ASTM D7128-18

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.
Designation:D7128 −18
Standard Guide for
Using the Seismic-Reflection Method for Shallow
Subsurface Investigation
This standard is issued under the fixed designation D7128; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope familiar with the relevant material in this guide, the references
cited in the text, and Guides D420, D653, D2845, D4428/
1.1 Purpose and Application:
D4428M, Practice D5088, Guides D5608, D5730, D5753,
1.1.1 Thisguidesummarizesthetechnique,equipment,field
D6235, and D6429.
procedures, data processing, and interpretation methods for the
1.2.2 This guide is limited to two-dimensional (2-D) shal-
assessmentofshallowsubsurfaceconditionsusingtheseismic-
low seismic-reflection measurements made on land. The
reflection method.
seismic-reflection method can be adapted for a wide variety of
1.1.2 Seismic reflection measurements as described in this
special uses: on land, within a borehole, on water, and in three
guide are applicable in mapping shallow subsurface conditions
dimensions (3-D). However, a discussion of these specialized
for various uses including geologic (1), geotechnical, hydro-
2 adaptations of reflection measurements is not included in this
geologic (2), and environmental (3). The seismic-reflection
guide.
method is used to map, detect, and delineate geologic condi-
1.2.3 This guide provides information to help understand
tions including the bedrock surface, confining layers
the concepts and application of the seismic-reflection method
(aquitards), faults, lithologic stratigraphy, voids, water table,
to a wide range of geotechnical, engineering, and groundwater
fracture systems, and layer geometry (folds). The primary
problems.
application of the seismic-reflection method is the mapping of
1.2.4 The approaches suggested in this guide for the
lateral continuity of lithologic units and, in general, detection
seismic-reflection method are commonly used, widely
of change in acoustic properties in the subsurface.
accepted, and proven; however, other approaches or modifica-
1.1.3 This guide will focus on the seismic-reflection method
tions to the seismic-reflection method that are technically
as it is applied to the near surface. Near-surface seismic
sound may be equally suited.
reflection applications are based on the same principles as
1.2.5 Technical limitations of the seismic-reflection method
those used for deeper seismic reflection surveying, but ac-
are discussed in 5.4.
cepted practices can differ in several respects. Near-surface
1.2.6 Thisguidediscussesbothcompressional(P)andshear
seismic-reflection data are generally high-resolution (dominant
(S) wave reflection methods.Where applicable, the distinctions
frequency above 80 Hz) and image depths from around6mto
between the two methods will be pointed out in this guide.
as much as several hundred meters. Investigations shallower
than 6 m have occasionally been undertaken, but these should 1.3 This guide offers an organized collection of information
be considered experimental. or a series of options and does not recommend a specific
course of action. This document cannot replace education or
1.2 Limitations:
experienceandshouldbeusedinconjunctionwithprofessional
1.2.1 This guide provides an overview of the shallow
judgment. Not all aspects of this guide may be applicable in all
seismic-reflection method, but it does not address the details of
circumstances. This guide is not intended to represent or
seismic theory, field procedures, data processing, or interpre-
replace the standard of care by which the adequacy of a given
tation of the data. Numerous references are included for that
professional service must be judged, nor should this document
purpose and are considered an essential part of this guide. It is
be applied without consideration for a project’s many unique
recommended that the user of the seismic-reflection method be
aspects. The word “Standard” in the title of this guide means
only that the document has been approved through the ASTM
consensus process.
This guide is under the jurisdiction ofASTM Committee D18 on Soil and Rock
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
1.4 The values stated in SI units are regarded as standard.
Characterization.
The values given in parentheses are inch-pound units, which
Current edition approved July 15, 2018. Published August 2018. Originally
approved in 2005. Last previous edition approved in 2010 as D7128–05(2010).
are provided for information only and are not considered
DOI: 10.1520/D7128-18.
standard.
The boldface numbers in parentheses refer to the list of references at the end of
this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7128−18
1.5 Precautions: Waste Contaminated Sites
1.5.1 It is the responsibility of the user of this guide to D6429 Guide for Selecting Surface Geophysical Methods
follow any precautions within the equipment manufacturer’s D6432 Guide for Using the Surface Ground Penetrating
recommendations, establish appropriate health and safety Radar Method for Subsurface Investigation
practices, and consider the safety and regulatory implications
when explosives or any high-energy (mechanical or chemical) 3. Terminology
sources are used.
3.1 Definitions:
1.5.2 If the method is applied at sites with hazardous
3.1.1 For definitions of common technical terms used in this
materials, operations, or equipment, it is the responsibility of
standard, refer to Terminology D653.
the user of this guide to establish appropriate safety and health
3.2 Definitions Specific to This Guide
practices and determine the applicability of any regulations
3.2.1 acoustic impedance, n—product of seismic compres-
prior to use.
