Gravimetry

 

GRAVITY

Gravity is attractive force acting between material bodies.

Newton - gravitačná sila

Gravitácia

Terrestrial gravimetric survey

Measurement of gravity defines the anomalous density inside the earth. The most common used terrestrial gravimetric survey is for accurate measurement of variations of the gravitational field of the earth at different points.
Gravity anomalies are calculated as the difference between the regional and the measured field and the result is correlated with the density variations in the Earth’s gravitational field.
Positive gravimetric anomalies are associated with high density bodies and negative anomalies with low density bodies.
For example, deposits of high-density chromite, hematite, and barium, and bearings with low density halite, kimberlite.
Gravimetric methods provide the opportunity of estimate the quantity (tonnage), which corresponds to the anomaly.

 

In gravimetric survey are interesting variations of gravity on the Earth’s surface, which are used to map variations in the density of rocks.For these small variations are used gravimetric units called miligals or m/s2, where 1 miligal = 10-3 Gals = 10-5 m/s2 and 1 Gal = 10-2 m/s2.

Družice GPS

Meranie gravimetrie v Bolívii

 

Gravimetry and GPS measurements

Gravimetric and GPS points are usually implemented in network measurement. Network density depends on the objectives of the survey. The measurements are usually performed in a daily loop, closed at the appropriate base stations with a maximum error of 0:05 mGal. Gravitative drift and corrections are performed every day.
The loop consists of sequences of gravimetric measurements, which begin and end at the gravimetric checkpoint.
The height of the device is also measured at each measuring point with high accuracy.
A certain amount of gravimetric and GPS points (around 5%) are repeated in order to secure the accuracy of the measurements. Gravimetric measurements usually consist of several independent measurements.
Data collection is stopped in case of seismic events, in bad weather conditions, the wind, which produce microseismic noise.

Gravimeter

 

Meranie gravimetrie v Indii

 

Quality control and initial processing of data

 

Quality control consists of the following basic procedures:

• Quality control of the accuracy of gravimetric data
• Quality control of the localization of measurements
• Quality control of GPS
• Quality control of Bouguer Anomaly

 

Gravimetric measurement accuracy is ensured by repeated measurements at selected points in the network.

 

Absolute gravity

ga = gB1 + (rH – rB1) (t – tB1)d

ga absolute gravity in miligal                  rB1 measurement on the basis of 1
gB1 base 1 absolute G in miligal               t measurement time
rh correction about the hight of the instrument    tB1 base 1 time measurement
d drift in miligal / hour

Kontrola uplnosti dát

Kontrola kvality dát a ich spracovanie

Final processing, interpretation, report

 The final processing is carried out by the following steps:

• Import of measured data to the database
• Application of basic corrections
• Assign the spatial coordinates
• Gridding and contouring
• Corrections:
Repair of the device operation
The tidal correction
Height of the instrument
Drift correction
Correction of the absolute gravity
Correction of the latitude
Free-air correction
The Bouguer correction
Terrain correction

 

Final Products:

Location of the gravimetric points (ST)
Digital model of the relief (ELEV)
Free air map of the anomaly (FA)
Bouguer map of the anomaly (BA)
Correction to the terrain (TC)
Map of the complete Bouguer anomaly (CBA)
Map of the first vertical derivation (VD), calculated from CBA

3D model rozloženia hmôt - Popradská kotlina

3D model of mass – Poprad basin

Úplna Bouguerova anomália - oblasť Myjava

Map of the complete Bouguer anomaly