Seismic data processing (Geometric Corrections

2. Presentation Topic;Presentation Topic;
Geometric CorrectionsGeometric Corrections
Presented to;Presented to;
Dr. Javed Iqbal TanoliDr. Javed Iqbal Tanoli
Presented by;Presented by;
Ali AhmadAli Ahmad
CIIT/SP17-R15-002/ATDCIIT/SP17-R15-002/ATD
Arsalan KhanArsalan Khan
CIIT/SP17-R15-003/ATDCIIT/SP17-R15-003/ATD

3. IntroductionIntroduction
In this section we will deal withIn this section we will deal with
ObjectivesObjectives
Geometric CorrectionsGeometric Corrections
CDP GathersCDP Gathers
Static CorrectionsStatic Corrections
Dynamic CorrectionsDynamic Corrections

4. ObjectivesObjectives
After studying this modules, we should know;After studying this modules, we should know;
a)a) How CDP gathers are made. How they areHow CDP gathers are made. How they are
corrected and stacked.corrected and stacked.
b)b) How to interpret stacking charts.How to interpret stacking charts.
c)c) How to apply weathering correction, elevationHow to apply weathering correction, elevation
correction i.e static corrections.correction i.e static corrections.
d)d) How to describe and apply NMO corrections.How to describe and apply NMO corrections.

5. Geometric Corrections
A seismic trace on a field monitor shows reflected energyA seismic trace on a field monitor shows reflected energy
bursts from subsurface rock layer interfaces.bursts from subsurface rock layer interfaces.
We will measure the travel times from source down toWe will measure the travel times from source down to
reflector and back to geophone and use them together withreflector and back to geophone and use them together with
average velocity to compute depths to the various reflectors.average velocity to compute depths to the various reflectors.
However before we use these reflected energy bursts andHowever before we use these reflected energy bursts and
their travel times, we must apply several corrections totheir travel times, we must apply several corrections to
compensate for geometric effects.compensate for geometric effects.
These corrections includeThese corrections include
– Static correctionsStatic corrections
– Dynamic correctionsDynamic corrections

6. TRACE GATHERING
(CDP GATHERS)

7. Common depth point defines as sum of traces whichCommon depth point defines as sum of traces which
correspond to the same subsurface reflection point butcorrespond to the same subsurface reflection point but
have different offset distances.have different offset distances.
At this step, we gather these CDP traces & then integrateAt this step, we gather these CDP traces & then integrate
all of these traces as one trace (Stacking).all of these traces as one trace (Stacking).
The main reason of using CDP method is to improve theThe main reason of using CDP method is to improve the
signal to noise ratio of data because when trace issignal to noise ratio of data because when trace is
summed, signals can be built where random noise can besummed, signals can be built where random noise can be
canceled.canceled.
It is also known as Common Mid Point (CMP)It is also known as Common Mid Point (CMP)
Common Depth Point (CDP)

8. Common Midpoint Method (CMP Method)

9. Shotpoint # 1Hydrophone groups
#1#2#3#4#5#6
Separation between midpoints is
1/2 separation between hydrophone groups
Midpoints
Common Midpoint Method (CMP Method)

10. Shotpoint # 2Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

11. Shotpoint # 3Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

12. Shotpoint # 4Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

13. Shotpoint # 5Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

14. Shotpoint # 6Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

15. Shotpoint # 7Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

16. Shotpoint # 8Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

17. Shotpoint # 8Hydrophone groups
#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)

18. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1
Common Midpoint Method (CMP Method)

19. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1
Shotpoint # 2
Shotpoint # 1
Shotpoint # 2
Common Midpoint Method (CMP Method)

20. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1
Shotpoint # 2
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 3
Common Midpoint Method (CMP Method)

21. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1
Shotpoint # 2
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 3
Shotpoint # 4
Shotpoint # 4
Common Midpoint Method (CMP Method)

22. Hydrophone groups
#
1
#
2
#
3
#4
#
5
#6
Midpoints
Shotpoints # 1-8
1
2
3
45 6 7 8 138
Common Midpoint Method (CMP Method)

23. Midpoints
1
2
3
45 6 7 8 13
Fold or Multiplicity is the number of times that the same midpoint isFold or Multiplicity is the number of times that the same midpoint is
sampled by different shots and different receivers.sampled by different shots and different receivers.
oror
It is number of reflections from one common depth.It is number of reflections from one common depth.
Fold
Common Midpoint Method (CMP Method)

24. Midpoints
1
2
3
45 6 7 8 138
Maximum Fold is achieved after the 6th shotMaximum Fold is achieved after the 6th shot
Fold
Common Midpoint Method (CMP Method)

25. When shot point spacing and group spacing areWhen shot point spacing and group spacing are
equal thenequal then
Maximum fold = number of geophones orMaximum fold = number of geophones or
hydrophoneshydrophones
Midpoint separation = 1/2 distance betweenMidpoint separation = 1/2 distance between
geophonesgeophones
Common Midpoint Method (CMP Method)

