Gravimeter

An Autograv CG-5 gravimeter being operated

A gravimeter is an instrument used in Gravimetry for measuring the local gravitational field of the Earth. A gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth. Though the essential principle of design is the same as in other accelerometers, gravimeters are typically designed to be much more sensitive in order to measure very tiny fractional changes within the Earth's gravity of 1 g, caused by nearby geologic structures or the shape of the Earth. Gravimeters display their measurements in units of gals (cm/s²), instead of more common units of acceleration.

Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and other geophysical research, and for metrology.

There are two types of gravimeters: relative and absolute. Absolute gravimeters measure the local gravity in absolute units, gals. Relative gravimeters compare the value of gravity at one point with another. They must be calibrated at a location where the gravity is known accurately, and then transported to the location where the gravity is to be measured. They measure the ratio of the gravity at the two points.

Memorize the words:

vary – мінятися, змінюватися though – однак, проте essential – істотний design – конструкція, проектувати sensitive – чутливий tiny – крихітний fractional – дробовий, частковий relative – відносний, порівняльний compare – порівнювати value – значення, величина calibrate – калібрувати, градуювати accurately – точно location – місце ratio – пропорція, співвідношення

Answer the questions:

1. What is a gravimeter?

2. How do gravimeters differ from other accelerometers?

3. What are changes in the Earth’s gravity caused by?

4. In what units do gravimeters display their measurements?

5. For what purposes are gravimeters used?

6. What are the two types of gravimeters?

7. What do absolute gravimeters measure?

8. What must be done before using a relative gravimeter?

9. What do relative gravimeters measure?

Absolute gravimeters

Absolute gravimeters, which nowadays are made compact so they can be used in the field, work by directly measuring the acceleration of a mass during free fall in a vacuum, when the accelerometer is rigidly attached to the ground.

The mass includes a retroreflector and terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the acceleration of the mass can be measured. A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion. This allows cancellation of some measurement errors; however "rise and fall" gravimeters are not in common use. Absolute gravimeters are used in the calibration of relative gravimeters, surveying for gravity anomalies (voids).

Memorize the words:

rigidly – жорстко attach – прикріпляти retro- – зворотної дії reflector – рефлектор, відбивач terminate – завершувати time – встановлювати тривалість, визначати час interference fringe – інтерференційна смужка toss – підкидати cancel – скасовувати void – порожнина

Answer the questions:

1. What does the mass used in absolute gravimeters include?

2. How is the acceleration of the mass measured?

3. How does the "rise and fall" version of gravimeter work?

4. What are absolute gravimeters used for?