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This is a Demo version of the e-course.
If you wish to take the e-course, you must acquire t he manual of Geofísica Práctica.



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GEOPHYSICAL NOTES- Introduction Course

2. GRAVIMETRIC METHODS
2,1 Important definitions.
* Gravimeter: Used sensor to measure the gravitational field strenght point to point.
It is common that these apparatuses give a relative quantification of g (for example, when comparing the elongación experienced by means of which it hangs a mass pattern).
* Gradiómetro of gravity: gravitational sensor that measures the gradient of the gravitational field instead of its absolute value.
According to its direction, gradiómetro can measure the gradient in X, and or Z (or combinations in diagonal).
In simple terms, the gradient corresponds to the fluctuation of the gravity by length unit.
* Spheroid: The normal spheroid corresponds to the form that would acquire our planet in the case of being a doughy mass submissive the simultaneous action of the force of gravity and the centrifugal force generated by the Earth spin.

2.2 TYPICAL GRAVITY SURVEY
A) Phase of Land.
To define a mesh and in each node to measure the acceleration of gravity.
B) Corrections.
i) It derives from the instrument: the gravimeter is not perfect, reason for which a mooring station is used to quantify the drift of the zero (it is assumed linear).
II) Complementary angle of site: a hill increases the acceleration of gravity, whereas a river basin makes diminish. All the data must take to flat topography
III) Other corrections: by latitude, of Faye, Bouguer, etc.
C) Final map: isoanómalas of gravity.
The resulting map shows the variations undergone by the acceleration of gravity like exclusive result of the different densities from rocks.
It can be advisable to make a statistical analysis of the gravity and to show to the deviations respect to background (“residual gravity”).

 


Anomalies of gravity according to GRACE.

2.3 Interpretation
The interpretation of the topographical datas along with makes use of the integral expression of the Law of Universal Gravitation the Gravitational Law of Gaussian:

For example, the Gravitational Law of Gaussian can be used to compare the topographical datas with the gravity generated by a flat distribution of mass of thickness h and density uniforms r, when the measurements are made to a distance r << h. In this case, as the system is equivalent to an infinite plane, we will have g (r) will be constant, analogous to the result that appears when using the Electrical Law of Gaussian for a condenser of parallel flat plates.

In case of being in a land with flat topography, we will have any anomaly of gravity will have to deviations of the density of the subsoil respect to the “density bases”. In other words, the gravitational anomalies are originated by variations in the distribution of the mass density point to point (3D). On the other hand, if the topography is complex, the gravity anomalies could be related or to the geometry of the land and/or the distribution of densities, since g = g (density, position).




QUESTIONNAIRE - Gravimetric Prospection

1) Select the correct alternative.
The free air correction

a) It reduces the value of the original data if these were moderate on the level of the sea.
b) Differential g is derived from the expression (R+h) = g (R) + dg (h)
 c) It is made discretizando the surface and seeing the effect of the topography on each compartment.

2) Alternative selects the two correct.
On the gravitational tension, we can affirm that:

a) It has nine independent components linearly
 b) gij is equal to gji
 c)
d) It has eight independent components linearly

3) Select the correct alternative.
A simple pendulum

a) It allows to measure the rate of space change of the gravity
 b) It allows to make qualitative measurements of the gravity
c) It allows to measure the absolute gravity

4) Select the correct alternative.
The gravity generated by the plateau (configuration of flat mass of density average r and thickness h) can be modeled in agreement with:

a) 2 p G r h
 b) 4 p G r h
 c) Another expression

5) Select the correct alternative.
On the acceleration of superficial gravity of the moon (mass of 7*1022 Kg and radio of 1.7*106 m) we can affirm that:

a) is a value superior to the 2 m/s2
 b) is equal to 0.78 m/s2
 c) is equal to 1.6 m/s2



This is a Demo version of the e-course.
If it wishes to make e-course it must acquire the manual of Geofísica Práctica.


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