gravity_notes1 - Exploration Geophysics and Seismology...

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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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Gravity 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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gravity_notes1 - Exploration Geophysics and Seismology...

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