GEOPYHSICS 5 Flashcards
1
Q
A field that satisfies the Laplace equation. The Laplace equation is equivalent in three dimensions to the inverse square law of gravitational or electrical attraction (in source-free regions; in regions with sources, it becomes Poisson’s equation). Examples of potential fields include the field of the gravity potential and static electric and magnetic fields.
A
potential field
2
Q
Seismic events whose energy has been reflected once. Multiples, in contrast, are events whose energy has been reflected more than once. A goal of seismic data processing is to enhance primary reflections, which are then interpreted as subsurface interfaces.
A
primary reflection
3
Q
In electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations.
A
Probe/ sound
4
Q
Alteration of seismic data to suppress noise, enhance signal and migrate seismic events to the appropriate location in space. Processing steps typically include analysis of velocities and frequencies, static corrections, deconvolution, normal moveout, dip moveout, stacking, and migration, which can be performed before or after stacking. Seismic processing facilitates better interpretation because subsurface structures and reflection geometries are more apparent.
A
Processing/ seismic processing
5
Q
The phase that occurs after successful exploration and development and during which hydrocarbons are drained from an oil or gas field.
A
Production
6
Q
A measure of the efficiency of seismic acquisition. Production can be expressed in terms of the number of lines, shots or lengths (km or miles) of data acquired in a given time.
A
Production
7
Q
To measure the lateral variation of a property, such as gravity or magnetic fields. Probing, in contrast, is the term used to describe the measurement of vertical variations of a property in electromagnetic and other nonseismic geophysical methods.
A
Profile
8
Q
Measuring thelateralvariation of a property, such as gravity or magnetic fields.Probing, in contrast, is the term used to describe the measurement of vertical variations of a property in electromagnetic and other nonseismic geophysical methods.
A
Profiling
9
Q
A property of a sinusoidal plane wave equal to twice pi divided by the wavelength. Also known as the wavenumber, the propagation constant is fundamental to the mathematical representation of wavefields. It is the spatial equivalent of angular frequency and expresses the increase in the cycle of the wave (measured in radians) per unit of distance. In nondispersive media, the wavespeed is the ratio of the angular frequency to the propagation constant. The propagation vector has magnitude equal to the propagation constant and points in the direction the wave is traveling.
A
Propagation constant
10
Q
A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a high seismic velocity (e.g., a salt layer or salt dome, or a carbonate reef) surrounded by rock with a lower seismic velocity causes what appears to be a structural high beneath it. After such features are correctly converted from time to depth, the apparent structural high is generally reduced in magnitude.
A
Pull up
11
Q
A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a low seismic velocity (e.g., a shale diapir or a gas chimney) surrounded by rock with a higher seismic velocity causes what appears to be a structural low beneath it. After such features are converted from time to depth, the apparent structural low is generally reduced in magnitude. Hydrocarbon indicators can display velocity push-downs because the velocity of hydrocarbon is slower than that of rock.
A
Push-down
12
Q
An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves.
A
P-wave/ Acoustic wave/ Compressional wave/ Dilatational wave
13
Q
The dimensionless quality factor. It is the ratio of the peak energy of a wave to the dissipated energy. As waves travel, they lose energy with distance and time due to spherical divergence and absorption. Such energy loss must be accounted for when restoring seismic amplitudes to perform fluid and lithologic interpretations, such as amplitude versus offset (AVO) analysis. Q is also described as the reciprocal of attenuation, but that is not strictly correct because the attenuation coefficient has units of inverse length.
A
Q
14
Q
A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation.
A
Q wave/ Love wave
15
Q
A subset of a 3D seismic survey comprising low fold or simplified processing (such as omitting dip moveout processing) that can be evaluated soon after acquisition.
