Lecture note on Geology

1. Lecture#10
GeophysicalExplorationMethods
PME 151
Geology for Petroleum And Mining Engineers
L-1,T-1 (20 Batch)
Wednesday, July 26, 2023

2. Gravity Surveying
Gravity method is a non-destructive geophysical technique
that measures the differences in Earth’s gravitational field at
specific locations, relatively cheap, non-invasive, remote
sensing method.
Principle-
 In gravity surveying, subsurface geology is investigated on
the basis of variations in the Earth’s gravitational field arising
from differences of density between subsurface rocks.
 It measures the variation in acceleration due to gravity.

3. Methodology
According to the Newton’s law of gravitation –
𝐹 =
𝐺𝑀𝑚
𝑑2
Here, F= Gravitational force between two point masses
M and m.
d= Distance between M and m.
G=Universal gravitational constant
=6.670 x 10−11 m3/kgs2

4. According to Newton’s second law of motion-
𝑎 =
𝐹
𝑀
=
𝐺𝑚
𝑟2
If M is considered as the mass of the Earth and r its radius,
then a is the gravitational acceleration of the Earth.
a is constant assuming Earth a true sphere of uniform
density.
a varies from place to place across the Earth, due to the
effect of latitude, altitude, topography and geology.

5. Procedure-
• This method measures minute variations in the Earth’s
gravity field. Based on this variation Earth’s subsurface field
and composition can be inferred.
• These variations can be determined by measuring the Earth’s
gravity field at numerous station along travers.
• Correlating the gravity data for elevation, tidal effects,
topography, latitude and instrument drift.

6. • Gravity field in not uniform everywhere in the earth’s
surface, it varies with the distribution of the mass materials
below
• A gravity survey means calculating the density property of
subsurface materials.
• The higher the gravity value, the denser the rock beneath.
• Modern instrument capable of rapid gravity measurements
are known as gravity meters or gravimeters.

7. Gravimeter-
• Spring balances carrying a
constant mass . Variation in
the weight of mass caused
by variation in gravity cause
the length of the spring to
vary and give a change in
the gravity.
• Relative gravimeter.
• Absolute gravimeter.

8. Gravity anomalies-
 An underlying concept is the idea of a causative body,
which is a rock unit of different density from its
surroundings.
 A causative body represents a subsurface zone of
anomalous mass and causes a localized fluctuation in the
gravitational field known as a gravity anomaly i.e.
Deviation from e predefined reference level, Geoid
(Surface over which the gravitational field has equal
value).

9. Gravity reduction-
Before interpreting the gravity surveys, it is necessary to
correct all the variations which do not cause due to the
variation of subsurface rock density.
 Drift correction
 Latitude correction
 Elevation correction
 Tidal correction

10.  Applications of Gravity methods-
i. Determine shape of the Earth
ii. Hydrocarbon exploration
iii.Regional geological studies
iv.Iso-static compensation determination
v. Detection of subsurface cavities
vi.Locations of buried rock valleys
vii.Determination of glacier thickness
viii.Tidal oscillations
ix.Basin geometry

11. Magnetic Surveying
 It aims at investigating Earth’s subsurface geology by
measuring the strength or magnitude of the Earth’s
magnetic field.
 Lateral variation in magnetic susceptibility and remanence
give rise to spatial variations in the magnetic field.
 It is expressed in magnetic anomalies i.e. deviations from
the Earth’s magnetic field.
 Unit of measurement is nanotesla ; I nT= 10−9 Tesla.
 Natural magnetic elements- iron, cobalt, nickel etc.

12. Magnetic susceptibility-
• Dimensionless property,
measures of how
susceptible a material is to
becoming magnetized.

13. The Earth possesses a magnetic
field caused primarily by
sources in the core.
Earth is a big magnet!!

14. Earth’s magnetic field-
Magnetic anomalies occur in local
field form magnetic rock below the
surface (Similar to gravity
anomaly).
 Magnetic materials below the
surface adds magnetism and
create a positive anomaly.
 Magnetic rocks include iron.
Grabbro , granite.

15. Removal magnetic materials
from near surface causes
negative anomaly.
Normal faulting.

17. Methodology
The Earth generates a magnetic field as if it were a dipole
magnet. Lines of force radiates from one magnetic pole and
converge at the other. The magnetic axis of the Earth moves
about within a circle of some 10-15 degrees of arc form the
rotational axis.
The force between two magnetic pole may be expressed as
follows
𝐹 =
𝐴𝑀𝑚
𝑑2
Here, F= Force; M and m strengths of the poles; D is the
distance between the poles.

18. Magnetometer
A magnetometer is a device that measures magnetic field or
magnetic dipole moment. Some magnetometers measure the
direction, strength, or relative change of a magnetic field at a
particular location.
 Disturbances in the Earth’s magnetic field are recorded by
the magnetometer.
 Data is processed by computer software to construct a
colored map.

19. Applications of Magnetic surveying-
i. Basin thickness estimation
ii. Determining fault types
iii. Locating mining prospect
iv. Finding buried elements
v. Evaluating ocean-floor spreading
vi. Mineral exploration