First Reflection seismic experiment in Oklahoma in 1921
Reflection Seismology
• Seismic Reflection is the most important tool we
have to image subsurface structure.
• Provides detailed imaging of approximately
horizontal layering in the earth
• Reflection seismology is echo or depth sounding
Reflection Seismology
• A ship is moving and fires airguns about every 10 s.
• The pulses travel downward
and are partially reflected back
up from the reflectors
• Waves are then recorded by
seismic receivers
Seismic Section
• The result is a
seismic section
• Provides a
picture of the
subsurface
structure
• The resulting
structure can be
interpreted as
stratigraphy
Seismic Section
• Yet, there are reasons that the
section is NOT true.
• Vertical scale is not depth, but
time
• Actual time is two-way traveltime (down and back).
• Can be converted to depth,
but must know velocity
structure.
Seismic Section
• If layering is not horizontal,
reflections will not come from
directly below the source.
• Problem solved using a
migration algorithm.
Seismic Section
• There may be multiple
reflections in addition to the
primary reflections
• This makes artifacts.
Algorithms exits to minimize.
Normal Moveout (NMO)
• The time to travel to a receiver a
distance x away from the source is
x: src—rec offset (m)
T0: vertical travel-time
∆t(x): moveout wrt offset
hi : ith layer thickness (m)
V: layer velocity
S: source coordinate
R: receiver coordinate
A: reflection point (midpoint of ray for flat
layers).
Normal moveout derivation
Normal Moveout
Derivation
2
2
t  2h / v L 
h  ( x / 2)
t 0  2h / v
h  t0v / 2
2
2
h 2  ( x / 2) 2 
h 2 (1  ( x / 2h) 2
v
v
2h
2h
1
t 
1  ( x / 2 h) 2 
(1  ( x / 2 h) 2 )
v
v
2
binomial approximation : x  1    1   / 2
t 
2h
2h
2hx 2
x2
2
2
t 
(1  x / 8h ) 

 t0 
2
v
v
8vh
4vh
2 x2
x2
t  t0 
 t 0  t
t ( x ) 
4vt0 v
2v 2 t 0
S
0
M
x/2
R
x
h
L
v
L
t: travel-time
L: right triangle hypotenuse
v: velocity of layer (km/s)
t0: two-way time
x: source-receiver offset
h: layer thickness (km)
Normal Moveout (NMO)
• Rearrange NMO equation for
the velocity of the layer
x
v1 
2t0 t
Layer Velocity
• to : two way travel-time (s)
• x: offset from src-rec (km)
• Δt(x): moveout at x-offset (s)
Multiple Layers
• What velocity should be used for two layers?
• Answer: Root Mean Square (RMS) velocity
• τi : one wave interval time (top to bottom time)
v2,rms
v  v 

1   2
v2,rms 
2
1 1
2
2 2
x
2t0,2 t 2
Can solve for v2
Can iterate this procedure
for deeper layers
Multiple Layers: Dix formula
• Another way to calculate velocity of any layer
•
•
•
•
•
•
•
vlayer 
B index: layer bottom
T index: layer top
vlayer : layer velocity
tB : 2-way tt to layer bottom
tT : 2-way tt to layer top
vrms,B RMS velocity to bottom
vrms,T RMS velocity to top
2
2
vrms
t

