This document provides an overview of gravity and seismic geophysical exploration methods. It begins with introductions to gravity, its units of measurement, and factors that cause gravity variations. It then discusses gravity data acquisition, processing steps like tidal and elevation corrections to derive anomaly maps, and interpretation. For seismic exploration, it describes data acquisition using common midpoint gathers and factors like fold, followed by processing steps like normal moveout correction and stacking to improve signal-to-noise ratio and imaging resolutions. It concludes with discussions on filtering, migration, and how these improve subsurface representations.

1. Surface and Subsurface
Exploration using Geophysical
Methods.
Presented By,
Ali Ahmad
SP17-R15-002

3. INTRODUCTION
WHAT IS GRAVITY?
standard gravity g= 9.80665 m/s² 9.81 m/s²
gravity unit, gu = micrometer per second,
μm/s2
c.g.s., unit of gravity, milligal
mgal = 10-3 gal= 10-3 cms-2 = 10 gu-3

4. Gravitational Acceleration
The gravity method is based on two laws derived by Sir Isaac Newton,
which he described in Philosophiæ Naturalis Principia Mathematica
(July 1967):
• Universal Law of Gravitation
• Second Law of Motion
Units
standard gravity g = 9.80665 m/s² = 9.81 m/s²
gravity unit, gu = micrometer per second, μm/s²
c.g.s. unit of gravity, milligal =1 mgal = 10-3 gal= 10-3 cm m/s²

5. Gravity variations over Earth’s surface
Variations are due to:
(A) Variations in gravity with latitude
1. Shape of the Earth (gE < gP)
2. Variations in mass distribution (gE > gP)
3. Rotation of the Earth (gE < gP)
(B) Topography (elevation)
The Earth is not smooth
(C) Heterogeneities within the Earth (local geology)
1.Local variations in mass distribution
2.Observable changes in surface g
3.Material property is density

6. Igneous and metamorphic rocks
Granite ρ = 2.52 – 2.75 g/cm³
Basalt ρ = 2.70 – 3.20 g/cm³
Gneiss ρ = 2.61 – 2.99 g/cm³
Sedimentary rocks
Water ρ = 1.00 – 1.05 g/cm³
Clay ρ = 1.63 – 2.60 g/cm³
Shale ρ = 2.06 – 2.66 g/cm³
Limestone ρ = 2.50 – 2.80 g/cm³
Dolomite ρ = 2.28 – 2.90 g/cm³
Sandstone
Cretaceous ρ = 2.05 – 2.35 g/cm³
Triassic ρ = 2.25 – 2.30 g/cm³
Carboniferous ρ = 2.35 – 2.55/cm³

7. Gravity Exploration
Methodology
1. Planning of gravity survey.
2. Data Acquisition
a. Gravity measurement
b. Elevation determination
c. Establishment of base station
d. Profile layout
3. Data Processing (Reduction)
4. Interpretation
5. Gravity modeling

8. Gravity data Processing (reduction)
• What to do with the gravity data that you have
collected?
• Series of processing steps to remove unwanted signals
from the data (i.e., gravity variations that are not
caused by local density variations)
1.Temporal corrections
a. Tidal effects
b. Instrument drift
2.Spatial corrections
a. Latitude
b. Elevation

9. 1. Tidal effects – gravitational pull of sun and moon cause bulges in
the Earth’s surface
- ocean tides (water) and Earth tides (land)
-effects are well-understood and can be accurately corrected
2. Instrument drift – due to temperature-induced changes in the
spring and temporal changes in the elastic properties of the spring

10. Latitude correction
• The theoretical value of gravity with latitude is given by the
Geodetic Reference System for 1967 (GRS67) equation:
g(θ) = 978031.846(1+ 0.0053024sin2 θ − 0.0000058sin2 2θ)

11. Elevation corrections
• g depends on the distance from the
centre of the Earth (r):
Free air correction
F.A.C = 0.3086 Δh
Bouguer Correction
B.C = 0.00004193 ρ Δ h
Summary of Elevation Corrections
Site elevation F.A.C B.C
Below reference level Subtract Add
Above reference level Add Subtract

12. Free Air Anomaly Determination
• Free air anomaly is the difference between Observed gravity
and theoretical gravity after applying necessary corrections
on observed gravity.
F.A.A = (gobs ±D.C ±F.A.C) – Th. Value of “g”
Bouguer Anomaly Determination
• Bouguer Anomaly is the difference between observed gravity
and theoretical gravity after applying necessary corrections
on observed gravity.
B.A = (gobs ± DC ±F.A.C ± L.C ±T.C) – Th. Value of g

