GEOPHYSICS (08/430/0012)
CRUST AND UPPER MANTLE SEISMIC SURVEYS (1)
OUTLINE
Seismic reflection surveys
Some points of interpretation
Reflection travel times: seismic velocities from reflection hyperbolae
Seismic data processing, the CMP method, stacked sections
Normal incidence ray paths: the effect of dip and faults, section migration
Examples of seismic reflection data and data processing
Results from deep reflection surveys e.g. reflective lower crust, Mohorovičić reflections, major fault and thrust zones
References
Klemperer, S. & Hobbs, R., 1991. The BIRPS Atlas - Deep seismic reflection profiles around the British Isles. Cambridge UniMuch recent BIRPS work has been abroad. (See http://www.esc.cam.ac.uk/Library/Journals/BIRPS/birps.html).
versity Press.
Open University Course Team, 1990. Lithosphere Geophysics in Britain. Study Unit S339 1B, Open University Educational
Review of seismic refraction and reflection results in Great Britain.
Enterprises.
Background reading: Fowler §4.5, Lowrie §3.6 & 3.7
GEOPHYSICS (08/430/0012)
SEISMIC REFLECTION SURVEYS (1)
Although it is much more elaborate, reflection surveying is similar to echo-sounding. Waves from a seismic source are fired
into the Earth and the reflections returning from layer boundaries within the Earth are recorded and can be displayed as a
vertical section through the Earth. The section normally displays the reflections with the vertical axis representing recording
time. This time is two-way time since each reflection has to travel down to the reflector and up again, thereby traversing the
travel path twice. The idea of a seismic section is deceptively simple. It is that the normal incidence two-way time is related
to the apparent depth by the equation
2H
= the normal incidence two − way time (twt)
t0 =
V
from Lecture 4. We say apparent depth because at normal incidence reflections bounce back at right angles to reflecting
boundaries. If the boundary is dips at angle α, the travel path is slanted at an angle α to the vertical.
When carried out at a large enough scale, seismic reflection surveys image the crust and the very top of the upper mantle.
Apart from their scale, they are similar to the seismic reflection surveys employed in prospecting for oil and gas. They require
a powerful seismic source and record reflections out to several kilometers offset and to two–way times of 15 s or more. Deep
marine reflection surveys use long (∼ 6 km) hydrophone streamers. A two ship operation would often be desirable but it is
rare because of the cost of hiring two ships.
Examples of deep reflection surveys
The class exercise presents an example of a deep reflection survey from the BIRPS deep reflection consortium for inspection
and interpretion. Refer also to your textbook and the references given earlier.
GEOPHYSICS (08/430/0012)
SEISMIC REFLECTION SURVEYS (2)
Some points of interpretation
These points outline concepts and terminology that are expanded in the succeeding pages.
• The seismic data go through several stages of processing and are usually presented as seismic sections (stacked sections)
that are effectively an enhanced echo–sounding profile. A stacked section approximates a profile of normal incidence
primary reflection signals from which can be produced a display of normal incidence two–way reflection times for the
most prominent reflections.
• A display of normal incidence reflection times is NOT an accurate image of reflector geometry (a) because the vertical
axis is time not depth and (b) because normal incidence reflections follow a vertical path only when reflectors are
horizontal and seismic velocity does not vary laterally. To amplify point (b) note that, whereas reflection times are
plotted vertically beneath their recording locations, the point of reflection from a dipping reflector is not vertically
below the recording location.
• Conversion of a reflection time profile to a depth profile is called time–to–depth conversion. It requires knowledge
of the seismic velocity field beneath the survey line and compensation for the ray path geometry of the reflections.
Seismic velocities can be estimated from the travel time curves of the reflections recorded as a function of increasing
source–to–receiver offset. The estimation accuracy decreases with increasing depth.
GEOPHYSICS (08/430/0012)
SEISMIC REFLECTION DATA PROCESSING
Seismic reflection data processing is a very highly developed technology. Its activities can be broadly classified as
1. preparing and organising the data: data input, demultiplexing, sorting;
2. quality control of the processing and the huge quantities of data processed;
3. isolating the reflection signal and enhancing the signal–to–noise ratio (noise attenuation): various filtering techniques,
stacking;
4. improving temporal definition: deconvolution;
5. improving spatial definition: migration;
6. estimating data parameters, especially “moveoutÔ velocities;
7. displaying results and identifying targets: advanced techniques of image processing and waveform analysis for extracting
key information from a huge mass of data.