v
,B B
rms ,T tT
t B  tT
Stacking
• Almost always the reflections are weak and are hard to
recognize because of inevitable noise
• To increase the signal-to-noise ratio, stacking is used
• Take repeated measurements and average
• Signal (reflections) add constructively and noise cancels
Stacking
• Finding layer velocities and stacking done simultaneously
• Try range of vrms
• NMO correct the times of the seismograms
• If the vrms is correct
strong reflections are found
• If vrms is incorrect
Reflections are not found.
Dipping Reflectors
• If a reflector is dipping, its
apparent position and dip are
wrong in an unmigrated section.
• Wave raypaths are least-time
paths and hence reflect from up
dip points.
• Unmigrated reflector is
shallower and with less dip.
• Travel-time hyperbola offset: h
(thickness), alpha (dip)
x= 2h sin
Curved Reflectors: Syncline
• If a interface is sufficiently
curved, there can be more than
one reflection returned from the
interface: multi-pathing.
• As the shot point is shifted from 1
to 10, three arrivals are produced
that make a ‘bow tie’ travel-time
pattern.
• Migration can ‘unwrap’ the bowtie to improve the quality of the
migrated image.
Curved Reflectors: Anticline
• Anticline has simpler response
wrt Syncline.
• An anticline seismic image is
broadened
• At edges of anticline two arrivals
exist.
Migration
• Correcting for the position
and shape of the reflector is
called migration
• Complicated and requires
large amounts of computer
time
• Be aware of possible
distortions in un-migrated
sections
Faulted Reflectors
• Consider a point source (reflector).
• The distance that a source-receiver
measures is on the arcs shown
• Multiple source-receivers produce
the reflection hyperbola shown
• A stepped reflector behaves
normally except near the step
• Produces a reflection hyperbola
• Difficult to tell position of fault
• Migration removes diffraction
effects and reveals features more
clearly
Faulted Reflectors
• Migration removes diffraction effects and reveals
features more clearly
Multiple Reflections
• The positions of multiples can be
anticipated from the position of
the primary reflectors
• However, sometimes it is
difficult to recognize a primary
reflector that comes in with the
same TWT as the multiples
• Can be distinguished
• Moveout for the primary is
less than for the multiple, so it
stacks using a higher velocity
Marine Surveys
• Most common source is an Air Gun
– Produces P-waves (no S-waves)
• Receiver is a hydrophone
– Microphone that responds to change in pressure
– Mounted at regular intervals and towed behind the
ship in a streamer
Land Surveys
• Most common source is an explosion
– Buried small charge fired by detonator
• Receiver is a geophone
– Often times in clusters to improve s/n ratio
• Moving the system on land is much harder resulting in
much more expense
Data Recording
• The output of each receiver is connected to an amplifier
• Data is recorded digitally
– sampled at regular intervals, often only 1 msec
Common Depth Point Stacking
• Common Depth Point Stacking (CDP) uses rays
that have all reflected from the same part of the
interface
• Uses pairs of shot points and receivers that are
symmetrical about the reflector point
• CDP stacking gives better data for computing
velocities and stacking
• Number of channels that are
added are the fold
240-fold stacking is common
Static Corrections
• In Land Surveys, significant topography has to be
corrected for
• Additionally, must take into account the effect of
the topsoil and other weathered layers
• Called Static Corrections
• puts data on convenient
horizontal plane
Data Display
• Deep reflections are weaker due to energy loss and
spreading
• Sometimes amplified to compensate- equalization
Vibroseis
• Vibroseis produces a continuous wave with changing
frequency
• To find the travel time, the recorded signal is shifted in
time until the entire waveform matches the source
• The energy required is small
• An advantage where non-intrusive is preferred
Vibroseis
• The waves are generated by a vehicle with a plate that
presses rhythmically against the ground
• instantaneous force of 15 tonnes
• 1 metric tonne =1000 kg or 2205 lb
• Frequency is swept from 10 to 100 Hz over 30 sec
• Sometimes several vehicles operate in unison
• increases energy
• has reached
Moho (30 km)
What is a Reflector?
• Rays are reflected when they meet an interface
• Abrupt change in seismic velocity
• How abrupt?
• We can define the acoustic impedance
Z  v
where
 kg 
 is the density  3 
m 
and
v is the  longitudinal  velocity of the material
Reflection Coefficient
• Reflection coefficient
R
areflected
aincident
2v2  1v1

2v2  1v1
• Transmission coefficient
atransmitted
2 1v1
T

aincident
2v2  1v1
• Only valid for normal rays
Reflection Coefficient
• Because the S-wave velocity and the P-wave velocities
are different, their coefficients of reflection can also
differ.
• Sometimes S-waves are reflected more strongly than Pwaves.
• Consider the ratio of reflected energy
Ereflected  R 2
Etransmitted  T
and
R2  T 2  1
2
Check on
this
Reflection Coefficient Example
• Calculate the reflection coefficient of sandstone
overlying limestone.
• Suppose that the properties of sandstone are at the
bottom of their ranges while those of limestone are at the
top
2v2  1v1  2.8  6    2.05  2 
R

 0.608
2v2  1v1  2.8  6    2.05  2 
Thus,
Ereflected  R
2
refle
  0.608  0.37 strong reflector
2
Reflection Coefficient Example
• Suppose instead we have materials with different
properties:
2v2  1v1  2.64  3   2.40  3.3
R

0
2v2  1v1  2.64  3   2.40  3.3
Thus,
2
Ereflected  Rrefle
 0!
• Lithological boundary, but no seismic boundary
• In reality, may produce a weak reflector
Bright Spot
• The interface in a hydrocarbon reservoir between a gas
layer and underlying oil or water produces a strong
reflection
• Called a ‘bright spot’
• A strong, horizontal ‘bright spot’
• Evidence for presence of gas
Vertical Resolution
• Suppose two interfaces could be brought progressively
closer together
• Reflected pulses would overlap more and more
• At some point, the pulses cannot be resolved
• The shape of the combined pulses also changes
Vertical Resolution
• Often the two interfaces are two sides of a thin layer
sandwiched within another
• Shale layer within Sandstone
• One interface has positive R and other negative
• Negative R means that the reflected pulse is
inverted
• Interference
• Can produce no reflected wave
Vertical Resolution
• Since the pulse reflected from the lower interface has to
travel further by twice the separation of the interfaces
• Difficult to resolve when they are less than half a
wavelength apart

