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 (
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.
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).
5.2.2Reflection 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.
5.3Equipment—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.
5.3.1Sources—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 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.
5.3.2Seismic 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.
5.3.3Seismographs—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.
5.3.4Source 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.
5.3.5Supporting 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.
5.4Limitations and Interferences:
5.4.1General 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.
5.4.2Limitations 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).
5.4.3Interferences 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.
5.4.4Interference 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 and3). 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.
5.5Alternative 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.
1. Scope
1.1Purpose 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
1.1.3 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.
1.2Limitations:
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 (1.3This 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.
1.5Precautions:
1.5.1It
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.
1.5.2If
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.
1.5.3This 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.
1.6This 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.
Standard Practice for Minimum
Requirements for Agencies Engaged in Testing and/or Inspection of
Soil and Rock as Used in Engineering Design and Construction
Standard Test Methods for Crosshole
Seismic Testing (Withdrawn 2023)
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