The exploration geophysics course includes an introduction to these techniques. For a qualitative appreciation of seismic
reflection sections it is important to understand the basics of the common mid–point (CMP) method, stacking and the
geometry of normal incidence ray paths.
GEOPHYSICS (08/430/0012)
THE COMMON MID–POINT (CMP) METHOD
Seismic reflection data is generally shot using uniform shot–point (SP) and receiver intervals. The data is collected shot by
shot. It is then sorted into trace gathers having a common mid–point. The main benefits from this procedure are:
1. the data are organised into the form required for velocity estimation;
2. reflections are recorded from approximately the same patch on a reflector; this allows their signal–to–noise ratio to be
enhanced by stacking.
The figure overleaf shows a simple CMP shooting scheme.
Fold of cover
Fold of cover is simply the number of traces in each CMP gather of traces. It also represents the number of replicated
recordings made of the reflection signal at any CMP. This replication, or redundancy, provides the basis for discriminating
the desired reflection signal from various types of noise. In practice the fold of cover may vary somewhat around a nominal
value.
Note: reflections recorded at different angles of incidence are not exact replicas but ignoring this makes a useful first
approximation.
Note: a CMP gather is not quite the same as a common reflection point (CRP) gather when there is dip in the section; a
process called dip moveout (or sometimes pre–stack migration) converts CMP gathers into CRP gathers. A common depth
point (CDP) is the same as a CRP which is sometimes found as an old and inaccurate terminology for CMP.
GEOPHYSICS (08/430/0012)
A SIMPLE CMP SHOOTING SCHEME
The figure below shows an off-end CMP shooting scheme of the kind employed in marine seismic reflection surveys. In a
marine survey a ship tows a source array (more commonly now two or three source arrays) behind which it tows a streamer
or streamers, often 3 km or more long, containing the recording arrays. In practice marine surveys normally employ at least
30–fold shooting; in 2D work much higher values, such as 96 and 192–fold, are common. Land surveys may be shot off-end
like marine surveys, or the shot may be placed within the spread, which is called split-spread shooting.
recording channels
∗ 24 ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦•4◦ ◦ ◦1
shot n + 6 ­
∗ 24 ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦•8◦ ◦ ◦ ◦ ◦ ◦ ◦1
shot n + 5 ­
◦• ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦1
∗ 24 ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦12
shot n + 4 ­
16
24
◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦• ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦1
∗
shot n + 3 ­
•◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦1
∗ 24 ◦ ◦ ◦ ◦20
shot n + 2 ­
∗ 24 •◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦1
shot n + 1 ­
@
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survey line
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reflector
GEOPHYSICS (08/430/0012)
SORTING FROM SHOT GATHERS INTO CMP GATHERS
The figure below illustrates sorting from 24-channel shot-point gathers into 12-fold CMP gathers. The shot spacing is one
group interval. Draw a diagram to check that Channel 2 from shot n + 1, channel 4 from shot n + 2, ... , through to channel
22 from shot n + 11 and channel 24 from shot n + 12 all have the same CMP. P denotes a primary reflection, and M a
multiple reflection (described later under stacking).
shot n+1
shot n+2
shot n+11
CMP gather
CMP gather
shot n+12
NMO correction
P
CMP sort
P
M
M
GEOPHYSICS (08/430/0012)
SEISMIC REFLECTION TRAVEL TIMES
A constant velocity layer over a horizontal reflector
In this case the travel times are given by
t2x
=
t20
x2
+ 2
V
or ∆t = tx − t0 =
r
t20 +
x2
− t0
V2
This comes from applying Pythagoras’ Theorem to the ray path of the reflection at source–receiver offset x.
x
offset x»
S
R
A
A
¡
¡ ¡µ
¡¡
A
¡¡
A
¡¡
A
AU
¡ ¡1
A
¡µ ¡ tx v
¡¡ 2
A
¡¡
A
¡¡
A
A ¡ ¡«
A¡
6
t0
∆t
6
H=
1
tv
2 0
tx
?
?
P
twt ?
