H:\EXCERC\GEOD98e.EX5.wpd



Department of Geophysics, Juliane Maries Vej 30, 2100 Copenhagen Ø.

Geodesy Course. Exercise 5.

The exercise has the purpose to show how a gravity instrument is used for a relative gravity measurement, and how the observations are treated afterwards.

Start in the Seismic-room in the basement, room 008. Here the first readings of the La Coste-Romberg gravimeter are done on the pillar. Remember to note down the station name (use 9999), time, and instrument reading. At least 3 readings must be made. (In practice also barometric pressure and temperature may be recorded).

After the first observation transport to Buddinge Batteri, where the gravimetric basic point of Denmark is located (see sketch). Every student must do at least one reading.

The latitude and longitude is determined by GPS. The altitude should be determined by levelling. Here we read the altitude from a 2 cm map.

Then we return to the Department of Geophysics, where the gravimeter is read again. (At least 3 readings).

The measurements are put into a file on a portable PC, see the format description in the program grredu.f. A file with the station coordinates must also be available.

The program grredu.exe is run on the PC. (See instructions in the program text). This program convert the readings to gravity units (mgal) and correct for tidal effects.

After this mean values are calculated for each station, and the gravity difference is calculated. From the known gravity in Buddinge (see sketch) we then can calculate gravity at the pillar in room 008. The observed gravity value is to be used in exercise 6.

Calculate the gravity anomaly by subtracting the normal gravity calculated in the altitudes read from the map. (See two formulas i Torge).