A
quicklook
16
Q
A log, or a display of several logs, that has been generated by a simple computation of log data. The quicklook is intended to make it easy to identify particular features in a section of log. The term is used for single curves designed, for example, to identify hydrocarbon zones, estimate porosity or identify lithology. Examples are Rwa, crossplot porosity, ratio method and apparent matrix density. The term is also used for a complete log containing some combination of quicklook curves, original logs and lithology display. In all cases the computations are based on simple models.
A
quicklook
17
Q
Borehole seismic data processed on site in the field
A
quicklook
18
Q
A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation.
A
Q-wave
19
Q
An array of sources or receivers radiating outward from a central point, usually a borehole.
A
radial array
20
Q
A borehole seismic method in which a surface source transmits seismic energy from various locations to a receiver in a wellbore to locate high-velocity features such as salt domes.
A
radial refraction
21
Q
A surveying technique used to identify local, high-velocity features such as salt domes, also called fan shooting.
A
radial refraction
22
Q
Disturbances in seismic data that are not coherent (they lack a phase relationship between adjacent traces, unlike air waves and ground roll) and cannot be correlated to the seismic energy source. Random noise can be reduced or removed from data by stacking traces, filtering during processing or using arrays of geophones during acquisition.
A
random noise
23
Q
A dilatation, or decrease in pressure and density of a medium as molecules are displaced by a P-wave. As P-waves pass through the Earth, the Earth undergoes compression and expansion. These changes in volume contribute to the positive and negative amplitudes of a seismic trace.
A
rarefaction
24
Q
A representation of the direction of travel of a seismic wave.
A
ray
25
The path or direction along which wave energy propagates through the Earth. In isotropic media, the raypath is perpendicular to the local wavefront. The raypath can be calculated using ray tracing. Seismic energy travels through media of variable anisotropy and can propagate by diffraction, factors that complicate determination of raypaths.
ray path
26
A technique for predicting or determining arrival times of waves at detectors using raypaths. Ray tracing requires a velocity model and the assumption that rays behave according to Snell's law. Ray tracing provides the traveltimes that are required for Kirchhoff migration.
ray tracing
27
A type of surface wave in which particles move in an elliptical path within the vertical plane containing the direction of wave propagation. At the top of the elliptical path, particles travel opposite to the direction of propagation, and at the bottom of the path they travel in the direction of propagation. Because Rayleigh waves are dispersive, with different wavelengths traveling at different velocities, they are useful in evaluation of velocity variation with depth. Rayleigh waves make up most of the energy recorded as ground roll.
Rayleigh wave
28
The path or direction along which wave energy propagates through the Earth. In isotropic media, the raypath is perpendicular to the local wavefront. The raypath can be calculated using ray tracing. Seismic energy travels through media of variable anisotropy and can propagate by diffraction, factors that complicate determination of raypaths.
raypath
29
A device that detects seismic energy in the form of ground motion or a pressure wave in fluid and transforms it to an electrical impulse.
receiver
30
To detect and measure energy.
record
31
In seismic data, the energy detected and measured by a receiver. Normally, most of the energy is provided by a seismic source. Noise records are obtained in the absence of a seismic source to measure background or ambient noise levels.
record
32
Generally, the return or rebound of particles or energy from the interface between two media. There are two laws of reflection, which state (1) that incident rays, reflected rays and the normal to the reflecting interface at the point of incidence are coplanar, and (2) that the angle of incidence is equal to the angle of reflection. In geophysics, reflection refers to the seismic energy or signal that returns from an interface of contrasting acoustic impedance, known as a reflector, according to Snell's law. Reflection seismic surveys are useful for mapping geologic structures in the subsurface, interpreting sedimentary environments and evaluating hydrocarbon accumulations that might occur as amplitude anomalies. Reflection surveys are complicated by the variation of velocity as well as the various types of wave energy that are propagated within the Earth. In electromagnetics, variation in electrical properties produces reflections.
