Significance and Use
Concepts:
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.
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 waves generated by a controlled seismic energy source propagate in the form of mechanical energy (particle motion) from the source through the ground or air to seismic sensors where the particle (ground) motion is converted to electrical voltage and transmitted to the seismograph.
Seismic energy travels away from the source both through the ground and air. In the ground, the energy travels as an elastic wave, with compressional waves (Eq 1) and shear waves (Eq 2) moving away from the source in a hemispherical pattern, and surface waves propagating away in a circular pattern on the ground surface.
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:
Snell’s law (Eq 4) describes the relationship between incident, refracted, and reflected seismic waves:
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. (6)).
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 multichannel seismograph simultaneously records the wave field at a number of seismic sensors as a function of time (Fig. 2). Multichannel seismic data are typically displayed as a time and source-to-seismic sensor distance representation of the source-induced particle motion propagating in the earth. This particle motion, also known as the elastic wave field, can be complex and is modified in a predictable way by the seismic sensors and instrumentation used for recording the seismic signal. A wave field is generally displayed in wiggle trace format, with the vertical (time) axis of the display typically referenced to the instant the seismic energy was released (t0) and the horizontal axis showing the linear source-to-seismic-sensor distance (Fig. 2). The arrivals of the wavefield at each seismic sensor are synchronized in time based on the selected digital sampling rate of the seismograph. Each seismic event of the wavefield represents different travel paths, particle motions, and velocities of the energy spreading outward from the seismic source. Fig. 2 shows data acquired from a shot in the center of a line of seismic sensors
Parameters Measured and Representative Values—Tables 1 and 2 provide generalized material properties related to the seismic-reflection method.
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).
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.
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.
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 (8-11). 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Limitations and Interferences:
General Limitations Inherent to Geophysical Methods:
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.
All surface geophysical methods are inherently limited by signal attenuation and decreasing resolution with depth.
Limitations Specific to the Seismic-Reflection Method:
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).
Successful imaging of geologic layers dipping at greater than 45 degrees may require non-standard deployments of sources and seismic sensors.
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 (12). 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 (13).
Interferences Caused by Natural and by Cultural Conditions:
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
(2) Geologic Sources
(3) Atmospheric Sources
(4) Cultural Sources
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.
Interference Caused by Source-Generated Noise:
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).
(1) Ground Roll
(2) Direct Waves
(3) Refractions
(4) Diffraction
(5) Air-coupled Waves
(6) Reflection Multiples
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.
Scope
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). 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 and health practices and determine the applicability of regulatory limitations prior to use.
