Geophysical field methods

Anomaly of gravitational force

Anomaly of gravitational force in gravimetric prospecting is called the difference between observed (measured) and theoretical (normal) gravitational force values.

(1)

where γ₀ and g- normal and observed gravitational force values

The observed value of gravitational force includes normal field and field created by non-homogeneous masses which are located inside the Earth. When we subtract normal field from observed one, we receive anomalies o gravitational force, which reflect the distribution of density heterogeneities inside the Earth, that is, include the information about its inner structure.

Measurement method of gravitational force.

Measurement method of gravitational force is subdivided into dynamic and static. In dynamic method, we observe the body movement under the effect of gravitational force. The proximate measured value in there is period of oscillation of the physical pendulum, or time of free acceleration fall, or frequency of the oscillation of string and so on. In static method, we observe the change of body equilibrium state under the effect of gravitational force and some balancing it force. The proximate measured value in there is linear or angular displacement of the constant mass.

The equipment for measuring the gravitational force is gravimeters. Gravimeters, in which dynamic principles of gravitational force measurement are used, are called dynamic, however gravimeters in which static principles are used, are called static. Determination of gravity force is done by absolute and relative methods. At absolute determination, the module of overall value of gravity force g is measured in the observational point. At relative determination the increment of gravity force ∆g = g₂ – g₁ is measured in two observational points. The dynamic methods of gravitational force measurement can be both absolute and relative, while static is only relative. In gravimetric prospecting all field measurements (gravimetric survey) are done in the form of relative determination of gravitational force. Absolute determination, necessary for calculation of the increment of gravity force in its complete values, is done only in the stationary geophysical observatories.

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  There are the following gravimeter types:
  

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  AQG - astatized quartz gravimeter (high sensitive), model 7T (accuracy 0.15 mGal). Astatized - artificial increase of sensitivity.
  
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  GNG – ground gravimeters, narrow-band quartz, model C(B) (accuracy 0.03-0.06 mGal).
  

Gravimeter consists of the following main parts:

1. 
   Sensitive element (pendulum, hung by the horizontal pendant). Pendulum is sustained by the main spring, which is affixed to the body. If g increases, pendulum is deviating and stretches the spring;
   
2. 
   measuring part consists of measuring and band spring and micrometer screw. Band spring serves for changing the instrument range. Measuring spring and micrometer screw serve for reading out. These springs are connected to the pendulum;
   
3. 
   temperature compensator compensates the errors of the instrument, which affects by the medium temperature. The amendments of the instrument indication in the similar measured points with the time are called “point zero” displacement or ”point zero” slide. All springs and pendant strings are produced from quartz, since quartz has the favorable elastic properties different from metals, and those elastic properties is less dependent on the medium temperature.
   
4. 
   Optic system is used for enlightening the scale and for transferring the mechanical displacement of the pendulum in the form of transmitting the light flare on the instrument scale. The optic system consists of the following parts: enlightening lamp, condenser, reflected mirrors, rotating prism, microscope and ocular.