reflection
33
The ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as: R = (ρ2V2 − ρ1V1) / (ρ2V2 + ρ1V1), Where R = reflection coefficient, whose values range from −1 to +1ρ1 = density of medium 1ρ2 = density of medium 2V1 = velocity of medium 1V2 = velocity of medium 2. Typical values of R are approximately −1 from water to air, meaning that nearly 100% of the energy is reflected and none is transmitted; ~0.5 from water to rock; and ~0.2 for shale to sand. At non-normal incidence, the reflection coefficient defined as a ratio of amplitudes depends on other parameters, such as the shear velocities, and is described as a function of incident angle by the Zoeppritz equations.
reflection coefficient/ reflectivity
34
A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers at the Earth's surface. In reflection tomography, space is divided into cells, each having a certain velocity and reflectivity. The final model is the one whose velocities and reflectivities best describe the data.
reflection tomography/ seismic reflection tomography
35
"Another term for reflection coefficient, the ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as: R = (ρ2V2 − ρ1V1) / (ρ2V2 + ρ1V1), Where R = reflection coefficient, whose values range from −1 to +1ρ1 = density of medium 1ρ2 = density of medium 2V1 = velocity of medium 1V2 = velocity of medium 2.
Typical values of R are approximately −1 from water to air, meaning that nearly 100% of the energy is reflected and none is transmitted; ~0.5 from water to rock; and ~0.2 for shale to sand. At non-normal incidence, the reflection coefficient defined as a ratio of amplitudes depends on other parameters, such as the shear velocities, and is described as a function of incident angle by the Zoeppritz equations.
"
reflectivity
36
An interface between layers of contrasting acoustic, optical or electromagnetic properties. Waves of electromagnetism, heat, light and sound can be reflected at such an interface. In seismic data, a reflector might represent a change in lithology, a fault or an unconformity. A reflector is expressed as a reflection in seismic data.
reflector
37
The change in the direction of travel of a wavefront, or the bending of a ray, as it passes from one medium to another, expressed mathematically by Snell's law. Refraction is a consequence of changes in wavelength and velocity of propagation of a wave produced by differences in refractive indices of the media. Refraction surveys where the incident and reflected angles are critical can be useful for evaluating increasing velocity gradients and locating features that have anomalously high velocities, such as a salt dome within surrounding rocks of lower velocities.
Refraction
38
The ratio of the speed of light in a vacuum to the speed of light in a given material, commonly symbolized by n. According to Snell's law, the refractive index is also the ratio of sine of the angle of incidence to the sine of the angle of refraction.
refractive index
39
A layer of rock that is sufficiently thick, areally extensive, and has a distinctly higher velocity than the rocks immediately above it such that it can transmit a head wave, or a wave transmitted at its critical incident angle.
refractor
40
The process of measuring, observing or analyzing features of the Earth from a distance. Satellite photography and radar are techniques commonly used for remote sensing. Many geophysicists do not consider seismic methods to be remote sensing because although seismic methods sense the subsurface remotely, the sources and receivers are in contact with the Earth.
remote sensing
41
An acoustic velocity value used during processing to produce static, vertical shifts in seismic and other time domain data in order to bring a specific point into alignment with some common elevation feature. Most often, the point in question is the 0.0 s time point, while the elevation feature is ground level. In other cases, the elevation feature may be arbitrary, such as 300 m above mean sea level.
replacement velocity
42
The ability to distinguish between separate points or objects, such as sedimentary sequences in a seismic section. High frequency and short wavelengths provide better vertical and lateral resolution. Seismic processing can greatly affect resolution: deconvolution can improve vertical resolution by producing a broad bandwidth with high frequencies and a relatively compressed wavelet. Migration can improve lateral resolution by reducing the size of the Fresnel zone.
resolution
43
A marine seismic data acquisition method using one or more seismic vessels to obtain a combination of multiazimuth and wide-azimuth geometries. A rich-azimuth seismic dataset can be formed by combining the data where multiple wide-azimuth surveys intersect.