13. Interpretation
1. Bouguer anomaly map
2. Free Air Anomaly Map
Gravity modeling

15. Seismic Exploration
1. Planning of Seismic survey.
2. Data Acquisition
Seismic data are recorded in the field
a. on magnetic tape.
b. Cartridge
c. Now a days on CD’s
3. Data Processing (Reduction)
4. Interpretation
5. Seismic modeling

16. Acquisition involves many
different reciever 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.
Seismic acquisition

17. Acquiring data (CMP Gather)

18. Shotpoint # 1Hydrophone groups
#1#2#3#4#5#6
Separation between midpoints is
1/2 separation between hydrophone groups
Midpoints
CMP Method

19. Shot point # 2Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

20. Shot point # 3Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

21. Shot point # 4Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

22. Shot point # 5Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

23. Shot point # 6Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

24. Shot point # 7Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

25. Shot point # 8Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

26. Shot point # 8Hydrophone groups
#1#2#3#4#5#6
Midpoints
CMP Method

27. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shot point # 1
CMP Method

28. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shot point # 1
Shot point # 2
Shot point # 1
Shot point # 2
CMP Method

29. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shot point # 1
Shot point # 2
Shot point # 1
Shot point # 2
Shot point # 3
Shot point # 3
CMP Method

30. Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shot point # 1
Shot point # 2
Shot point # 1
Shot point # 2
Shot point # 3
Shot point # 3
Shot point # 4
Shot point # 4
CMP Method

31. Hydrophone groups
Midpoints
Shot points # 1-8
1
2
3
45 6 7 8 138
CMP Method

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

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

34. Fold
 The number of recorded channels
 The number of station intervals between geophones
groups and source points
 The nominal fold and stacking density of the resulting
CDP section
 The nominal fold of a given fold geometry can be
derived from the equation;
F=1/2 C . (dx)/(dg) . (dx)/(ds)
Where F = Fold
C = Number of recording channels
dx = Spacing between Stations
dg =Spacing between geophone groups
ds = Spacing between source points

35. Seismic data processing
Processing seismic data consists of
applying a sequence of computer programs,
each designed to achieve one step
along the path from field tape to record section.
Ten to twenty programs are
usually used in a processing sequence,
wisely selected from a library of several two hundred programs.
In each Processing Software library
there may be several programs designed to
produce the same effect by different approaches.
For example one
program may operate in time domain
while another works in frequency domain,
yet they may yield similar results.

36. Objectives
 To improve the signal to noise ratio
 Isolation of the wanted signals
 Reflections isolated from multiples and surface
waves
 To obtain a higher resolution
 by adapting the waveform of the signals
 To obtain a realistic image
 by geometrical correction
 To obtain information about the subsurface
 A near to Geological Section

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

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

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

41. • By definition, the
weathering for the
emerging wave path in
fig.
WC = - (dw)/(Vw) +
(dw)/(Vc)
Or
WC = - dw (VC-
Vw)/VwVc
Since Vc>Vw the
weathering correction is
always negative.

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

43. Elevation Correction (Datum Correction)

44. ECs = Ed – Es / Vc
ECr = Ed - Er / Vc

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

47. CDP Gather Hyperbola

48. 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.

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

50. The common
reflecting point on a
reflector, or the halfway
point when a wave
travels from a source to a
reflector to a receiver, is
shared by numerous
locations. Move out
corrections and stacking,
or summing of traces,
result in redundancy of
the data that improves
the signal-to-noise ratio.
Stacking

51. After Gather After NMO After Stacking

52. Filtering of Seismic Data
Seismic data, in general,
• Contain noise signals along with seismic reflection signals.
These noise signals interfere
• With the interpretation of the seismic signals
• Degrade the quality of the subsurface images
• That can be obtained by further processing.
It is, therefore,
• Very desirable to suppress the noise
• That is present in the recorded data
• Before processing it for imaging.

53. Types of Filters
10 HZ 125 HZ
50/60 HZ
50/60 HZ

54. Migration
Seismic migration is the process by which seismic
events are geometrically re-located in either space or
time to the location the event occurred in the
subsurface rather than the location that it was
recorded at the surface.
 Thereby creating a more accurate image of
the subsurface.
This process is necessary to overcome the limitations
of geophysical methods imposed by areas of complex
geology.
 Such as: fault, salt bodies, folding, etc.

55. Time Section Before Migration

56. Time Section After Migration