(two–way time)
A plot of tx against x is a hyperbola. A plot of t2x against x2 is a straight line with intercept t20 and slope V −2 . The velocity
V can be estimated by fitting a hyperbola to the travel time curve (tx –x plot) or by plotting t2x against x2 and measuring
the slope. For rough calculations only, the hyperbolic moveout equation can be approximated by the parabolic equation:
∆t '
x2
2V 2 t0
GEOPHYSICS (08/430/0012)
NORMAL MOVEOUT
Some jargon
The normal moveout (nmo) of a reflection recorded at a source–to–receiver offset x is the difference between its travel–time
tx at x and the normal incidence (zero offset) travel–time t0 of the reflection; i.e. normal moveout = ∆t = tx − t0 .
Moveout curves over dipping and layered structures
After the reflection data have been sorted into common mid–point gathers, the travel times from a dipping reflector beneath
a constant velocity layer still obey the hyperbolic equation given above but the velocity V is replaced by V / cos δ where δ is
the dip of the reflector.
The hyperbolic moveout equation remains a reasonably good approximation at short source–receiver offsets for reflections
from layered structures with small to moderate dip. For practical purposes short offsets are those smaller than the depth to
the reflector. The common methods of velocity analysis (velocity estimation) and nmo correction (signal alignment) assume
hyperbolic moveout curves.
GEOPHYSICS (08/430/0012)
THE STACKED SEISMIC SECTION
Seismic sections aim to display the primary (once down, once up) reflections and to attenuate multiple reflections and other
types of noise. Stacking (or sometimes an elaboration of stacking) is the principal means of enhancing the primary seismic
reflection signal. The primary reflections on the section present an image of the geological structure in two–way reflection
time on which dips, faults, anticlines and synclines are distorted to some degree. A process called (time) migration tries
to unravel the worst of this distortion. Representation of the structure in depth through time–to–depth conversion requires
good knowledge of the seismic velocities in the subsurface.
Illustration of primary and multiple reflections
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GEOPHYSICS (08/430/0012)
STACKING
Stacking, more precisely CMP stacking, is the summing of seismic traces following alignment of the reflection signal by nmo
correction. The process is illustrated in the figure on the next page. Recall that CMP shooting introduces redundancy
into recording the reflections such that reflections from any subsurface location are recorded several times, at different
angles of incidence. Sorting the data into CMP (common mid–point) gathers collects together data having approximately
common reflection points. Within a CMP gather reflections follow hyperbolic travel time curves whose apexes are the
normal incidence reflection time, the time at zero source–to–receiver offset. Normal moveout corrections align the primary
reflections by applying continuously varying time shifts that remove the normal moveout; that is, a reflection at time tx
on offset x is moved to the normal incidence reflection time t0 . This leaves multiple reflections and other forms of noise
unaligned. Summing therefore enhances the primary reflections and produces a trace that approximates the zero–offset or
normal incidence reflection signal. Thus a stacked seismic section approximates a normal incidence reflection
section.
Note: In velocity analysis it can be important to distinguish stacking velocity, the velocity employed in nmo corrections
prior to stacking, from the actual best–fit normal moveout velocity but we need not dwell on this here. Normal moveout
velocity is a kind of weighted average velocity down to the depth of the reflector. It is generally somewhat higher than the
simple average velocity down to the depth of the reflector.
GEOPHYSICS (08/430/0012)
ILLUSTRATION OF STACKING
The following figures illustrate the effectiveness of stacking in attenuating (left) correlated noise, such as multiple reflections,
and (right) correlated and uncorrelated or random noise, such as wind noise and traffic noise. In each figure the stack, or
average trace, is trace (or offset) 26.
aligned primary + multiple + random noise
2000
2050
2050
2100
2100
2150
2150
two−way time
two−way time
aligned primary + multiple reflection
2000
2200
2200
2250
2250
2300
2300
2350
2350
2400
0
5
10
15
offset
20
25
2400
0
5
10
15
offset
20
25
GEOPHYSICS (08/430/0012)
THE GEOMETRY OF NORMAL INCIDENCE RAY PATHS
A stacked section closely approximates a normal incidence or echo–sounding section that enhances primary reflections.
Reflectors on such sections appear down–dip of their actual lateral position. The figure below illustrates the geometry of
normal incidence ray paths to a dipping reflector CD through a constant velocity overburden and the apparent position C0 D0
that it would appear on a depth–converted stacked section. The apparent depths, AC0 and BD0 , on the normal incidence
section are half the product of normal incidence two–way time and the average velocity to the depths of C and D respectively.