Rich-azimuth towed-streamer acquisition
44
A zero-phase wavelet commonly convolved with a reflectivity trace to generate a synthetic seismogram.
Ricker wavelet
45
The study of the physical characteristics and behavior of rock. Rock mechanics can include analysis of and relationships between properties such as velocity, density, porosity, permeability, shear strength, and bending and crushing behavior, as well as the greater geological context of forces that deform strata and produce geological structures.
rock mechanics
46
The value of the square root of the sum of the squares of the velocity values divided by the number of values, symbolized by vrms. The root-mean-square velocity is that of a wave through subsurface layers of different interval velocity along a specific raypath, and is typically several percent higher than the average velocity. The stacking velocity and the root-mean-square velocity approach equality when source-receiver offset approaches zero and layers are horizontal and isotropic.
root-mean-square velocity
47
An elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson's ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques.
S wave
48
A type of refraction survey to help define a salt-sediment interface near a wellbore. The source is typically placed directly above the top of a salt dome and the receivers are placed at a number of locations within the borehole. This technique takes advantage of the fact that sound travels faster through the salt than the surrounding soft sediments, such as in the US Gulf Coast. This survey measures the fastest travel path, with part of its path through the salt. The resultant traveltimes are then inverted via a model to obtain a profile of the salt flanks relative to the borehole.
salt proximity survey
49
A type of reflection survey to help define a salt-sediment interface near a wellbore.
salt-proximity VSP/ salt-proximity vertical seismic profile
50
The number of data points or measurements per unit of time or distance.
sample frequency
51
The distance or time between data points or measurements.
sample interval
52
The number of measurements per unit of time, or the inverse of the sample interval.
sample rate
53
A type of receiver that can be positioned on the seafloor to acquire seismic data.
seabed geophone
54
Multiply reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data, and are suppressed by seismic processing.
secondary reflection
55
Pertaining to waves of elastic energy, such as that transmitted by P-waves and S-waves, in the frequency range of approximately 1 to 100 Hz. Seismic energy is studied by scientists to interpret the composition, fluid content, extent and geometry of rocks in the subsurface. "Seismic," used as an adjective, is preferable to "seismics," although "seismics" is used commonly as a noun.
seismic
56
The generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate, and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing.
Seismic acquisition/ acquisition
57
Another term for acoustic impedance, the product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient.
seismic impedance/ acoustic impedance
58
Analysis of seismic data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists.
seismic interpretation/ seismic modeling
59
A display of seismic data along a line, such as a 2D seismic profile or a profile extracted from a volume of 3D seismic data. A seismic section consists of numerous traces with location given along the x-axis and two-way traveltime or depth along the y-axis. The section is called a depth section if the section has been converted from time to depth and a time section if this has not been done.
seismic line/ seismic section
60
The comparison, simulation or representation of seismic data to define the limits of seismic resolution, assess the ambiguity of interpretation or make predictions. Generation of a synthetic seismogram from a well log and comparing the synthetic, or modeled trace, with seismic data is a common direct modeling procedure. Generating a set of pseudologs from seismic data is the process known as seismic inversion, a type of indirect modeling. Models can be developed to address problems of structure and stratigraphy prior to acquisition of seismic data and during the interpretation of the data. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given data set.
Seismic modeling/ modeling
61
Alteration of seismic data to suppress noise, enhance signal and migrate seismic events to the appropriate location in space. Processing steps typically include analysis of velocities and frequencies, static corrections, deconvolution, normal moveout, dip moveout, stacking, and migration, which can be performed before or after stacking. Seismic processing facilitates better interpretation because subsurface structures and reflection geometries are more apparent.
seismic processing/ processing
62
Traces recorded from a single shotpoint. Numerous seismic records are displayed together in a single seismic section.
seismic record/ seismogram
63
A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers at the Earth's surface. In reflection tomography, space is divided into cells, each having a certain velocity and reflectivity. The final model is the one whose velocities and reflectivities best describe the data.