1.5.3 This standard does not purport to address all of the sionalwavevelocityanddensity.Compressionalwavevelocity
of a material is dictated by its bulk modulus, shear modulus,
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- and density. Seismic impedance is the more general term for
priate safety, health, and environmental practices and deter- the product of seismic velocity and density.
mine the applicability of regulatory limitations prior to use.
3.2.2 automatic gain control (AGC), n—trace amplitude
1.6 This international standard was developed in accor-
adjustment that varies as a function of time and the amplitude
dance with internationally recognized principles on standard-
of adjacent data points. Amplitude adjustment changing the
ization established in the Decision on Principles for the
output amplitude so that at least one sample is at full scale
Development of International Standards, Guides and Recom-
deflection within a selected moving window (moving in time).
mendations issued by the World Trade Organization Technical
3.2.3 blind seismic deconvolution, n—a very challenging
Barriers to Trade (TBT) Committee.
and yet common seismic deconvolution problem is where the
source wave is unknown and has the potential for time
2. Referenced Documents
variation. Identifies the case where we have one known
2.1 ASTM Standards:
(measured seismogram with additive noise) and two unknowns
D420 Guide for Site Characterization for Engineering De-
(source wave and reflection coefficients).
sign and Construction Purposes
3.2.4 body waves, n—P-and S-wavesthattravelthroughthe
D653 Terminology Relating to Soil, Rock, and Contained
body of a medium, as opposed to surface waves which travel
Fluids
along the surface of a half-space.
D2845 Test Method for Laboratory Determination of Pulse
Velocities and Ultrasonic Elastic Constants of Rock 3.2.5 bulk modulus, n—the resistance of a material to
(Withdrawn 2017) change its volume in response to the hydrostatic load. Bulk
D3740 Practice for Minimum Requirements for Agencies modulus (K) is also known as the modulus of compression.
Engaged in Testing and/or Inspection of Soil and Rock as
3.2.6 check shot survey (downhole survey), n—direct mea-
Used in Engineering Design and Construction
surement of traveltime between the surface and a given depth.
D4428/D4428M Test Methods for Crosshole Seismic Test-
Usually sources on the surface are recorded by a seismic
ing
receiver in a well to determine the time-to-depth relationships
D5088 Practice for Decontamination of Field Equipment
at a specified location.
Used at Waste Sites
3.2.7 coded source, n—a seismic energy-producing device
D5608 Practices for Decontamination of Sampling and Non
thatdeliversenergythroughoutagiventimeinapredetermined
Sample Contacting Equipment Used at Low Level Radio-
or predicted fashion.
active Waste Sites
D5730 Guide for Site Characterization for Environmental 3.2.8 common mid-point (CMP) or common depth point
Purposes With Emphasis on Soil, Rock, the Vadose Zone (CDP) method, n—a recording-processing method in which
and Groundwater (Withdrawn 2013) each source is recorded at a number of locations and each
D5753 Guide for Planning and Conducting Geotechnical
location is used to record from a number of source locations.
Borehole Geophysical Logging
3.2.8.1 Discussion—After corrections, these data traces are
D5777 Guide for Using the Seismic Refraction Method for
then combined (stacked) to provide a common-midpoint sec-
Subsurface Investigation
tion approximating a coincident source and receiver at each
D6235 Practice for Expedited Site Characterization of Va-
location. The objective is to attenuate random effects and
dose Zone and Groundwater Contamination at Hazardous
events whose dependence on offset is different from that of
primary reflections.
3.2.9 compressional wavevelocity(P-wavevelocity),n—the
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
propagation rate of a seismic wave without implying any
Standards volume information, refer to the standard’s Document Summary page on
direction, that is, velocity is a property of the medium. Particle
the ASTM website.
displacement of a compressional wave is in the direction of
The last approved version of this historical standard is referenced on
www.astm.org. propagation.
D7128−18
3.2.10 dynamic range, n—the ratio of the maximum reading 3.2.23 seismic convolution, n—the convolution between the
to the minimum reading which can be recorded by and read reflection series and source wave.
from an instrument without change of scale.
3.2.24 seismic deconvolution, n—the process of removing
3.2.11 fold (redundancy), n—the multiplicity of common- the characteristics of the source wave from the recorded
midpoint data or the number of midpoints per bin. seismic time series so that one is ideally left with only the
reflection coefficients.
3.2.11.1 Discussion—Wherethemidpointisthesamefor12
source/receiver pairs, the stack is referred to as “12-fold” or
3.2.25 seismic impedance, n—product of seismic wave ve-
1200 percent.
locity and density.
3.2.12 G-force, n—measure of acceleration relative to the 3.2.25.1 Discussion— The seismic impedance includes
gravitational force of the earth. shear waves and surface waves, whereas acoustic impedance,
by strict definition, includes only compressional waves.
3.2.13 impedance contrast, n—ratio of the seismic imped-
ance across a boundary or seismic impedance of the lower 3.2.26 seismicsensor,n—receiversdesignedtocoupletothe
layer divided by the seismic impedance of the upper layer. earth and record vibrations (for example, geophones,
accelerometers, hydrophones).
