1. First Reflection seismic experiment in Oklahoma in 1921

2. 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

3. Reflection Seismology
A ship is moving and fires air-guns about every 10 s.
The pulses travel downward and are partially reflected back up from the reflectors
Waves are then recorded by seismic receivers

4. Seismic Section The result is a seismic section
Provides a picture of the subsurface structure
The resulting structure can be interpreted as stratigraphy

5. Seismic Section Yet, there are reasons that the section is NOT true.
Vertical scale is not depth, but time
Actual time is two-way travel-time (down and back).
Can be converted to depth, but must know velocity structure.

6. Seismic Section
If layering is not horizontal, reflections will not come from directly below the source.
Problem solved using a migration algorithm.

7. Seismic Section
There may be multiple reflections in addition to the primary reflections
This makes artifacts. Algorithms exits to minimize.

8. 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 (mid-point of ray for flat layers).

9. Normal moveout derivationh v
S M R
x/ x 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)

10. Normal Moveout (NMO)
Rearrange NMO equation for the velocity of the layer
to : two way travel-time (s)
x: offset from src-rec (km)
Δt(x): moveout at x-offset (s)

11. 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)
Can solve for v2
Can iterate this procedure for deeper layers

12. Multiple Layers: Dix formula
Another way to calculate velocity of any layer
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

13. 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

14. 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.

15. 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)

16. 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 bow-tie to improve the quality of the migrated image.

17. Curved Reflectors: Anticline
Anticline has simpler response wrt Syncline.
An anticline seismic image is broadened
At edges of anticline two arrivals exist.

18. 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

19. 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

20. Faulted Reflectors
Migration removes diffraction effects and reveals features more clearly

21. 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

22. 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

23. 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

24. Data Recording
The output of each receiver is connected to an amplifier
Data is recorded digitally
sampled at regular intervals, often only 1 msec

25. 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

26. 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

27. Data Display
Deep reflections are weaker due to energy loss and spreading
Sometimes amplified to compensate- equalization

28. 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

29. 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)

30. 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

31. Reflection Coefficient
Transmission coefficient
Only valid for normal rays

32. 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 P-waves.
Consider the ratio of reflected energy
Check on this and compare with problem 10
Ken says (R+T)2 = 1
Check on this

33. 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

34. Reflection Coefficient Example
Suppose instead we have materials with different properties:
Lithological boundary, but no seismic boundary
In reality, may produce a weak reflector

35. 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

36. 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

37. 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
May want to add some discussion of a pulse on a rope here

38. 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
Vertical resolution can be improved by using a shorter pulse
Shorter pulses are more rapidly attenuated
Must compromise between resolution and depth penetration

39. 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

40. 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’

41. 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

42. 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

43. 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

44. 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

45. 3D Surveying
May want to add the image from plate 6 of the book here and comment on the use of porosity

46. 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

47. 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

48. Hydrocarbon Traps Structural Traps result from tectonic processes
Folds, domes, faults, etc

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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

57. 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

58. 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

59. 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.