CMP
A
CMP
B
"
φ"" TT
T
θ = apparent dip
(from C0 D0 on stacked section)
φ = true dip
(from CD in depth section)
AC0 - BD0 = D0 B0 = AB tan θ
AC - BD = AA0 = AB sin φ
Hence
tan θ = sin φ
T
"
T
"
T
"
"
T "" 0
TA
T
T
T
T
T
T
T
T
T
"T
"
T
TT""
D
T
0
"
D
T
"
¨
¨¨
"
T ¨¨
"
T¨¨ " φ
¨¨
"
¨
¨
TT"
¨
¨
"
¨
¨
¨¨
C
¨¨
θ
¨¨
0 ¨
B0
C
Note also: the lateral (x direction) displacement of C0 is AC0 × sin φ, and
its vertical displacement is AC0 × (1 − cos φ).
GEOPHYSICS (08/430/0012)
MIGRATION
Because of the way that stacked sections distort dips, anticlines on a stacked section appear larger than they are. On the other
hand synclines whose centres of curvature lie above the recording plane appear tighter than they are, while synclines with
centres of curvature below the recording plane become inverted into “bow–tieÔ features. Another characteristic of stacked
sections is that faults do not end abruptly but show diffraction tails.
Migration processing attempts to restore reflections to their correct lateral position. Consequently it reconfigures a “bow–
tieÔ reflection as the buried syncline from which it originated. It also collapses diffractions to a point, thereby improving
the definition of faults. Section migration is a two–dimensional (2D) process that should strictly be applied only to dip–line
stacked sections. Migration in three dimensions requires 3D seismic data. The figure on the next page illustrates the geometry
of migrating a dip–line stacked section. It swings the normal incidence segment C0 D0 up dip until the angle of incidence at
the reflector is 90◦ .
Section migration of deep reflection profiles is not usually very satisfactory; instead the two–way times of prominent reflections
can be migrated and modelled. The figure overleaf illustrates how the two-way times from stacked deep reflection sections
can be migrated. First the two-way times Ttwt are converted to apparent depth Dapp using Dapp = 0.5 × Vave × Ttwt where
Vave is the average velocity down to Ttwt . An apparent depth section (with the same horizontal and vertical scales) is drawn
plotting Dapp against recording location. The actual reflection point could be anywhere on a circle of radius Dapp centred
on this location. Drawing these circles for a set of locations identifies the reflecting surface as the envelope tangential to the
circles.
GEOPHYSICS (08/430/0012)
MIGRATION BY GEOMETRICAL CONSTRUCTION
As an exercise measure the apparent and true dips of the stack and migrated times in the figure and check that they agree
with the equation tan θ = sin φ. Check also the vertical and horizontal displacements from P to Q with the formulae given
in "The Geometry of Normal Incidence Ray Paths" earlier.
MIGRATION OF A DIPPING REFLECTION
−0.5
0
:stacked section
:migrated section:
depth(km)
0.5
1
1.5
2
Q
P
2.5
3
0
0.5
1
1.5
2
2.5
location (km)
3
3.5
4
4.5
5
GEOPHYSICS (08/430/0012)
EXAMPLES OF SEISMIC REFLECTION DATA AND DATA PROCESSING
The following pages show samples of seismic reflection data recorded in the field and at various stages of their data processing.
The data come from a 3D survey in the southern North Sea, prospecting for gas in the Rotliegendes sandstone. The
Rotliegendes sandstone underlies a relatively thin shale, above which is the Zechstein evaporites. In the class exercise you
will work on a section from a deep crustal reflection survey. However deep crustal reflection data are considerably noisier
than prospecting data and prospecting data provides a clearer view of the progression through data processing.
1. Field reflection data (3 shot-point gathers).
These data come from a survey shot with two source arrays 50 m apart, firing into three 2000 m streamers, each 100
m apart. The streamers had 160 12.5 m hydrophone recording groups. Adjacent traces from the field data have been
summed, following corrections for the differential moveout between traces, in order to improve their signal-to-noise
ratio, thereby halving the number of traces. A gain (amplification) increasing with time has also been applied in order
to give shallow and deeper reflections similar amplitudes.
Note the wedge of refractions and strong linear-moveout noise ahead of the reflection data.
2. Common mid-point gathers.
The refractions and linear-moveout noise have been removed. This and the sorting into CMP gathers has enhanced
the reflection signal.