3.2.13.1 Discussion—A value of 1 implies total transmit-
tance. Values increase or decrease from 1 as the contrast
3.2.27 seismic sensor group (spread), n—multiple receivers
increases, that is, more energy reflection from a boundary.
connected to a single recording channel, generally deployed in
Values less than 1 are indicative of a negative reflectivity or
an array designed to enhance or attenuate specific energy.
reversed reflection wavelet polarity.
3.2.28 shear modulus (G) (rigidity modulus), n—the ratio of
3.2.14 normal moveout (NMO), n—the difference in
shear stress to shear strain of a material as a result of loading
reflection-arrivaltimeasafunctionofshot-to-receiverdistance
3.2.28.1 Discussion—G is equivalent to the second Lamé
because the receiver is not located at the source point.
constant. For small deformations, Hooke’s law holds and strain
3.2.14.1 Discussion—It is the additional traveltime required
is proportional to stress.
because of offset, assuming that the reflecting bed is not
3.2.29 shear wave velocity (S-wave velocity), n—speed of
dipping and that raypaths are straight lines. This leads to a
energy traveling with particle motion perpendicular to its
hyperbolic shape for a reflection.
direction of propagation.
3.2.15 normal moveout velocity (stacking velocity),
3.2.30 shot gather (field files), n—a side-by-side display of
n—velocity to a given reflector calculated from normal-
seismic traces that have a common source location.
moveout measurements, assuming a constant-velocity model.
3.2.31 source to seismic sensor offset, n—the distance from
3.2.15.1 Discussion—Because the raypath actually curves
the source-point to the seismic sensor or to the center of a
as the velocity changes, fitting a hyperbola assumes that the
seismic sensor (group) spread.
actual velocity distribution is equivalent to a constant NMO
velocity, but the NMO velocity changes with the offset.
3.2.32 source wave, n—seismic source wave generated to
However, the assumption often provides an adequate solution
travel thorough stratigraphic profile under investigation.
for offsets less than the reflector depth. Used to calculate NMO
3.2.33 stacking, n—adding seismic traces from different
corrections to common-midpoint gathers prior to stacking.
records to reduce noise and improve overall data quality.
3.2.16 Nyquist frequency, n—also known as the aliasing or
3.2.34 takeout, n—a connection point on a multiconductor
folding frequency, is equal to half the sampling frequency or
cable where seismic sensors can be connected.
rate.
3.2.34.1 Discussion—Takeouts are usually physically polar-
3.2.17 optimum window, n—range of offsets between source
ized to reduce the likelihood of making the connection back-
and receiver that provide reflections with the best signal-to-
wards.
noise ratio.
3.2.35 tap test, n—gently touching a receiver while moni-
3.2.18 raypath, n—a line everywhere perpendicular to
toring on real-time display, to qualitatively appraise sensor
wavefronts (in isotropic media).
response.
3.2.19 reflection, n—the energy or wave from a seismic
3.2.36 twist test, n—light rotational pressure applied to each
source that has been reflected (returned) from an acoustic-
seismic sensor to ensure no motion and, therefore, a solid
impedance contrast (reflector) or series of contrasts within the
ground coupling point.
earth.
3.2.37 wavetrain (wavefield), n—(1) spatial perturbations at
3.2.20 reflection series, n—the reflection coefficients defin-
a given time that result from passage of a wave; and (2) all
ing a stratigaphic profile.
components of seismic energy traveling through the earth as
3.2.21 reflector, n—an interface having a contrast in physi- the result of a single impact.
cal properties (elasticity and/or density) that reflects seismic
3.2.38 wide-angle reflections, n—reflections with an angle
energy.
of incidence near or greater than the critical angle.
3.2.22 roll-along switch, n—a switch that connects different 3.2.38.1 Discussion—The critical angle is defined as the
geophone groups to the recording instruments, used in unique angle of incidence at which rays incident to a boundary
common-midpoint recording. (boundary defined as an abrupt vertical increase in velocity)
D7128−18
“refract” and travel in the lower, higher velocity media parallel wave on the ocean moves toward shore. Surface waves
to the boundary. Wide-angle reflections become asymptotic to penetrate into the earth to a depth that is a function of their
refractions at increasing offset and can possess exceptionally wavelength.
large amplitudes. If they are included in CMPstacked sections
4.1.4 The seismic-reflection method requires contrasts in
they can disproportionately contribute to the stacked wavelet.
the physical properties of earth materials, much like ground
penetrating radar (GPR) (see Guide D6432). The measurable
3.2.39 wiggle trace, n—a single line display of seismic
physical parameters (seismic velocity and density) upon which
sensor output as a function of time.