2  h  <
2

 h<
4
• Vertical resolution can be improved by using a
shorter pulse
• Shorter pulses are more rapidly attenuated
• Must compromise between resolution and depth
penetration
Vertical Resolution
• Another situation where there is no reflection is when an
interface is a gradual change of velocities and densities
extending over more than about half a wavelength, rather
than being abrupt
• Can be thought of as many thin, sandwiched layers
• Interference causes cancelation
• Lithological boundary with no seismic reflector
Synthetic Reflection Seismograms
• Can be used to improve interpretation
• First, choose a pulse that corresponds to the source
• Next, calculate the reflection and transmission
coefficients
• Based on borehole data
• Propagate the pulse
‘down through
the layers’
Synthetic Reflection Seismograms
• If there are several interfaces closer together than the
length of the average wavelength of the pulse, the
reflected pulses often combine to give peaks that do no
coincide with any of the interfaces
3D Surveying
• We are often interested in the form of structures
perpendicular to the seismic sections we have been
studying
• There can be reflections from dipping interfaces outside
the plane of the section
• Sideswipe
• This can be addressed by using a regular grid on the
surface (3D survey)
• Often times the grid is on 50 m spacing
3D Surveying
• A CDP gather comprises pairs of shot points and
receivers from all around the CDP
• Stacking , migration follow the same principles
• Obviously more difficult and computer intensive
3D Surveying
• We can think of the reflectors revealed as being
embedded in a block
• Can take ‘slices’ in any direction
• More sophisticated processing can reveal properties of
the rock such as porosity
• Determines the amount
of hydrocarbon that a
rock can hold
3D Surveying
• May want to add the image from plate 6 of the book here
and comment on the use of porosity
Time-Lapse Modeling
• By repeating surveys at intervals, the extraction of
hydrocarbons can be followed and remaining pockets of
oil detected
• Though 3D surveying is much more expensive, both for
data acquisition and reduction, it can pay for itself in the
increased understanding of the structure of hydrocarbon
reservoirs
Forming Hydrocarbon Reservoirs
• Organic material (minute plants & animals) is buried in a
source rock that protects it from oxidation (often clays in
a sedimentary basin)
• Bacterial action operating at temperatures of 100 to
200°C changes the organic matter into droplets of oil
• The droplets are squeezed out of the source rock
• Being lighter than water, usually move upward
• Deformation can cause them to move sideways
• Impervious cap rock (shale) prevents leakage
• To be extractable, reservoir rock must be porous and
permeable
• To be commercially viable, must be concentrated
Hydrocarbon Traps
• Structural Traps result from tectonic processes
• Folds, domes, faults, etc
Hydrocarbon Traps
• Stratigraphic Traps are formed by lithological variation
in the strata at the time of deposition
• Lens of permeable and porous sandstone or a
carbonate reef, surrounded by impermeable rocks
Hydrocarbon Traps
• Combination Traps have both structural and stratigraphic
features
• Where low density salt is squeezed upwards to form a
salt dome, both tilting strata and causing
hydrocarbons to concentrate as well as blocking off
their escape
Hydrocarbon Traps
• How can hydrocarbon traps be located with seismic
reflection?
• Easiest to recognize are structural ones.
• Stratigraphic traps which have tilted reservoir rcoks
terminating upwards in an unconformity are also
fairly easy to spot
• Bright spots show presence of gas-oil or gas-water
interface
• Smaller and harder to recognize traps are being exploited
• High quality surveys necessary
• Closely spaced stations, high resolution sources
• Precise processing of data
Sequence Stratigraphy
• Sequence stratigraphy is the building up of a stratigraphy
using seismic sections
• Can provide constraints on global sea-level change
• Used as a dating tool
• Possible because chronostratigraphic boundaries
(surfaces formed within a negligible interval of time)
are also often reflectors
• Stratigraphic sequence
• Sequence of strata of common genesis bounded by
unconformities
Sequence Stratigraphy
• How a sequence is built up and terminated depends on
the interplay of deposition and changes in sea level
(eustatic changes)
• Complex, but all we are concerned with is the relative
sea level changes while the sediments are being
deposited
Sequence Stratigraphy
• Consider deposition along a
coastline where sediments are
being supplied by a river, and
sea level is constant
• Prograding succession
• Thinning at top
• Top lap
Sequence Stratigraphy
• Consider deposition along a
coastline where sediments are
being supplied by a river, and
sea level is rising steadily
• Successive near horizontal
layers
• Onlap where they butt up
to the coastline
Sequence Stratigraphy
• Slow rise in sea level (and
constant sea level) results in
progradation (lateral buildup
of stratigraphy)
• Rapid rise of sea level results
in aggradation (vertical
buildup of stratigraphy
Sequence Stratigraphy
• An unconformity
• defines the boundary of a
sequence
• occurs when sea level falls
fast enough for erosion
rather than deposition to
occur
• Moderate sea level retreat
• Truncates tops of beds
• Rapid sea level retreat
• Erosion cuts laterally
Sequence Stratigraphy
• Beds that end are
termed discordant
• Appear in
seismic
sections as
reflectors that
cease laterally
• Can be used for
working out the
history of
deposition and
erosion
Sequence Stratigraphy and Eustasy
• Sequence stratigraphy provides a record of the changes
in local relative sea level
• Types and volumes of various facies within the sequence
provides constraints on the amounts of rise and fall of
sea level
• Fossils taken from boreholes can be used to estimate
their times
• Correlations between widely separated basins can be
used to determine global sea level changes.