26. CMP Gather
The travel times differ sinceThe travel times differ since
the path for a near offsetthe path for a near offset
trace is less than the path fortrace is less than the path for
a far offset trace.a far offset trace.
with the correct velocity, wewith the correct velocity, we
can correct for the differencecan correct for the difference
in travel time for each trace.in travel time for each trace.
The curvature of this hyperbola is a
function of the average velocity down to
the depth of the reflection
CMP Gather
Offset Distance

27.  The process of selecting a set of traces to be stackedThe process of selecting a set of traces to be stacked
is called sorting. The set of traces sorted for particularis called sorting. The set of traces sorted for particular
CDP stack is called the CDP gather.CDP stack is called the CDP gather.

28. STACKING

29. The common reflecting point on a reflector, or the halfwayThe common reflecting point on a reflector, or the halfway
point when a wave travels from a source to a reflector to apoint when a wave travels from a source to a reflector to a

30. After Gather After NMO After Stacking

31. Advantages of Stacking chartsAdvantages of Stacking charts
A stacking chart illustrates the relationship betweenA stacking chart illustrates the relationship between
 The number of recorded channelsThe number of recorded channels
 The number of station intervals between geophonesThe number of station intervals between geophones
groups and source pointsgroups and source points
 The nominal fold and stacking density of the resultingThe nominal fold and stacking density of the resulting
CDP sectionCDP section
The nominal fold of a given fold geometry can be derived from theThe nominal fold of a given fold geometry can be derived from the
equation;equation;
F=1F=1//2 C . (dx)2 C . (dx)//(dg) . (dx)(dg) . (dx)//(ds)(ds)
WhereWhere F = FoldF = Fold
C = Number of recording channelsC = Number of recording channels
dx = Spacing between Stationsdx = Spacing between Stations
dg =Spacing between geophone groupsdg =Spacing between geophone groups
ds = Spacing between source pointsds = Spacing between source points

32. Static Correction
Often called statics, a bulk shift of a seismic trace in timeOften called statics, a bulk shift of a seismic trace in time
during seismic processing.during seismic processing.
Removing near surface effects requires two corrections;Removing near surface effects requires two corrections;
A weathering correctionA weathering correction
An elevation correctionAn elevation correction
A common static correction is the weathering correction,A common static correction is the weathering correction,
which compensates for a layer of low seismic velocitywhich compensates for a layer of low seismic velocity
material near the surface of the Earth.material near the surface of the Earth.
Other corrections compensate for differences inOther corrections compensate for differences in
topography and differences in the elevations of sources andtopography and differences in the elevations of sources and
receivers (Datum Correction or elevation correction).receivers (Datum Correction or elevation correction).

33. Weathering Correction
A weathering correction replaces the actual travel timeA weathering correction replaces the actual travel time
through the weather layer by a computed travel time.through the weather layer by a computed travel time.
A method of compensating for delays in seismicA method of compensating for delays in seismic
reflection or refraction times induced by low-velocityreflection or refraction times induced by low-velocity
layers such as the weathered layer near the Earth'slayers such as the weathered layer near the Earth's
surface.surface.
The weather layer does vary in thicknessThe weather layer does vary in thickness
If the thickness of the weathered layer is not known, thenIf the thickness of the weathered layer is not known, then
it can be determined by one or more uphole surveys.it can be determined by one or more uphole surveys.

34. A. Uphole Method
This method is used to estimate the thickness & velocityThis method is used to estimate the thickness & velocity
of weathered layer.of weathered layer.
This method involves drilling a hole into the weatheringThis method involves drilling a hole into the weathering
layer (up to 300ft).layer (up to 300ft).
An uphole geophone placed near the hole & a seismicAn uphole geophone placed near the hole & a seismic
source (usually charges of dynamite) are set in the hole.source (usually charges of dynamite) are set in the hole.
The geophone records seismic waves at each depth. TheseThe geophone records seismic waves at each depth. These
depths & times can be plotted on Time-distance curve.depths & times can be plotted on Time-distance curve.
From time-distance curve, we can estimate the thickness &From time-distance curve, we can estimate the thickness &
velocity of LVL (low velocity layer).velocity of LVL (low velocity layer).

36. By definition, theBy definition, the
weathering for theweathering for the
emerging wavepath inemerging wavepath in
fig.fig.
WC = - (dw)WC = - (dw)//(Vw) + (dw)(Vw) + (dw)//(Vc)(Vc)
OrOr
WC = - dw (VC-Vw)WC = - dw (VC-Vw)//VwVcVwVc
Since Vc>Vw the weatheringSince Vc>Vw the weathering
correction is always negative.correction is always negative.