the seismic-reflection method depends are quite different from
the physical parameters (conductivity and dielectric constant)
4. Summary of Guide
on which GPR depends, but the concept of reflected energy is
4.1 SummaryoftheMethod—Theseismic-reflectionmethod
analogous. The similarities between seismic reflection and
utilizes seismic energy that propagates through the earth,
electrical methods (resistivity, spontaneous potential), electro-
reflects off subsurface features, and returns to the surface. The
magnetic (EM), or potential fields (gravity or magnetics) are
seismicwavestravelfromasourcetoseismicsensorsdeployed
substantially less.
in a known geometry. Sound waves traveling downward will
reflect back to the surface wherever the velocity or density of 4.2 Complementary Data—Geologic and hydrogeologic
subsurface materials increases or decreases abruptly (for data obtained from borehole logs, geologic maps, data from
example, water table, alluvium/bedrock contact, limestone/ outcrops, or other surface and borehole geophysical methods
shale contact). are generally necessary to uniquely interpret subsurface con-
ditions from seismic-reflection data. The seismic-reflection
4.1.1 Images of reflectors (velocity or density contrast) are
method provides a non-unique representation of the subsurface
used to interpret subsurface conditions and materials. Reflec-
that, without supporting or complementary data, cannot be
tions returning from reflectors to seismic sensors will follow
definitively interpreted.
travel paths determined by the velocities of the materials
through which they propagate. Reflection arrivals on seismic
data recorded with multiple seismic sensors at different offsets 5. Significance and Use
(distance between source and seismic sensor) from the source
5.1 Concepts:
can be collectively used to estimate the velocity (approxi-
5.1.1 This guide summarizes the basic equipment, field
mately average) of the material between the reflection point
procedures, and interpretation methods used for detecting,
and seismic sensor. Reflections can be used to characterize
delineating, or mapping shallow subsurface features and rela-
properties of the subsurface such as continuity, thickness, and
tive changes in layer geometry or stratigraphy using the
depth of layers and changes in velocity and material type.
seismic-reflection method. Common applications of the
4.1.2 The seismic-reflection method depends on the pres-
method include mapping the top of bedrock, delineating bed or
ence of discrete seismic-velocity or mass-density changes in
layer geometries, identifying changes in subsurface material
the subsurface that represent acoustical impedance changes.
properties, detecting voids or fracture zones, mapping faults,
Mathematically, acoustic impedance is proportional to the
defining the top of the water table, mapping confining layers,
product of mass density and acoustic wave velocity. Reflection
andestimatingofelastic-wavevelocityinsubsurfacematerials.
may or may not occur at natural boundaries between geologic
Personnel requirements are as discussed in Practice D3740.
layers or at manmade boundaries such as tunnels and mines.
5.1.2 Subsurface measurements using the seismic-reflection
The classic use of the seismic reflection method is to identify
method require a seismic source, multiple seismic sensors,
boundaries of layered geologic units. However, the technique
multi-channelseismograph,andappropriateconnections(radio
can also be used to search for localized anomalies such as sand
or hardwire) between each (Fig. 1, also showing optional
or clay lenses and faults.
roll-along switch).
4.1.3 Seismic energy in the earth travels in the form of body
5.1.3 Seismic waves generated by a controlled seismic
wavesandsurfacewaves.Bodywavespropagatingthroughthe
energy source propagate in the form of mechanical energy
earth behave similarly to sound waves propagating in air.
(particle motion) from the source through the ground or air to
When sound waves traveling in air from voices, explosions,
seismic sensors where the particle (ground) motion is con-
horns, etc., come in contact with a wall, cliff, or building (all
verted to electrical voltage and transmitted to the seismograph.
acoustic contrasts), it is common to hear an echo, which is
reflected sound. When a body wave propagating in the subsur- 5.1.3.1 Seismic energy travels away from the source both
face comes in contact with a volume of material with a
through the ground and air. In the ground, the energy travels as
different acoustical impedance in the subsurface, reflections an elastic wave, with compressional waves (Eq 1) and shear
(echoes) are also generated. In the subsurface, the situation is waves (Eq 2) moving away from the source in a hemispherical
complex because some of the body wave energy arriving at an
pattern, and surface waves propagating away in a circular
acoustic interface can be transmitted, refracted, or converted to
pattern on the ground surface.
othertypesofseismicwavesattheinterface.Surfacewavesare
V 5 =@~K14G/3!/ρ# (1)
p
the dominant (in total energy) part of a seismic energy pulse
1/2 1/2
and propagate along the free surface of the earth much like a V 5 G/ρ 5 E/ 2ρ 11µ (2)
~ ! $ @ ~ !#%
s
D7128−18
FIG. 1Schematic of Equipment and Deployment of Equipment for a Seismic Reflection Survey
where: where:
V = compressional wave velocity, i = incident angle,
p
K = bulk modulus, r = reflected angle, and
G = shear modulus, t = refracted angle.
