this problem in some instances ( 11 ) . 5.4.3 Interferences Caused by Natural and by Cultural Conditions: 5.4.3.1 The seismic refraction method is sensitive to ground vibrations (time-variable noise) from a variety of sources. Geologic and cultural factors also produce unwanted noise. 5.4.3.2 Ambient Sources— Ambient sources of noise include any vibration of the ground due to wind, water movement (for example, waves breaking on a nearby beach), natural seismic activity, or by rainfall on the geophones. 5.4.3.3 Geologic Sources— Geologic sources of noise include unsuspected variations in travel time due to lateral and vertical variations in seismic velocity of subsurface layers (for example, the presence of large boulders within a soil). 5.4.3.4 Cultural Sources— Cultural sources of noise include vibration due to movement of the field crew, nearby vehicles, and construction equipment, aircraft, or blasting. Cultural factors such as buried structures under or near the survey line also may lead to unsuspected variations in travel time. Nearby powerlines may induce noise in long geophone cables. 5.4.3.5 During the course of designing and carrying out a refraction survey, sources of ambient, geologic, and cultural noise should be considered and its time of occurrence and location noted. The interference is not always predictable because it depends upon the magnitude of the noises and the geometry and spacing of the geophones and source. 5.5 Alternative Methods— The limitations discussed above may prevent the use of the seismic refraction method, and other geophysical or non-geophysical methods may be required to investigate subsurface conditions (see Guide D5753 ).