Gravity and magnetic methods are an essential part of oil exploration. They do not replace seismic. Rather, they add to it. Despite being comparatively low-resolution, they have some very big advantages. These geophysical methods passively measure natural variations in the earth’s gravity and magnetic fields over a map area and then try to relate these variations to geologic features in the subsurface. Lacking a controlled source, such surveys are usually environmentally unobjectionable.

1. Methods of PETROLEUM exploration
Topic :-
Gravity survey
Presented By-
Manjil Nath (500065767)
M.Tech. Petroleum Engineering
University of Petroleum and
Energy Studies

2. Gravity
• The natural force that tends to cause physical things to move towards each
other the force that causes things to fall.
• Differences in rock density produce small changes in the Earth’s gravity field
that can be measured using portable instruments known as gravity meters or
gravimeters.
• Gravity method are uses in:
i. Petroleum and Mineral Prospecting
ii. Seismology
iii. Geodesy
iv. Metrology

3. Theories
• The gravity method is based on two laws derived by Sir Isaac Newton,
a. Universal Law of Gravitation
b. Second Law of Motion
• Units
standard gravity ɡ0 or ɡn = 9.80665m/s² ≈ 9.81m/s²
gravity unit, gu = micrometer per second sq., μms-2
c.g.s. unit of gravity, milligal = 1mgal = 10-3gal = 10-3 cms-2
= 10 gu-3
• Geoid is the equipotential surface that would coincide with the mean ocean
surface of the Earth if the oceans and atmosphere were in equilibrium, at rest
relative to the rotating Earth and extended through the continents.
• Reference ellipsoid is the idealized geometrical representation of the Earth.

4. • Gravity anomaly is the difference between gravity measured at a point and a
model value at that point that is based on the normal gravity of a reference
ellipsoid.
+ gravity anomaly = Geoid surface is higher than reference ellipsoid
- gravity anomaly = Geoid surface is lower than reference ellipsoid

5. Concepts
Densities of Sedimentary Rocks
• Sedimentary rocks exhibit the greatest range of density variation due to factors
such as:
Mineral composition, Cementation, Porosity, Pore fluid type
• Typically the contrast between adjacent sedimentary layers is less than
0.25 Mg m-3
• Density is increased by depth of burial:
Sandstones and Limestone : Density is increased by infilling of the pore space,
not by volume change.
Shales : Density increased by compaction, and ultimately recrystallization into
minerals with higher densities.

6. Densities of Igneous Rocks
• Igneous rocks tend to be denser than sedimentary rocks, with the density
controlled primarily by silica content:
Mafic rocks are thus more dense than felsic.
Ultramafic rocks are most dense.
• The range of density variation tends to be less than in sediments as porosities
are typically lower.
Densities of Metamorphic Rocks
• The densities of metamorphic rocks tends to increase with decreasing acidity
and with increasing grade of metamorphism.
• However, variations in density within metamorphic rocks are far more erratic
and can vary considerably over short distances.

7. Gravity Survey-
• Measurements of the gravitational field at a series of different locations over an
area of interest. The objective in exploration work is to associate variations with
differences in the distribution of densities and hence rock types.
• The primary goal of studying detailed gravity data is to provide a better
understanding of the subsurface geology.
• The gravity method is a relatively cheap, non - invasive, non - destructive
method.
• It is also passive – that is, no energy need be put into the ground in order to
acquire data; thus, the method is well suited to a populated setting such as urban
areas and a remote setting such as Mars and the Moon.

8. Materials
Gravity- measuring equipment -
• Falling bodies :- Directly computing the acceleration of a body undergoing
free-fall drop by carefully measuring distance and time as the body falls;
absolute
• Pendulum :- The gravitational acceleration is estimated by measuring the
period oscillation of a pendulum; absolute, relative
• Gravimeters :- They are basically spring balances carrying a constant mass.
Variations in the weight of the mass caused by variations in gravity cause the
length of the spring to vary and give a measure of the change in gravity;
absolute, relative.
Positioning equipment -
• Theodolites, GPS receivers

9. Relative gravimeters –
• A gravimeter that expresses measured g as the difference in g between two sites.
a. Survey: Measurement of relative gravity as a function of position on, or
under, the surface of the earth or on the sea floor.
b. Stationary: Continuous measurement of changes in gravity or low-
frequency earth motion as a function of time at a fixed location on, or
under, the surface of the earth or on the sea floor.
c. Dynamic: Measurement of relative gravity as a function of position from a
moving platform such as an aircraft or a ship.
Absolute gravimeters –
• An instrument used to measure gravity absolutely, traceable to time and length
standards.

10. Advancements
Exploration Gravity Gradiometer -
• utilizes the concept of superconductivity and operates at 4 degrees above
absolute zero (-269°C), which allows greater sensitivity and stability.
Integrated quantum sensors -
• Uses ultra cold atoms.
• Can be used in making very compact, highly sensitive gravimeter.

11. Limitations
• A sufficient density contrast between the background conditions and the feature
being mapped must exist for the feature to be detected.
• Some significant geologic or hydro geologic boundaries may have no field -
measurable density contrast across them, and consequently cannot be detected
with this technique.
• Ambiguity of the interpretation of the anomalies almost always present.
Accurate determination usually requires outside geophysical or geological
information.
• Each station has to be precisely surveyed for elevation and latitude control. This
could be costly and time consuming, especially in surveys covering large areas

12. Thank
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