ρ = density,
At each boundary represented by a change in the product of
E = Young’s modulus,
velocity and density (acoustic impedance), the incident seismic
µ = Poisson’s ratio, and
wave generates a reflected P, reflected S, transmitted P, and
V = shear wave velocity.
s
transmitted S wave. This process is described by the Zoeppritz
Seismic energy propagation time between seismic sensors
equations (for example, Telford et al. (4)).
depends on wave type, travel path, and seismic velocity of the
5.1.3.2 Analysis and recognition of seismic energy arrival
material. The travel path of reflected body waves (compres-
patternsatdifferentseismicsensorsallowsestimationofdepths
sional (P) and shear (S) waves) is controlled by subsurface
to reflection coefficients (reflectors) and average velocity
material velocity and geometry of interfaces defined by acous-
between the reflection coefficient and the earth’s surface.
tic impedance (product of velocity and density) changes. A
Analog display of the seismic waves recorded by each seismic
difference in acoustic impedance between two layers results in
sensor is generally in wiggle trace format on the seismogram
an impedance contrast across the boundary separating the
(Fig. 2) and represents the particle motion (velocity or accel-
layers and determines the reflectivity (reflection coefficient) of
eration) consistent with the orientation and type of the seismic
the boundary; for example, how much energy is reflected
sensor (geophone or accelerometer) and source.
versus how much is transmitted (Eq 3). At normal incidence:
5.1.4 A multichannel seismograph simultaneously records
ρ V 2 ρ V ρ V
2 2 1 1 2 2 the wave field at a number of seismic sensors as a function of
R 5 and A 5 (3)
ρ V 1ρ V ρ V
2 2 1 1 1 1 time(Fig.2).Multichannelseismicdataaretypicallydisplayed
as a time and source-to-seismic sensor distance representation
where:
of the source-induced particle motion propagating in the earth.
R = reflectivity = reflection coefficient,
This particle motion, also known as the elastic wave field, can
V V = velocity of layers 1 and 2,
1 2
becomplexandismodifiedinapredictablewaybytheseismic
ρ ρ = density of layers 1 and 2,
1 2
sensors and instrumentation used for recording the seismic
Vρ = acoustic impedance, and
signal. A wave field is generally displayed in wiggle trace
A = impedance contrast.
format, with the vertical (time) axis of the display typically
Snell’s law (Eq 4) describes the relationship between
referenced to the instant the seismic energy was released (t )
incident, refracted, and reflected seismic waves:
and the horizontal axis showing the linear source-to-seismic-
V V V sensor distance (Fig. 2). The arrivals of the wavefield at each
1 1 1
5 5 (4)
sin i sin r sin t seismic sensor are synchronized in time based on the selected
D7128−18
TABLE 2 Approximate Reflectivity of Interfaces Between
Common Materials
Material Middle Material Bottom Approximate
A B C
Layer Layer Reflectivity
Dry Sand Dry Sand 0.0
Dry Sand Dry Clay / Saturated Clay 0.14 / 0.5
Dry Sand Gravel −0.08
Dry Sand Saturated Sand 0.43
Dry Sand Limestone 0.75
Dry Sand Shale 0.72
Dry Sand Sandstone 0.63
Dry Sand Granite 0.84
Saturated Sand Granite 0.66
Clay Dry Sand −0.14
Clay Clay 0.0
Clay Gravel −0.17
Clay Saturated Sand −0.27
Clay Limestone 0.71
Clay Shale 0.66
Clay Sandstone 0.54
A
Layer 1 on Fig. 1.
B
Layer 2 on Fig. 1.
C
R in Eq 3, Absolute value R = 1 total reflectance.
NOTE 1—Shows the entire wavefield.
NOTE 2—Acquired with vertical geophones.
FIG. 248-Channel Seismograph Record Acquired with a Seismic
weathered, competent), saturation (fluid or gas content),
Source 7.5 m Away from the Nearest Seismic Sensors
porosity, geologic structure (geometric distortion), or density
(compaction).
5.2.2 Reflection Coeffıcient or Reflectivity—Reflectivity is a
digitalsamplingrateoftheseismograph.Eachseismiceventof
measure of energy expected to return from a boundary (inter-
thewavefieldrepresentsdifferenttravelpaths,particlemotions,
face) between materials with different acoustic impedance
and velocities of the energy spreading outward from the
values. Materials with larger acoustic impedances overlying
seismic source. Fig. 2 shows data acquired from a shot in the
materials with smaller acoustic impedances will result in a
center of a line of seismic sensors
negative reflectivity and an associated phase reversal of the
5.2 Parameters Measured and Representative Values—
reflected wavelet. Intuitively, wavelet polarity follows reflec-
Tables 1 and 2 provide generalized material properties related
tion coefficients that are negative when faster or denser layers
to the seismic-reflection method.
overlie slower or less dense (for example, clay over dry sand)
5.2.1 The seismic-reflection method images changes in the
layers and positive when slower or less dense layers overlie
acoustic(seismic)impedanceofsubsurfacelayersandfeatures,
faster or denser (for example, gravel over limestone) layers.A
which represent changes in subsurface material properties.
reflectivity of one means all energy will be reflected at the
While the seismic reflection technique depends on the exis-
interface.