seismic reflection tomography
64
A seismic aquisition method in which the incident and reflected angles are critical. These refraction surveys can be useful for evaluating increasing velocity gradients and locating features that have anomalously high velocities, such as a salt dome within surrounding rocks of lower velocities.
seismic refraction method
65
A display of seismic data along a line, such a 2D seismic profile or a profile extracted from a volume of 3D seismic data. A seismic section consists of numerous traces with location given along the x-axis and two-way traveltime or depth along the y-axis. The section is called a depth section if the section has been converted from time to depth and a time section if this has not been done.
seismic section
66
Another term for sequence stratigraphy, a field of study in which basin-filling sedimentary deposits, called sequences, are interpreted in a framework of eustasy, sedimentation and subsidence through time in order to correlate strata and predict the stratigraphy of relatively unknown areas. Sequences tend to show cyclicity of changes in relative sea level and widespread unconformities, processes of sedimentation and sources of sediments, climate and tectonic activity over time. Sequence stratigraphic study promotes thorough understanding of the evolution of basins, but also allows for interpretations of potential source rocks and reservoir rocks in both frontier areas (having seismic data but little well data) and in more mature hydrocarbon provinces. Prediction of reservoir continuity is currently a key question in mature hydrocarbon provinces where sequence stratigraphy is being applied. The field originated during the 1960s with the study of the stratigraphy of the continental USA, where numerous unconformities could be correlated widely, and led to the proposal that major unconformities might mark synchronous global-scale events. Through sequence stratigraphy, widely-separated sediments that occur between correlatable unconformities could be compared with each other. Studies of outcrops and seismic lines bore out these concepts, which initially were called "Seismic Stratigraphy" and first published widely in 1977. Further study of seismic lines led to the interpretation of the geometry or architecture of seismic events as representing particular styles of sedimentation and depositional environments, and the integration of such interpretations with well log and core data. Because of the simultaneous, competitive nature of the research, numerous oil companies and academic groups use the terminology of sequence stratigraphy differently, and new terms are added continually.
seismic stratigraphy
67
A seismic data set measured and recorded with reference to a particular area of the Earth's surface, to evaluate the subsurface.
seismic survey
68
The seismic data recorded for one channel. A seismic trace represents the response of the elastic wavefield to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from a source through the subsurface to a receiver or receiver array.
seismic trace/ trace
69
The rate at which a seismic wave travels through a medium, that is, distance divided by traveltime. Seismic velocity can be determined from vertical seismic profiles or from velocity analysis of seismic data. It can vary vertically, laterally and azimuthally in anisotropic media and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant-velocity (homogeneous) medium.
seismic velocity
70
A periodic vibrational disturbance in which energy is propagated through or on the surface of a medium without translation of the material. Waves can be differentiated by their frequency, amplitude, wavelength and speed of propagation. Seismic waves are waves of elastic energy, such as that transmitted by P-waves and S-waves, in the frequency range of approximately 1 to 100 Hz. Seismic energy is studied by scientists to interpret the composition, fluid content, extent and geometry of rocks in the subsurface. Wavelength is defined as: λ = v/f, where λ = wavelength v = speed of propagation f = frequency.
seismic wave
71
A technique using a seismic source on the surface and receivers in the borehole to acquire a vertical seismic profile (VSP) during pipe connections. Waveforms are transmitted to the surface during drilling operations and can be processed in time to yield reliable time-depth information and sometimes reflection information. Because the survey and analysis can be performed while a well is being drilled, the data can be considered in decisions during drilling operations.
seismic while drilling vertical seismic profile
72
A technique using a seismic source on the surface and receivers in the borehole to acquire a vertical seismic profile (VSP) during pipe connections. Waveforms are transmitted to the surface during drilling operations and can be processed in time to yield reliable time-depth information and sometimes reflection information. Because the survey and analysis can be performed while a well is being drilled, the data can be considered in decisions during drilling operations.