37. B.B. Computation at Geophone StationComputation at Geophone Station
The simplest and often the bestThe simplest and often the best
method of computing themethod of computing the
weathering correction at aweathering correction at a
geophone station is to use thegeophone station is to use the
uphole time at a nearby shortuphole time at a nearby short
point.point.
tu = ds -dwtu = ds -dw//Vc + dwVc + dw//VwVw
tu = dstu = ds//Vc + dw(Vc-Vw)Vc + dw(Vc-Vw)//VwVcVwVc
tu = dstu = ds//Vc - WCVc - WC
So that: WC=dsSo that: WC=ds//Vc-tuVc-tu

38. Vc is known as the correction velocity.Vc is known as the correction velocity.
Vc may beVc may be
Known from previous experience.Known from previous experience.
Measured by uphole surveys.Measured by uphole surveys.
Determined from first arrival refracted along theDetermined from first arrival refracted along the
base of the weathered layer.base of the weathered layer.
The normal routine is to compute a weathering correctionsThe normal routine is to compute a weathering corrections
at every shot hole and to estimate corrections for otherat every shot hole and to estimate corrections for other
stations by interpolation.stations by interpolation.

39. C.C. Energy source above and below theEnergy source above and below the
weathered layerweathered layer
If the energy source is above the base of the weatheredIf the energy source is above the base of the weathered
layer, weathering correction should be applied at the bothlayer, weathering correction should be applied at the both
ends of the reflection path.ends of the reflection path.
Computing weathering corrections from uphole timesComputing weathering corrections from uphole times
requires a shot, below the weathered layer.requires a shot, below the weathered layer.
When a surface source such as vibroseis is used, weatheringWhen a surface source such as vibroseis is used, weathering
corrections are some time found by statistical methods, arecorrections are some time found by statistical methods, are
they may be computed from first arrival.they may be computed from first arrival.

40. Elevation Correction
(Datum Correction)
The effects of topography on a trace’s reflection times are removedThe effects of topography on a trace’s reflection times are removed
by applying this correction which, in effect move both source andby applying this correction which, in effect move both source and
receiver vertically to a pre-selected datum surface. This surface isreceiver vertically to a pre-selected datum surface. This surface is
usually (but not always) a flat plane. It is also called datum correction.usually (but not always) a flat plane. It is also called datum correction.
DatumDatum
An agreed and known value, such as the elevation of a benchmark orAn agreed and known value, such as the elevation of a benchmark or
sea level, to which other measurements are corrected.sea level, to which other measurements are corrected.
In seismic data, the term refers to an arbitrary planar surface toIn seismic data, the term refers to an arbitrary planar surface to
which corrections are made and on which sources and receivers arewhich corrections are made and on which sources and receivers are
assumed to lie to minimize the effects of topography and near-surfaceassumed to lie to minimize the effects of topography and near-surface
zones of low velocity.zones of low velocity.

42. ECs = Ed – EsECs = Ed – Es // VcVc
ECr = Ed - ErECr = Ed - Er // VcVc

43. Routine (Brute) Static correctionsRoutine (Brute) Static corrections
Routine static correction for each field trace can beRoutine static correction for each field trace can be
calculated by the computer.calculated by the computer.
The necessary information is usually read from the traceThe necessary information is usually read from the trace
header block on the demultiplexed tape.header block on the demultiplexed tape.
depth of sourcedepth of source
uphole timeuphole time
correction velocitycorrection velocity
source elevationsource elevation
receiver elevationreceiver elevation
datum elevationdatum elevation

45. Dynamic CorrectionDynamic Correction
One of the steps of processing the data is to rearrange theOne of the steps of processing the data is to rearrange the
traces to make CDP gathers (Fig).traces to make CDP gathers (Fig).
The traces from different record which correspond to sameThe traces from different record which correspond to same
depth point location are collected together into a singledepth point location are collected together into a single
record.record.
The traces are normally arranged with in this gather recordThe traces are normally arranged with in this gather record
in order of increasing offset distance.in order of increasing offset distance.
Then the reflected signals from a single horizontal interfaceThen the reflected signals from a single horizontal interface
align along a hyperbola as shown in figure.align along a hyperbola as shown in figure.

46. CDP Gather Hyperbola

47. Normal Move Out (NMO)Normal Move Out (NMO)
The term normal move out or NMO means the variation in reflection
arrival time with offset distance from source to receiver.
Before stacking, the traces must be shifted to its original place by
NMO.
A reflection typically arrives first at the receiver nearest the source.
The offset between the source and other receivers induces a delay in
the arrival time of a reflection from a horizontal surface at depth.
A plot of arrival times versus offset has a hyperbolic shape.
Move out correction is time correction applied to each offset.

48. a) Gather Data b) Gather Data after NMO correction

49. After Gather After NMO After Stacking