tence of non-zero reflection coefficients, it is the interpreter
5.3 Equipment—Geophysical equipment used for surface
who, based on knowledge of the local conditions and other
seismic measurement can be divided into three general catego-
data, must interpret the seismic-reflection data and arrive at a
ries: source, seismic sensors, and seismograph. Sources gener-
geologically feasible solution. Changes in reflected waveform
ate seismic waves that propagate through the ground as either
canbeindicativeofchangesinthesubsurfacesuchaslithology
an impulsive or a coded wavetrain. Seismic sensors can
(rock or soil type), rock consistency (that is, fractured,
measure changes in acceleration, velocity, displacement, or
pressure. Seismographs measure, convert, and save the electric
TABLE 1 Approximate Material Properties
signal from the seismic sensors by conditioning the analog
A A
P-Wave S-Wave signal and then converting the analog signal to a digital format
Density Acoustic
Material Velocity Velocity
3 B
(kg/m ) Impedance (A/D). These digital data are stored in a predetermined
(m/s) (m/s)
C 6 standardized format. A wide variety of seismic surveying
Dry sand/gravel 750 200 1800 1.35 × 10
Clay 900 300 2000 1.80 × 10 equipment is available and the choice of equipment for a
Saturated sand 1500 350 2100 3.15 × 10
seismicreflectionsurveyshouldbemadetomeettheobjectives
Saturated clay 1800 400 2200 3.96 × 10
6 of the survey.
Shale 3500 1500 2500 8.75 × 10
Sandstone 2850 1400 2100 5.99 × 10
5.3.1 Sources—Seismic sources come in two basic types:
Limestone 4000 2200 2600 10.4 × 10
impulsive and coded. Impulsive sources transfer all their
Granite 6000 3500 2600 15.6 × 10
energy (potential, kinetic, chemical, or some combination) to
A
Velocities are mean for a range appropriate for the material (5).
the earth instantaneously (that is, usually in less than a few
B
Acoustic impedance is velocity multiplied by density, specifically for compres-
sional waves; the equivalent for shear waves is referred to as seismic impedance milliseconds). Impulsive source types include explosives,
(units of kg/s·m ).
weight drops, and projectiles. Coded sources deliver their
C
Subsonic velocities have been reported by researchers studying the ultra-
energy over a given time interval in a predetermined fashion
shallow near surface .
(swept frequency or impulse modulated as a function of time).
D7128−18
Source energy characteristics are highly dependent on near- 5.3.2.3 Hydrophones are used when measuring seismic
surface conditions and source type (6-9). Consistent, broad signals propagating in liquids. Because shear waves are not
transmitted through water, hydrophones only respond to com-
bandwidth source energy performance is important in seismic
reflection surveying. The primary measure of source effective- pressional waves. However, shear waves can be converted to
ness is the measure of signal-to-noise ratio and resolution compressional waves at the water/earth interface and provide
an indirect measurement of shear waves. Hydrophones are
potential as estimated from the recorded signal.
pressure-sensitive devices that are usually constructed of one
5.3.1.1 Selection of the seismic source should be based
or more piezoelectric elements that distort with pressure.
upon the objectives of the survey, site surface and geologic
5.3.2.4 Geophones and accelerometers can be used for
conditions and limitations, survey economics, source
compressional or shear wave surveys on land. Orientation of
repeatability, previous source performance, total energy and
the seismic sensor determines the seismic sensor response and
bandwidth possible at survey site (based on previous studies or
sensitivity to different particle motion. Some seismic sensors
site specific experiments), and safety.
areomnidirectionalandaresensitivetoparticlemotionparallel
5.3.1.2 Coded seismic sources will generally not disturb the
to the motion axis of the sensor, regardless of the sensor’s
environment as much as impulsive sources for a given total
spatial orientation direction. Others seismic sensors are de-
amount of seismic energy. Variable amplitude background
signed to be used in one orientation or the other (P or S). Shear
noise (such as passing cars, airplanes, pedestrian traffic, etc.)
wave seismic sensors are sensitive to particle motion perpen-
affects the quality of data collected with coded sources less
diculartothedirectionofpropagation(linebetweensourceand
than for impulsive sources. Coded sources require an extra
seismic sensors) and are sensitive to vertical (SV) or horizontal
processing step to compress the time-variable signal wavetrain
(SH) transverse wave motion. Compressional wave seismic
down to a more readily interpretable pulse equivalent. This is
sensors are sensitive to particle motion parallel to the direction
generally done using correlation or shift and stack techniques.
of propagation (line between source and seismic sensor) and
5.3.1.3 In most settings, buried small explosive charges will
thus the motion axis of the seismic sensor needs to be in a
result in higher frequency and broader bandwidth data, in
vertical position.
