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Exploration Geophysics and Seismology Course Notes
Gravity
Gravity is defined as the force of attraction between two masses. Geophysicists can use
gravity measurements to help them determine the internal structure of the earth. Unlike
other exploration methods, the gravitational potential field already exists in and around the
earth. Thus, ideally, measurements can be made quickly and efficiently anywhere on, in or
above its surface. A large subsurface body of a different density from its surroundings,
having a density contrast, will attract a mass on the surface to a different extent than the
surrounding. Therefore, by analyizing the change in gravitational attraction along the
surface of the earth, geophysicists can detect subsurface features. As you might guess the
added or reduced attraction due to a subsurface body can be very slight, therefore the
instruments geopysicists use must be very sensitive. Gravity measurements alone are very
difficult to analyze, for there are many solutions to the observed surficial measurements. A
small body, with a low density constrast located near the surface can have the same effect
on surface gravity measurements as a large body of a larger density contrast located
deeper in the subsurface. Thus, to accurately analyze field measurements, other
exploration, other exploration methods are also used, including geological mapping of the
surface. Used in conjunction with other methods, gravity exploration becomes a very
valuable tool in studying the inner structures of the earth.
Fundamentals
Gravitational Potential and Acceleration
Two point masses m1 and m2 at a distance r apart are attracted to each other by a force
deduced by Sir Isaac Newton, and is known as Newtons First Law of Gravitational
Attraction :
where G is the universal gravitation constant with a value of:
The above equation assumes that the earth was a perfect shpere. If a ball and a feather are
thrown from the same elevation they will both have the same gravitational acceleration. The
earth exerts the same force on the objects. This concept was applied the moon's orbit of
the earth.
where M is the mass of the earth and R is the radius of the earth.
Newton's second law of motion (F=ma) is also applied. The acceleration due to gravity on
earth is defined as
g
.
. Solving for
g
:
1
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View Full DocumentGravity
can
be
expressed
as
a
potential
field:
The potential field is defined by:
For a point mass, the work done by the potential field moving a point mass from infinity to a
distance
R
is given by:
Gravity has units of m/s
2
. 1 g = 9.8 m/s
2
= 980 cm/s
2
. Changes in measured gravity are
often quite small. Measurements are most often given in mGal. 1 mGal = 10
3
g. Typical
resolution requirements for gravity surveys are 0.1
mgal
.
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 Spring '09
 ES

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