Specific aspects of above-ice gravimetric observations

Today, gravimetry is actively used in solving various detailed engineering problems, in researches of the underground fluid dynamics, etc. It is worth noting that the areas of large rivers and lakes are an interference in creating a regular network of observations, which is necessary to the above problems solution. For now the results of satellite, marine or aero surveys do not allow obtaining materials with the necessary resolution and accuracy parameters. The solution to this problem may be surveying in the winter period on frozen water reservoirs.

Gravimetric surveys were carried out on the surface of the ice covering the Ugra River as part of the field course of the Geological Faculty of Lomonosov Moscow State University in Kaluga region. Two high-precision relative gravimeters CG-5 Autograv by Scintrex Ltd were used.

During above-ice gravimetric observations, many factors influence the gravimeter. They can be divided into two groups: natural and human cause. The first group includes gusts of wind, melting ice, the flow of the river. The second group includes operator’s movements, interference of people passing by, etc.

The completed studies made it possible to evaluate the parameters of the standard deviation (SD) of the gravimeter records and the influence of the above factors on its tilt. An analysis of the observation results showed that high-precision relative gravimeters allow above-ice surveying with an accuracy of no worse than 5 μGal, which corresponds to the current level of ground gravity survey’s accuracy.

To minimize the influence of interfering factors, a special observation technique is required:

• For an independent assessment of the accuracy of observations at each point, several gravimeters should be applied;
• At each point, at least 10 measurements with one gravimeter should be performed;
• The time of one measurement should be at least 60 seconds;
• To minimize the influence of external factors on the measurements of gravimeters, it should be ensured that near the operator there are no outsiders, equipment, etc., creating oscillations of the surface which the device is installed on. At the same time, the operator of the gravimeter must constantly check the records of the gravimeter, the SD parameter and its levels.