3. Velocity analysis display.
The velocities are estimated by applying an extensive set of hyperbolic moveout curves to the CMP data. The set of
hyperbolae come from 11 pre-set velocity-time functions. The left hand panel displays a colour-coded contour plot of
the semblance, a measure of trace-to-trace similarity computed along the hyperbolae from the data. A high semblance
(red) indicates that reflections have been aligned. The moderately high semblances at the lower left come from multiple
reflections. The next panel shows the CMP data aligned using the picked velocities (white line and dots). Then comes
a small panel of data around this CMP stacked using the picked velocity, followed by 11 stack panels corresponding to
the 11 pre-set velocity functions.
GEOPHYSICS (08/430/0012)
EXAMPLES OF SEISMIC REFLECTION DATA AND DATA PROCESSING (CTD)
4. Moveout-corrected common mid-point gathers.
These are the same CMP gathers as before but now the primary reflections have been aligned using the picked velocities.
The strong reflection at 1600 ms comes from the base Zechstein.
5. Near-offset section.
This is a simple echo-sounding (single-fold) section obtained by plotting the trace nearest each shot from part of a
single line from the 3D survey . Note how noisy the section is.
6. Near-offset section repeated in colour display.
These are the same data as in the previous figure but now they are plotted in colour.
7. Stacked section.
The data corresponds to the same part in-line shown as a near-offset section. Comparing the two shows that stacking
has greatly improved the signal-to-noise of the data. Nonetheless the structures are poorly focussed. The faults are
obscured by diffractions: these have a hyperbolic shape. There is a "bow-tie" at 1000 ms beneath cross-line 650; this
is seen to be a small syncline after migration.
8. Migrated section.
The faults and structures are better focussed and appear in close to the correct position. However the vertical scale
is still two-way time. Because velocity tends to increase with depth, this makes a linear fault appear to be concave
upwards (Can you understand why this is so?).
GEOPHYSICS (08/430/0012)
3 SHOT-POINT GATHERS
Field reflection data (after adjacent trace summing and some scaling)
GEOPHYSICS (08/430/0012)
COMMON MID-POINT GATHERS
(with refractions and noise removed at far offsets)
GEOPHYSICS (08/430/0012)
VELOCITY ANALYSIS DISPLAY
The velocities are estimated by applying numerous trial hyperbolic reflection trajectories.
GEOPHYSICS (08/430/0012)
MOVEOUT-CORRECTED COMMON MID-POINT GATHERS
(The strong reflection at 1600 ms comes from the base Zechstein)
GEOPHYSICS (08/430/0012)
NEAR-OFFSET SECTION
A simple echo-sounding (single-fold) section displayed as a wiggle-variable area section.
GEOPHYSICS (08/430/0012)
NEAR-OFFSET SECTION
A simple echo-sounding (single-fold) section displayed in colour. The colour varies from red (positive) to black (negative).
GEOPHYSICS (08/430/0012)
STACKED SECTION
Note the poorly focussed structures
GEOPHYSICS (08/430/0012)
MIGRATED SECTION
The faults and structures are better focussed
GEOPHYSICS (08/430/0012)
RESULTS FROM DEEP REFLECTION SURVEYS
Because deep reflection seismic surveys are expensive and demanding experiments, they are conducted by consortia organised
on a national or international basis. The first recordings of deep crustal reflections and faulting at depth in the crust were
made in the U.S.A. by COCORP, the Consortium for Continental Reflection Profiling. Following the success of COCORP,
consortia were organised in other countries. In the U.K. the surveys were conducted offshore, building on the experience
of the North Sea exploration industry. Marine surveys generally yield better quality data and are significantly more cost
effective than land surveys. The U.K. consortium is called BIRPS, the British Institutions Reflection Profiling Syndicate.
The first survey (MOIST: Moine and Outer Isles Seismic Traverse) was conducted off north-east Scotland.
Significant results from BIRPS surveys include:
1. reflections from the Moho (Mohorovičić) discontinuity at the base of the crust (∼ 10 s two-way time);
2. a relatively non-reflective upper crust and reflective lower crust;
3. major fault and thrust zones in the crust and upper mantle.
MOIST mapped the extensions of the Moine and Outer Isles thrusts offshore and the Flannan thrust was found to penetrate
through the crust into the upper mantle. The Flannan thrust is interpreted as a remnant of the subduction zone which closed
during the Caledonian orogeny (∼ 400 Ma) when the Iapetus ocean between North America and Scotland closed.