Calculation of Building Correction for urban gravity surveys. A case study of Athens metropolis (Greece)

In gravity surveys, many unwanted effects are produced by geological or non-geological sources. These calculable effects have to be removed through the data reduction procedure. Common corrections in gravity measurements are those for the instrument drift, the tide, the Free Air, the Bouguer and the terrain effect. However, when we deal with gravity campaigns carried out in cities, we also have to take into consideration the so-called Building Correction. This concerns the correction of the gravitational effect caused by the existence of buildings and anthropogenic constructions (stadiums, bridges etc.) close to a gravity measurement. This process can become quite demanding sometimes. Because of that, in this paper we discuss a calculation method for the Building Correction of the gravity measurements. Two types of data are crucial in that procedure. The first one is the mean building density and the other one is the volume of the existing buildings, which is related to the spatial distribution and the buildings height. The mean building density has been calculated in this paper, based on percentage contribution of the building materials (concrete, bricks etc.) of the whole building volume. The calculated mean building density was equal to 0.44 g/cm3. A Building Height Map has been produced, based on the Digital Elevation Model and the Digital Surface Model. Taking into account the building volume and their density, a simulation of the terrain correction procedure has been carried out, for the Building Correction calculation. The calculated Building Correction values range from almost zero (in the suburbs) to 0.25 mGal. A comparison for the Residual Anomaly values (affected by the Building Correction) has also been made. Differences up to 0.19 mGal revealed are considered to be quite significant for the credibility of the final data.