seismic-while-drilling vertical seismic profile
73
Traces recorded from a single shotpoint. Numerous seismograms are displayed together in a single seismic section.
seismogram/ seismic record
74
A device or system that records the ground oscillations that make up exploration seismic data or earthquakes, sometimes used incorrectly as a synonym for geophone. A seismograph can include amplifiers, receivers and a recording device (such as a computer disk or magnetic tape) to record seismograms. A crude seismograph was built in 1855 by Italian physicist Luigi Palmieri (1807 to 1896). The modern seismograph, which used a pendulum, was invented in 1880 by James Ewing, Thomas Gray and Sir John Milne.
seismograph
75
The study of seismic or elastic waves, such as from earthquakes, explosions or other causes. Interpretation of the structure and composition of the Earth from artificially created seismic waves is a chief concern of seismologists exploring for hydrocarbons and other resources. English physicist John Mitchell (1724 to 1793) is known as the founder of seismology in part because of his observation that one can determine an earthquake's epicenter, or point of origin in the subsurface, by measuring the arrival time of earthquake waves at different locations. The invention of the modern seismograph in 1880 promoted further studies of earthquakes.
seismology
76
A device that records seismic energy in the form of ground motion and transforms it to an electrical impulse.
seismometer/ geophone/ jug/ receiver
77
A quantitative measure of the coherence of seismic data from multiple channels that is equal to the energy of a stacked trace divided by the energy of all the traces that make up the stack. If data from all channels are perfectly coherent, or show continuity from trace to trace, the semblance has a value of unity.
semblance
78
The smallest change in a measurement that can be recorded by an instrument.
sensitivity
79
In matrix stimulation, a characteristic of rock that indicates the degree of reaction between the rock minerals and a given treating fluid. A formation is described as sensitive if a given stimulating fluid damages the formation. The detrimental reactions include disaggregation and collapse of the matrix, release of fines or formation of precipitates. Sensitivity depends on the overall reactivity of the formation minerals with the fluid; reactivity depends on the structure of the rock and the distribution of minerals within the rock. Sandstone sensitivity also depends on permeability; low-permeability formations are normally more sensitive than high-permeability sandstones for a given mineralogy because certain types of damage, such as formation of precipitates, are more harmful in small pore throats (as in low-permeability formations).
sensitivity
80
A shear wave that is polarized so that its particle motion and direction of propagation are contained in a horizontal plane.
SH wave
81
Generally, an area of the Earth from which waves do not emerge or cannot be recorded. In seismology, the term is used to more specifically describe regions of the subsurface where P-waves and S-waves are difficult to detect, such as regions of the core at certain distances from the epicenter of an earthquake, or the point on the Earth's surface directly above an earthquake. Such zones were first observed in 1914 by Beno Gutenberg (1889 to 1960), an American geologist born in Germany. Because of the molten nature of the outer core, S-waves are especially difficult to detect at 103 to 142 degrees from the epicenter of an earthquake and not observable from 142 to 180 degrees from the epicenter. Areas below salt features are also called shadow zones because the high velocity of salt bends and traps energy, so seismic data quality beneath salt is generally poor unless special seismic processing is performed.
shadow zone
82
An explosive device that utilizes a cavity-effect explosive reaction to generate a high-pressure, high-velocity jet that creates a perforation tunnel. The shape of the explosive material and powdered metal lining determine the shape of the jet and performance characteristics of the charge. The extremely high pressure and velocity of the jet cause materials, such as steel, cement and rock formations, to flow plastically around the jet path, thereby creating the perforation tunnel.
shaped charge/ perforating charge
83
Explosives designed to affect a certain direction preferentially. Shaped charges are most commonly used to perforate wells, but can be an energy source for seismic acquisition.
shaped charge/ perforating charge
84
A type of vertical seismic profile in which the source is a shear-wave source rather than a compressional-wave source. Shear waves travel through the Earth at about half the speed of compressional waves and respond differently to fluid-filled rock, and so can provide different additional information about lithology and fluid content of hydrocarbon-bearing reservoirs.