comparison to surface sources. However, explosive sources
5.3.3 Seismographs—Seismographs measure the voltages
generally come with use restrictions, regulations, and more
generated by seismic sensors as a function of time and
safety considerations than other sources. Most explosive and
synchronize them with the seismic source. Seismographs have
projectile sources are designed to be invasive, while weight
differing numbers of channels and a range of electronic
drop and most coded sources are generally in direct contact
specifications. The choice of an appropriate seismograph
with the ground surface and therefore are non-invasive.
should be based on survey objectives. Modern multichannel
5.3.1.4 Sources that shake, impact, or drive the ground so
seismographs are computer based and require minimal fine-
that the dominant particle motion is horizontal to the surface of
tuning to adjust for differences or changes in site characteris-
the ground are shear-wave sources. Sources that shake, impact,
tics. Adjustable seismograph acquisition settings that will
or drive the ground so that the dominant particle motion is
affect the accuracy or quality of recorded data are generally
vertical to the surface of the ground are compressional sources.
limited to sampling rate, record length, analog filter settings,
Many sources can be used for generating both shear and
pre-amplifier gains, and number of recording channels. There
compressional wave energy.
is limited need for selectable analog filters and gain adjust-
5.3.2 Seismic Sensors—Seismic sensors convert mechanical
ments with modern, large dynamic range (>16 bits) seismo-
particle motion to electric signals. There are three different
graphs.Seismographsstoredigitaldatainstandardformats(for
typesofseismicsensors:accelerometers,geophones(occasion-
example, SEGY, SEGD, SEG2) that are generally dependent
ally referred to as seismometers), and hydrophones.
on the type of storage medium and the primary design
5.3.2.1 Accelerometers are devices that measure particle
application of the system. Seismographs can be single units
acceleration. Accelerometers generally require pre-amplifiers
(centralized), with all recording channels (specifically analog
to condition signal prior to transmission to the seismograph.
circuitry and A/D converters) at a single location, or several
Accelerometers generally have a broader bandwidth of sensi-
autonomousseismographscanbedistributedaroundthesurvey
tivity and a greater tolerance for high G-forces than geophones
area. Distributed seismographs are characterized by several
or hydrophones. Accelerometers have a preferred direction of
small decentralized digitizing modules (1–24 channels each)
sensitivity.
located close to the geophones to reduce signal loss over
long-cable seismic sensors. Digital data from each distributed
5.3.2.2 Geophones consist of a stationary cylindrical mag-
module are transmitted to a central system where data from
net surrounded by a coil of wire that is attached to springs and
multiple distributed units are collected, cataloged, and stored.
free to move relative to the magnet. Geophones measure
particle velocity and therefore produce a signal that is the 5.3.4 Source and Seismic Sensor Coupling—The seismic
derivative of the acceleration measured by accelerometers.
sensors and sources must be coupled to the ground. Depending
Geophones are generally robust, durable, and have unique on ground conditions and source and seismic sensor
response characteristics proportional to their natural frequency
configuration, this coupling can range from simply resting on
and coil impedance. The natural frequency is related to the the ground surface (for example, land streamers, weight drop,
spring constant and the coil impedance is a function of the
vibrator) to invasive ground penetration or burial (for example,
number of wire windings in the coil. spike, buried explosives, projectile delivery at bottom of a
D7128−18
hole). Hydrophones couple to the ground through submersion generated seismic noise are all factors in defining the practical
in water in a lake, stream, borehole, ditch, etc. limitations of seismic-reflection method.
(1) Highly attenuative near-surface materials such as dry
5.3.5 Supporting Components—Additional equipment in-
sand and gravel, can adversely affect the resolution potential
cludes a roll-along switch, cables, time-break system (radio or
and signal strength with depth of seismic energy (10). Attenu-
hardwire telemetry between seismograph and source), quality
ation is rapid reduction of seismic energy as it propagates
control (QC) and troubleshooting equipment (seismic sensor
through an earth material, usually most pronounced at high
continuity,earthleakage,cableleakage,seismographdistortion
frequencies. Attenuative materials can prevent survey objec-
and noise thresholds, cable and seismic sensor shorting plug),
tives from being met.
and land surveying equipment.
(2) While it is possible to enhance signal not visible on raw
5.4 Limitations and Interferences:
field data, it is safest to track all coherent events on processed
5.4.1 General Limitations Inherent to Geophysical Meth-
seismic reflection sections from raw field data through all
ods:
processing steps to CMP stack. Noise can be processed to
5.4.1.1 Afundamental limitation of all geophysical methods
appear coherent on CMP stacked sections.
is that a given set of data does not uniquely represent a set of
(3) Differencesinwaterqualitydonotappeartochangethe
subsurface conditions. Geophysical measurements alone can-
velocity and density sufficiently that they can be detected by
not uniquely resolve all ambiguities, and some additional
the seismic-reflection method (11).
information, such as borehole measurements, is required.
