Gravity and magnetic surveying are non-destructive geophysical techniques. Gravity surveying measures variations in the gravitational field to infer subsurface density variations, while magnetic surveying measures variations in the magnetic field to detect magnetic materials below the surface. Both methods work by measuring spatial anomalies - deviations from a predefined reference level for the gravitational or magnetic field. These anomalies are interpreted to determine properties of subsurface rock layers and structures.

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