Shear
85
An elastic constant for the ratio of shear stress to shear strain. The shear modulus is one of the Lamé constants. It can be expressed mathematically as follows: μ = τ / γ = (ΔF/A) / (ΔL/L), where μ = Shear modulusτ = Shear stress = ΔF/AΔF = Increment of shear forceA = Area acted on by the shear forceγ = Shear strain = ΔL/LΔL = Increment of transverse displacement parallel to AL = Original length.
Shear modulus
86
Also known as S-wave, an elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson's ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques.
shear wave
87
To perforate a wellbore in preparation for production.
shoot a well
88
To acquire a type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore.
shoot a well
89
Multiply-reflected seismic energy with a shorter travel path than long-path multiples. Short-path multiples tend to come from shallow subsurface phenomena or highly cyclical sedimentation and arrive soon after, and sometimes very near, the primary reflections. Short-path multiples are less obvious than most long-path multiples and are less easily removed by seismic processing.
short-path multiple
90
The location of an explosive seismic source below the surface. Before acquisition of surface seismic data onshore using explosive sources such as dynamite, holes are drilled at shotpoints and dynamite is placed in the holes. The shotholes can be more than 50 m [164 ft] deep, although depths of 6 to 30 m [20 to 98 ft] are most common and depth is selected according to local conditions. With other "surface" sources, such as vibrators and shots from air shooting, the shots occur at the Earth's surface.
shot depth
91
One of a number of locations or stations at the surface of the Earth at which a seismic source is activated.
shotpoint
92
A shear wave that is polarized so that its particle motion and direction of propagation are contained in a horizontal plane.
SH-wave
93
A system for acoustic surveying most commonly deployed in marine environments and towed by a ship. The side-scan sonar generates a pulse on the order of 30 to 120 kHz that is reflected from the seafloor. Side-scan sonar records yield an image of the seafloor and shallow sediments.
side-scan sonar
94
A type of event in 2D seismic data in which a feature out of the plane of a seismic section is apparent, such as an anticline, fault or other geologic structure. A properly migrated 3D survey will not contain sideswipes.
sideswipe
95
The portion of the seismic wave that contains desirable information. Noise is the undesirable information that typically accompanies the signal and can, to some extent, be filtered out of the data.
signal
96
The ratio of desirable to undesirable (or total) energy. The signal-to-noise ratio can be expressed mathematically as S/N or S/(S+N), although S/N is more commonly used. The signal-to-noise ratio is difficult to quantify accurately because it is difficult to completely separate signal from noise. It also depends on how noise is defined.
signal to noise ratio
97
The ratio of desirable to undesirable (or total) energy. The signal-to-noise ratio can be expressed mathematically as S/N or S/(S+N), although S/N is more commonly used. The signal-to-noise ratio is difficult to quantify accurately because it is difficult to completely separate signal from noise. It also depends on how noise is defined.
signal-to-noise-ratio
98
A distinguishing feature of a waveform in a seismic event, such as shape, polarity, amplitude, frequency or phase. The signature of the seismic source waveform is of particular interest to geophysicists.
signature
99
A step in seismic processing by which the signature of the seismic source in the seismic trace is changed to a known, shorter waveform by using knowledge of the source waveform. If the source waveform is known for each shot, then the process also minimizes variations between seismic records that result from changes in the source output.
signature deconvolution
100
An event in which one deeper and one near-surface reflector, such as the base of weathering or the ocean floor, are involved. The seismic energy bounces twice from the deep reflector and only once from the shallow reflector, causing the multiple to appear at roughly twice the traveltime of the primary reflection.
simple multiple