5.4.3 Interferences Caused by Natural and by Cultural
Because of this inherent limitation in geophysical methods, a
Conditions:
seismic-reflection survey will not completely represent subsur-
5.4.3.1 The seismic-reflection method is sensitive to me-
face geological conditions. Properly integrated with other
chanical and electrical noise from a variety of sources.
geologic information, seismic-reflection surveying can be an
Biologic, geologic, atmospheric, and cultural factors can all
effective, accurate, and cost-effective method of obtaining
produce noise.
detailed subsurface information.All geophysical surveys mea-
(1) Biologic Sources—Biologic sources of noise include
sure physical properties of the earth (for example, velocity,
vibrations from animals both on the ground surface and
conductivity, density, susceptibility) but require correlation to
underground in burrows as well as trees, weeds, and grasses
the geology and hydrology of a site. Reflection surveys do not
shaking from wind. Examples of animals that can cause noise
directly measure material-specific characteristics (such as
include mice, lizards, cattle, horses, dogs, and birds. Animals,
color,texture,andgrainsize),orlithologies(suchaslimestone,
especially livestock, can produce seismic vibrations several
shale, sandstone, basalt, or schist), except to the extent that
orders of magnitude greater than seismic signals at longer
these lithologies may have different velocities and densities.
offset traces on high-resolution data.
5.4.1.2 All surface geophysical methods are inherently lim-
(2) Geologic Sources—Geologic sources of noise include
itedbysignalattenuation and decreasing resolutionwithdepth.
rockslides, earthquakes, scattered energy from fractures, faults
5.4.2 Limitations Specific to the Seismic-Reflection Method:
or other discontinuities, and moving water (for example, water
5.4.2.1 Theoretical limitations of the seismic-reflection
falls, river rapids, water cascading in wells).
method are related to the presence of a non-zero reflection
(3) Atmospheric Sources—Atmospheric sources of noise
coefficient, seismic energy characteristics, seismic properties
include wind shaking seismic sensors or cables, lightning, rain
(velocity and attenuation), and layer geometries relative to
falling on seismic sensors, snow accumulations melting and
recording geometries. In a homogenous earth, no reflections
falling from trees and roofs, and wind shaking surface struc-
are produced and therefore none can be recorded. When
tures (for example, buildings, poles, signs).
reflection measurements are made at the surface of the earth,
(4) Cultural Sources—Cultural sources of noise include
reflections can only be returned from within the earth if layers
power lines (that is, 50 Hz, 60 Hz, and related harmonics),
with non-zero reflection coefficients are present within the
vehicles (for example, cars, motorcycles, trains, planes,
earth. Layers, for example, defined by changes in lithology
helicopters, ATVs), air conditioners, lawn mowers, small
without measurable changes in either velocity or density
engine-powered tools, construction equipment, and people—
cannot be imaged with the seismic reflection method. Theo-
both crew members and pedestrians—moving in proximity to
retical limits on bed or object-resolving capabilities of a
the seismic line. Radio Frequency (RF) and other electromag-
seismicdatasetarerelatedtofrequencycontentofthereflected
netic (EM) signals transmitted from radar installations, radio
energy (see 8.4).
transmitters, or beacons can appear on seismic data at ampli-
5.4.2.2 Successful imaging of geologic layers dipping at tudes several times larger than source-generated seismic sig-
greater than 45 degrees may require non-standard deployments nals.
of sources and seismic sensors.
5.4.3.2 During the design and operation of a seismic reflec-
5.4.2.3 Resolution (discussed in 8.4) and signal-to-noise tion survey, sources of biologic, geologic, atmospheric, and
ratios are critical factors in determining the practical limita- cultural noise and their proximity to the survey area should be
tions of the seismic-reflection method. Source configuration, considered,especiallythecharacteristicofthenoiseandsizeof
source and seismic sensor coupling, near-surface materials, the area affected by the noise. The interference of each is not
specification of the recording systems, relative amplitude of always predictable because of unknowns associated with earth
seismic events, and arrival geometry of coherent source- coupling and energy attenuation.
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5.4.4 Interference Caused by Source-Generated Noise: to the seismic velocity of the material below the velocity
5.4.4.1 Seismic sources generate both signal and noise. contrast. Refractions are generally the first (in time) coherent
Signal is any energy that is to be used to interpret subsurface seismic energy to arrive at a sensor, beginning a source-to-
conditions. Noise is any recorded energy that is not used to sensor offset beyond those where direct wave energy arrives
interpret subsurface conditions or diminishes the interpretabil- first. For a more detailed discussion of refractions and their use
ity of signal. Ground roll (surface waves), direct waves, as a geophysical imaging tool, see Guide D5777.
refractions, diffractions, air-coupled waves, and reflection mul- (4) Diffraction—Diffractions are energy scattered from dis-
tiplesareallcommontypesofsource-generatednoiseobserved continuous subsurface layers (faults, fractures) or points where
on a seismogram recorded during seismic reflection profiling subsurface layers or objects terminate (lens, channel, boulder).
(Fig. 3). Diffractions are generally considered seismic noise when
(1) GroundRoll—Groundrollisatypeofsurfacewavethat undertaking a reflection survey.
appears on a re
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